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Anthropogenic activities have raised nitrogen (N) input worldwide with profound implications for soil carbon (C) cycling in ecosystems. The specific impacts of N input on soil organic matter (SOM) pools differing in microbial availability remain debatable. For the first time, we used a much‐improved approach by effectively combining the 13C natural abundance in SOM with 21 years of C3–C4 vegetation conversion and long‐term incubation. This allows to distinguish the impact of N input on SOM pools with various turnover times. We found that N input reduced the mineralization of all SOM pools, with labile pools having greater sensitivity to N than stable ones. The suppression in SOM mineralization was notably higher in the very labile pool (18%–52%) than the labile and stable (11%–47%) and the very stable pool (3%–21%) compared to that in the unfertilized control soil. The very labile C pool made a strong contribution (up to 60%) to total CO2 release and also contributed to 74%–96% of suppressed CO2 with N input. This suppression of SOM mineralization by N was initially attributed to the decreased microbial biomass and soil functions. Over the long‐term, the shift in bacterial community toward Proteobacteria and reduction in functional genes for labile C degradation were the primary drivers. In conclusion, the higher the availability of the SOM pools, the stronger the suppression of their mineralization by N input. Labile SOM pools are highly sensitive to N availability and may hold a greater potential for C sequestration under N input at global scale. [ABSTRACT FROM AUTHOR]

China’s economy has made great strides since 1949, and especially since China initiated economic reforms and the open-door policy in the 1980’s. The growth in agricultural production has been one of the main national accomplishments. By 1999 China was feeding 22 % of the global human population with only 9 % of the world’s arable land and per capita food availability reached the levels of developed countries. The use of fertilizers has played a crucial role, accounting for about 50 % of the yield increase. However, rapid economic growth has led to unprecedented resource exhaustion and environmental degradation. China’s ‘grain security’ will face multiple pressures stemming from resource limitation, environmental pollution and population growth. The Chinese government regards agriculture as the primary field of development of the national economy in the 21st century. The optimal agricultural developmental path for China is to improve the ratio of resource utilization and to protect the environment while guaranteeing the grain supply. This paper summarizes the trends in crop production and crop yields, fertilizer use and soil quality in China, and presents the approach of integrated nutrient management (INM) for improving crop productivity with efficient resource utilization and environmental protection. In the INM approach, the strategy is to emphasize the integrated use of nutrients from fertilizers, wastes and soil and environmental sources; managing nutrients according to different nutrients’ specific characteristics; and integrated nutrient management with sound soil management practices and other farming techniques. These new nutrient management systems can, on average, reduce N fertilizer inputs by 26%, save P fertilizer inputs by 20%, raise grain yields by 8%, and reduce N loss by 47 % compared to conventional agricultural practice.

Global agriculture is facing unprecedented challenges and risks. Rates of yield growth have slowed since the 1980s (ref. 4), and even stagnated in many areas (5-7). Meanwhile, agriculture incurs substantial [...], Agriculture faces great challenges to ensure global food security by increasing yields while reducing environmental costs (1,2). Here we address this challenge by conducting a total of 153 site-year field experiments covering the main agro-ecological areas for rice, wheat and maize production in China. A set of integrated soil-crop system management practices based on a modern understanding of crop ecophysiology and soil biogeochemistry increases average yields for rice, wheat and maize from 7.2 million grams per hectare (Mg [ha.sup.-1]), 7.2 Mg [ha.sup.-1] and 10.5 Mg [ha.sup.-1] to 8.5 Mg [ha.sup.-1], 8.9 Mg [ha.sup.-1] and 14.2 Mg [ha.sup.-1], respectively, without any increase in nitrogen fertilizer. Model simulation and life-cycle assessment (3) show that reactive nitrogen losses and greenhouse gas emissions are reduced substantially by integrated soil-crop system management. If farmers in China could achieve average grain yields equivalent to 80% of this treatment by 2030, over the same planting area as in 2012, total production of rice, wheat and maize in China would be more than enough to meet the demand for direct human consumption and a substantially increased demand for animal feed, while decreasing the environmental costs of intensive agriculture.

Author(s): Zhenling Cui [1]; Hongyan Zhang [1]; Xinping Chen [1]; Chaochun Zhang [1]; Wenqi Ma [2]; Chengdong Huang [1]; Weifeng Zhang [1]; Guohua Mi [1]; Yuxin Miao [1]; Xiaolin Li [...]

We investigated the effects of land use conversion from cereal grain fields (GF) to greenhouse vegetable fields (VF) on total soil organic C (SOC) and total N (TN) and the labile soil organic matter pools dissolved organic C (DOC), permanganatc-oxidizable C (POC), dissolved organic N (DON), mineral N, and microbial biomass C (MBC) in the top 30 cm of the soil profile. The soil samples were collected in 20 sets of paired soils (greenhouse vegetable vs. wheat [Triticum aestivum L.]-maize [Zea mays L.] rotation fields) in northeast China. Compared with those in GF, the SOC and TN concentrations in VF increased by 28 and 58%, respectively,. The accumulation rate of TN vs. SOC, however, led to a significant decrease in soil C/N ratio in VF compared with GF systems. The greatest difference in C/N ratios between GF and VF was observed in the large macroaggregates (>2000 [micro]m). Conversion to VF led to increased DOC, POC, and DON concentrations and an increase in the percentage of SOC as DOC and POC. The pH, MBC, and percentage of SOC as MBC, however, were lower in the VF than GF systems. Furthermore, a significant positive relationship was found between pH and MBC in the VF systems, while MBC was positively related to total SOC and POC in the GF systems. In conclusion, the results show tradeoffs between increasing SOC sequestration when agricultural land is converted from wheat-maize to vegetable systems and decreased C/N ratios and soil acidification. Improvements in soil and crop management for vegetable production systems in China, particularly the avoidance of excess N fertilizer and manure applications, are recommended. doi: 10.2136/sssaj2009.0222