Scholarly article on topic 'Cropping system innovation for coping with climatic warming in China'

Cropping system innovation for coping with climatic warming in China Academic research paper on "Agriculture, forestry, and fisheries"

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Abstract of research paper on Agriculture, forestry, and fisheries, author of scientific article — Aixing Deng, Changqing Chen, Jinfei Feng, Jin Chen, Weijian Zhang

Abstract China is becoming the largest grain producing and carbon-emitting country in the world, with a steady increase in population and economic development. A review of Chinese experiences in ensuring food self-sufficiency and reducing carbon emission in the agricultural sector can provide a valuable reference for similar countries and regions. According to a comprehensive review of previous publications and recent field observations, China has experienced on average a larger and faster climatic warming trend than the global trend, and there are large uncertainties in precipitation change, which shows a non-significantly increasing trend. Existing evidence shows that the effects of climatic warming on major staple crop production in China could be markedly negative or positive, depending on the specific cropping region, season, and crop. However, historical data analysis and field warming experiments have shown that moderate warming, of less than 2.0°C, could benefit crop production in China overall. During the most recent warming decades, China has made successful adaptations in cropping systems, such as new cultivar breeding, cropping region adjustment, and cropping practice optimization, to exploit the positive rather than to avoid the negative effects of climatic warming on crop growth. All of these successful adaptations have greatly increased crop yield, leading to higher resource use efficiency as well as greatly increased soil organic carbon content with reduced greenhouse gas emissions. Under the warming climate, China has not only achieved great successes in crop production but also realized a large advance in greenhouse gas emission mitigation. Chinese experiences in cropping system innovation for coping with climatic warming demonstrate that food security and climatic warming mitigation can be synergized through policy, knowledge, and technological innovation. With the increasingly critical status of food security and climatic warming, further efforts should be invested in new agricultural policy, knowledge and technology creation, and popularization of climate-smart agriculture, and more financial investments should be made in field infrastructure development to increase cropping system resilience in China.

Academic research paper on topic "Cropping system innovation for coping with climatic warming in China"

CJ-00191; No of Pages 15

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Cropping system innovation for coping with climatic warming in China

Aixing Denga,1f Changqing Chenb'1, Jinfei Fengc, Jin Chend, Weijian Zhanga'*

aInstitute of Crop Science, Chinese Academy of Agricultural Sciences/Key Laboratory of Crop Physiology and Ecology, Ministry of Agriculture, Beijing 100081, China

bInstitute of Applied Ecology, Nanjing Agricultural University, Nanjing 210095, China

cChina National Rice Research Institute, Chinese Academy of Agricultural Sciences, Hangzhou 310006, China

dSoil and Fertilizer & Resources and Environmental Institute, Jiangxi Academy of Agricultural Science, Nanchang 330200, China

ARTICLE INFO ABSTRACT

Article history: Received 23 April 2016 Received in revised form 23 June 2016 Accepted 17 July 2016 Available online 29 July 2016

Keywords: Global warming Food security Grain production Response and adaptation Carbon emission mitigation

China is becoming the largest grain producing and carbon-emitting country in the world, with a steady increase in population and economic development. A review of Chinese experiences in ensuring food self-sufficiency and reducing carbon emission in the agricultural sector can provide a valuable reference for similar countries and regions. According to a comprehensive review of previous publications and recent field observations, China has experienced on average a larger and faster climatic warming trend than the

global trend, and there are large uncertainties in precipitation change, which shows a non-significantly increasing trend. Existing evidence shows that the effects of climatic warming on major staple crop production in China could be markedly negative or positive, depending on the specific cropping region, season, and crop. However, historical data analysis and field warming experiments have shown that moderate warming, of less than 2.0 °C, could benefit crop production in China overall. During the most recent warming decades, China has made successful adaptations in cropping systems, such as new cultivar breeding, cropping region adjustment, and cropping practice optimization, to exploit the positive rather than to avoid the negative effects of climatic warming on crop growth. All of these successful adaptations have greatly increased crop yield, leading to higher resource use efficiency as well as greatly increased soil organic carbon content with reduced greenhouse gas emissions. Under the warming climate, China has not only achieved great successes in crop production but also realized a large advance in greenhouse gas emission mitigation. Chinese experiences in cropping system innovation for coping with climatic warming demonstrate that food security and climatic warming mitigation can be synergized through policy, knowledge, and technological innovation. With the increasingly critical status of food security and climatic warming, further efforts should be invested in new agricultural policy, knowledge and technology creation, and popularization of

* Corresponding author. E-mail address: zhangweijian@caas.cn (W. Zhang).

Peer review under responsibility of Crop Science Society of China and Institute of Crop Science, CAAS. 1 These authors contributed equally to this study.

http://dx.doi.Org/10.1016/j.cj.2016.06.015

2214-5141/© 2016 Crop Science Society of China and Institute of Crop Science, CAAS. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

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climate-smart agriculture, and more financial investments should be made in field infrastructure development to increase cropping system resilience in China. © 2016 Crop Science Society of China and Institute of Crop Science, CAAS. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction

There is evidence of genuine world climatic warming [1]. The global annual average surface temperature in 2015 is set to reach 1 °C above the pre-industrial average and is predicted to increase by 1.3-1.7 °C by 2050 [2]. Even if the world can reduce global carbon emissions by 2020 to a lower level than that of 1990, the temperature will still increase by 2.0 °C by 2100. Simultaneously, there is another critical issue, food security, associated with global population increase and economic development. For ensuring global food security, world grain production will need to increase by more than 60% from its 2005-2007 levels by 2050 [3]. However, global total crop production increased by only 28% between 1985 and 2005 [4].

