Scholarly article on topic 'Nitrous oxide emission and nitrogen use efficiency in response to nitrophosphate, N-(n-butyl) thiophosphoric triamide and dicyandiamide of a wheat cultivated soil under sub-humid monsoon conditions'

Nitrous oxide emission and nitrogen use efficiency in response to nitrophosphate, N-(n-butyl) thiophosphoric triamide and dicyandiamide of a wheat cultivated soil under sub-humid monsoon conditions Academic research paper on "Earth and related environmental sciences"

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Academic research paper on topic "Nitrous oxide emission and nitrogen use efficiency in response to nitrophosphate, N-(n-butyl) thiophosphoric triamide and dicyandiamide of a wheat cultivated soil under sub-humid monsoon conditions"

Biogeosciences Discuss., 11, 13571-13603, 2014 www.biogeosciences-discuss.net/11/13571/2014/ doi:10.5194/bgd-11-13571-2014 © Author(s) 2014. CC Attribution 3.0 License.

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This discussion paper is/has been under review for the journal Biogeosciences (BG). Please refer to the corresponding final paper in BG if available.

Nitrous oxide emission and nitrogen use efficiency in response to nitrophosphate, N-(n-butyl) thiophosphoric triamide and dicyandiamide of a wheat cultivated soil under sub-humid monsoon conditions

W. X. Ding1, Z. M. Chen1, H. Y. Yu1, J. F. Luo2, G. Y. Yoo3, J. Xiang1, H. J. Zhang1, and J. J. Yuan1

1 State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing 210008, China Land and Environment, AgResearch, Hamilton 3240, New Zealand 3College of Engineering, Kyung Hee University, Yongin-si 446-701, Republic of Korea

Received: 21 August 2014 - Accepted: 3 September 2014 - Published: 19 September 2014 Correspondence to: W. Ding (wxding@issas.ac.cn)

Published by Copernicus Publications on behalf of the European Geosciences Union.

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Abstract

A field experiment was designed to study the effects of nitrogen (N) source and urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) or nitrification inhibitor di-cyandiamide (DCD) on nitrous oxide (N2O) emission and N use efficiency (NUE) in a sandy loam soil. Six treatments including no N fertilizer (control), N fertilizer urea alone (U), urea plus NBPT (NBPT), urea plus DCD (DCD), urea plus NBPT and DCD (NBPT + DCD), and nitrate-based fertilizer nitrophosphate (NP) were designed and implemented separately during the wheat growth period. Seasonal cumulative N2O emissions with urea alone amounted to 0.49 ± 0.12 and were significantly (P < 0.05) reduced to 0.28 ± 0.03, 0.31 ± 0.01 and 0.26 ± 0.01 kgN2O-N ha-1 by application of DCD, NBPT and NBPT + DCD, respectively. Cumulative N2O emissions from NP were 0.28 ± 0.01 kgN2O-N ha-1. A single N2O flux peak was identified following basal fertilization, and DCD and/or NBPT inhibition effects mainly occurred during the peak emission period. The NP application significantly (P < 0.05) increased wheat yield by 12.3% and NUE from 28.8% (urea alone) to 35.9%, while urease and/or nitrification inhibitors showed a slight increase effect. Our results clearly indicated that the application of urea as basal fertilizer, but not as supplemental fertilizer, together with DCD and NBPT is an effective practice to reduce N2O emissions. The application of NP instead of urea would be an optimum agricultural strategy for reducing N2O emissions and increasing crop yield and NUE for wheat cultivation in soils of the North China Plain.

1 Introduction

Nitrous oxide (N2O) is a potent and long-lived atmospheric greenhouse gas, with an annual increasing rate of 0.26 % over the past decades and a contribution of 7 % to the annual increase in radiative forcing (IPCC, 2007). Agricultural soils are identified as the major source of atmospheric N2O, contributing 4.1 Tg N yr-1 (IPCC, 2013) to the global atmospheric N2O budget of ~ 14TgNyr-1 (Fowler et al., 2009). Field management

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practices and soil and climatic factors are recognized as being determinants of N2O emissions from agricultural soils (Stehfest and Bouwman, 2006; Gagnon et al., 2011). Among management practices, the large inputs of industrially fixed N in agriculture are a major perturbation to terrestrial N cycling and a major contribution to accelerating N2O emissions (Galloway et al., 2008). During the period 1990-2005, agricultural N2O emissions were globally estimated to have increased by 17% (USEPA, 2006), and are projected to increase by 35-60% by 2030 due to the continuous increase of global N fertilizer consumption and animal manure production (FAO, 2003).

China is a major agricultural producer (West et al., 2014) and the amount of applied N fertilizer has increased from 7.07 to 26.21 Tg N yr-1 over the period from 1977 to 2005 (Ju et al., 2009). The North China Plain, primarily containing low organic carbon (C) calcareous soils (6.40 vs. 9.60gC kg-1 for national upland soils) (Xie et al., 2007), is an intensive agricultural region. It covers ~ 300000 km and produces up to one-fourth of the total annual grain yield in China (Liu et al., 2001). A winter wheat and summer maize rotation is a commonly used cropping system, and the annual application rates of synthetic N fertilizers have amounted to 600 kg N ha-1 or more (Zhao et al., 2006; Ju et al., 2009). However, a low proportion of fertilizer N is taken up by crops (< 30 %) and it is estimated that up to 41 % of N applied during the growth season is subject to losses via leaching, nitrification, denitrification and ammonia volatilization (Cai et al., 2002). At present, up to 33% of fertilizer N was over-applied to the fields in China, resulting in China's contribution to 28% of the global annual N2O emissions from croplands (West et al., 2014). Hence, it is urgent to develop optimum methods for enhancing the recycling of N in the agricultural ecosystem and reducing the fertilizer N-induced N2O emissions.

