Modeling the Impacts of Soil Organic Carbon Content of Croplands on Crop Yields in China

Agricultural Sciences in China

April 2009

2009, 8(4): 464-471

Modeling the Impacts of Soil Organic Carbon Content of Croplands on Crop Yields in China QIU Jian-jun1, WANG Li-gang1, LI Hu1, TANG Hua-jun1, LI Chang-sheng2 and Eric Van Ranst3 Key Laboratory of Resources Remote Sensing and Digital Agriculture, Ministry of Agriculture/Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences (CAAS), Beijing 100081, P.R.China 2 Institute for the Study of Earth, Ocean and Space, University of New Hampshire, Durham, NH 03824, USA 3 Department of Geology and Soil Science (WE13), Laboratory of Soil Science, Ghent University, Krijgslaan, 281 (S8), B-9000 Gent, Belgium 1

Abstract This study quantified the impacts of soil organic carbon (SOC) content on the grain yield of crops using a biogeochemical model (DNDC, denitrification-decomposition). Data on climate, soil properties, and farming management regimes of cropping systems were collected from six typical agricultural zones (northeast, north, northwest, mid-south, east and southwest regions of China, respectively) and integrated into a GIS database to support the model runs. According to the model, if the initial SOC content in the cropland was increased by 1 g C kg-1, the crop yield may be increased by 176 kg ha-1 for maize in the northeast region, 454 kg ha-1 for a maize-wheat rotation in the north region, 328 kg ha-1 for maize in the northwest region, 185 kg ha-1 for single-rice in the mid-south region, 266 kg ha-1 for double-rice in east region, and 229 kg ha-1 for rice and wheat rotation in southwest region. There is a great potential for enhancing the crop yield by improving the SOC content in each region of China. Key words: soil organic carbon, grain yield, carbon sequestration, DNDC model

INTRODUCTION Food security is usually an important issue for Chinese agriculture, especially in recent years, China is under a great pressure to meet the demands of a huge population for food supply with limited cropland and water resources. Since the total grain production reached 510 million ton in 1998, the amounts of production and yield have been declining continuously, with a sharp decreasing of the rice and wheat planting area and a huge annual variation in yields for maize and soybean (National Bureau of Statistics 2004). According to the forecast from “The White Paper on Chinese Food

Problem”, by 2030, China will have a population of 1.6 billion, the total food demand will reach 640 million ton (State Council Information Office 1996). The need to increase food production by about 120-210 million ton is an unprecedented challenge to the Chinese agriculture. A great number of studies indicated that soil organic matter (SOC) is one of the key factors that affect agricultural production, nutrient availability, soil stability, and the flux of greenhouse gases (Wardle et al. 2004; Li 2000; Su et al. 2002). It represents a major pool of carbon within the biosphere and acts both as a source and a sink for carbon and nutrients (Wang et al. 2004). Loss of SOC may cause soil degradation, which not only undermines sustainable agricultural development

This paper is translated from its Chinese version in Scientia Agricultura Sinica. Correspondence QIU Jian-jun, Tel: +86-10-82106231, Fax: +86-10-82106231, E-mail: [email protected]

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Modeling the Impacts of Soil Organic Carbon Content of Croplands on Crop Yields in China