Great efforts have been made in the assessment of climatic warming impacts on global crop production. For example, one study showed that even a 1 °C increase in daily minimum temperature would reduce rice yield by 10% [5]. However, other studies show that the effects of climatic warming on crop production will differ by cropping region and crop [6-8]. Global maize and wheat production may decline by 3.8% and 5.5%, respectively, while soybean and rice may remain unchanged owing to a global balance between winners and losers [3]. In some regions at high latitude, climatic warming may benefit crop production [9]. To our knowledge, there are still large uncertainties in the assessment of climatic warming effects on global grain production. Reducing the uncertainty will greatly benefit the development of strategy for innovation in crop production technology and policy for coping with climatic warming. Further efforts still need to be made in the evaluation of climatic warming effects on crop production in specific areas.

China is the largest food-producing and -consuming country, owing to having the highest rates of population increase and economic development. Total grain (rice, wheat, and corn) production in China was 557.2 Mt in 2014, accounting for 19.7% of global grain production. However, total grain and soybean imports were 19.5 and 71.4 Mt, respectively, in the same year [10]. China is becoming the largest grain-importing country in the world. Obviously, a reduction in Chinese grain production will affect food security not only in the country but also in the world. However, a large proportion of farmland in China is highly vulnerable to climate change, owing to limited arable land area and available fresh water, so that even moderate warming may exert a severe effect on crop production [11]. During the past decades, increases in air temperature warming and extreme weather occurrence frequency in China have exceeded the average levels in the world [12]. Thanks to improvement in crop cultivars and innovations in agronomic practices, China has achieved great successes in grain production with a sustained increase rate during the latest, warmest years. For developing strategies for crop production aimed at ensuring food security, it is very helpful to study historical experiences in coping with climatic warming [11]. We have accordingly conducted a

comprehensive review based on previous publications and our own investigations of changes in cropping systems in China in recent decades. Our objectives were to summarize climatic warming trends, effects of warming on major staple food crop production, and adaptations of cropping systems in the country, with the aim of providing references for other regions and countries faced with maintaining food security under climatic warming.

2. Evidences and trends of climatic warming in China

2.1. Changes in air temperature

Increasing evidence indicates that climate change in China shows considerable similarity to global change, though there are still some marked differences between the two [12,13]. The country-averaged annual mean surface air temperature has markedly increased over the past 100 years and the change ranged between 0.5 and 0.8 °C [14], slightly higher than the global temperature increase during the same periods. Showing a trend similar to the global warming trend, northern China in the winter season has experienced the greatest increases in surface air temperature.

Based on records of the Chinese Meteorological Administration [15], the annual mean surface air temperature has increased by 1.2 °C over the past 55 years (Fig. 1-a), a greater increase than that of a previous assessment [16]. Especially during the past 20 years over 1994-2014, the mean air temperature increased rapidly, with values of 0.65 °C. Spatial interpolation of mean air temperature trends also shows that northern and northwestern China, high-latitude regions, experienced the greatest—of more than 0.5 °C—increases in surface air temperature over 1960-2014 (Fig. 2-a). However, no clear trend was found in southwestern China, and there was only a slight increase in other parts of southern China. Although there are great differences in warming rates among different cropping regions in China during the past decades, most previous studies and present records show that China has experienced pronounced warming, especially in the last 20 years [12]. Changes in air temperatures may have strongly influenced and will continue to affect cropping systems via effects on crop growth period and the formation of grain yield and quality.

2.2. Changes in annual precipitation

There are large spatial differences in water resource and precipitation availability in China [12]. Southern China receives abundant precipitation and has experienced a moderate increase in air temperature, whereas northern China has a severe lack of effective precipitation and has experienced a strong warming trend. Given that temperature is the key driving force of the atmospheric hydrologic cycle, climatic warming

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Fig. 1 - Anomaly changes in annual mean air temperature and precipitation in China during 1960-2014. Data are from the Chinese Meteorological Administration. (a) Air temperature; (b) annual precipitation.

could positively or negatively affect spatiotemporal patterns and amounts of precipitation [17,18]. Based on available observations and records, however, changes in precipitation trends for the last 50 and 100 years have been negligible, in contrast to the large increase in air temperature, but since 1956 it has shown a weak increasing trend in China [19]. In temperature and precipitation records from 753 national meteorological stations for the period 1966-2005, the most obvious change was the eastern movement of the boundary between an arid and a subhumid temperate zone [20], indicating an increase in winter precipitation and extreme precipitation in the region. The changes in precipitation may also have strongly affected cropping systems.

Based on historical meteorological records [15], there were high variations in annual precipitation with no significant trend during 1960-2014, though precipitation showed a slight decreasing trend over the past 55 years (Fig. 1-b). In addition to high interdecadal variation in precipitation (Fig. 1-b), marked differences and obvious trends in annual precipitation on regional scales are detectable (Fig. 2-b). During the past 55 years, northeast China, northwest China or the Tibetan plateau and the middle and lower Yangtze River basin have shown a clear increasing

trend, while northern China and southwestern China have experienced a decrease in precipitation (Fig. 2-b). Although there has been a slight increasing trend in precipitation amounts on average [19], climatic warming could reduce precipitation frequency while increasing precipitation intensity, suggesting less effective precipitation for crop production [12].

2.3. Changes in extreme weather occurrence

Over the past several decades, China has experienced an increase in the frequency and intensity of main extreme weather and climate events. In 1998, for example, a severe flood inundated more than 20 Mha of land and five million houses in the Yangtze basin [12]. Winter-spring drought and summer heating stress occurred more frequently and strongly, causing severe losses in wheat yield in major winter wheat cropping areas in China. Similarly, increasingly severe heat stress affected rice growth in rice-wheat cropping systems in the region. Interdecadal variation in heat waves during 1961-2005 showed that the frequency and intensity of heat waves would increase dramatically with climatic warming [21]. However, interannual changes in climate zones showed

Longitude (°E) Longitude (°E)

Fig. 2 - Spatial differences in increases in annual mean surface air temperature and annual precipitation in China during 1960-2014. Data are from the Chinese Meteorological Administration. (a) Air temperature; (b) annual precipitation.