In the past decade, a number of field measurements of N2O emissions have been conducted in the North China Plain (Meng et al., 2005; Ding et al., 2007; Ju et al., 2011). A 3year field measurement showed that the direct N2O emission factors of synthetic N applied to the wheat-maize cropping systems was 0.82% (Cai et al., 2013), which was higher than the 0.6 % reported from fertilizer N-treated upland soils in China

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(Xing, 1998). Nitrification is found to be the main process for the N2O emission because low availability of easily degradable organic C limits denitrification in this region (Ding et al., 2007; Ju et al., 2011). Nitrification inhibitors such as DCD help to retard the oxidation of NH+ to NO- by inhibiting the activities of Nitrosomonas bacteria in soil (Prasad and Power, 1995), resulting in the reduction of N2O emissions directly by decreasing nitrification or indirectly by reducing the availability of NO3- for denitrification and leaching. As a consequence, DCD can increase NUE by increasing plant growth and N uptake (Asing et al., 2008). Similarly, a urease inhibitor like NBPT can slow the conversion of urea to NH+, thereby reducing N losses by NH3 volatilization (Manunza et al., 1999; Zaman et al., 2009) and potentially reducing nitrification and subsequent denitrification rates. So the use of NBPT with urea-based fertilizers may be a potential management strategy to mitigate N2O emissions (Menéndez et al., 2009). A combined application of nitrification inhibitor and urease inhibitor with urea can maintain N as NH+ for a longer time with more chance of the fertilizer-derived N being taken up by the crops or immobilized by the organic or mineral component of the soil, thereby reducing the gaseous loss (Xu et al., 2002). Though application of inhibitors to reduce N2O emissions has attracted more attention recently and has already been investigated in many areas (Menéndez et al., 2009; Zaman et al., 2009), their effect on N2O emissions in the North China Plain has not been fully investigated.

Soil N2O emissions are also influenced by the source of fertilizer N. Gagnon et al. (2011) found that N2O emissions from urea ammonium nitrate were drastically than those from anhydrous ammonia during the maize growth season in a poorly drained clay soil of Canada. In contrast, Venterea et al. (2005, 2010) reported N2O emissions from soils amended with anhydrous NH3 to be 2- to 4-fold greater than that from soils receiving urea ammonium nitrate in a silt loam of the United States. Based on the analysis of published data in the literature, Stehfest and Bouwman (2006) concluded that the N2O emissions from nitrate-based fertilizers were on average lower than those from ammonium-based fertilizers. During the winter wheat growth season in the North China Plain, limited precipitation occurs. Therefore, it is likely that apply-

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ing nitrate-based fertilizer instead of urea will not accelerate the leaching of NO- but reduce N2O emissions and increase NUE.

In this study, we hypothesize that application of urease inhibitor and/or nitrification inhibitor with urea will lower N2O emission and increase wheat yield by suppressing the nitrification rate and increasing NUE in the North China Plain. We also hypothesize that use of a nitrate-based fertilizer nitrophosphate by replacing urea will have similar effects. The objectives of this study were: (1) to evaluate the influence of application of urea with NBPT, DCD and NBPT + DCD on N2O emissions and (2) to investigate whether the use of nitrophosphate instead of urea reduces N2O emissions from an intensively cultivated calcareous soil during the wheat growth season.

2 Materials and methods

2.1 Experimental site and soil characteristics

The field experiment was conducted at the Fengqiu State Key Agro-ecological Experimental Station, Chinese Academy of Sciences, Henan Province, China (35°00' N, 114°24' E), a typical region of the North China Plain. The region has a sub-humid temperate continental monsoon climate with dry cold winters and wet hot summers. A winter wheat (Triticum aestivum L.) and summer maize (Zea mays L.) rotation is selected as an intensively managed double-cropping system. The 30 year mean annual temperature was 13.9°C, with a range varying from -1.0 °C in January to 27.2 °C in July. The mean annual precipitation is 615 mm, two thirds of which falls between June and September. The soil is derived from alluvial sediments of the Yellow River and is classified as aquic inceptisol. The physicochemical properties of the soil are summarized in Table 1.

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Inhibitors and nitrophosphate effects on N2O emissions

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2.2 Treatment and crop management

The field experiment was carried out during the winter wheat growth season and included six fertilization treatments: (1) no N fertilizer (control), (2) N fertilizer urea alone (U), (3) urea plus N-(n-butyl) thiophosphoric triamide (NBPT), (4) urea plus dicyan-diamide (DCD), (5) urea plus NBPT and DCD (NBPT + DCD), and (6) nitrate-based fertilizer nitrophosphate (NP). The plots were arranged in a randomized complete block with three replicates and the plot size was 5m x 5 m. Urea and nitrophosphate (Jinkai chemical, Kaifeng, China), totaling 200 kg N ha-1, were added in two applications: 120 kg N ha-1 as basal fertilizer and 80 kg N ha-1 as supplemental fertilizer. Calcium superphosphate was applied as basal fertilizer at a rate of 125 kg P2O5 ha-1 for all treatments. For the NP treatment, calcium superphosphate was added as the basal fertilizer to ensure the same application rate of phosphate between the treatments. The NBPT (Hengshuo Chemical, Wuhan, China) and DCD (Sunnyfield Chemicals, Ningxia, China) were applied ata rate of 0.2% and 10% of the applied N (w/w), respectively. Inhibitors) and urea were thoroughly mixed. All basal fertilizers were evenly spread onto the soil surface by hand and immediately incorporated into the surface soil (0-20 cm) by plowing before sowing on 15 October 2009. The supplemental urea and inhibitor(s) or nitrophosphate were surface applied by hand, then integrated into the plowed layer with irrigation water (40 mm) on 6 March 2010. The mature wheat was harvested on 10 June 2010.

