Scholarly article on topic 'Alternate wetting and moderate drying increases rice yield and reduces methane emission in paddy field with wheat straw residue incorporation'

Alternate wetting and moderate drying increases rice yield and reduces methane emission in paddy field with wheat straw residue incorporation Academic research paper on "Agriculture, forestry, and fisheries"

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Academic research paper on topic "Alternate wetting and moderate drying increases rice yield and reduces methane emission in paddy field with wheat straw residue incorporation"

ORIGINAL RESEARCH

Alternate wetting and moderate drying increases rice yield and reduces methane emission in paddy field with wheat straw residue incorporation

Guang Chu1, Zhiqin Wang1, Hao Zhang1, Lijun Liu1, Jianchang Yang1 & Jianhua Zhang2

1Jiangsu Key Laboratory of Crop Genetics and Physiology/Co-Innovation Center for Modern Production Technology of Grain Crops, Yangzhou University, Yangzhou, Jiangsu, China

2School of Life Sciences and State Key Laboratory of Agrobiotechnology, The Chinese University of Hong Kong, Hong Kong, China

Keywords

Alternate wetting and drying, grain yield, methane, nitrous oxide, rice (Oryza sativa), wheat straw

Correspondence

Jianchang Yang, Jiangsu Key Laboratory of Crop Genetics and Physiology/Co-Innovation Center for Modern Production Technology of Grain Crops, Yangzhou University, Yangzhou, Jiangsu, China. Tel: +86 514 87979317 Fax: +86 514 87324276; E-mail: jcyang@yzu. edu.cn

Funding Information

We are grateful for grants from the National Natural Science Foundation of China (31461143015; 31271641, 31471438, 91317307), the National Key Technology Support Program of China (2011BAD16B14; 2012BAD04B08; 2014AA10A605), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Hong Kong Research Grant Council (AoE/M-05/12) and Shenzhen Overseas Talents Innovation & Entrepreneurship Funding Scheme (The Peacock Scheme).

Received: 19 March 2015; Revised: 7 May 2015; Accepted: 22 July 2015

Food and Energy Security 2015 4(3): 238-254

Abstract

Wheat residue incorporation into the rice paddy field is becoming a popular practice in rice production in China's main rice-growing area but risks an increased emission of greenhouse gases. This study investigated if an alternate wetting and moderate drying (AWMD) irrigation regime in rice production reduces CH4 emission and increases grain yield when wheat straw residues are incorporated into rice paddy field. One super rice variety was field-grown in 2012 and 2013 and subjected to four irrigation and straw incorporation treatments: continuously flooded (CF) without straw incorporation (-S), AWMD without straw incorporation (AWMD-S), then CF with straw incorporation (CF + S) and AWMD + S. When compared with the CF, the AWMD regime increased grain yield and water use efficiency (WUE, grain yield over the amount of water used) by 2.7% and 27.6%, respectively, under -S, and by 18.0 and 50.0%, respectively under +S. The AWMD + S treatment also significantly increased nitrogen use efficiency (NUE) compared with the CF + S treatment. The increase in grain yield, WUE and NUE in the AWMD regime, especially under +S, were attributed mainly to a greater root oxidation activity, deeper root distribution and increases in productive tillers, crop growth rate and non-structural carbohydrate remobilization during grain filling. There was a total of 0.49 kg N2O-N ha-1 more loss in the AWMD than in the CF regime. However, the AWMD regime substantially decreased seasonal CH4 emissions, global warming potential (GWP, including both CH4 and N2O) and greenhouse gas intensity (grain yield over GWP) by 49.8%, 45.2% and 46.7%, respectively, under -S, and by 57.5, 55.9% and 62.6%, respectively, under +S, when compared with the CF regime. The results demonstrate that the AWMD is an effective practice to increase grain yield and resource-use efficiency and reduce environmental risks especially, when wheat straw is incorporated into paddy field.

doi: 10.1002/fes3.66

Introduction

Global agriculture in the 21st century faces the tremendous challenge of providing enough food for a growing population under increasing scarcity of water resources, while minimizing environmental consequences (Bouman 2007; Linquist et al. 2015). Rice (Oryza sativa L.) is one of the

most important food crops in the world and consumed by more than 3 billion people (Fageria 2007). It is estimated that, by the year 2025, it will be necessary to produce about 60% more rice than what is currently produced to meet the food needs (Fageria 2007; GRiSP 2013). About 75% of total rice production comes from irrigated lowlands (Maclean et al. 2002; GRiSP 2013).

© 2015 The Authors. Food and Energy Security published by John Wiley & Sons Ltd. and the Association of Applied Biologists. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Irrigated rice accounts for about 80% of the total fresh water resources used for irrigation in Asia (Bouman and Tuong 2001). Fresh water for irrigation, however, is becoming increasingly scarce because of population growth, increasing urban and industrial development, and the decreasing availability resulting from pollution and resource depletion (Belder et al. 2005; Bouman 2007). On the other hand, rice fields have been identified as an important source of atmospheric methane (CH4), one of the major potent greenhouse gases (GHG), and contribute approximately 15-20% of the global total anthropogenic CH4 emission (Aulakh et al. 2001; Yan et al. 2005). Several recent experiments have shown that nitrous oxide (N2O), another potent GHG, could be emitted from rice fields, which may be attributed to the combined effect of nitrogen (N) fertilization and water management (Zou et al. 2007; Shan and Yan 2013; Li et al. 2014). It would have great significance to develop technologies and practices for reducing GHG emissions and water use while increasing grain yield in rice.

China is one of the largest rice producing countries, accounting for 18.6% of the world rice harvested area and 30% of total world production of rice grain (GRiSP 2013). The rice-wheat rotations, with the acreage of 13 million hectares, are a major cropping system in the Yangtze River Valley in this country (Ma et al. 2009). Crop production inevitably results in large amounts of straw residues. Farmers usually burn wheat residues particularly when they want to establish rice crop rapidly while labor is limited. This leads to loss of most organic carbon and large losses (up to 80%) of N (Raison 1979), 25% of phosphorus (P), and 21% of potassium (K) (Ponnamperuma 1984) as well as significant air pollution and death of beneficial soil fauna and microorganisms. Therefore, incorporating crop residues into the field has currently been highly recommended in China as a measure to promote organic matter recycling and environmental friendly, sustainable agricultural production (Yao et al. 2013). However, this measure undoubtedly provides the readily available carbon and N substrate, inducing greater CH4 release from rice paddy fields and also influencing N2O emissions (Ma et al. 2009; Wang et al. 2010; Shan and Yan 2013; Yao et al. 2013; Li et al. 2014). Ways must be sought to reduce CH4 emissions when the crop residues are incorporated into rice fields.