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Fig. 3 - Spatial distribution of daily mean, maximum, and minimum temperature increases under the 1.5 °C global-warming target by 2100 in China. Data are from the Chinese Meteorological Administration. (a) Daily mean temperature; (b) daily maximum temperature; (c) daily minimum temperature.

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Fig. 4 - Spatiotemporal changes in total grain production in China and yield over the period 1960-2014. Data are from the National Bureau of Statistics of China. (a) Temporal changes in grain production and yield; (b) spatial distribution of total grain production; (c) spatial distribution of grain yield.

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that China has been warming, but not becoming drier or wetter [22]. Moreover, a recent study showed that the frequencies of abnormally cold and heat temperatures could simultaneously increase in different regions and seasons [23]. Although there are increasing numbers of studies of the changes of extreme weather occurrence in China, our understanding of this subject is still weak owing to the limited number of records. Given the severe effects of extreme weather on crop production, more efforts need to be made to identifying trends of change in extreme weather and its potential effects, using multidisciplinary methods.

2.4. Climatic projected trends

According to long-term meteorological record analysis with global and regional climate models, surface air temperature will continue to increase and annual precipitation will likely increase simultaneously in China by the end of the 21st century [13]. In agreement with global warming patterns, the highest rates of air temperature increase will likely occur largely at nighttime, during winter months, and in northern regions of the country. Climate modeling has projected that the country-averaged annual mean temperature would increase by 1.5-2.1 °C by 2020, 2.3-3.3 °C by 2050, and 3.9-6.0 °C by 2100 depending on the emission scenario. Although no pronounced trend has been found in country-averaged annual precipitation, most models forecast a 10%-12% increase in annual precipitation in China by 2100, indicating a positive effect on prolonging crop growth period or extending cropping area northward.

At the Paris summit in December 2015, a goal to limit global warming to 2 °C or less by 2100 was established by more than 100 countries. It is thus imperative to curb global greenhouse gas generation to ensure that the goal is reachable. Given that the global warming threshold has been set to 1.5 °C on average as comparison with 1980-2010, an increase of 2.5 °C in daily mean temperature will be particularly evident in northeastern and northwestern China, but one of only 0.5 °C will be evident in southern China by 2100 (Fig. 3-a). There was no marked upward trend in daily maximum temperature, which showed an increase of less than 1 °C (Fig. 3-b). However, clear increasing trends in daily minimum temperature appear, particularly in northeastern and northwestern China, with increases averaging 4-5 °C (Fig. 3-c). Because nighttime warming may stimulate crop respiration, there may be great differences among different warming scenarios in warming impacts on cropping systems.

3. Impacts of climatic warming on crop production in China

3.1. Impacts on crop biological phenomena

Plant phenology is very sensitive to climate changes, especially warming. Based on long-term phenological observations, much evidence shows that climatic warming has caused great changes in plant phenological periods in grassland, forest, and agricultural ecosystems. Theoretically,

climatic warming can abolish low-temperature limitations to crop growth and development, in turn increasing growth duration. However, warming can worsen high-temperature stress on crop growth and development, advancing crop development and reducing growth duration. Warming can also increase or decrease the length of the crop growth period by affecting precipitation. Thus, the impacts of climatic warming on crop biological phenomena may depend on the specific crop, season, and region [24]. Owing to the unclear trends in spatiotemporal patterns of precipitation changes in China, more attention should be paid to investigating precipitation changes in specific cropping regions and during specific growing seasons.

The actual effects of climatic warming on crop phenology have been widely investigated in China. For example, Wang et al. [25] reported that increased temperature reduced the growth duration of winter wheat, mainly by shortening the growth period from sowing to jointing in the North China plain. Xiao et al. [26] found that single rice transplanting, heading, and maturity dates could generally advance, but the heading and maturity dates of single rice in the middle and lower reaches of the Yangtze River and the Northeast China plain could be delayed by climate change. Based on long-term field observation and meteorological records, the theoretical sowing and harvest dates of crops maybe five days earlier and five days later, respectively, in the 2000s than the 1970s, resulting in an increase in theoretical growth duration by 10 days in northeastern China [9]. Our recent observations also showed that an increase of air temperature about 1.0 °C at nighttime could markedly advance wheat flowering and maturity dates, by 5.9 days and 1.8 days, respectively, on average in the North China plain (unpublished data). This change will likely result in a reduction in the pre-flowering period by 6.0 days and an increase in the post-flowering period by 4.3 days in the region. Based on a novel field warming experiment using Free Air Temperature Increase, an increase of less than 1.5 °C in mean air temperature can affect wheat and rice growing period length in eastern China, mainly by shortening the crop pre-anthesis period and prolonging or maintaining the post-anthesis period [27,28]. Warming effects on crop phenology may depend on the warming scenario and the ambient temperature background of the cropping regions. Greater warming effects on crop growth period and grain yield can be found in cold than in warm areas and seasons.

3.2. Impacts on main staple food crop yields

Climatic warming can increase or decrease crop yield, depending on crop type, warming extent, and background temperature during the crop growing season, especially during the post-flowering grain-filling period. Given that warming can shorten the crop growing period and stimulate plant respiration, it might lead to a tremendous decrease in crop yield [5,29]. On the other hand, warming might benefit crop growth by allowing higher yield by mitigating low temperature/chill injury and relieving heat stress by shifting the post-anthesis phase to more favorable temperature conditions [24]. Modeling has shown that climatic warming without CO2 elevation could reduce rice, maize, and wheat

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yields by up to 37% [30]. Based on crop and climate data from representative meteorological stations across China during 1981-2000, Tao et al. [31] also concluded that rice and wheat yields would decline by 3.7% and 10.2%, respectively, for each 1 °C increase in daily mean air temperature during the rice growing season, while maize yield would remain unchanged. However, modeling results have been challenged by field observations and recent field warming experiments. In fact, great increases in crop yields have been observed during the warming years, though agronomic technology has contributed greatly [32-34].

