Spatial-temporal variation of nitrogen balance and partial factor productivity of nitrogen in rice region of China over the past 30 years

Objectives There are complex and diverse rice cultivation models and regional conditions in China. Hence, it has a great importance to assess the effects of nitrogen (N) fertilizer rate on N use efficiency of rice and soil N balance in each rice growing region.

Methods Based on the national paddy soil long-term monitoring platform from 1988 to 2017, the spatio-temporal variation characteristics of N fertilizer application level, partial factor productivity of N (PFPN) and soil N balance in five main rice growing regions (322 soil monitoring sites), i.e. northeast (NE), southwest (SW), middle reaches of the Yangtze River (MYR), south China (SC) and the Yangtze River delta (YRD) were analyzed, and the reasonable application rate of N fertilizer in each rice growing region was proposed.

Results Over the past 30 years, the average N fertilizer inputs of rice in NE, SW, MYR (early rice season), SC (double cropping) and YRD were N 159, 173, 179, 284 and 279 kg/hm², respectively. The average nitrogen fertilizer input across the rice area of China was N 215 kg/hm². In SW and YRD, the fertilizer inputs were kept increasing significantly, with the increasing rates of 0.4 and 2.5 kg/(hm²·a), respectively. The apparent balance of N was significant different among the rice regions over the past 30 years (P < 0.05). The average annual N surplus in NE, SW, MYR (early rice season), SC (double cropping) and YRD rice regions were 35, 5, 20, 69 and 109 kg/hm², respectively, which were accounted for 22%, 3%, 11%, 24% and 39% of their corresponding N application rates. The N surplus increased significantly with the increase of N fertilizer application rate (P < 0.0001). The partial factor of productivity of nitrogen in rice were 54, 51, 42, 44 and 35 kg/kg in NE, SW, MYR (early rice season), SC (double cropping) and YRD rice regions, respectively, which increased significantly with the prolonged fertilization years in the NE and MYR (early rice season) (P < 0.05), but decreased with the increasing of N surplus in the whole country (P < 0.0001).

Conclusions In general, nitrogen fertilizer is over applied in all the rice production regions, except for southwest region in China. Comprehensively considering the maintenance of soil N supply and PFPN, the recommended N application rates are 131, 167, 156, 244 and 151 kg/hm² in NE, SW, MYR (early rice season), SC (double cropping) and YRD rice region, respectively.