Comprehensive Index Evaluation of Soil Fertility in Rain Fed Farmland with Carbon and Nitrogen Addition

The semi-arid Loess Plateau is one of the regions with the lowest soil fertility in China. Over cultivation combined with crop straw removal and uneven distribution of precipitation jointly lead to soil quality degradation, which seriously affects the sustainable development of agriculture and environment in this area. The purpose of this study is to determine the optimal improvement rate of different carbon sources (wheat straw and biochar) and different amounts of nitrogen fertilizer combined application, and to provide scientific basis for efficient fertilization and improvement of soil fertility in rain fed farmland. The straw and biochar were applied alone or combined with N fertilizer (50 and 100 kg/hm2), whilst the no carbon resource was combined with N fertilizer 0, 50 and 100 kg /hm2. Through 3 years field positioning trail, 20 physical, chemical and biological indicators reflecting soil quality were selected as evaluation indicators, and the soil fertility quality was comprehensively evaluated by principal component analysis and weighted comprehensive model, then verified by wheat yield. 3 common factors extracted from the initial 20 indices could reflect 98.29% of the original information quantity without losing any original variable. Soil physical structure and total organic carbon level are the primary contributing factors of soil. The second common factor was regarded as the active organic carbon, followed by the soil nutrient index. The addition of biochar, straw and nitrogen is conducive to the improvement of soil comprehensive fertility quality, and the comprehensive fertility index of soil quality of rain-fed farmland with different treatments varies greatly, ranking as BN100 > BN50 > SN100 > SN50 > BN0 > SN0 > CN100 > CN50 > CN0. This result is consistent with the change trend of yield. Under the experimental conditions of this study, the soil physical structure and total organic carbon level are the most important factors affecting the comprehensive soil fertility, and 15 t/hm2 biochar combined with 100 kg/hm2 nitrogen is the best fertilization mode.