To counter water shortage and increase water use efficiency (WUE), alternate wetting and drying (AWD) irrigation in rice has been developed as a novel water-saving technique and adopted in many countries such as China, Bangladesh, India, and Vietnam (Bouman and Tuong 2001; Belder et al. 2004; Yang et al. 2007; Zhang et al. 2008; Yao et al. 2012; Liu et al. 2013). This technique could substantially reduce irrigation water by introducing

alternation of periods of soil submergence with periods of nonsubmergence during the growing season (Belder et al. 2004). It is reported that AWD practices could reduce both water use and GHG emissions without seriously sacrificing grain yield in rice systems (Linquist et al. 2015). Our earlier work (Yang et al. 2007; Zhang et al. 2009, 2010, 2012; Liu et al. 2013) has shown that an alternate wetting and moderate drying (AWMD) regime could not only save water but also maintain or even increase rice yield. However, little is known whether an AWMD regime could substantially reduce GHG emissions meanwhile markedly increase grain yield and WUE under the condition of wheat straw incorporation into paddy fields.

The objective of this study was to test the hypothesis that an AWMD regime may decrease global warming potentials (GWP) through reducing CH4 emissions and increase grain yield through enhancing shoot and root growth when wheat straw is incorporated into paddy fields. Both CH4 and N2O emissions, WUE and N use efficiency (NUE) were determined. Tiller number, root and shoot biomasses, root oxidation activity (ROA), leaf photosyn-thetic rate, crop growth rate (CGR), and nonstructural carbohydrate (NSC) remobilization were investigated to understand the biological process in which water and straw management affects rice growth.

Materials and Methods

Plant materials and growth conditions

The experiment was conducted at a research farm of Yangzhou University, Jiangsu Province, China (32o30'N, 119o25'E, 21 m altitude) during the rice growing season (May to October) of 2012, and repeated in 2013. The soil was a sandy loam (Typic fluvaquents, Etisols [U.S. taxonomy]) with 24.3 g kg-1 organic matter, 101 mg kg-1 alkali hydrolysable N, 34.5 mg kg-1 Olsen-P, and 65.6 mg kg-1 exchangeable K. The field capacity soil moisture content was 0.188 g g-1, and bulk density of the soil was 1.33 g cm-3. The average air temperature, precipitation, and sunshine hours during the rice growing season across the two study years measured at a weather station close to the experimental site are shown in Fig. S1.

A "super" rice (Oryza sativa. L) variety Yangjing 4038 (japonica), currently used in local production, was grown in the field. Seedlings were raised in the field with sowing date on 15 May and transplanted on 10 June at a hill spacing of 0.16 m x 0.25 m with two seedlings per hill. In both years, N (60 kg ha-1 as urea), P (30 kg ha-1 as single superphosphate) and K (40 kg ha-1 as KCl) were applied and incorporated just before transplanting. Nitrogen as urea was also applied at early tillering (8 days

after transplanting (DAT) (40 kg ha-1), panicle initiation (45 DAT, 50 kg ha-1) and the initial of spikelet differentiation (62 DAT, 50 kg ha-1). The total N application was 200 kg ha-1 which is within the recommended range. The variety (50% of plants) headed on 27-28 August, and was harvested on 15-16 October.

Treatments

The experiment was laid out in a complete randomized block design with four replicates. Plot dimensions were 6.4 m x 5 m and plots were separated by an alley 1 m wide with plastic film inserted into the soil to a depth of 50 cm to form a barrier. Treatments consisted of four water and straw management combinations including continuously flooded (CF) without wheat straw incorporation (-S) (CF-S), alternate wetting and moderate drying (AWMD) without wheat straw incorporation (AWMD-S), CF with wheat straw incorporation (+S) (CF + S), and AWMD with wheat straw incorporation (AWMD + S). In the +S plots, wheat straw from the preceding season was chopped to approximately 10 cm in length and incorporated freshly to a soil depth of 0-15 cm during the tillage, prior to rice transplantation. The amount of the wheat straw incorporated was approximately 6500 kg ha-1 in dry weight containing 36 kg N ha-1, 6.1 kg P ha-1 and 72 kg K ha-1, on average. Wheat straw was removed from the -S plots. Except drainage at the mid season, the field was continuously flooded with 2-3 cm water level until 1 week before harvest in CF regimes. In AWMD regimes, plots were kept a 2-3 cm water level during the first 12 days after transplanting (DAT), an thesis, and the timing for N top dressing. At other growth stages, fields were not irrigated until the soil water potential reached -15 kPa (soil moisture content 0.167 g g-1) at 15-20 cm depth. Soil water potential of -15 kPa in the AWMD regime was chosen as our earlier work (Yang et al. 2007) has shown that a mild soil drying (soil water potential -15 kPa at 15-20 cm depth) during the growing season could not reduce grain yield when compared the CF regime. Soil water potential was monitored at 15-20 cm soil depth with a tensiometer consisting of a sensor of 5 cm length (Soil Science Research Institute, Nanjing, China). Four tensiometers were installed in each plot, and readings were recorded at 1200 h each day. When soil water potential reached the threshold, a flood with 2.0-3.0 cm water depth was applied to the plots. The amount of irrigation water was monitored with a flow meter (LXSG-50 Flow meter, Shanghai Water Meter Manufacturing Factory, Shanghai, China) installed in the irrigation pipelines. Both irrigation and drainage systems were built between blocks. Each plot was irrigated or drained independently.

Soil redox potential, leaf water potential, and photosynthetic rate measurement

Soil redox potential (Eh) was measured at 10, 19, 39, 55,

78, 90, 104, and 127 DAT in 2012 and at 10, 20, 40, 56,

79, 91, 105, and 128 DAT in 2013. The growth stages corresponding above dates were early tillering, mid tillering, late tillering, the initial of spikelet differentiation, heading time, early grain filling, mid grain filling, and maturity, respectively. Soil Eh was monitored at 10 cm soil depth by using Pt-tipped electrodes (Hirose Rika Co. Ltd. Niwa-Gun, Aichi, Japan) and an oxidation-reduction potential meter with a reference electrode (Toa PRN-41), with four replications for each treatment.

Leaf water potentials of flag leaves were measured at 2-h intervals from 0600 h to 1800 h at 72 and 73 DAT when days were clear and soil water potential was approximately -15 kPa in the AWMD regime (Fig. S2). A pressure chamber (Model 3000, Soil Moisture Equipment Corp., Santa Barbara, CA) was used for leaf water potential measurement, with eight leaves for each treatment.

The photosynthetic rate of the flag leaves were determined at 72 and 73 DAT (D1) in both years and 93 and 98 DAT (D2), respectively, in 2012 and 2013 when soil water potential was approximately -15 kPa in the AWMD regime and at 74 and 75 DAT (W1) in both years and at 95 and 100 DAT (W2), respectively, in 2012 and 2013 when plants were rewatered. A gas exchange analyzer (Li-Cor 6400 portable photosynthesis measurement system, Li-Cor, Lincoln, NE) was used for measurement of the photosynthetic rate during 0900 to 1100 h when photo-synthetic active radiation above the canopy was 13001500 |imol m-2 s-1. The measurement was made on the upper surface of the flag leaf using eight leaves from each treatment.