2.3 N2O flux measurement

In situ soil-surface fluxes of N2O were measured using the static chamber-gas chromatograph (GC) method. Flux measurements were taken over the period from 16 October 2009 to 8 June 2010 (235 days) during the wheat growth season. Immediately after sowing, a PVC chamber base (30cm x 30cm x 10cm) was inserted into the soil about 5 cm deep between wheat rows in the center of each plot. The PVC chamber (30 cm x 30 cm x 15 cm) was tightly fitted to the top of the base by inserting the flange

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of the chamber into the water trough at the upper end of the chamber base. The chamber was equipped with two ports: a small, silicon-sealed vent for sampling and a second port for measuring chamber temperature. Gas samples were initially taken twice a week and later reduced weekly then fortnightly over the winter. Sampling was done in the morning between 09:00 LT and 12:00 LT in order to minimize diurnal variation in flux patterns. Each time, four samples of the chamber air were manually pulled into 50 mL syringes at 0,10, 20 and 30 min after closure, injected into 20 mL pre-evacuated vials fitted with butyl rubber stoppers and taken to our laboratory for analysis. The air temperature inside the chamber was simultaneously measured with a mercury thermometer.

N2O concentrations were analyzed on a gas chromatograph (Agilent 7890, Santa Clara, CA, USA) equipped with an electron capture detector. The interfering oxygen contained in the injected gas sample (1.0 mL) was separated by a pre-column (1 m) in combination with an analytical column (3 m). Both columns, packed with Porapak Q (80/100 mesh), were attached directly to the 6-port valve to control the backflush. The temperatures of column oven, injector and detector were 40°C, 100°C and 300°C, respectively. The flow rate of carrier gas (95% Argon + 5% CH4) was 40mLmin-1. The standard N2O gas was provided by the National Institute for Agro-Environmental Sciences, Japan. The N2O fluxes were calculated using the following equation:

F = p x (P/760) x (V/A) x {AC/At) x [273/(273 + T)]

where F is the N2O flux ^g N2O-N m 2 h 1), p is the density of N2O at 0 °C and 760 mm

— 3 3

Hg (kg m ), V is the chamber volume (m ), A is the area from which N2O was emitted into the chamber (m2), AC/At is the rate of N2O accumulation in the chamber (ppbv N2O-N h-1), T is the chamber air temperature in Celsius, and P is the air pressure of the experimental site (mm Hg). The altitude of the experimental site for this study is very close to sea level, so P/760 « 1. Few sample sets were discarded when they yielded a linear regression value of R2 greater than 0.90.

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2.4 Grain yield and aboveground N uptake

After crops reached physiological maturity (10 June 2011), grain and straw were manually harvested from each plot. Grain and straw were air-dried, then further dried for 3 days at 65 °C and weighed to obtain dry matter yields. Subsamples were ground with a ball mill and analyzed for N concentration with an elemental N analyzer (VarioMax, Elementar, Hanau, Germany). Total N content in aboveground biomass was calculated from the sum of N masses harvested in grain and straw from each plot.

2.5 Auxiliary variables

Soil temperatures were measured, simultaneously with gas sampling, at vertical depths of 5, 10 and 15 cm with a digital thermometer (Model 2455, Yokogawa, Japan). Soil moisture was measured at 5 cm depth at three different positions in the vicinity of each chamber using time domain reflectometry probes and was expressed as water-filled pore space (WFPS) by the equation:

WFPS [%] = (volumetric water content [%]/total soil porosity [%]) x 100 (2)

where total soil porosity = 1 - (soil bulk density/2.65), with 2.65 [gcm-3] being the assumed particle density of the soil. The precipitation and air temperature were monitored at a neighboring meteorological station 100 m away from the experimental field.

During the growth season, six soil samples were taken from the 0-20 cm soil layer at different positions in each plot just after flux measurement using a 5 cm diameter stainless steel soil sampler and then all samples from each plot were thoroughly mixed to form a composite. After visible roots and litter materials were removed, soil samples were passed through a 2 mm sieve and then extracted with 2M KCl (soil/KCl suspension of 30 : 100 ratio) for 1 h on a rotary shaker. The extracted solutions were filtered and stored in a deep freezer (-18 °C) until analysis. The NH+-N and NO--N concentrations were measured using a colorimetric method on a Skalar segmented flow analyzer (SAN++, the Netherlands).

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2.6 Data analysis and statistics

Average fluxes and standard errors of the N2O fluxes were calculated from triplicate plots. Seasonal cumulative N2O emissions were calculated using the following equation:

Cumulative N2O emission = ^ (F + F+1)/2 x (t¡+1 -t¡) x 24

where F is the N2O flux ^gN2O-Nm h ), i is the ith measurement, the term of (tj+1 - ti) is the number of days between two measurements, and n is the total number of the measurements. The N2O direct emission factor (%) of fertilizer N applied to the soil with background adjustment was calculated as follows:

Emission factor = ((N2Ü-NfertiNzer - N2Ü-NC0ntr0|)/NfertiNzer) x 100

where N2O-Nfertilizer and N2O-Ncontrol are the cumulative N2O emissions (kgN2O-N ha-1) in the N-fertilized treatment and the control treatment, respectively, and Nfertilizer is the amount of fertilizer N applied (kg N ha-1). Yield-scaled N2O emissions were calculated by dividing cumulative N2O emission by grain yield for each plot. NUE was calculated by dividing differences of the N amount in the aboveground biomass between N-fertilized plots and control plots within the same block by the N application rate (200 kg N ha-1). Soil inorganic N intensities were calculated separately for NH+ (NH4I), NO- (NO3I) and the sum of NO- + NH+ (IONI) as the summation of daily NH+-N, NO--N or (NO- + NH+)-N concentrations in the 0-20 cm layer over the same period as for cumulative N2O emissions using linear interpolation between sampling dates, and presented in units of g d kg-1, the index being what is commonly reported (Zebarth et al., 2008; Engel et al., 2010).

