[Effects of fertilization and planting patterns on soil aggregate and carbon distribution in farmland of the Loess Plateau, Northwest China].

Based on a long-term experiment in the Changwu Agro-ecological Experimental Station in Xianyang, Shaanxi, China, we examined the effects of fertilization and planting patterns on soil aggregate quantity, aggregate stability and total carbon and organic carbon distribution in different aggregate fractions through dry and wet sieving methods, as well as the TOC combustion method. There were ten treatments, including uncultivated (R), wheat continuous cropping (CK/W), wheat-corn rotation (L), and nitrogen fertilizer (N), phosphorus fertilizer (P), nitrogen and phosphorus fertilizer (NP), organic fertilizer (M), nitrogen and organic fertilizer (NM), phosphorus and organic fertilizer (PM), nitrogen and phosphorus and organic fertilizer (NPM) for CK/W. The results showed that fertilizer application and planting patterns affected soil aggregate distribution and stability, the contents and contribution rates of total C and organic C. Force-stable aggregate was mainly constituted by >0.25 mm aggregate (>67%), which was reduced by fertilization. Continuous cropping decreased micro-aggregate while rotation facilitated it and the effect was larger than fertilization. Water-stable aggregate was mainly comprised of micro-aggregate (<0.25 mm), the contribution of which was larger than 61%. Both fertilizer application and planting pattern reduced water-stable micro-aggregate. Fertilizer application and planting pattern decreased the percentage of aggregate destruction rate (PAD) and increased macro-aggregate (>0.25 mm, R) content. Organic fertilizer significantly improved total C and organic C concentrations in all the fractions of force-stable aggregates. Continuous cropping and rotation cropping increased total C concentration in all the aggregate fractions while rotation cropping significantly decreased organic C concentration. Single N and P fertilization decreased soil total C concentration, while mixed application of N and P fertilizers, and organic fertilizer significantly increased soil total C concentration. The effect of planting patterns on soil total C was lower than that of fertilization. Both continuous cropping and rotation cropping increased soil total C. Mixed application of N and P fertilizers, and organic fertilizer signifi-cantly increased soil organic C concentration while single N and P fertilization decreased it. The effect of planting patterns on soil organic C was lower than that of fertilization, while rotation cropping did not facilitate soil organic C. Micro-aggregate was the most notable size fraction to total carbon and organic C, with the contribution being 21.2%-33.6%. Fertilization and planting pattern increased the contribution rate of micro-aggregate in soil total C. NP and NPM significantly increased the contribution rate of micro-aggregate in soil total C and soil organic C. The effect of rotation cropping was most obvious in driving the contribution rate of micro-aggregate in soil total C and soil organic C.