Fertilizer nitrogen use efficiency and fates in maize cropping systems across China: Field N-15 tracer studies

Maize (Zea mays L.) is a staple crop that is grown worldwide. The heavy use of nitrogen (N) fertilizers in maize cropping systems has resulted in low N use efficiency (NUE) and even caused N pollution in some regions of China. To evaluate the environmental impacts of over-fertilization, it is essential to reveal NUE and the fates of the applied N fertilizers, which can be accurately quantified only by the field N-15 tracer technique. In this study, we conducted six on-farm N-15 tracer experiments with four in Northeast (NE) China where maize is extensively cropped. We combined the results from these field experiments with previous N-15 tracer results (most in the North-Central (NC) region) to estimate the fates of N fertilizer in maize cropping systems throughout China. In total, there were 23 site-year field experiments. We found that, on average, 34%, 35% and 31% of the applied N fertilizers (222 kg N ha(-1) on average) was taken up by aboveground biomass, retained in the soil and lost to the environment, respectively. The NUE, as the percentage of N-15 removal by aboveground biomass, was much higher in NE China than in NC China (47% vs. 28%, n = 6 and 16, respectively). The regional NUE differences suggested that the overall NUE in the Chinese maize cropping system would be underestimated if only data from NC China were considered. Additionally, NE China had a higher crop N uptake (260 vs. 192 kg N ha(-1)) and a lower N loss proportion (21% vs. 34%) than NC China. These regional differences were controlled more by soil properties than by climatic factors. In addition to fertilizer N, our N-15 results indicated that, on average, 64% of the maize N was derived from soil, implying that native soil N is also an important N source for crop N uptake. Based on the mass balance of N input and N output, exogenous N replenishment to soil N pool consumption is a vital mechanism for maintaining the long-term fertility of the soil. To evaluate the long-term fates and use efficiency of N fertilizer, future research needs to quantify the contribution of N fertilizer to soil N consumption replenishment.