All data were statistically analyzed using the SPSS software package for Windows (Version 13.0, SPSS inc, Chicago, IL, USA). The effects of fertilization management

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on N2O emissions, emission factor, and grain yields were evaluated using one-way ANOVA, followed by the least significant difference (LSD) test at P < 0.05. All dependent variables were evaluated for normality and were log-transformed to normalize the distributions if necessary prior to statistical analysis. Correlation and nonlinear regression analyses were used to test relationships between N2O fluxes and other factors.

3 Results

3.1 Wheat yield and nitrogen use efficiency

Grain yield in the urea alone treatment was 4652 kg ha-1 and this was increased by 1.3%, 1.8% or 1.8% when NBPT, DCD or both were added with urea fertilizer (Table 2). Compared with the urea alone, the increase in the grain yield in the NP treatment was 12.3% (Table 2). The N process inhibitors, NBPT, DCD or the combination of both, slightly increased the amount of N uptake by wheat plants. However, this increase was not statistically significant (P > 0.05). A significant increase in the plant N uptake was observed in the NP treatment compared with urea only treatment (P < 0.05). The NUE was calculated at 28.8% for the urea alone treatment and this was slightly increased to 29.2-31.2% when urea was applied with NBPT, DCD or the combination of NBPT and DCD. However, the NUE was increased to 35.9% for the NP treatment, which was significantly higher than that for all the urea treatments (P < 0.05).

3.2 Soil temperature and moisture

Temporal variations of air temperature, precipitation, and soil WFPS and soil temperature at 5 cm depth over the experimental period are presented in Fig. 1. The cumulative rainfall over the wheat growth season was 97.6 mm which was lower than the long-term average. Soil moisture levels were highly variable, with WFPS values varying from 10.7% to 80.4%. Periods with high soil moisture (> 75% WFPS) were observed

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following heavy rainfall or irrigation events. Soil temperature at 5 cm depth was below zero in early January and increased to 23°C in early June.

3.3 N2O emissions

Variations of the N2O fluxes over the wheat growth season are illustrated in Fig. 2. N2O fluxes from all fertilizer-incorporated treatments showed almost the same pattern with peak N2O fluxes being observed soon after application of basal fertilizers. The fluxes from all the N fertilizer treatments were generally low on most of the other sampling dates and these fluxes were not significantly different from those from the control. On several occasions in the winter the fluxes were negative. No significant increases in the N2O fluxes were found following the supplemental fertilization coupled with irrigation.

N2O fluxes did not increase after heavy rainfall events either.

The peak N2O flux was 120.4 ^ N2O-N m h in the urea alone treatment. Compared with the urea only, application of NBPT, DCD or the combination of both reduced the peak fluxes by 41.1 %, 75.0 % and 61.2 %, respectively. Application of NP reduced peak fluxes by 69.1 % compared with application of urea alone. Analysis showed that the natural logarithms of the N2O fluxes were weakly, but not significantly, correlated with soil WFPS in all treatments except the DCD treatment, but significantly (P < 0.05) correlated with soil temperature in all treatments except the NBPT + DCD treatment (Table 3).

Cumulative N2O emissions from the different treatments are listed in Table 4. Total N2O emissions from the control, urea alone, urea + NBPT, urea + DCD, urea + NBPT + DCD and NP treatments were 0.16 ± 0.02, 0.49 ± 0.12, 0.31 ± 0.01, 0.28 ± 0.01, 0.26 ± 0.01 and 0.28 ± 0.03kgN2O-N ha-1, respectively, over the wheat growth season. The highest total N2O emission was found from the plot which only received urea. These emissions mainly occurred during the 18 day peak emission period following basal fertilizer application from 16 October to 3 November. Application of NBPT, DCD or the combination of both significantly reduced the seasonal N2O emissions from urea by 36.7%, 42.9% or 46.9%, respectively (P < 0.05). Compared with

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the emissions from the urea alone treatment, significantly lower N2O emissions were also observed from the NP treatment (P < 0.05) (42.9% less than those from the urea alone treatment). The direct N2O emission factor for urea application alone was 0.17%, and the addition of NBPT, DCD or the combination of both reduced the emission factor for urea to 0.05-0.08%. These reductions were statistically significant (P < 0.05). The direct N2O emission factor for NP was 0.06%, which was also significantly lower than that for urea application alone (P < 0.05).

The grain yield-scaled N2O emission from the NP treatment was significantly lower than that from the urea alone treatment (P < 0.05), but not different from those from the NBPT, DCD or NBPT + DCD treatments during the wheat growth season (Table 4).