but also affects environmental health. So it is of prime importance to study the impact of SOC sequestration on the crop yield and soil environment in croplands of China. Most of the studies about the impacts of soil fertility change on crop yields may be carried out under microand macro-processes. In the micro-process, a great amount of experimental data were used to form the mathematic models which were suitable for a ecological region. In the other macro-process, a lot of biogeochemical models have been designed for estimating soil fertility and its impacts on the grain yield, such as CQESTR, Century, Rothamsted, and so on. More and more studies showed that it may improve the arable soil productivity through recovering the arable soil fertility (Wang et al. 2002; Lal 2004). In fact, Chinese agriculture is facing the scarcity of arable land and shortage of soil organic carbon (SOC) (Pan et al. 2005). The arable land per capita in China is relatively close to the warning line of 0.053 ha per capita reported by the FAO. The SOC content in China, which is 0.8-1.2% in South China, 0.5-0.8% in North China, 1-1.5% in Northeast China, and even less than 0.5% in the Northwest China, is over 30% lower than the world average and over 50% lower than the European (Huang 2005). The studies on the impacts of soil fertility change on the grain yield under different land use and different regions have increased somewhat, but lack of experimental data and unsuitable models have resulted in the inadequate demands and obvious differences comparing with overseas studies, especially the researches on large scale and multi-interface were not enough. This paper aims to quantify the impacts of improvement or decline of SOC on the grain yield and soil environment by using a biogeochemical model (DNDC, denitrification-decomposition). Data on climate, soil properties, and farming management regimes of cropping systems were collected from six typical agricultural zones and integrated into a GIS database to sup-

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port the model runs.

MATERIALS AND METHODS Selection of simulating points Based on the regional characteristics of Chinese agriculture, typical points with special cropping systems were selected for model simulation according to six regions, namely, northeast, north, northwest, south central, east and southwest (Table 1). Each selected point represents the soil, climate, cropping system of each region. Information on farming practices was collected from field experimental observations and local records. Weather data (daily maximum and minimum temperature, and daily precipitation) in the selected points were collected from the National Meteorological Bureau of China. All these supporting data were integrated into a GIS database.

DNDC model The DNDC model is a biogeochemical model originally developed for predicting carbon sequestration and nitrogen cycling in agro-ecosystems (Li et al. 1992, 1994, 2003; Li 2001; Tang et al. 2006). It consists of six sub-models, which simulate soil climate, plant growth, decomposition, nitrification, denitrification and fermentation, respectively. DNDC simulates SOC dynamics by tracking the turnover of four SOC pools, namely plant residue (or litter), microbial biomass, humads (or active humus), and passive humus. Each pool consists of two or three sub-pools with specific decomposition rates subject to temperature, moisture, redox potential and N availability in the soil. As soon as fresh crop residue is incorporated into the soil, DNDC will partition the residue into very labile, labile, and resistant litter pools based on C/N ratio of the residue.

Table 1 Characteristics of the represent field sites in different agricultural regions in China Regions Northeast North Northwest Mid-South East Southwest

Sites (County, Province)

Latitude

Cropping system

SOC (kg C kg-1)

Qiqihar, Heilongjiang Quzhou, Hebei Pingliang, Gansu Zhijiang, Hunan Jingdezhen, Jiangxi Yanting, Sichuan

47.22°N 36°N 37°N 25°N 29.3°N 30°N

Spring maize Winter wheat/Summer maize Spring maize Single rice Rice-Rice Rice/Winter wheat

0.0400 0.0097 0.0040 0.0114 0.0060 0.0155

Bulk density (g cm -3) 1.20 1.50 1.40 1.26 1.35 1.12

pH 6.8 7.2 8.0 6.3 6.0 8.56

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The lower the C/N ratio, the more the residue will be partitioned into very labile or labile pool. Each of the SOM pools has a specific decomposition rate subject to temperature, moisture and N availability. The organic matter in the litter pools will be broken down by the soil microbes. When the microbes die, their biomass will turn into humads pool. Humads can be further utilized by the soil microbes and turned into passive humus. During the sequential decomposition processes, a part of the organic C becomes CO2, and a part of the organic N becomes ammonium. By tracking the processes, DNDC quantifies SOM turnover in soils. By precisely simulating the soil microbial activities, DNDC links C sequestration to soil fertility and crop yields, and also climate change. During the past decade, with the support of more international researchers getting involved in the development of the model, DNDC has been substantially enhanced and has become a generic agro-ecosystem model for predicting crop yield, C sequestration, and greenhouse gas emissions for both upland and wetland crops. The DNDC model is available free of charge (downloadable) via the website (http://www.dndc.sr. unh.edu). Detailed descriptions of the model’s development and its structure were published by Li et al. (1992, 1994). Currently the model has been applied in many countries. At the International Workshop on Global Change for Asia Pacific Region in 2000, the DNDC model was also designated as the first choice of biogeochemical models to be extended in the Asia Pacific regions.