Recently, some field warming experiments showed that an increase of less than 2.0 °C in air temperature would increase winter wheat yield by more than 15% in major winter wheat cropping areas in China without water limitation [28]. Similar field warming experiments were also conducted in major rice cropping regions in China [27], showing that an increase of less than 2 °C in daily mean temperature would reduce the early rice yield of a double rice cropping system and the middle rice yield of a rice-wheat cropping system, and increase grain yields of later rice in a double cropping system and in the northeast in a single rice cropping system. For the northeastern region, warming will benefit all crop production by limiting low temperatures [9]. Historical warming has led to large annual increases of 16.6, 15.5, and 3.2 kg ha-1 in rice, corn and soybean yields, respectively, in northeastern China over the period 1970-2009. As evidence increases, most studies show that moderate warming, of less than 2.0 °C, will benefit most crop production in China if there is no severe precipitation shortage. For a more accurate assessment of climate change impacts on grain production, more studies of warming effects on crop growth in specific cropping regions, crops and growing seasons in situ are needed. Moreover, it is necessary to integrate the results from field warming experiments, historical data analysis and modeling analysis to improve our knowledge base.

3.3. Effect on crop quality

Compared to warming effects on crop hysteresis phenomena and productivity, there is little knowledge about warming effects on crop quality, although most researchers believe that high temperature during grain filling may lower crop quality. As for protein components, Lin et al. [35] found that high temperature increased rice glutelin while decreasing prolamin and globulin. Jiang et al. [36] found increased amounts of long-chain rice amylopectin at high temperature. Lin et al. [35] showed that climatic warming would reduce amylase content and flour gel consistency of rice grains. Tian et al. [37] found that field warming at different times tended to increase the amylose-to-amylopectin content ratio but decrease the protein content of wheat. Zhang et al. [38] reported that high field temperature increased grain crude protein content, while decreasing ether extract and starch content of corn. Although there is increasing evidence of climatic warming effects on crop quality, great efforts still need to be made to increase our understanding of these effects.

4. Cropping system innovations for food security

4.1. Success in crop production in China

Thanks to innovations in crop production technology and economic development, grain (rice, wheat and maize) production in China has achieved great success during the last two decades, the warmest years. Total grain production and yield in the country have increased by 322.0% and 358.3%, respectively, during 1960-2014 (Fig. 4-a). Total grain production and yield were 607.0 Mt and 5.6 t ha-1, respectively, the highest values on record in China. Although there were decreasing trends in total grain production and yield during 1999-2003, they have increased greatly since 2003.

Fig. 5 - Changes in the spatial distribution of rice cropping areas in Heilongjiang, China. Data are from the National Bureau of Statistics of China. Unit of color is the area ratio between sown and cultivated land. (a) 1978; (b) 2010.

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Interestingly, the highest rates of increase in total production and yield have been found in northern areas (Fig. 4-a, b), which are also at high latitudes and show the greatest warming. In addition to grain, other agricultural products have simultaneously greatly increased during the past decade. For example, total production of cotton, meat, eggs, and milk in China were 6.3, 85.4, 28.8, and 35.3 Mt, respectively, in 2013, which are 42.6%, 41.9%, 31.8%, and 326.8% higher, respectively, than those in 2000. The patterns of spatiotemporal change in major grain production in China and yield suggest that historical warming has promoted crop production especially in the regions with the highest warming rates.

Chinese success in grain production can be attributed to several factors, including progress in agricultural science and technology, policy innovations, and increases in agricultural production inputs. To ensure self-sufficiency in grain production, the Chinese government has made great efforts in agricultural science and technology innovation, especially in new crop cultivar breeding and improvements in agronomic practice for coping with the changing environment. Agricultural science and technology progress, especially in crop cultivar improvement and agronomic innovations, has made the largest contribution to grain production improvement in China during recent decades [39]. The contributions of agricultural science and technology progress to yield gains during 1984-2007 were 66%, 44%, 42.1%, and 60% for rice in the south, rice in the north, wheat, and corn [40]. Recently, the Chinese government announced that agricultural science and technology progress contributed 56% of the continuous 11-year increase in grain yield and farmer household income.

4.2. Staple crop cultivar improvement

Climatic warming will exert severe adverse effects on crop production by increasing the frequency and intensity of some abiotic andbiotic stresses, such as heat, drought, and diseases [12]. Breeding new crop cultivars with high resistance to these stresses cannot only exploit warming-led positive effects of prolongation of growing period but also avoid warming-led negative impacts [41]. For example, crop breeders can develop new crop cultivars to accurately match warming-led changes inmoisture and temperature and escape or avoid warming-led occurrences of abiotic stresses at critical periods in the crop growing season.

To cope with changing environments, crop scientists of China have made pronounced progress in developing new crop cultivars and breeding strategies over recent decades [39]. Although climatic warming is predicted to shorten the wheat growth period, Chinese scientists have not focused on extended the growth period of new wheat cultivars. However, great progress has been achieved in increasing wheat grain filling rate and grain weight [42]. Meanwhile, the resistance of new wheat cultivars to drought has been greatly increased to allow it to tolerate warming-led severe winter moisture limitation and summer heat stress [43]. Over the period 1981-2008 in China, the average annual genetic gain in annual grain yield was 51.3 kg ha-1 This genetic improvement in grain yield is attributed directly to increased 1000-grain weight in major wheat cropping areas in China, although there has been negligible change in the length of the growing period from old to new cultivars. The breeding strategy of

improving wheat 1000-grain weight is identical to the positive effects of warming on wheat grain filling, as shown in recent field warming experiments [28].