Soil NH+ and NO3 concentrations

Soil NH+ and NO- concentration drastically increased after application of basal N fertilizers compared with the control. The levels of NO- in the NBPT and NBPT + DCD treatments were relatively low for one week after basal fertilizer application compared with those in the other treatments. However, the levels of NO- in the NBPT and NBPT + DCD treatments gradually increased, this was probably due to degradation of NBPT and its subsequent loss of effectiveness. Following application of supplemental fertilizer urea, no apparent increase in soil NO- levels was observed, and NO- concentration kept at a relatively constant level. In contrast, soil NO- concentration following application of supplemental fertilizer NP showed a rapidly decreasing trend. In all urea added treatments, soil NO- concentration sharply decreased to less than 10 mg N kg from 15 April onwards.

Soil NH+ concentration increased from 2 to 10mgNkg-1 after application of basal fertilizer; however it sharply decreased soon afterwards. Application of DCD or NBPT + DCD sustained soil NH+ concentrations at higher levels compared with urea application alone. In the NP treatment, soil NH+ concentrations were always at low levels. The natural logarithms of the N2O fluxes were more correlated with NH+ con-

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centrations than with NO- concentrations in the soil, despite the fact that a significant relationship was only observed in the urea alone treatment.

Mean soil NH4I levels in the NP treatment were the lowest among all N-added treatments and tended to be higher in the DCD and NBPT + DCD treatments compared with urea alone (Table 5). Mean soil NO3I levels showed a similar trend among the treatments and were ranked in the order of NBPT, NBPT + DCD > DCD > urea alone, NP > control. Mean soil IONI levels were also similar among the treatments and were ranked as NBPT + DCD > NBPT, DCD > urea alone, NP > control.

4 Discussion

4.1 Nitrous oxide emissions as affected by nitrogen sources

Compared with the urea alone, application of NP significantly reduced N2O emissions by 42.9% during the wheat growth season (Table 4), and increased wheat yield by 12.3% and NUE by 24.7% (Table 2). Ju et al. (2011) obtained a similar result in the North China Plain, finding that emissions of N2O derived from Ca(NO3)2 were lower than those from NH4(SO4)2 during the maize growth season (0.38-0.81 vs. 1.31 — 3.52kgN2O-N ha-1). A lower N2O emission for urea ammonium nitrate than for anhydrous ammonia was also reported in a silt loam of the United States (Venterea et al., 2005). In contrast, Gagnon et al. (2011) measured a significantly higher N2O emission following application of urea ammonium nitrate or calcium ammonium nitrate compared with anhydrous ammonia in a poorly drained clay soil of Canada. In a German grassland ecosystem, Müller and Sherlock (2004) found that the emissions for ammonia-based fertilizer were lower than those for nitrate-based fertilizer. These researchers suggested that higher emissions from nitrate-based fertilizers were because of the propensity of the fine-textured clay soil to become anaerobic following rainfall and a strong fixation of NH+ in clay lattices reducing NH+ available for N2O production (Chantigny et al., 2004). In this study, mean soil NO- intensities (NO3I) were not signif-

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icantly different between the NP and urea alone treatments (Table 5), and mean NO-concentrations (Fig. 3) were higher than the suggested threshold value for denitrifica-tion of 5 mg N kg-1 (Dobbie and Smith, 2003) during the growth season except for the period from 24 April to 10 June. These results imply that soil NO- concentration was not the only limiting factor affecting denitrification and N2O emission in the test soil.

The notable difference in the seasonal N2O emissions between the NP and urea alone treatments occurred mainly during the 18 day peak emission period following the basal fertilizer application and concurrent irrigation from 16 October to 3 November. It has been reported that application of ammonium-based fertilizers emitted more N2O than nitrate-based fertilizers under aerobic soil conditions, while application of nitrate-based fertilizers induced a greater increase in N2O production when soil conditions were anoxic (Pathak and Nedwell, 2001; Tenuta and Beauchamp, 2003). For cultivated soils, the primary mechanism of N2O production is generally believed to be the nitrification process when soil WFPS levels are between 30% and 70% and the denitrification process when soil WFPS levels were between 70% and 90% (Granli and Bockman, 1994). Some other studies also suggest that denitrification in general could produce more N2O compared with nitrification (eg. Dobbie et al., 1999). According to the studies of Ding et al. (2007) and Wan et al. (2009), N2O in sandy loam soils of the North China Plain was primarily produced by nitrification unless soil WFPS reached 75% or more. Pihlatie et al. (2004) reported that even at 100 % WFPS in a loamy sand soil with 24 g organic C kg-1, nitrification was still the dominant N2O production process. In this study, the highest soil WFPS measured during the peak emission period was ~ 65%; thus we suggest that low soil moisture limited denitrification and N2O production from the nitrate-based fertilizer in the test soil.

In the North China Plain, the addition of starch to soil treated with nitrate-based fertilizers in the field stimulated N2O production through denitrification, but wheat straw amendment did not do so (Wan et al., 2009; Ju et al., 2011). Previous studies demonstrated that denitrification was not only controlled by soil moisture and nitrate, but also by organic C supply, and increasing organic C availability could reduce the minimum

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soil moisture threshold for denitrification (van Groenigen et al., 2004; Chantigny et al., 2013). Yu et al. (2012) found that the mass proportion of macroaggregates in a NPK-treated soil with 6.0 g organicC kg-1 only accounted for 8.8%, while this proportion amounted to 30.8% in an 18 year compost-added soil with 10.0 g organic C kg-1 in the North China Plain. This change significantly increased the proportion of pores with a neck diameter < 4^ by reducing the proportion of pores with a neck diameter of 15-60 which in turn lowered the effective diffusion coefficient of oxygen in the soils and the ratio of monounsaturated to branched phospholipid fatty acids (PLFAs) i.e. aerobic to anaerobic microorganisms (Zhang et al., 2014). According to results found by Myrold and Tiedje (1984), only large aggregates have anaerobic microsites. Thus, it is likely that the relatively low organic C concentration in the test soil retards macroag-gregation and slows formation of anaerobic microsites, which in turn results in rise of the minimum moisture threshold required for denitrification. Consequently, the denitrification process is of much less importance than nitrification for N2O production and emissions in soils of the North China Plain.

