Winter wheat grain yield associated with precipitation distribution under long-term nitrogen fertilization in the semiarid Loess Plateau in China

Nitrogen (N) and precipitation are the main limiting factors in food production under rain-fed cropping systems in arid and semiarid regions. A long-term field experiment was conducted from 1984 to 2009 in the Changwu State Key Agro-Ecological Experimental Station of the Loess Plateau, China, to study interactions between precipitation and N fertilization. Winter wheat (Triticum aestivum L) was grown with N fertilization at 0, 45, 90, 135, and 180 kg N ha(-1). With 25-year N fertilization, soil organic carbon and total N were increased by 18% and 26%, respectively. Mean yields and precipitation use efficiency (PUE) levels with N fertilization were increased by onefold to twofold compared with unfertilized soils. Generally, during the period of 1984-2009, grain yields with 45-180 kg N ha(-1) increased at the rate of 38-49 kg ha(-1) year(-1) (p<0.05), whereas PUE increased at the rate of 0.071-0.088 kg mm(-1) year(-1). These increases over time are largely attributable to improvements in soil organic carbon and total N. Variations in yield were proportionally reduced by N fertilization, but heavily depended on the fluctuation of precipitation. Grain yields with 45-180 kg N ha(-1) decreased by approximately 10%-45% (p<0.05) in the dry years (drought index<-035) compared with the normal years, whereas significant increases were not detected in the wet years (drought index>035). Grain yields with 90-180 kg N ha(-1) were positively correlated (p<0.05) with fallow season precipitation, but insignificantly correlated with growing season precipitation. The optimum N fertilization rates in the dry, normal and wet years increased following an increasing sequence of 45, 135 and 180 kg N ha(-1). These results have significant implications for optimizing N fertilization and maximizing yield and PUE in rain-fed cropping systems in arid and semiarid regions. (C) 2012 Elsevier B.V. All rights reserved.