Estimating soil nitrogen balance at regional scale in China's croplands from 1984 to 2014

Estimating regional soil nitrogen (N) balance in croplands is critical to improve management practices, reduce environmental risks and develop sustainable agriculture. In this study, spatial and temporal variations of soil N balance were evaluated from 1984 to 2014 in China's croplands. Results indicated that the total soil N balance was in surplus and increased by 7.3 Million tonne N (130.4%) between 1984 and 2014, which was attributed to the increased N input of 29.3 Million tonne N, compared with the increased N output of 22.1 Million tonne N. Soil N balance continually increased from the 1980s (1984–1989) to the 2000s (2000–2009), and then decreased in the 2010s (2010–2014). Meanwhile, N use efficiency decreased gradually from the 1980s to the 2000s, but it increased in the 2010s. The N loss from greenhouse gases (N2, N2O), ammonia (NH3) and nitrate (NO3) leaching and runoff) increased significantly from the 1980s to the 1990s, while the increasing trend gradually reduced from the 1990s to the 2010s. The spatial-temporal distribution of the N balance at the regional scale showed that the total highest and lowest N balance was in the middle and lower reaches of Yangtze River (2.1–3.7 Million tonne N) and northeast of China (0.3–1.0 Million tonne N), but the highest and lowest N balance per cropping area was in the southeast (93.4–129.7 kg N ha−1) and northeast (19.6–43.9 kg N ha−1) regions respectively from the 1980s to the 2010s. The N balance decreased for all regions from the 2000s to the 2010s, excluding the southeast and southwest of China due to higher increased rate of N input than the lower increased rate of N output. Reducing the use of chemical fertilizer N would improve cop productivity, decrease soil surplus N and environmental risks of N gas emissions, nitrate leaching and runoff.