For rice cultivar breeding, great efforts have been made to prolong the growing period and enhance low-temperature resistance rather than increase resistance to heat and drought stresses. Owing to climatic warming, farmers plant rice increasingly early and delay harvesting. Although air temperature is increasing, low-temperature stress is occurring more frequently during the early spring and late autumn. In northeast China, all crop growing periods have been prolonged by development of new cultivars for more accurate matches to climatic warming effects. For example, the growth periods of newly approved varieties of rice, corn, and soybean have been prolonged since the 1950s by 14.0, 7.0, and 2.7 days, respectively, in the region [9]. New approved crop cultivars display greater resistance to low temperature than the old cultivars in the region. In fact, Chinese crop scientists have adopted a progressive and active adaptation strategy of crop breeding for coping with climatic warming by exploiting positive rather than avoiding negative effects on crop growth and development. For winter wheat, crop breeding scientists have made great efforts to increase grain weight rather than prolong wheat growth period, although warming can markedly shorten wheat growth duration. In northeast China, the largest warming benefit for crop production has been the extension of crop growth period and cropping area. The main efforts in crop breeding have focused on enhancing low-temperature resistance and prolonging the crop growth period.

4.3. Agronomic practice innovations

Agronomic practice innovations have played an important role in adapting cropping systems to higher yield under climatic warming. For example, farmers have extended crop growth periods by adjusting crop sowing and harvest timing according to increasing trends in air temperature. In northeast China, all crop sowing dates have been advanced and harvesting dates have been delayed during the period 1970-2009 [9]. As a result, the growing periods of rice and corn in the region have been extended by respectively 6 and 4 days since the 1990s, resulting in increasing trends in corn and rice yield of 15.5 and 16.6 kg ha-1 year-1 [9]. Another study assessing adaptation showed that use of earlier sowing dates has increased yields by up to 4% and that adoption of longer-season cultivars has led to a substantial (13-38%) increase in yield over the past 27 years [44]. Based on an extensive rice phenology dataset in China during the 1980s-2000s, Zhang et al. [23] found that the sowing dates of single rice in the northeast and early and late rice in the south were advanced by 0.8, 0.9, and 0.3 days °C-1, respectively. In north China, however, dates of sowing, emergence, and dormancy of winter wheat were delayed on average by 1.5, 1.7, and 1.5 days per decade during the period 1981-2009, whereas the sowing date of corn has advanced with a large delay in harvest date [26]. The length of the early rice growing period in south China and that of the winter wheat growing period have been greatly reduced, while the later rice growing period in the south and that of middle rice in rice-wheat cropping systems and summer corn in wheat-corn cropping systems

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2005 2010 2015

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Fig. 6 - Increase in conservation cropping areas during 2002-2015 and soil carbon stock changes in arable lands during the 1990s-2000s in China. (a) Conservation farming area; (b) soil organic carbon stock.

have been prolonged [45,46], resulting in a large gain in annual crop yield [46].

In addition to optimization of sowing and harvesting timing, other agronomic innovations have been introduced to respond to climatic warming in major cropping areas in China. As an alternative to conventional tillage, low/no-tilling plus subsoiling has been widely recommended for water saving, especially in dry areas. The Chinese government has provided a subsidy of about $50 ha-1 to disseminate subsoiling since 2011, and the cropping area with subsoiling increased to 13.33 Mha in 2015. Other practices, such as conservation farming, crop straw incorporation, and organic fertilizer application, have also been implemented to strengthen the resilience of the cropping system to climatic warming, and corn yields under straw incorporation and organic fertilizer application have shown significant annually increasing trends, with rates of 0.137 and 0.194 t ha-1, respectively [47]. Meanwhile, the central and local governments have initiated many projects to build cropping system resilience to climatic change by improving field operations, such as by optimizing irrigation, introducing protected fields for seedling nursery, and film mulching. All innovations have been developed for exploiting the potential positive effects of climatic warming on crop production as well as for mitigating the negative effects of air temperature increase.

northward by about 100 km in 2010 in comparison with the regions in 1970 (Fig. 5). In Heilongjiang province, the northernmost province in China, the rice cropping area has been extended to 13.33 Mha, against only 0.11 Mha in 1949. Owing to the spatial distribution optimization of cropping systems, northeast China has become the area with highest grain production in China, accounting for 23.8% of total grain production in China in 2015.

Yield gains of spatial distribution adaptations have also been found in other major cropping areas in China during recent decades. Based on long-term field observations, Yang et al. [48] found that northern limits of multiple cropping systems in China have shifted markedly northward, resulting in a 2.2% (~ 8,000,000 t) increase in national production of grain crops (maize, wheat, and rice) during the period 1981-2010. Most of the spring wheat cropping areas have been replaced by winter wheat cropping areas in north China, resulting in a great increase in national wheat yield. Based on both historical climate records and climate SRES-A1B emission scenario data for China, the area of cultivated land under triple-cropping systems may expand greatly during the 21st century [9,48]. Thus, it may be concluded that climatic warming at a moderate level, less than 2.0 °C, may benefit crop production in China if concomitant adjustments are made in multiple-cropping systems.