Measurements of tiller number, root and shoot biomass and root activity

Twenty plants in each plot were tagged for observation of tiller number. The observation was made at transplanting, jointing stage, heading time, and maturity. The percentage of productive tillers was defined as the number of panicles developed from tillers as a percentage of the number of tillers at the jointing stage.

Root and shoot biomass and ROA were determined at 20-21, 43-44 (panicle initiation), 78-79 and 127-128 DAT. Shoot biomass was also measured at transplanting, and ROA was determined at mid grain filling (104-105 DAT) instead of maturity. When root biomass and ROA were measured, soil water potential was about -15 kPa at 20-21 DAT, 104-105 DAT, and 127-128 DAT and was 0 kPa at 43-44 DAT and 78-79 DAT in the AWMD regime.

To maintain canopy conditions, the vacant spaces left after sampling for measurements of root and shoot biomasses were immediately replaced with hills taken from the borders and these replanted hills were not subjected to sampling any more.

For each root sampling, a cube of soil (25 cm in length x 16 cm in width x 20 cm in depth) around each individual hill was removed up using a sampling core. Such a cube contains approximately 95% of total root biomass (Kukal and Aggarwal 2003; Yang et al. 2008). Plants of four hills from each plot formed a sample at each measurement. The cube of soil was cut into two parts, with 10 cm depth for each part. The roots in each cube of soil were carefully rinsed with hydropneumatic elutriation device (Gillison's Variety Fabrications, Benzonia, MI). After combining roots of four hills and recording fresh weight, portions of each root sample were used for measurements of root oxidation activity (ROA). The rest of the roots were dried in an oven at 70°C to constant weight and were weighed. The ROA was determined by measuring oxidation of alpha-naphthylamine (a-NA) according to the method of Ramasamy et al. (1997), and was expressed as fig a-NA per gram DW per hour (ig a-NA g-1 DW h-1). Before root sampling, aboveground plants were sampled and separated into leaves, stems, panicles (at heading time and maturity) and dead shoot parts, and were dried in an oven at 70°C to constant weight for determining shoot biomass. The amount of nonstructural carbohydrate (NSC) in the stem (culm + sheath) was determined at heading time and maturity according to the method described by Yoshida et al. (1976). Crop growth rate (CGR) was calculated using the following formula:

CGR (grn-2d~1) = (W2 - W,)/^ - t,)

where W1 and W2 are the first and second measurement of shoot biomass (g m-2), respectively, and t1 and t2 present the first and second time (d), respectively, of the measurement.

Greenhouse gas flux measurements

Fluxes of CH4 and N2O were measured using static vented flux chamber technique (Hutchinson and Livingston 1993). The chamber included a permanent base that was inserted into the soil (with rice plants growing inside); extensions of varying length to accommodate the growing plants; and a lid which was equipped with vent tube, fan and thermocouple wire. The base was made of PVC frame (0.5 m x 0.5 m) and inserted to a depth of 15 cm which left about 10 cm above the soil line. Holes drilled in the base above and below the soil line allowed for relatively free root and water movement. During sampling, holes

above the water line were plugged with rubber stoppers when the water level was below the holes to ensure chambers were airtight. One chamber was employed in each plot and positioned at least 1 m inside the plots and sampling locations were connected using board walks to prevent soil disturbance when sampling. The chamber was wrapped with a layer of sponge and aluminum foil to minimize the air temperature changes inside the chamber during the period of sampling.

Gas flux measurements were conducted at daily during the entire growing season. For each flux measurement, gas samples were collected from 0900 to 1100 h by a 20-mL syringe at 0, 10, 20, and 30 min after the chamber closure. Gas samples were analyzed for CH4, N2O, and CO2 concentrations by a gas chromatograph (Agilent 7890A, Agilent Technologies, Palo Alto, CA) equipped with two detectors. N2O was detected by an electron capture detector (ECD), and CH4 was detected by a hydrogen flame ionization detector (FID). CO2 was reduced with hydrogen to CH4 in a nickel catalytic converter at 375°C and then detected by the FID. The carrier gas was argon-methane (5%) at a flow rate of 40 mL min-1. The temperatures for the column and ECD detector were maintained at 40°C and 300°C, respectively. The oven and FID were operated at 50°C and 300°C, respectively. Sample sets were rejected unless they yielded a linear regression value of r2 greater than 0.90. The average fluxes and standard deviations (SDs) of CH4 and N2O were calculated from four replicates. The seasonal amounts of CH4 and N2O emissions were calculated from the daily measurement.

The GWP of N2O and CH4 was calculated in mass of CO2 equivalents (kg CO2 eq ha-1) over a 100-year time horizon. A radiative forcing potential relative to CO2 of 298 was used for N2O and 25 for CH4 (Ma et al. 2013).

Final harvesting

Plants were hand-harvested on 15 October in 2012 and 16 October in 2013. The measurement of grain yield and yield components was followed the procedure as described by Yoshida et al. (1976). Plants in two rows on each side of the plot were discarded to avoid border effects. Grain yield was determined from a harvest area of 6.0 m2 in each plot (not including plants in the chamber) and adjusted to 14% moisture. Aboveground biomass and yield components, i.e., the number of panicles per square meter, number of spikelets per panicle, percentage of filled grains, and grain weight, were determined from plants of 0.6 m2 (excluding the border ones and those in the chamber) sampled randomly from each plot. The percentage of filled grains was defined as the filled grains (specific gravity > 1.06 g cm-3) as a percentage of total number of spikelets.

Aboveground plants sampled at maturity were separated into straw, filled and unfilled grains, and rachis. Dry weight of each component was determined by oven-drying at 70°C to constant weight and weighed. Tissue N content was determined by micro Kjeldahl digestion, distillation and titration to calculate aboveground N uptake (Yoshida et al. 1976). The methods for calculating NUE were according to Xue et al. (2013), i.e., the internal N use efficiency (IEN) = grain yield/the total amount of N uptake in plants at maturity, and N partial factor productivity (PFPn) = grain yield/the amount of N applied. The WUE was calculated from and grain yield and amount of irrigation water and precipitation (grain yield over the amount of irrigation water and precipitation, kg m-3), and GHG intensity (GHGI) was expressed as GWP per unit mass of rice grain (kg CO2 eq kg-1 grain) (Mosier et al. 2006).

Statistical analysis

Analysis of variance was performed using SAS/STAT statistical analysis package (version 6.12, SAS Institute, Cary, NC). The statistical model included sources of variation due to replication, year (Y), irrigation regime (I), straw incorporation (S) and the interaction of Y x I, Y x I, Y x S, and I x S. Data from each sampling date were analyzed separately. Means were tested by least significant difference at P < 0.05 (LSD0 05).

