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Abstract: 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 180kgNha−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–`180kgN ha−1 increased at the rate of 38–49kgha−1 year−1 (p <0.05), whereas PUE increased at the rate of 0.071–0.088kgmm−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 45–180kgN ha−1 decreased by approximately 10%–45% (p <0.05) in the dry years (drought index<−0.35) compared with the normal years, whereas significant increases were not detected in the wet years (drought index>0.35). Grain yields with 90–180kgN 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 180kgNha−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. [Copyright &y& Elsevier]

Many ecosystem functions and processes depend on biodiversity, however, the effect of root diversity in agroecosystems on soil bacterial communities and processes remained largely unknown. Our objectives were to examine the importance of increased root diversity through crop rotation on soil bacterial community composition and its relationship with soil carbon (C) and nitrogen (N) accumulation, which play an important role in soil fertility. In a field experiment with 30-year crop rotation, where there was no difference in root biomass input in top soil, soil C and N accumulation rates, soil microbial activities and bacterial community composition were investigated. Soil C and N accumulation rates and microbial biomass content were generally increased after rotation, with a greater increase in legume-cereal rotation than in cereal-cereal rotation. Crop rotation also increased soil microbial activity (soil respiration, potential N mineralization), but did not affect soil bacterial diversity. The increased bacterial abundance and changes in bacterial community structure and abundances of dominant bacterial phyla in rotation soils were related to increases in soil C and N accumulation and microbial activity. Our results suggest that increased root diversity through rotation can influence soil bacterial community structure and increase soil fertility by enhancing C and N accumulation rates, and cause positive effects on soil organic C and fertility. The influence of legume-cereal rotation was greater than that of cereal-cereal rotation. [ABSTRACT FROM AUTHOR]