In a German silt loam soil, similar to that tested in this study, Rover et al. (1998) reported that winter was a key period for N2O emissions from arable crops in the temperate climate zone, contributing ~ 70% of the annual N2O losses during the thawing from December to February. Wolf et al. (2010) also verified that N2O pulses due to spring thaw dominated total annual N2O emission in a steppe grassland of Inner Mongolia, China. At our site, spring thawing of the soil at the fertilized plots only caused minor N2O emission pulses, which were considerably lower than those reported earlier for other arable soils (Syvasalo et al., 2004; Teepe et al., 2000). It is suggested that reduced oxygen supply through alteration of pore structure during thawing, and high soil water contents in the winter, would promote microbial denitrification (Edwards and Killham, 1986; M0rkved et al., 2006). Our present study, together with previous measurements (Ding et al., 2007; Ju et al., 2010; Cai et al., 2012; Cui et al., 2012), showed that the highest soil WFPS was no more than 70% during the spring thawing period, a value that was lower than the threshold value of 80 % for thawing N2O pulses in a silt

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loam found by Rover et al. (1998). The cumulative rainfall during the winter period from December 2009 to February 2010 was only 4.4 mm and no apparent snow cover was observed at our study site. So the warm temperate monsoon zone, with cold and dry winter in the North China Plain, which is distinctly different from other climatic zones such as western Europe (Dobbie and Smith, 2003) and inner Mongolia of China (Wolf et al., 2010), would not induce thawing N2O pulses from arable soils, as found in our study. Our results confirm that N fertilizer sources influence soil N2O emissions, but that this effect probably depends on soil properties and especially climate conditions. Our study also suggests that, compared with urea or ammonium-based fertilizer, applying nitrate-based fertilizer is an effective management strategy to mitigate N2O emissions and to increase NUE and wheat yield in the North China Plain.

4.2 Nitrous oxide emissions as affected by inhibitors

The application of basal fertilizer urea followed by flooding irrigation resulted in N2O emission pulses for 18 days. This finding is in agreement with those of other studies for arable fields (Bouwman et al., 2002; Ding et al., 2007; Cui et al., 2012). The presence of inhibitors NBPT and/or DCD significantly lowered N2O peak fluxes, and cumulative N2O emissions during the 18 day peak emission period were reduced by 50.0% by NBPT, 78.6% by DCD and 67.9% by NBPT + DCD, compared with application of urea alone. Our results indicate that the addition of DCD alone or in combination with NBTP effectively reduced N2O emissions from application of urea. In other sites of the North China Plain, Liu et al. (2013) also reported that nitrification inhibitors DCD and DMPP (3,4-dimethylpyrazole phosphate) could reduce N2O emissions from application of N fertilizers by 30 % and 21 %, respectively, during the wheat growth season. Ju et al. (2011) observed no apparent differences in cumulative N2O emissions between zero N control and urea with DMPP during the maize growth season, suggesting strong nitrification inhibition effectiveness of DMPP.

N2O emission is directly related to the amount of mineral N available in the soil and application of inhibitors with urea can effectively regulate the NO- and NH+ concen-

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trations (Li et al., 2009; Zaman et al., 2009). Recently, Maharjan and Venterea (2013) demonstrated that N2O emissions were more correlated with soil NO- intensity rather than NO- or NH+ intensity, and that inhibitors controlled N2O production by adjusting soil NO- intensity. In this study, soil NH+ concentration slightly increased in the presence of DCD and, in contrast, relatively low NH+ concentration was found after NBPT application following application of basal fertilizer. NBPT delays urea hydrolysis, thereby lowering soil pH elevation and NH+ production, which can in turn reduce NH3 toxicity effects on nitrite-oxidizing bacteria (NOB). DCD slows oxidation of NH+ to NO- mainly by inhibiting activities of ammonia-oxidizing bacteria (AOB), which allows NOB to use NO- at the rate closely matched to its production rate (Zaman et al., 2008; Maharjan and Venterea, 2013). Both NBPT and DCD could additively attenuate formation of N2O from urea in the soil. Thus, the reduction of N2O emissions by inhibitors is probably due to both low oxidation rate of NH+ and low NO- concentration, thereby reducing N2O "leaking" as a by-product of nitrification (Firestone and Davidson, 1989).

Following supplemental fertilization with or without inhibitors, no distinct N2O flux peaks were found in our study. This may be attributable to no significant increase of soil NH+ and NO- concentrations after urea top-dressing. Cui et al. (2012) ascribed low increases in mineral N concentrations to large losses of urea-derived ammonia via volatilization. However, a field measurement at our study site showed that < 1 % of the N applied was lost via volatilization following urea top-dressing in March (Ni et al., 2009); so a large amount of NH3 loss would not occur at our site. Milchunas et al. (1988) suggested that urea hydrolysis is primarily affected by soil moisture. An incubation at 13°C demonstrated that lowering soil moisture level from 60% to below 40 % water holding capacity produced a longer lag before ammonia evolution and considerably retarded urea hydrolysis (Foster et al., 1980). The range of soil WFPS between 40 and 60 % during the period following urea top-dressing with subsequent irrigation suggested that soil moisture could partly have affected N2O production. Suter et al. (2011) observed that lowering incubation temperature from 25 °C to 5 °C greatly retarded the hydrolysis of urea when WFPS was below 60%, especially for an alka-

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line soil with low urease activity. In contrast, the temperature decrease increased the inhibitory effectiveness of NBPT on urea hydrolysis. In this study, soil temperature measured in the field after urea top-dressing varied from 2 °C to 9 °C, close to or just above the thresholds for nitrification (above 5.0 °C) (Anderson et al., 1971) and urea hydrolysis (~ 2 °C) (Xu et al., 1993; Yadav et al., 1987). It is obvious that low soil temperature led to the absence of fertilizer N-induced N2O flux peaks following the supplemental fertilization and urease or nitrification inhibitors should not necessarily be applied with supplemental fertilizers during the wheat growth season.