Results

Soil and leaf water potentials and soil Eh

The difference in total rainfall during the growing season was rather small between the two study years (561.6 mm in 2012 and 516.6 mm in 2013), especially during the mid and late growing stages (40-128 DAT) (Fig. S1c). Changes in soil water potentials were similar in both years (Fig. S2a and b). It took 6-10 days to reach soil water potential of -15 kPa for the AWMD regime under either without (-S) or with (+S) wheat straw incorporation. The CF regimes received 18-20 times of irrigation, whereas AWMD regimes were applied 11-13 times of irrigation, from transplanting to maturity. The differences in soil water potentials were not significant between -S and +S when the irrigation regime was the same (Fig. S2).

The amount of irrigation water from land preparation to rice harvest was 371 mm for the AWMD-S treatment and 417 mm for the AWMD + S treatment, which was 62.5% and 61.9% of that for the CF-S (594 mm) and CF + S (674 mm) treatments, respectively (Fig. 1). More water use under +S than under -S was mainly resulted from more irrigation water for land preparation for the

CF-S AWMD-S CF+S AWMD+S Treatment

Figure 1. Irrigation water under various irrigation and wheat straw incorporation treatments in 2012 and 2013. CF, AWMD, -S and +S represent continuously flooded, alternate wetting, and moderate drying, without straw incorporation and with straw incorporation, respectively. Vertical bars represent ± standard error of the mean (n = 4), where these exceed the size of the symbol. Different letters above the column indicate statistical significance (P < 0.05).

former, such as soaking the field and puddling before rice transplanting, due partly to more water to rehydrate the dry straw and partly to more water to flood the increased field level, when wheat straw was incorporated into soil.

Figure 2 shows diurnal changes of leaf water potentials when soil water potentials were approximately -15 kPa in AWMD regimes. The leaf water potential ranged from -0.28 to -0.32 MPa at predawn (0600 h) to -0.53 to -0.56 MPa at mid-day (1200 h) for the plants in CF regimes. It was greatly reduced for the plants in AWMD regimes during the day, and reached -0.87 to -0.91 MPa at mid-day (Fig. 2A and B). However, the differences in leaf water potentials in the morning (0600 h and 0800 h) were very small between the plants in CF and AWMD regimes, indicating that plants in AWMD regimes could rehydrate overnight. No significant difference was observed in leaf water potentials between -S and +S in the same irrigation regime (Fig. 2).

As shown in Figure 3, soil Eh was very low at 10 DAT (before the start of AWMD treatments) and was -75 to -98 mV under -S and -174 to -185 mV under +S. It was substantially increased by the AWMD treatment, and ranged from 121 to 233 mV during the soil drying period and from -4.5 to -32.5 mV during the wetting period, and showed no significant difference between AWMD-S and AWMD + S treatments (Fig. 3A and B). When compared with CF-S treatment, the CF + S treatment showed much lower soil Eh during the flooding period. Soil Eh in CF regimes was markedly increased during mid season drainage and the drainage before the final harvesting (Fig. 3A and B).

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Figure 2. Diurnal changes in leaf water potentials of rice under various irrigation and wheat straw incorporation treatments in 2012 (A) and 2013 (B). CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively. Measurements were made on the flag leaves at 2-h intervals from 0600 h to 1800 h on the 72 and 73 day after transplanting when soil water potentials were about -15 kPa in AWMD regimes. Vertical bars represent ± standard error of the mean (n = 8), where these exceed the size of the symbol.

Tiller number, leaf photosynthesis, and crop growth rate

The number of tillers varied with treatments (Table 1). The CF-S treatment always showed more, whereas the CF + S treatment exhibited less, tiller number than any other treatment. The percentage of productive tillers under both AWMD-S and AWMD + S treatments were greater than that under the CF-S or CF + S treatment, and showed no significant difference between the treatments of AWMD-S and AWMD + S or between CF-S and CF + S (Table 1).

Similar to the tiller number, the photosynthetic rate of leaves was significantly smaller for the CF + S treatment than for any other treatment at all the measurement times (Fig. 4A and B). It showed no significant difference among the CF-S, AWMD-S and AWMD + S treatments

200 100 0

-100 -200 -300

0 20 40 60 80 100 120 140 Days after transplanting

Figure 3. Soil redox potential (Eh) under various irrigation and wheat straw incorporation treatments in 2012 (A) and 2013 (B). CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively. Vertical bars represent ± standard error of the mean (n = 4), where these exceed the size of the symbol. Dotted and dash arrows indicate the start of soil drying and during the soil drying period, respectively, in the AWMD treatments. Solid arrows represent mid season drainage and the drainage before the final harvesting for all the treatments.

during the soil drying period in AWMD regimes (D1 and D2), but was significant greater for both AWMD-S and AWMD + S treatments than for the CF-S treatment when plants in AWMD regimes were rewatered (W1 and W2), indicating a beneficial effect of the AWMD irrigation.

Consistent with the number of tillers and leaf photo-synthetic rate, the crop growth rate (CGR) of the CF + S treatment was the smallest among the four treatments during the whole growing season (Fig. 5A and B). It was significantly greater under the CF-S treatment than the AWMD-S or AWMD + S treatment from mid tillering to panicle intimation, and showed no significant difference among the three treatments from transplanting to mid tillering and from panicle initiation to heading time. When compared with the CF regime, the AWMD regime significantly increased CGR from heading to maturity under either

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Table 1. Number of tillers and the percentage of productive tillers of rice under various Irrigation and wheat straw incorporation treatments1.

Year/Treatment Number of tillers per m2 Productive tillers (%)2

Mid tillering Jointing Heading Maturity

CF-S 219a3 338a 278a 248a 73.4b

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Analysis of variance

Year (Y) NS4 NS NS NS NS3

Irrigation (I) 9.8** 4.8* 6.1* 8.7** 23.9**

Straw incorp. (S) 17.4** 49.6** 42.4** 12.5** NS

Y x I NS NS NS NS NS

Y x S NS NS NS NS NS

I x S 13.3** 56.7** 47.5** 10.6** NS

1CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively.

2The number of panicles developed from tillers (tillers at maturity)/the maximum number of tillers at the jointing stage. 3Different letters indicate statistical significance at the P < 0.05 level within the same column and the same year. 4NS, not significant (P > 0.05).

♦Significant at the P < 0.05 level; **Significant at the P < 0.01 level.

-S or +S, suggesting that the AWMD regime is more favorable to plant growth during the grain filling period.

When compared with those in the CF regime, NSC accumulation in the stem at the heading time and NSC remobilization during the grain filling period were greater in the AWMD regime under either -S or +S (Table 2). The AWMD-S treatment showed the greatest, while the CF + S treatments exhibited the smallest, NSC accumulation and remobilization among the four treatments, indicating an interaction between the irrigation regime and straw incorporation on NSC accumulation and remobilization.