4.4. Spatial distribution optimization of major cropping systems

Much evidence have shown that climatic warming effects on crop production depends on specific cropping area and that cropping areas at high latitude may increase strong yield gains [3]. Thus farmers can optimize the spatial distribution of cropping areas to match temperature changes. For example, rice cropping regions in northeast China have been extended

5. Innovations in cropping systems for climate change mitigation

5.1. Greenhouse gas emissions from arable soils

Cropping systems are an important anthropogenic source of greenhouse gas emission (GHG). Total GHG, including CH4, N2O, and CO2, emissions, from arable soil in China were

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estimated to be 447.8 Tg CO2-eq in 2012, accounting for 21.2% of world GHG emissions from agricultural soils [49]. Rice cultivation is the primary source of CH4 emission, with a wide range of 3.5 to 1146.5 kg ha-1 in the seasonal cumulative emission rate, depending on climate, soil, and management factors. The mean seasonal CH4 emission rates for the three major rice cropping systems in China are, in descending order, double rice cropping system (573.3 kg ha-1), rice-upland crop rotations (209.3 kg ha-1), and single rice cropping system (65.4 kg ha-1) [50]. The total amount of CH4 emission was estimated as 7.67 Tg year-1, ranging from 5.82 to 9.57 year-1, owing to uncertainties in areas receiving organic inputs and intermittent irrigation [51]. The annual CH4 emission rate differed among the four primary rice cropping regions in China, with the highest in the southern and the lowest in the northeastern regions.

The emission of N2O is attributed mainly to the inorganic N fertilizer applied in agricultural soils. Thus, arable lands with high N fertilizer application, such as rice, wheat, maize, and vegetable fields, are the main contributors to N2O emission. The emission factor of N2O emission from agricultural soils ranges from 0.002 to 0.050 kg N2O-N kg-1 N, with a total amount of 432.1 Gg year-1 (ranging from 92.7 to 1016.7 Gg year-1) [52]. About 47% of total N2O is emitted from permanent upland fields cultivated with wheat, maize, and other non-vegetable crops, 20% was released from vegetable fields, and the rest is attributed mainly to rotation fields of paddy rice and upland crops and/or dry fallow. Given that alternative wetting-drying irrigation is widely recommended for rice cropping, N2O emission may increase in rice cropping systems. Thus, technological innovations for increasing rice yield with lower CH4 and N2O emissions are necessary and urgent.

Besides CH4 and N2O, agricultural soil is also an anthropogenic source of CO2 emission to the atmosphere. The amount of CO2 emission is usually evaluated by the change in content of soil organic carbon (SOC). Based on national soil survey data, the top soil SOC density of cultivated soils in China has been estimated as 35 ± 32 t C ha-1 [53]. The SOC density of paddy field was higher than that of upland soils [54]. The total SOC for cultivated soils amounted to 5.1 Pg; however, on average, cultivation of China's soils has led to a decrease of 15 t C ha-1 in SOC density and a reduction of 2.0 Pg in the soil carbon pool [53]. The decrease in SOC is not only a critical issue in carbon emission but also a serious problem for soil fertility and will pose a challenge to food security under climatic warming. For this reason, much effort has been devoted to increasing SOC in China during the last two decades.

5.2. Innovations for carbon sequestration in arable soils

Soil carbon loss from Chinese cropland is due mainly to the reduction in carbon input (by, for example, harvest of biomass) and breaking down of the soil aggregate that protects soil carbon from decomposition. For this reason, much effort has been made in the past decades to increase soil carbon content, including conservation farming (CF), crop straw incorporation, and organic fertilizer application. The Chinese government has made great efforts to disseminate and implement CF, and the total cropping area under CF has increased to 8.6 Mha in 2015 from 0.33 Mha in 2002 (Fig. 6-a). Meanwhile, the government is

imposing strict regulations and policies to promote organic matter amendment of farmland to increase soil fertility. For example, the government has provided a large subsidy to disseminate formula fertilizer application based on soil testing and organic fertilizer application.

It is well known that low/no tillage with straw incorporation or mulching and manure application can greatly increase SOC stocks [55]. Based on a 30-year experiment conducted in a wheat-maize cropping system in northwest China, crop straw and farmyard manure application increased SOC content by 23.0% and 39.9%, respectively, as compared with input of no organic materials [56]. Similarly, 35-year green manure and straw application increased SOC content by 8.7%-31.9% in paddy fields in southern China [57]. Using a meta-analysis, Rui et al. [58] found that crop straw incorporation and animal manure application increased SOC in paddy fields in eastern China by respectively 0.41 and 0.34 Mg ha-1 year-1 on average during the experimental period. However, low/no tillage alone without straw incorporation or mulching led to negligible increase in SOC stock and crop yield [55,59]. Thanks to conservation farming and crop productivity increase, SOC in Chinese crop land has increased from 54 (16-90) Tg CO2-eq year-1 in the early 1980s to 117 (56-172) Tg CO2-eq year-1 in the late 2000s [60]. According to soil survey data, SOC content has increased greatly in most cropping areas in China, although not in the northeast (Fig. 6-b). This finding means that Chinese success in grain production has also contributed greatly to carbon emission mitigation by improving soil fertility during the past two or three decades. However, crop straw incorporation in the field is difficult in the Northeast because of the limited extent of straw decomposition at low temperature. More efforts need to be made for soil fertility improvement in this region to promote carbon sequestration in farmland soils.

