Changes of crop yield and soil fertility under long-term fertilization and nutrients-recycling and reutilization on a black soil:Ⅳ.Soil organic carbon and its fractions
黑土长期施肥及养分循环再利用的作物产量及土壤肥力变化Ⅳ.有机碳组分的变化

A long-term experiment was conducted on a black soil of Northeast China to study the effects of applying chemical fertilizers and recycled organic manure (ROM) on the changes of soil organic carbon and its fractions. The results showed that from 1985 to 2004, soil total organic carbon (TOC) decreased by 7.83% in control,4.56% in N application, 1.61% in N + P application, and 5.56% in ROM application, but increased by 0.33% in N + P + K application. Comparing with single application of ROM, its application with chemical fertilizers, i. e., N + ROM, N + P + ROM, and N + P + K + ROM, increased the TOC concentration by 0.35%, 1.05% and 0.64%, respectively. The readily oxidized carbon (ROC) in fertilization treatments was increased by 8.64% to approximately 28.4%, and the increment was higher in treatments of chemical fertilizers plus ROM than in treatments of chemical fertilizers. The ROC was significantly correlated with soil TOC (Y = 14.192X + 23.9, R2 = 0.802) and stalk yields (Y = 19032X - 7950.6, R2 = 0.759). Light fraction organic carbon (LF-C) had the same trends with ROC. After 20 years fertilization, the organic carbon in soil humic acid and fulvic acid was decreased by 1.64% to approximately 26.23% and 2.33% to approximately 28.68%, respectively, but in treatments of chemical fertilizers plus ROM, the decreasing trend was slowed down.