Change of organic carbon content and its fractions in black soil under long-term application of chemical fertilizers and recycled organic manure

Soil organic matter (SOM) is an important indicator for soil quality and sustainable agriculture, and agricultural practices may strongly affect SOM content and chemistry. Thus, a long-term experiment was conducted in northeast China to study the effect of chemical fertilizers and recycled organic manure on the contents of SOM along its fractions of black soil (0-20-cm depth). Eight treatments were used: 1) control, 2) recycled organic manure (ROM), 3) nitrogen (N) alone, 4) N+ROM, 5) N+phosphorous (P), 6) N+P+ROM, 7) N+P+potassium (K), and 8) N+P+K+ROM. The results showed that from 1985 to 2002; soil total organic carbon (TOC) decreased by 6.5% over the 18 years in control (33.25 g/kg in 1985), 5.6% in ROM treatment, and 5.1% in N-alone treatment, and 1.5% in the N+P treatment but increased by 0.3% in the N+P+K treatment. In the three treatments of chemical fertilizers together with ROM (i.e., N+ROM, N+P+ROM, and N+P+K+ROM), soil TOC content in 2002 increased by 0.3%, 1.3%, and 2.8%, respectively, when compared with control in 1985. Compared with control in 2002, light fraction organic carbon (LFOC) content increased by 23% in N+ROM treatment, 24% in N+P+ROM, and 28% in N+P+K+ROM, and readily oxidized carbon (ROC) content increased by 24% 31%, and 39%, respectively, in these three treatments. The ROC was well correlated with soil TOC. Compared with the data from 18 years ago, in all eight treatments organic carbon of soil in the humic acid fraction decreased by 5.0-13% and in fulvic acid decreased by 1.4-14%; however, with chemical fertilizers and ROM applied together, the rate of decrease was reduced. Overall, ROM is effective for increasing the content of soil TOC and thus, its different active carbon fractions in black soil would help to maintain or increase soil productivity.