scenario is based on the actual cropping management practices, with no manure applications and only 10% of above crop residue returned to the field. Four other scenarios were designed implementing 75, 50, 125, and 150%, respectively, of the initial SOC content in the baseline scenario. For each scenario, the DNDC model was run considering a 20 years’ period to obtain the average effects for that period. Climate data from 1983 till 2003 in each point were chosen as a representative for the upcoming 20 years to support model’s running.

Simulation scenarios

Impacts of different initial SOC content on the crop yields

RESULTS AND DISCUSSION Tests of DNDC model The DNDC model has been validated throughout the world by using long-term and short-term experimental data, testing the modeling behavior and sensitivity of the carbon and nitrogen biogeochemical processes in agricultural soil. In order to quantify the impacts of SOC on the grain yield and soil environment, experimental data sets from 3 long-term studies conducted in northeast, north, and southwest (Shi et al. 2002; Niu 1999; Sun et al. 2002) have been selected to validate the model, test the modeling behavior and sensitivity of the crop yield. The test results showed that the DNDC model generally showed a good performance in simulating crop yields at the 3 experimental sites, with the simulated yields being well consistent with the observed yields (Table 2, Fig.1).

In order to recognize the impacts of improvement or decline of SOC on the grain yield and soil environment in different regions, a baseline and alternative scenarios were designed to represent the current and altered cropping practices for the model predictions. The baseline

Northeast region Northeast China is the main grainproduction area in China. The highest SOC amounts occur in the northeast because of the dominantly fertile soils with high organic matter contents. At present

Table 2 Characteristics of the sites for validations of DNDC in China Sites Gongzhuling Quzhou Chongqing

Aver. Temp. (°C) 4.5 13.1 18.4

Prec. (mm) 550 650 1100

Simulated yield Observed yield (kg ha-1) (kg ha-1) 7 772 10 722 7 718

7 720 9 699 8 422

Cropping system

Soil texture

Initial SOC (kg C kg-1)

Time span

Corn Winter wheat/Summer maize Winter wheat/Rice

Silty soil Loam Silty soil

0.015 0.0066 0.013

1990-1995 1995-2001 1992-1999

Simulated yield and observed yield are all the average value during the experimental span.

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Modeling the Impacts of Soil Organic Carbon Content of Croplands on Crop Yields in China

however, the SOC content in this region is decreasing (Liang et al. 2000; Qiu et al. 2004). According to DNDC model simulation, the SOC content contributes greatly to the crop yields. When the initial SOC content is decreased by 50% in comparison with the baseline scenario, a decline of the maize yield of 1 223 kg ha -1 will occur. When the initial SOC content is increased by 50%, the maize yield will raise by 1 850 kg ha-1 (Fig.2, Qiqihar). The regression analysis (y = 176 428x + 4 699.6, R2 = 0.912) confirms that an increase of initial SOC content by 1 g C kg -1 may cause maize yield to be raised by 176 kg ha-1 in the northeast region. North region The north of China is also one of the main grain-production areas in China. The double-cropping systems are prevailing and the SOC content is less than 0.01 kg C kg-1 at present. The model simulated results show that when the initial SOC is decreased by 25%, the average yield for winter wheat/summer maize may decrease with 464 kg ha-1, and with 712 kg ha-1