Application of urea with NBPT and/or DCD slightly increased wheat yields, which differs from application of NP. Similar results were also obtained by Ju et al. (2011) and Liu et al. (2013) in the North China Plain. A meta-analysis of data measured in Germany showed that N fertilizers with nitrification inhibitors did not significantly influence the yields of all investigated crops (Hu et al., 2014). The absence of inhibitor effects on crop yields might have been ascribed to the following three reasons. Firstly, low precipitation during the wheat growth season reduced the risk of N leaching and resulted in low N losses. This is evidenced by the significant stimulation of NP on wheat yields. Secondly, it is well known that the application rate of N fertilizers is far above optimum for crops (West et al., 2014). The overloading of N fertilizer might mask the influence on crop yields of increased mineral N in soils caused by inhibitors. The result of Sharma and Prasad (1996) supported the hypothesis that application of DCD significantly increased maize yield when the application rate of fertilizer N was as low as 60 kg N ha-1. It should be noted that the increase in NH+ concentration in the test soil due to DCD application alone following the basal fertilization may stimulate NH3 volatilization, resulting in higher N losses compared with urea alone, since NH3 volatilization accounted for ~ 13% of N applied (Ni et al., 2009). Mahmood et al. (2011) demonstrated that application of DCD to an alkaline calcareous soil increased fertilizer N losses. Finally, also more importantly, application of DCD with supplemental fertilizer slowed the nitrification rate and then lowered NO- supply for wheat growth when it was at the rapid growth stage. A lower soil NO- concentration in the NP treatment than in the urea-added

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treatments following the supplemental fertilizer in this study supports this speculation. Based on this study, it is not necessary to apply DCD with supplemental fertilizer urea and a combination of urease and nitrification inhibitors would be a better approach to reduce N2O emission than urease or nitrification inhibitor application alone with basal 5 fertilizer urea for wheat cultivation.

5 Conclusions

The present field study provided an insight into N2O emissions from a calcareous soil during the wheat growth season in the North China Plain, as affected by application of urease or nitrification inhibitors and nitrate-based fertilizer nitrophosphate. A single 10 N2O flux peak was found following basal fertilization during the wheat growth period. Application of urea with NBPT, DCD or NBPT + DCD significantly reduced N2O emissions from urea by 36.7%, 42.9% or 46.9%, respectively. Application of nitrophosphate also resulted in reduction of total N2O emissions by 42.9%, compared with application of urea alone. NBPT and/or DCD were effective in reducing N2O emissions following 15 basal fertilization. Compared with urea application alone, application of inhibitors with urea, either individually or combined together, slightly increased wheat yield and NUE, while nitrophosphate significantly increased wheat yield by 12.3% and increased NUE from 28.8% (urea alone) to 35.9%. N2O flux was primarily affected by soil temperature and low temperature at the study site minimized fertilizer N-induced N2O peaks 20 following application of supplemental fertilizer. Based on our findings, the combination of NBPT and DCD with basal fertilizer urea would be an effective practice for reducing N2O emission. As well, this study suggests that application of nitrophosphate, instead of urea, is an optimum agricultural strategy for reducing N2O emission and for increasing crop yield and NUE for wheat cultivation in the soils of the North China Plain.

25 Acknowledgements. This work was supported by the National Basic Research Program of China (2012CB417102), Strategic Priority Research Program of the Chinese Academy of

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Sciences (XDB15020101) and National Natural Science Foundation of China (41171191, 41471207).

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Zaman, M., Saggar, S., Blennerhassett, J. D., and Singh, J.: Effect of urea and nitrification inhibitors on N transformation, gaseous emissions of ammonia and nitrous oxide, pasture yield and N uptake in grazed pasture system, Soil Biol. Biochem., 41, 1270-1280, 2009.

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Table 1. Soil properties.

Soil depth pH Bulk density Organic C Total N C/N NO--N NH+-N Particle size distrubution (%)

(cm) (H2O) (Mgm-3) (gCkg-1) (gNkg-1) (mgNkg-1) (mgNkg-1) Sand Silt Clay

0-20 8.60 1.40 12.0 1.50 8.0 14.70 2.48 17.0 72.0 11.0

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Table 2. Effects of urease and/or nitrification inhibitors and nitrophosphate on wheat biomass, amount of N uptake by crops and N use efficiency.