5.3. Innovations for CH4 emission mitigation from paddy fields

CH4 is produced from the anaerobic decomposition of organic compounds in flooded paddy soil. Optimizing water management decisions to shorten flooding duration is the recommended strategy for mitigating CH4 emission as well as increasing rice yield. Qin et al. [61] reported that midseason drainage and intermittent irrigation could mitigate seasonal CH4 emission by respectively 51.9% and 60.6% under an organic rice cropping system and 37.6% and 42.3% under a conventional rice cropping system. Optimizing water management in rice seeding or nurse stages could also mitigate CH4 emission. For example, dry direct seeding significantly reduced CH4 emission from paddy fields compared with wet direct seeding and conventional transplanting both with and without straw incorporation [62]. Compared with a flooded nursery, moist and dry nursery reduced CH4 emission by respectively 49.6%-74.2% and 78.6%-99.2% under different rice cropping systems [63]. Organic matter amendments such as crop straw and green manure can greatly enhance soil fertility, but also strongly stimulate CH 4 emission. However, crop straw incorporation in field with alternative wetting-drying irrigation (AWD) did not stimulate CH4 emission [62]. Moreover, the rice cultivar also plays an important role in regulating paddy CH4 emission. Recent

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ClimateSmart Agriculture: Coping with Climatic Warming for Food Security

Rebuilding resilience to warming

Mitigating climatic warming

Objectives: Producing more grain with less GHG emissions

Fig. 7 - Framework of climate-smart agriculture associated with cropping system innovations in China.

studies [64,65] have shown that a rice cultivar with high productivity and root activity could show high yield with low CH4 emission, indicating potential in cultivar breeding for CH4 emission mitigation.

To produce more rice grain with lower CH4 emission, Chinese scientists have studied rice cultivar breeding and agronomic innovations. During the last two decades, the Chinese government has initiated a long-term project in "Superior Rice Breeding" and the Ministry of Agriculture has approved 141 Superior Rice cultivars over 1996-2015. This progress has greatly increased rice yield and may have greatly reduced paddy CH4 emission in China [65]. The government has also devoted major efforts to disseminating AWD irrigation for rice cropping, and the cropping area under flood irrigation has declined to less than 15% of the total rice cropping area in China in 2015. Simultaneously, the flooded nursery area has decreased to less than 10% of the total rice nursery area in China since the 1970s [63]. Moreover, the migration of the major rice cropping area in China to the northeast from the south may also have contributed a large part of CH4 emission mitigation, given that the northeast shows the lowest CH4 emission by area and yield in China [50]. For example, the rice cropping area of Heilongjiang province increased from 1.0% in 1960 to 10.6% in 2014 of the total rice cropping area in China, while the proportional area in Guangdong province declined from 14.2% to 6.2% during the same period. All agronomic innovations in rice cropping have greatly enhanced rice production as well as reduced CH4 emission in China.

5.4. Innovations for N2O emission mitigation

N2O is produced by the microbial processes of nitrification and denitrification. N2O emission from agricultural soil is attributed mainly to the application of inorganic N fertilizer. However, such fertilizer remains essential for high crop

production. The response of N2O emission to N addition rates is regulated by N competition between crop uptake and soil microbe utilization. For this reason, increasing N use efficiency by crops is a key issue for mitigating N2O emission. In China, many high-efficiency fertilizer application options have been investigated with the aim of reducing N2O emission from cultivated soil. For example, Chen et al. [66] developed an integrated soil-crop management system based on modern understanding of crop ecophysiology and soil biogeochemistry. They conducted a total of 153 site-year field experiments covering the main agroecological areas of rice, wheat and maize production in China to test the efficiency of this system. The results showed that the average yields of rice, wheat and maize were increased by 18.1%, 23.6%, and 35.2%, respectively, without any increase in nitrogen fertilizer and that nitrogen losses, including by N2O emission and NH3 volatilization, were reduced substantially.

Given an N use efficiency increase from the present level of 35% to the target of 40% in 2020, N fertilizer application in China can be reduced by 20%-30% under optimal formula fertilizer application based on soil testing, resulting in a 40.0% reduction in N2O emission without wheat yield loss [67]. Partially replacing chemical N fertilizer with crop straw and manure compost can markedly reduce N2O emissions, and optimized irrigation with reduced N fertilizer could also reduce N2O emissions by 42.6% without yield loss in a wheat-maize rotation system [68]. Accordingly, the Chinese government is devoting huge efforts to implementing formula fertilizer application and crop straw incorporation in the field. The grain cropping area using formula fertilizer has increased to 100 Mha in 2015 from 5.33 Mha in 2005. According to the action plan of zero increase in chemical fertilizer application in 2020, more than 60% and 65% of nutrients in the livestock waste and crop straw, respectively, should be incorporated into the field. This result will not only improve soil fertility for crop production but also

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greatly reduce N2O emissions because of the reduction of chemical N fertilizer application. Soil fertility improvement will also contribute to building cropping system resilience to climatic warming, in turn benefiting crop production for food security in China.

6. Prospects for coping with climate warming in China

During the last two decades, China has achieved great success in ensuring food security as well as in mitigating greenhouse gas emissions from major cropping systems. To guarantee more than 90% self-sufficiency in grain production under increasingly critical conditions of limited resource availability and changing climate, cropping systems in China are confronted with substantial challenges of sustainable development. First, although China produces more than 20% of total global grain with less than 9% of the total arable land in the world, more than 35% of the chemical fertilizer is consumed in the country, with a low use efficiency of 35%. There are enormous nutrient losses from crop straw burning on farms and animal waste discharge into the environment. The recycling use rate of crop straw and animal waste as fertilizer for crop production was less than 35% and 50%, respectively, of the total amounts in 2013. Owing to population increase and economic development, moreover, availability of resources such as water and arable land for crop production will be scarcer than ever before. The most feasible approach for Chinese to ensure self-sufficiency in grain production is to further increase grain yield in a rapidly warming climate. Given that China is becoming the country of largest grain consumption and carbon emission in the world, it is very important to further increase grain production and reduce carbon emissions for both food security and world climatic warming mitigation. In fact, all the above challenges also suggest high potential to increase grain yield and reduce greenhouse gas emission via innovations in Chinese cropping system techniques.