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for an initial SOC decrease of 50%. When the initial SOC content is increased by 25%, the average yields will increase with 349 kg ha -1, and only with 626 kg ha-1 for an initial SOC increase of 50% (Fig.2, Quzhou). The regression analysis (y = 454 103x + 7 482.7, R 2 = 0.9913), confirms that an increase of initial SOC content by 1 g C kg-1 may cause a raise in crop yield of 454 kg ha -1 for the winter wheat/ summer maize rotation in this area. Northwest region In the northwest region, infrequent precipitation and insufficient irrigation facilities limit the crop yields and biomass production, which lowers the SOC equilibrium. According to the model runs, an initial SOC content increase of 25% results in an increase of the average yield for spring maize of 216 kg ha-1, and of 623 kg ha-1 for the scenario where the initial SOC is increased by 50% (Fig.2, Pingliang). The regression analysis indicates that an increase of initial SOC content by 1 g C kg-1 may cause a rise in spring maize yield of 328 kg ha-1.

Fig. 1 Tests of the DNDC in modeling crop yield using 3 sets of long-term experimental data. A, from location Gongzhulin, Liaoning Province, with a spring corn crop for 5 years and a fertilizer application of 165 kg N ha-1; B, from location Quzhou, Hebei Province, with a wheat-corn rotation for 7 years and a fertilizer application of 200 kg N ha-1; C, from location in Chongqing City, with a wheat-rice rotation for 8 years and a fertilizer application of 150 kg N ha-1.

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Mid-south region In the mid-south region, cropping systems with rice are prevailing. According to the model runs, an initial SOC content increase of 50%, results in an increase of the single rice yield of 519 kg ha-1 in comparison with the baseline scenario, and yields will be decreased by 780 kg ha-1 when the SOC content is reduced by 50% (Fig.2, Zhijiang). The regression analysis (y = 185 302x + 4 226, R 2 = 0.9479) indicates that in this region, an increase of initial SOC content by 1 g C kg-1 may cause a single paddy rice yield rise of 185 kg ha-1. Eastern region Based on the DNDC model runs, an

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initial SOC content increase of 25 and 50% results in an increase of the yield of double rice of 328 and 731 kg ha-1, respectively; and when the initial SOC is dropped by 25 and 50%, the yields will decrease with 243 and 588 kg ha-1, respectively (Fig.2, Jingdezhen). The regression analysis (y = 265 582x + 1 532.9, R2 = 0.9936) indicates that in this region, an increase of initial SOC content by 1 g C kg-1 may cause a rise of the double paddy rice yield of 266 kg ha-1. Southwest region In this area, rice and winter wheat rotation is the dominant cropping system. The model simulated results show that when the initial SOC con-

Fig. 2 Impacts of different SOC contents on crop yields in different croplands in China.

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Modeling the Impacts of Soil Organic Carbon Content of Croplands on Crop Yields in China

tent is increased by 25 and 50%, the yields of rice and winter wheat will increase with 857 and 525 kg ha-1 respectively. When the initial SOC drops by 25 and 50%, the yields will relatively decrease with 568 and 1 350 kg ha-1, respectively (Fig.2, Yanting). The regression analysis (y = 229 712x + 5 558.3, R2 =0.9949) indicates that in this region, an increase of the initial SOC content by 1 g C kg-1 may cause a rise in yields of rice/winter wheat rotation of 229 kg ha-1.

DISCUSSION The simulated results suggest that in each region, there is a high potential for raising crop yields by improving the SOC content (Table 3). The potentials of raising crop yields depend on regional conditions. Higher increases (13.3%) occur in the northwest where the initial SOC are low, while the lower increases of only 2.28% can be obtained in the northeast, where the initial SOC contents are high. It implies that slight changes of the initial SOC will result in major changes in crop yields. Generally, an increase of the SOC content by 1 g C kg-1 may cause a yield increase of 231 kg C ha-1 in the whole nation, which is much higher than the result of 40.8 kg ha-1 by Wang et al. (2002), as a result of different farm management practices and cropping systems. Chinese government highlighted intensively on the food security for many years, and adopted a great number of effective measures in protecting the food supply. Over the last three years, top priority is given to maintain food production, enhance land productivity and develop modern farming technologies. The key factors to improve crop yields in China, where the arable land area is hard to expand, should be the improvement of soil productivity. The increase of crop yields per unit area, appropriately increasing the investment to agriculture, and improving traditional farming techniques