Shoot and root biomass and root oxidation activity

At each growth stage, both shoot and root biomasses showed no significant difference among the CF-S, AWMD-S, and AWMD + S treatments (Fig. 6A-D). The CF + S treatment exhibited the smallest shoot and root biomasses among the four treatments, in good agreement with the tiller number and CGR (Table 1 and Fig. 5). The root/root ratio had no significant difference among the four treatments at the same growth state (Fig. 6E and F).

Although the difference in total root biomass in 0-20 cm soil layer was not significant among the CF-S, AWMD-S, and AWMD + S treatments, both AWMD-S and

AWMD + S treatments exhibited significantly greater root dry weight in 10-20 cm soil layer than the CF-S or CF + S treatment at all growth stages (Table 3). A similar observation was made on ROA (Fig. 7A and B). The ROA was the greatest under the AWMD-S or AWMD + S among the four treatments, followed by the CF-S, and the smallest under the CF + S treatment (Fig. 7A and B).

Grain yield and N and water use efficiencies

There was a very significant interaction between the irrigation regime and the straw incorporation on grain yield (Table 4). When compared with that in CF regimes, grain yield in AWMD regimes was increased by 2.7% under -S and by18.0% under +S (Table 4). The significant increase in grain yield under the AWMD + S than under the CF + S was due mainly to increases in the panicle number per m2, percentage of filled grains and grain weight. A low grain yield under the CF + S treatment was mainly attributed to a decreased panicle number per m2, which was closely associated with the decreased tillering ability resulted from a strong soil reduction condition when wheat straw was incorporated into soil and the field was continuously flooded (Table 1 and Fig. 3).

As shown in Table 4, both AWMD-S and AWMD + S treatments showed a higher harvest index than the CF-S or CF + S treatment. A higher harvest index in AWMD

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Figure 4. Photosynthetic rate of the flag leaf of rice under various irrigation and wheat straw incorporation treatments in 2012 (A) and 2013 (B). CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively. D1 and D2 are measurement times at 72-73 and 93-98 days after transplanting (DAT), respectively, when soil water potential was about -15 kPa in the AWMD plot, and W1 and W2 are measurement times at 74-75 and 95-100 DAT, respectively, when plants were rewatered. Vertical bars represent ± standard error of the mean (n = 8), where these exceed the size of the symbol. Different letters above the column indicate statistical significance (P < 0.05) within the same measurement date.

regimes may be attributed partly to more NSC remobi-lization during the grain filling period (Table 2) and partly to a greater CGR during maturity (Fig. 5).

Although N content in plants at maturity showed no significant difference among the four treatments, NUE varied with treatments (Table 5). When wheat straw was incorporated into the field, the total N uptake in plants at maturity, IEn and PEPn were significantly higher in the AWMD regime than in the CF regime. They showed no significant difference between CF and AWMD regimes when wheat straw was not incorporated. Both AWMD-S and AWMD + S exhibited the highest, whereas the CF + S treatment showed the smallest, WUE among the four treatments (Table 5), in good agreement with harvest index (Table 4).

18 15 12

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AWMD+S g S

I-1 CF-S

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b b T c

MT-PI PI-HT

Growth period

Figure 5. Crop growth rate of rice in 2012 (A) and 2013 (B). CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively. Tr-MT, from transplanting to mid tillering; MT-PI, from mid tillering to panicle initiation; PI-HT, from panicle initiation to heading time; HT-Mu, from heading time to maturity. Vertical bars represent ± standard error of the mean (n = 4), where these exceed the size of the symbol. Different letters above the column indicate statistical significance (P < 0.05) within the same growth period.

GHG emissions and GHG intensity

Fluxes of CH4 were highly dependent on water management and straw incorporation conditions (Fig. 8A and B). In all treatments CH4 emissions were generally higher during the continuous flooding period at the early stage of rice growth and were fluctuated in AWMD regimes and peaked 24-25 DAT in CF regimes. They were markedly decreased during the mid season drainage in CF regimes and during the soil drying period in AWMD regimes. The peak of CH4 fluxes also appeared 47-48 DAT under the all treatments which may be attributed to higher temperature (Fig. S1a) and vigorous rice growth during this period, and then CH4 fluxes maintained at a very low level. Wheat straw incorporation substantially increased, while the AWMD regime greatly reduced, CH4

Table 2. Nonstructural carbohydrate (NSC) accumulation and remobili-zation of rice under various irrigation and wheat straw incorporation treatments1.

Year/Treatment NSC at heading (g m-2) NSC at maturity (g m-2) NSC remobilization during grain filling (%)2

CF-S 268b3 125a 53.4b

AWMD-S 297a 124a 58.2a

CF+S 213c 119a 44.1c

AWMD+S 292a 126a 56.8a

CF-S 274b 131a 52.2b

AWMD-S 302a 126a 58.3a

CF+S 217c 121a 44.2c

AWMD+S 298a 130a 56.4a

Analysis of variance

Year (Y) NS4 NS NS

Irrigation (I) 445** NS 56.6**

Straw incorp. (S) 139** NS 18.7**

Y x I NS NS NS

Y x S NS NS NS

I x S 101** NS 8.7**

1CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively.

2(NSC in stems at heading time - NSC in stems at maturity)/NSC in stems at heading time x100.

3Different letters indicate statistical significance at the P < 0.05 level within the same column and the same year. 4NS, not significant (P > 0.05). **Significant at the P < 0.01 level.

emissions. During the entire rice growing season, the average CH4 flux was 5.7, 2.8, 29.2, and 6.2 mg CH4 m-2 h-1, respectively, under CF-S, AWMD-S, CF + S and AWMD + S treatments.

Nitrous oxide emissions were only observed when soil water potential <-10 kPa, and exhibited a pulse-like pattern which was coincided with the mid season drainage in CF regimes and the soil drying period in AWMD regimes (Fig. 8C and D). The AWMD regime increased, whereas wheat straw incorporation showed some reduction in, N2O emissions. The average N2O flux was 10.0, 26.6, 8.1, and 23.2 fig N2O m-2 h-1, respectively, under CF-S, AWMD-S, CF + S and AWMD + S treatments during the whole growing season.

During the experimental period, the GWP of CH4 and N2O under various treatments varied between 2.3 and 11.2 t CO2-eq ha-1 in 2012 and between 2.5 and 11.4 t CO2-eq ha-1in 2013 (Table 6). Compared with CF regimes, AWMD regimes decreased the GWP by 45.2% and 55.9%, respectively, under -S and +S. Similarly, AWMD regimes decreased the GHGI, i.e., yield-scaled GWP, by 46.7%

under -S and by 62.6% under +S, when compared with CF regimes (Table 6). As shown in Table 6, the GWP of N2O accounted for 2.1%, 10.1%, 0.7%, and 4.3% of the total GWP of CH4, and N2O, respectively, under the treatments of CF-S, AWMD-S, CF + S, and AWMD + S, indicating that CH4 emissions are the dominant in the total GWP from rice fields in either CF or AWMD regimes.

