Nitrogen surplus benchmarks for controlling N pollution in the main cropping systems of China

Nitrogen (N) surplus is a useful indicator for improving agricultural N management and controlling N pollution. Few studies have developed benchmarkvalues for cropping systemsin China, a country with the largest N fertilizer use in the world. We established N surplus benchmarksfor 13 main cropping systems, at optimal N management, using results from >4,500 on-farm field experiments and a soil surface balance approach. These cropping systemsaccounted for about 50% of total N fertilizer consumption in Chinese agriculture in 2009. The results showed that N surplus benchmarksfor single cropping systemsranged from 40 to 100 kg N ha-1 yr-1 (average 73 kg N ha-1 yr-1), and for double cropping systemsfrom 110 to 190 kg N ha-1 yr-1 (average 160 kg N ha-1 yr-1), roughly twice that of single cropping systems. These N surplus benchmarksmay be further refined, following further decreases in N deposition rates and reactive N losses as a result of strict implementation of “4R-nutrient stewardship” and improvemen...