Spatiotemporal distribution characteristics of synthetic nitrogen fertilizer use efficiency in maize fields in China

Maize (Zea mays L.) is one of the three staple food crops in China. Synthetic nitrogen fertilizer (SNF) significantly contributes to maize yield. Nevertheless, SNF could also cause problems such as environmental pollution and resource waste. SNF use efficiency of maize in China has become a significant concern in maize production. The negative consequences of SNF have been far more serious in China in recent years due to unscientific applications, including excessive use and outdated management practices. Based on climatic conditions and crop management systems, maize production area in China has been divided into six cropping regions. These included the northern spring maize region, Huang-Huai-Hai Plain spring and summer maize region, southwest mountain maize region, southern hilly maize region, northwest irrigated maize region and Qinghai-Tibet Plateau maize region. Nitrogen use efficiency in the different regions varied greatly based on local production conditions. The objective of this study was to outline the spatial and temporal distribution patterns of nitrogen use efficiency via calculations of SNF application rate (AR), recovery efficiency (RE), agronomic efficiency (AE) and partial factor productivity (PFP) of maize in the different ecological regions of China for the last decades using the up-scaling average method. The results showed that national average AR increased gradually from 93.3 kg·hm^-2 in the 1970s to 238.2 kg·hm^-2 in the 2010s at the rate of 50.0 kg·hm^-2 per decade. The national average of RE and AE decreased from 42.1% and 17.0 kg·kg^-1 in the 1970s to 26.4% and 9.5 kg·kg^-1 in the 2010s, respectively. Also while the national average of RE decreased rapidly during 1990s, that of AE decreased sharply during 2000s. Unlike RE and AE, the national average PFP remained relatively stable, falling to a narrow range of 40 45 kg·kg^-1. The variations in AR and nitrogen use efficiency were a little more intricate in the different regions. Northern spring maize region showed a relatively less increase in AR compared with the other regions. Thus AR rate in this region remained within a reasonable range, only 217.8 kg·hm^-2 even in 2000s. Nitrogen use efficiency in the region was relatively higher, and RE and AE reached 46.4% in the 1970s and 43.8% in the 1980s before decreasing rapidly to 33.5% in 1990s and 28.6% in 2000s; higher than the national average for all the periods. The southwest mountain maize region displayed a similar trend, relatively lower AR and higher nitrogen use efficiency than the northern spring maize region. The Huang-Huai-Hai Plain spring and summer maize region was increasingly crucial for Chinese maize production. Both RE and AE in the Huang-Huai-Hai Plain spring and summer maize region were lower than the national average due to excessive fertilizer application. This was especially obvious during 2000s when RE and AE in the region were only 23.5% and 6.7 kg·kg^-1 respectively, both much lower than other regions. Moreover, balanced fertilizer application, high-efficiency nitrogen genotype and improved agricultural management were main alternatives to enhancing SNF use efficiency, reducing resources waste and alleviating environmental pollution in China.