Residual soil nitrogen estimation of black soil farmland at county scale in Heilongjiang province

Based on the theory of nitrogen “input – output” balance, the residual soil nitrogen (RSN) of the farmland in Heilongjiang province from 1980 to 2002 was estimate. The data were obtained from Heilongjiang Statistical Yearbooks, and parameters were acquired from recent references. Results showed that the N inputs (fertilizer, manure-N, biological fixation, and atmospheric deposition) increased by 77.8% from 1982 (83.7 kg/hm2) to 2002 (148.8 kg/hm2), mean value was 118.1 kg/hm2. N outputs (N removed in crop harvest, N lost from ammonia volatilization denitrification) increased by 72.1% from 49.8 to 85.7 kg/hm2, mean value was 74.8 kg/hm2, and RSN increased by 86.2% from 33.9 to 63.2 kg/hm2 in the same period. Regional difference was evident, for example, the RSN in west region of Heilongjiang province was higher than that in the east. The results were consistent with previous studies and actual situation. During the non-growing season, the surplus RSN could change to NO3, and lost by runoff, leached into rivers and groundwater or emitted into atmosphere as N2O or N2, which caused the waste of nitrogen resources and potential environmental risk. Quantification study on RSN with spatial and temporal scales in Heilongjiang province can provide a new method for the macro-management of N resources.