Discussion

Integrative effect of AWMD and straw incorporation on grain yield and WUE

Although the effect of AWD technology or straw application on rice yield, WUE or GHG emissions has been studied previously (Ma et al. 2009; Yao et al. 2012; Zhang et al. 2012; Li et al. 2014; Linquist et al. 2015), information on the integrative effect of AWMD and straw incorporation on grain yield, WUE, NUE, and GHG emissions is unavailable. Our results showed that, when compared with the CF regime, the AWMD regime increased grain yield by 2.7% and 18.0%, reduced irrigation water by 37.7% and 38.2% and increased WUE by 27.6% and by 50.0%, respectively, under -S and +S (Tables 4 and 5, Fig. 1). The results suggest that there is an interaction between AWMD and straw incorporation on rice yield and WUE, and adoption of the AWMD technology is more effective to increase grain yield and resource-use efficiency if wheat straw is incorporated into paddy fields.

How could an AWMD regime lead to higher rice yield and WUE, especially under wheat straw incorporation? The physiological mechanism is not understood. It has been observed that a long period of flooding in the paddy field could produce high concentrations of toxic reduction products such as Fe2+, H2S, and organic compounds (Ramasamy et al. 1997). These toxic reduction products are more aggravated when wheat straw is incorporated into soil, and thereby seriously inhibit root growth (Yao et al. 2013; Li et al. 2014). In this study, we observed that soil Eh was much lower under the CF + S treatment than under the CF-S treatment (Fig. 3), indicating a strong soil reduction condition when wheat straw was incorporated into soil and the field was continuously flooded. A strong soil reduction condition could inhibit root growth, which was evidenced by much smaller root biomass and much lower ROA under the CF + S treatment than those under the CF-S treatment (Figs. 6, 7). Drainage during mid season, especially an AWD regime, could greatly improve soil redox conditions and remove toxic reduction substances (Ramasamy et al. 1997; Yang et al. 2007; Liu et al. 2013), and therefore benefit root growth, which was evidenced by our observations that

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Ezzzzi AWMD-S кчччч CF+S pwra AWMD+S

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Figure 6. Shoot dry weight (A and B), root dry weight (C and D) and root/shoot ratio (E and F) of rice in 2012 (A, C, and E) and 2013 (B, D, and F). CF, AWMD, -S and +S represent continuously flooded, alternate wetting, and moderate drying, without straw incorporation and with straw incorporation, respectively. MT, PI, HT and Mu represent mid tillering, panicle initiation, heading time and maturity, respectively. Vertical bars represent ± standard error of the mean (n = 4), where these exceed the size of the symbol. Different letters above the column indicate statistical significance (P < 0.05) within the same measurement stage, and NS means not significant (P > 0.05).

soil Eh and ROA were much higher under both AWMD-S and AWMD + S treatments than under either CF-S or CF + S treatments (Figs. 3, 7), and that root biomass was greater in AWMD regimes than in CF regimes when

wheat straw was applied (Fig. 6). Furthermore, there was a greater root biomass in 10-20 cm soil depth throughout the growing season in AWMD than in CF regimes irrespective of -S and +S (Table 3), suggesting a deeper

Table 3. Root dry weight of rice in 10-20 cm soil layer under various irrigation and wheat straw incorporation treatments1.

Year/Treatment Mid Panicle Heading Maturity

tillering initiation time (g m-2)3

(g m-2) (g m-2) (g m-2)

CF-S 3.58b2 25.4b 42.7b 17.2b

AWMD-S 4.85a 30.9a 48.5a 20.3a

CF+S 2.49c 20.5c 35.2c 13.9c

AWMD+S 4.61a 28.8a 46.4a 19.6a

CF-S 3.67b 26.3b 43.5b 18.1b

AWMD-S 4.96a 31.5a 48.8a 21.5a

CF+S 2.53c 21.8c 36.4c 14.2c

AWMD+S 4.74a 29.1a 47.3a 20.0a

Analysis of variance

Year (Y) NS3 NS NS NS

Irrigation (I) 386** 204** 89.5** 57.6**

Straw incorp. (S) 58.9** 43.7** 45.6** 26.8**

Y x I NS NS NS NS

Y x S NS NS NS NS

I x S 25.5** 21.8** 16.4** 14.5**

1CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively.

2Different letters indicate statistical significance at the P < 0.05 level within the same column and the same year. 3NS, not significant (P > 0.05). "Significant at the P < 0.01 level.

root distribution in the soil. There is a proposal that plants that a deeper root distribution in the soil could maximize soil moisture capture and thereby maintain a high plant water status under drought conditions, and consequently contributes to a higher crop yield and WUE (Garnett et al. 2009; Luo et al. 2011; Lynch 2013; Chu et al. 2014). Therefore, we conclude that a deeper root distribution contributes, at least partly, to higher grain yield and WUE in AWMD regimes.

It is proposed that an interdependent relationship exists between the root and shoot (Osaki et al. 1997; Yang et al. 2004), that is, active shoots can ensure a sufficient supply of carbohydrates to roots and maintain active root functions; the activation of root functions can improve shoot growth by supplying a sufficient amount of nutrients, water and phytohormones to shoots, thus ensures an increase in crop productivity (Osaki et al. 1997; Yang et al. 2004; Zhang et al. 2009). Our earlier work (Yang et al. 2002, 2003) has shown that a moderate soil drying during grain filling of rice and wheat (Triticum asetivum L.) could increase abscisic acid levels in root exudates, and consequently, enhance NSC remobilization from vegetative tissues to grains and increase grain filling rate. The present results showed that, when compared with the CF + S treatment, the AWMD + S treatment markedly increased

Y7777\ AWMD-S

CF+S KTOi AWMD+S a

PI HT Grwoth stage

Figure 7. Root oxidation activity of rice in 2012 (A) and 2013 (B). CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively. MT, PI, HT, and MGF represent mid tillering, panicle initiation, heading time and mid grain filling, respectively. Vertical bars represent ± standard error of the mean (n = 4), where these exceed the size of the symbol. Different letters above the column indicate statistical significance (P < 0.05) within the same measurement stage.

tiller number, the percentage of productive tillers, leaf photosynthetic rate, crop growth rate, NSC accumulation in stems before heading, NSC remobilization during grain filling and shoot biomass during the whole growing season (Tables 1 and 2, Figs. 4, 5 and 6). We speculate that the AWMD regime, especially under +S, improves root growth which benefits other physiological processes, leading to higher grain yield and better WUE.

There are reports showing that an increase in WUE usually accompanies a yield penalty under an AWD compared with that under the CF (Bouman and Tuong 2001; Belder et al. 2004; Yao et al. 2012; Linquist et al. 2015). The results herein demonstrated that the AWMD regime could not only increase WUE but also increase grain yield when compared the CF regime under either -S or +S (Tables 4 and 5). The discrepancies between previous studies and our work are probably attributed to many reasons, such as variations in soil hydrological conditions, climate

Table 4. Effect of water and straw management on grain yield, yield components and harvest index of rice under various irrigation and wheat straw incorporation treatments1.