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all will require attention (Feng et al. 2000; Fu et al. 2001). The croplands will benefit most with the strategies that change concerns about the short-term profits gained from the land use to the long-term fertility. It is the foundation for protecting Chinese food security. The SOC plays an important role in the fertility by affecting the soil stability and nutrient availability. As we indicate in this paper, the increase of the SOC content has great effects on the biomass productivity, thereby influencing the security of the Chinese food supply, while at the same time affecting the changes of atmospheric CO2 concentration which leads to the global warming, so to improve the SOC content is recognized as a win-win strategy. It has been estimated that the amounts of carbon sequestration from farmlands of China were 0.19 Pg with 10 million ton sequestration of CO2-C per year, more than 20% of total biotic pool (Chen et al. 2004). Especially in paddy field, the rate of carbon sequestration in China ranges from 0.1 to 2 t C ha-1 yr -1, which is even beyond that in North America forest (Zhang et al. 2004; Pan et al. 2003), thereby carbon sequestration in Chinese arable lands has significant effects on the global carbon cycles. Uncertainties in the study mainly originate from three aspects. Firstly, the cropping systems in each region are very complex. Even in a small area, there are lots of cropping patterns with a very particular cultivation. As such, there is a certain degree of discrepancy between the selected cropping systems and the regional reality. Secondly, in this study, climate data from 1983 to 2003 are taken as representative for the future 20 years’ to support the model running, without consideration of the possible global climate change in this period due to global warming. We take North China region as an example, two scenarios are designed which are A2 (temperature rise 1.4°C) and B2 (temperature rise 0.9°C) according to IPCC (Intergovernmental Panel on Climate Change) and SRES (Special Report of Emission Scenario) (Houghton et al. 2001). The model re-

Table 3 Response of crop yield to the increase of 1g C ha-1 of initial SOC content in different regions Sites Qiqihaer, Heilongjiang Quzhou, Hebei Pingliang, Gansu Zhijiang, Hunan Jingdezhen, Jiangxi Yanting, Sichuan

Aver. yield (kg ha-1) 7 754 9 920 2 540 5 706 4 204 9 246

Initial SOC (g C kg-1) 40.0 9.7 4.0 11.4 6.0 15.5

Increase of yield (kg ha-1) 177 490 331 185 266 257

Increase rate (%) 2.28 4.94 13.03 3.24 6.33 2.78

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sults indicate that under the 2 scenarios, the crop yields increase by 2 and 12 kg, respectively. It shows slight difference by comparison with the baseline scenario. Therefore it may consider the crop yield results of this paper as the future yield even if the climate changes. Thirdly, some of the data supporting the model runs are derived from authoritative literature and predications by experts, but they will inevitably differ considerably from the real situation in future. Also, the model runs without considering the effects of any extreme weather events.

CONCLUSION In this paper, the simulated results showed that there is a great potential for increasing the yields by improving the SOC content in each region. If the initial SOC content in the each cropping area is increased by 1 g C kg-1, the crop yield may be increased by 176 kg ha-1 for maize in the northeast region, 454 kg ha-1 for a maize-wheat rotation in the north region, 328 kg ha-1 for maize in the northwest region, 185 kg ha-1 for single-rice in the midsouth region, 266 kg ha-1 for double-rice in east region, and 229 kg ha-1 for rice-wheat rotation in southwest region. The increase rates of crop yield were also discussed in these regions. If the initial SOM content was low, even if it changes a little, would result in higher change rate of crop yield, i.e. in northwest region; whereas, the change of SOM content would result in little change rate of crop yield when the initial SOM content was high, i.e. in northeast region.

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Acknowledgements This work was supported by a bilateral scientific cooperation project financed by UGent-BOF, Belgium, and the Ministry of Science and Technology, China (20052), as well as by the Non-profit Research Foundation for Agriculture of China (200803036).

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