Recently, the Food and Agriculture Organization of the United Nations has promoted the new agricultural development mode of Climate Smart Agriculture (CSA), which is also strongly recommended in China as Climate/Environment Friendly Agriculture. CSA is a practical approach to developing agricultural strategies for sustainable food security under climatic warming. It aims to tackle three main goals: (1) to sustainably increase agricultural productivity and farmer income, (2) to adopt and build agroecosystem resilience to climatic warming, and (3) to reduce or remove greenhouse gas emissions from agroecosystems [69]. For this triple objective, the world needs to devote great effort to creating new policy, knowledge, and technology for agricultural development. Widespread use of sustainable agricultural practices can help by reducing risks to food production and farmer incomes and by decreasing GHG emissions and resource degradation of cropping systems. Investments and policy changes are needed from local to global scales [29]. According to Chinese situations in cropping systems, further scientific and technological efforts need to be made for coping with climatic warming (Fig. 7).

6.1. Strengthening scientific research into climatic warming impacts on crop production

Despite much study of the impacts of climate change on crop production in China, there are a many uncertainties in impact assessment. This knowledge gap will hinder strategy development and new technology creation for coping with climatic warming to ensure food security. The major factor contributing to the uncertainties associated with cropping systems is unclear understanding of the actual effects of climatic warming in the field, its effects in combination with other changes (e.g. CO2 and O3 elevation, solar radiation, and precipitation changes), and their specific effects in different regions and seasons. Moreover, assessment methods and modeling need to be improved for higher accuracy based on the actual responses of cropping systems to climatic warming in situ, so as to scale up plot-scale findings to the large-region scale. In addition to the assessment of climatic warming effects on cropping systems, evaluation of the contribution of crop production to climatic warming should be also performed, so that feasible new agronomic practices can be developed and disseminated for higher crop yield with lower GHG emissions. Without accurate understanding of climatic warming effects, we will lose ourselves in new crop cultivar breeding, cropping system optimization, and agronomic practice improvement. Coping with climatic warming needs more new theoretical and technological support in cropping systems.

6.2. Strengthening agricultural policy innovation for coping with climatic warming

Coping with climatic warming is not only a scientific and technological but also an agricultural policy issue. The government needs to develop new policies to promote investments in crop production improvement for enhancing system resilience to warming and to stimulate stake holders (e.g. farmers, cooperatives, and companies) to apply environment friendly techniques. Some regulations must also be developed for limiting crop straw burning on farms and for overuse of chemical fertilizer and irrigation water, or to make new incentive policies to promote conservational farming and formula fertilizer application based on soil testing. More crop straw and animal waste need to be applied in the field to enhance soil fertility and mitigate GHG emissions, operations that need support from new incentive policies. Other policies need to be created for large-scale farm and cooperative farm development, so as to increase the use efficiency of chemical inputs and mechanized cropping efficiency. This activity also contributes strongly to GHG emission by energy consumption.

6.3. Strengthening knowledge creation and popularization of climate-smart agriculture

Knowledge creation and popularization of climate-smart agriculture is very important for coping with climatic warming. Since most farmers know little about climatic warming and GHG emissions, they normally ignore the effects of cropping practices on CH4 or N2O emissions. Without an understanding of production by the younger generation, they will not care about efficient fertilizer application and water

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use. They may not care about crop straw burning on farms if they do not know the global warming potential of carbon emissions. Although farmers are very concerned about air quality, they do not know that gaseous N emissions from agricultural ecosystems have contributed about 1/4 of PM2 5 as a secondary pollutant in the atmosphere. Consequently, they pay little attention to increasing N fertilizer use efficiency and the recycling of animal waste as fertilizer. To promote voluntary stake holder participation, green-development ideals and climate-smart agriculture knowledge need to be popularized in rural areas.

6.4. Strengthening investments in building cropping system resilience to climatic warming

The most feasible approach to cope with climatic warming for sustainable crop production and GHG emission mitigation is increase in investment in building cropping system resilience. To enhance cropping system resistance to warming-led drought, more investments need to be made in development of field infrastructure, especially irrigation. Good field infrastructure can not only reduce warming-led drought stress to crop growth but can also reduce CH4 emission via water-controlling irrigation. Increasing investment in crop breeding can provide more new cultivars with higher resource use efficiency and greater resistance to biotic and abiotic stresses for farmers, especially in cropping areas with high vulnerability to climate change. Investments also need be strengthened for development of systems for climate change forecasting and early warning, so that farmers can take precautions against natural calamities or extreme weather, such as intense rainfall, extreme temperatures, and drought. As mentioned above, more funding needs to be invested into scientific and technological research on climate change, to enhance our ability to cope with climatic warming for food security.

7. Conclusions

The earth has experienced and continues to face a rapid warming trend, while global population and economic development will simultaneously increase at increasing speed during the next 50 years. Even if we take strong measures to reduce carbon emissions by 2020 to a level less than that of 1990, the earth surface's mean temperature will still increase by about 2.0 °C, suggesting a more critical status of global food security by the end of this century. Thus, the most feasible approach to ensure sustainable food security under a warming climate is to make innovations in cropping systems for higher yield with lower GHG emissions. Although China has experienced severe climatic warming and resource limitation, agriculture in China has achieved great success in grain production. The present comprehensive review shows that Chinese government, scientists, and farmers have made many active adaptations to cope with climatic warming for sustainable increase in

grain yield rather than passively avoiding warming-led limitations to crop production. The main adaptation strategies in cropping systems have included crop cultivar improvement, and crop growth season and cropping region adjustment. All of the new practices have greatly increased grain yield with higher resource use efficiency by improving soil quality, resulting in a large increase in soil organic carbon stock and a tremendous reduction in GHG emissions. China has the largest food consumption, population, and economy, and its experiences in coping with climatic warming can provide valuable references for other developing countries.

Acknowledgments

This work was supported by the State Key Program of China (No. 2016YFD0300903), the National Key Technology R&D Program of China (No. 2015BAC02B02), the Special Fund for Agro-scientific Research in the Public Interest (Nos. 201503122, 201503118), and the Agricultural Science and Technology Innovation Program of CAAS.

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