Year/Treatment Grain yield (t ha-1) Panicles per m2 Spikelets per panicle Filled grains (%) Grain weight (mg) Harvest index2

CF-S 9.01a3 298a 136a 85.9b 26.2b 0.484b

AWMD-S 9.32a 295a 133a 88.7a 26.9a 0.503a

CF+S 8.05b 274b 137a 84.5b 26.2b 0.478c

AWMD+S 9.48a 293a 135a 89.2a 27.1a 0.499a

CF-S 9.36a 309a 134a 88.9b 26.5b 0.486b

AWMD-S 9.54a 304a 132a 91.3a 27.3a 0.502a

CF+S 8.19b 274b 135a 85.5c 26.4b 0.479c

AWMD+S 9.67a 305a 133a 91.6a 27.1a 0.501a

Analysis of variance

Year(Y) NS4 NS NS 7.4* NS NS

Irrigation (I) 53.8** 7.1* NS 23.4** 26.4** 162**

Straw incorp. (S) 15.7** 14.5** NS NS NS 8.6**

Y x I NS NS NS NS NS NS

Y x S NS NS NS NS NS NS

I x S 27.2** 13.6** NS NS NS NS

1CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively. Values of grain yield are means of plants of 6 m2 harvested from each plot. Values of panicles per m2, spikelets per panicle, filled-grain percentage, and 1000-grain weight are means of plants harvested from 0.6 m2 from each plot in each treatment. 2Total grain weight (dry weight)/total aboveground biomass (dry weight).

3Different letters indicate statistical significance at the P < 0.05 level within the same column and the same year. 4NS, not significant (P > 0.05).

♦Significant at the P < 0.05 level; **Significant at the P < 0.01 level.

Table 5. Nitrogen content and uptake at maturity, nitrogen use efficiency and water use efficiency (WUE) of rice under various irrigation and wheat straw incorporation treatments1.

Year/Treatment N content (%) N uptake (kg ha-1) IEn2 (kg kg-1) PFPN3 (kg kg-1) WUE4 (kg m-3)

CF-S 1.05a5 168a 53.6ab 45.1a 0.79b

AWMD-S 1.04a 166a 56.1a 46.6a 1.01a

CF+S 1.06a 154b 52.4b 40.3b 0.66c

AWMD+S 1.05a 172a 55.2a 47.4a 0.98a

CF-S 1.03a 171a 54.6ab 46.8a 0.84b

AWMD-S 1.02a 167a 57.2a 47.7a 1.07a

CF+S 1.05a 154b 53.0b 41.0b 0.68c

AWMD+S 1.04a 172a 56.1a 48.4a 1.03a

Analysis of variance

Year (Y) NS6 NS NS NS 5.4*

Irrigation (I) NS 45.4** 10.9** 52.5** 209**

Straw incorp. (S) NS 20.2** NS 14.8** 21.6**

Y x I NS NS NS NS NS

Y x S NS NS NS NS NS

I x S NS 88.9** NS 25.6** 8.1**

1CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively.

2IEn, internal N use efficiency: grain yield (kg)/N uptake of plants (kg).

3PFPn, N partial factor productivity: grain yield (kg)/N rate (200 kg ha-1).

4WUE, water use efficiency: grain yield (kg)/(amount of irrigation water + precipitation) (m3).

5Different letters indicate statistical significance at the P < 0.05 level within the same column and the same year.

6NS, not significant (P > 0.05).

♦Significant at the P < 0.05 level; **Significant at the P < 0.01 level.

T -C cm 80

7 si 600

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cd x 200

CF-S AWMD-S CF+S AWMD+S

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CF-S AWMD-S CF+S AWMD+S

CF-S AWMD-S CF+S AWMD+S

20 40 60 80 100 120 140 Days after transplanting

20 40 60 80 100 120 140 Days after transplanting

Figure 8. Fluxes of CH4 (A and B) and N2O (C and D) during the rice growing season in 2012 (A and C) and 2013 (B and D). CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively. Vertical bars represent ± standard error of the mean (n = 4), where these exceed the size of the symbol.

during rice growing season, rice varieties used, N nutrition and the timing of the irrigation method applied (Belder et al. 2004; Yang et al. 2004; Zhang et al. 2008, 2009). In this study, the soil drying in the AWMD regime was very mild which was evidenced that the difference in leaf water potentials in the morning (0600-0800 h) was not significant between CF and AWMD regimes, and the midday leaf water potential was not lower than -0.91 MPa under the AWMD regime (Fig. 4). There is a report showing that mid day leaf water potential > -1.2 MPa could not seriously affect leaf photosynthesis (Yang et al. 2007). The results herein also demonstrated that the leaf photo -synthetic rate was not inhibited by the soil drying, but it was greatly enhanced when plants were rewatered in AWMD regimes (Fig. 4). We suggest that the diagnosis that midday leaf water potential is not lower than -1.0 MPa and plants could rehydrate overnight could be used as an index for soil drying in a safe AWD to increase both grain yield and WUE.

Integrative effect of AWMD and straw incorporation on GHG emissions and NUE

It is generally believed that rice systems produce more GWP of GHG (mainly CH4) than other cereal crops, because flooded paddy soils create an anaerobic environment favorable for methanogenesis (Yan et al. 2005; Linquist et al. 2012). Straw incorporation in the paddy fields could provide a source of readily available C and a predominant source of methanogenic substrates, and straw decomposition in anaerobic flooded soils could not only result in accumulation of acetate, one of the most important substrates for methanogens in flooded soils but also acted as an electron donor, helping to stimulate soil reduction and create strict reductive conditions for CH4 production (Gao et al. 2004; Minamikawa and Sakai 2006; Lee et al. 2010; Li et al. 2014). We observed that CH4 emissions in CF regimes were very high during the early and mid growing periods, and were greatly aggravated

Table 6. Seasonal CH4 and N2O emissions, global warming potential (GWP) and greenhouse gas intensity (GHGI) under various irrigation and wheat straw incorporation treatments1.

Year/Treatment CH4 (kg CH4-C ha-1) N2O (kg N2O-N ha-1) GWP2 (kg CO2 eq ha-1) GHGI3 (kg CO2 eq kg-1 grain)

CF-S 171.4c4 0.316c 4379c 0.486b

AWMD-S 83.6d 0.833a 2338d 0.251c

CF + S 442.8a 0.265d 11 149a 1.385a

AWMD + S 186.4b 0.718b 4874b 0.514b

CF-S 177.2c 0.298c 4519d 0.483b

AWMD-S 91.7d 0.807a 2533b 0.266c

CF + S 452.9a 0.231d 11 391a 1.391a

AWMD + S 194.6b 0.710b 5077c 0.525b

Analysis of variance

Year (Y) NS5 NS NS NS

Irrigation (I) 1039** 2120** 955** 1818**

Straw incorp. (S) 1243** 60.3** 1213** 2058**

Y x I NS NS NS NS

Y x S NS NS NS NS

I x S 256** 4.9* 254** 625**

1CF, AWMD, -S and +S represent continuously flooded, alternate wetting and moderate drying, without straw incorporation and with straw incorporation, respectively.