Treatment Biomass (kg ha 1) Amount of N uptake (kg N ha 1 ) N use efficiency (%)

Grain Straw Total Grain Straw Total

Control 2297 ± 150 c 2215 ± 134 b 4513 ± 283 c 24.4 ± 1.6 c 11.9 ± 0.7 b 36.4 ± 2.3 c -

U 4652 ± 11b 4075±81 a 8727 ±85 b 59.6 ± 0.1 b 34.2 ± 0.7 a 93.9 ± 0.7 b 28.8 ± 0.8 b

NBPT 4711 ± 126 b 4098 ±356 a 8809 ±472 b 60.4 ± 1.6 b 34.4 ± 3.0 a 94.8 ± 4.5 b 29.2 ± 1.0 b

DCD 4736 ± 103 ab 4080 ±52 a 8816 ± 86 b 60.7 ± 1.3 b 34.3 ± 0.4 a 95.0 ± 1.1 b 29.3 ± 0.9 b

NBPT + DCD 4735 ± 290 ab 4535 ±503 a 9271 ± 764 ab 60.7 ± 3.7 b 38.1 ± 4.2 a 98.8 ± 7.6 ab 31.2 ± 1.2 b

NP 5225 ± 142 a 4906±251 a 10131 ± 370 a 67.0 ± 1.8 a 41.2 ± 2.1 a 108.2 ± 3.7 a 35.9 ± 1.1 a

Mean ± standard error (n = 3).

Different letters within the column indicate significantly difference between treatments at P < 0.05.

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Table 3. Correlation between ln[N2O flux + 1] and soil WFPS, soil temperature at depths of 5 (T5cm), 10 (T10cm) and 15cm (T15cm), ammonium (NH+-N), nitrate (NO--N) or inorganic nitrogen (NH+-N plus NO--N) concentration.

Treatment WFPS T5 cm T10cm T15cm NH+-N NO--N Inorganic N

Control 0.095 0.413** 0.376* 0.392* -0.153 0.140 0.109

U 0.023 0.381** 0.340* 0.346* 0.274* 0.365** 0.380**

NBPT 0.118 0.275* 0.264 0.274 0.215 0.206 0.222

DCD 0.323* 0.282* 0.180 0.189 0.104 -0.127 -0.092

NBPT + DCD 0.021 0.216 0.252 0.272 0.074 0.155 0.156

NP 0.084 0.301* 0.370* 0.403** 0.105 -0.056 -0.037

: P < 0.05, ** P < 0.01.

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Table 4. Effects of urease and/or nitrification inhibitors and nitrophosphate on cumulative N2O emissions, fertilizer N-induced N2O emission factors and yield-scaled N2O emissions.

Treatment Cumulative N2O emission (kgN2O-Nha 1) Total Peak Ratio of peak to total emissions (%) Emission factor (% of applied N) Yield-scaled N2O emission (gN2O-Nkg-1 grain)

Control 0.16 ± 0.02c 0.03 ± 0.00 d 18.8 ± 2.3d - 0.068 ± 0.006 b

U 0.49 ± 0.12 a 0.28 ± 0.10a 57.1 ± 4.2 a 0.17 ± 0.05 a 0.105 ± 0.026 a

NBPT 0.31 ± 0.01 b 0.14 ± 0.01 b 45.2 ± 3.9 b 0.08 ± 0.00 b 0.065 ± 0.003 b

DCD 0.28 ± 0.01 b 0.06 ± 0.01 c 21.4 ± 2.1 d 0.06 ± 0.00 b 0.060 ± 0.004 b

NBPT + DCD 0.26 ± 0.01 b 0.09 ± 0.00 bc 34.6 ± 3.2 c 0.05 ± 0.00 b 0.056 ± 0.003 b

NP 0.28 ± 0.03 b 0.11 ± 0.03 bc 39.3 ± 3.7 c 0.06 ± 0.01 b 0.053 ± 0.008 b

Mean ± standard error (n = 3).

Peak emission denotes cumulative emissions during the 18 days' period following the basal fertilizer application from 16 October to 3 November. Different letters within the column indicate significantly difference between treatments at P < 0.05.

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Table 5. Effects of urease and/or nitrification inhibitors and nitrophosphate on soil ammonium (NH4I), nitrate (NO3I) and inorganic N (IONI) intensities.

Treatment

(gNdkg-1)

(gNdkg-1)

(gNdkg-1)

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U 0.40 ± 0.03 d 4.75 ± 0.13 c 5.15 ± 0.16 c

NBPT 0.61 ± 0.02 c 6.18 ± 0.08 a 6.79 ± 0.08 b

DCD 0.96 ± 0.01 b 5.74 ± 0.01 b 6.70 ± 0.01 b

NBPT +DCD 1.07 ± 0.01 a 6.11 ± 0.16 a 7.17 ± 0.16 a

NP 0.36 ± 0.02 d 4.69 ± 0.09 c 5.05 ± 0.07 c

Mean ± standard error (n = 3).

Different letters within the column indicate significantly difference between treatments at P < 0.05.

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Figure 1. Temporal variation of daily precipitation and air temperature, and mean soil moisture and water-filled pore space (WFPS) at time of N2O sampling in the control, urea alone (U), urea with NBPT (NBPT), urea with DCD (DCD), urea with NBPT + DCD (NBPT + DCD) and nitrophosphate (NP) treatments during the wheat growth season. The standard errors of soil temperature and moisture were not shown for figure clarity.

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Figure 2. Temporal variation of nitrous oxide fluxes in the control, urea alone (U), urea with NBPT (NBPT), urea with DCD (DCD), urea with NBPT + DCD (NBPT + DCD) and nitrophos-phat e (NP) treatments during the wheat growth season. Flux values are mean values ± standard errors for three replicates. Arrows indicate date of fertilizer application.

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Figure 3. Temporal variation of ammonium and nitrate concentrations in samples from the 0-20cm depth in the control, urea alone (U), urea with NBPT (NBPT), urea with DCD (DCD4, urea with NBPT + DCD (NBPT + DCD) and nitrophosphate (NP) treatments during the wheat growth season. Vertical bars denote the standard error of the means (n = 3).

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Inhibitors and nitrophosphate effects on N2O emissions

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