2GWP = 25 x CH4 + 298 x N2O. 3GHGI = GWP/grain yield.

4Different letters indicate statistical significance at the P < 0.05 level within the same column and the same year. 5NS, not significant (P > 0.05).

♦Significant at the P < 0.05 level; **Significant at the P < 0.01 level.

when wheat straw was incorporated in to the soil (Fig. 8A and B), consistent with the previous reports (Shan and Yan 2013; Yao et al. 2013; Li et al. 2014). The present results showed that the AWMD regime could substantially decrease CH4 fluxes, and the seasonal CH4 emissions in such a regime were reduced by 86.7 kg CH4-C ha-1 under -S and by 258 kg CH4-C ha-1 under +S when compared with those in the CF regime (Table 6). The mechanism that the AWMD regime reduces CH4 emissions is not clear. A probably explanation is that an alternate wetting and drying regime could greatly improve soil redox conditions (Fig. 3), which prevent CH4 formation by inhibiting methanogenic bacteria and hence reduce CH4 emissions (Li et al. 2014). The results suggest that the AWMD irrigation is a good practice to increase grain yield meanwhile reduce CH4 emissions in rice production, especially under wheat straw incorporation.

Flooded rice systems generally emit less N2O than dryland crops because flooding results in most N being lost as N2 rather than N2O (Firestone and Davidson 1989). Mid season drainage or an AWD could increase N2O emissions because of the introduction of aerobic periods (Akiyama et al. 2005; Zou et al. 2007; Li et al. 2014). We also observed that AWMD regimes markedly increased N2O emissions when compared with CF regimes under either -S or +S (Table 6, Fig. 8C and D). However, the N2O emission was only detected when soil water

potentials < -10 kPa (Figs. S2, 8), implying that N2O emissions would not be increased if the threshold of soil water potential for rewatering is set at > -10 kPa in AWD. Moreover, the proportion of GWP of N2O was rather small in the total GWP of CH4 and N2O, and was 1.4% and 7.4% under CF and AWMD regimes, respectively (Table 6). Although N2O emissions were increased in AWMD regimes, the total GWP of CH4 and N2O was decreased by 50.1% and the GHGI decreased by 52.9% in these regimes when compared with those in CF regimes (Table 6). The results suggest that reducing CH4 emissions in rice systems is a major approach to reduce GHG emissions and GHGI.

We observed that wheat straw incorporation reduced N2O emissions in both CF and AWMD regimes when compared with no straw incorporation (Table 6 and Fig. 8). Similar observations were also made in previous studies (Shan and Yan 2013; Yao et al. 2013; Li et al. 2014). The mechanism in which straw incorporation reduces N2O emissions is not well known. One possible reason is that straw incorporation could substantially increase C:N ratio in the soil (Yao et al. 2013), and the increase in C:N ratio decreases mineral N availability (Millar et al. 2004). Thus, incorporating wheat straw into rice fields stimulates a net immobilization of plant-available N, thereby lowering the N substrate availability for N2O production (Yao et al. 2013). 2

There are reports showing that the adoption of AWD-based technologies could reduce total cumulative plant N and NUE by stimulating N losses through increases in nitrification and denitrification (Sah and Mikkelsen 1983; Eriksen et al. 1985). In present study, the sum of 0.49 kg N2O-N ha-1 was more lost in the AWMD than in the CF regime (Table 6), which accounted for 0.25% of the total amount N application, indicating that N loss due to N2O emissions is negligible in the AWMD regime. We observed that the total N uptake, internal N use efficiency (IEn) and N partial factor productivity (PFPn) showed no significant differences between CF and AWMD regimes when wheat straw was not incorporated, whereas they were significantly higher in the AWMD than in the CF regimes when wheat straw was applied (Table 5). The results demonstrate that the AWMD regime could increase, rather than decrease, the total cumulative plant N and NUE when wheat straw is incorporated in the paddy field. Further research is needed to determine nitrification and denitrification to exactly estimate N loss in the AWMD regime.

It should be noted that we only presented CH4 and N2O fluxes, and did not consider CO2, another source of GHG, although we measured CO2 fluxes. The first reason for this is that CO2 emissions are estimated to contribute less than 1% to the GWP of agriculture (Linquist et al. 2015). The second reason is that the net balance between C respiration and fixation in a cropping system is reflected by changes in soil organic C over time (West and Post 2002; Stewart et al. 2007) which is difficult to detect in short-term experiments due to the relatively small change and high degree of spatial variability in soil organic C (Post et al. 2001; Conant et al. 2011; Linquist et al. 2012). In addition, in the rice-dryland crops rotation systems, such as rice-soybean and rice-wheat rotations, an AWD in rice season may not alter soil organic C as soil C stocks are already degraded in these systems (Ma et al. 2013; Linquist et al. 2015).

Conclusion

An AWMD regime, that is, mid-day leaf water potential was not lower than -1.0 MPa and plants could rehydrate overnight during the soil drying period, could increase grain yield, WUE and NUE and decrease GWP and GHGI when compared with the CF regime. The increases or decreases were more remarkable when wheat straw was incorporated in the paddy field. Increases in grain yield, WUE and NUE in the AWMD regime, especially under straw incorporation, were attributed mainly to a greater ROA, deeper root distribution, and increases in the percentage of productive tillers, leaf photosynthetic rate, crop growth rate and NSC remobilization during grain filling

and harvest index. While the reduction in GWP and GHGI in the AWMD regime was due primarily to the substantial decrease in CH4 emissions. The loss of N due to N2O emissions was negligible in the AWMD regime. There was an interaction between the AWMD and straw incorporation on rice yield, WUE, NUE, and GHG emissions. Adoption of the AWMD technology was more effective to increase grain yield and resource-use efficiency and reduce environmental risks when wheat straw was incorporated into paddy fields.

Acknowledgments

We are grateful for grants from the National Natural Science Foundation of China (31461143015; 31271641, 31471438, 91317307), the National Key Technology Support Program of China (2011BAD16B14; 2012BAD04B08; 2014AA10A605), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Hong Kong Research Grant Council (AoE/ M-05/12), and Shenzhen Overseas Talents Innovation & Entrepreneurship Funding Scheme (The Peacock Scheme).

Conflict of Interest

None declared. References

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Supporting Information

Additional supporting information may be found in the online version of this article at http://onlinelibrary.wiley. com/doi/10.1002/fes3.66/suppinfo

Figure S1. The mean temperature, sunshine hours, and precipitation during the growing season of rice in 2012 and 2013 at the experiment site of Yangzhou, Southeast China.

Figure S2. Soil water potentials under various irrigation and wheat straw incorporation treatments in 2012 and 2013.