Effects of chicken manure application methods on soil organic carbon content and its fractions

Soil organic carbon (SOC) content and its fractions are critical indicators of soil quality and function. The impacts of the addition of exogenous organic fertilizer on SOC and its fractions in black soil with high SOC remains unclear. Based on a experiment with five years treatments in the Hailun Agro-ecological Experimental Station of the Chinese Academy of Sciences, we explored the effects of equal amount of chicken manure (30 t·hm^-2·a^-1) on SOC content and physical fractions (free light fraction carbon fLFC, occluded light fraction carbon oLFC, heavy fraction carbon HFC) and chemical fractions (easily oxidized organic carbon EOC, dissolved organic carbon DOC, humic acid carbon HAC, fulvic acid carbon FAC, humin carbon HMC) in 0-50 cm soil profile. There were three different methods of surface coverage (CM), shallow incorporation 15 cm (ST) and deep incorporation 35 cm (DT). The results showed that the SOC content of CM, ST, and DT was increased by 0 to 19.2%, 1.7% to 15.0% and 2.1% to 22.4%, respectively. The SOC storage was increased by 3.4%, 5.7% and 9.1%, respectively. The SOC content and storage of each treatment decreased with the deepening of soil layer. The organic carbon conversion rate of DT treatment was significantly higher than that of ST and CM. Compared with CK, CM treatment increased fLFC, oLFC and HFC by 39.5%, 30.0% and 17.6% in 0-10 cm soil layer, and the chemical fractions DOC and HAC were increased by 100.0% and 30.7%, respectively. The contents of fLFC, oLFC and HFC in 10-20 cm soil layer in ST treatment were significantly increased by 45.5%, 22.2%, and 10.8%, respectively, compared with CK, while the contents of DOC and HMC in 20-30 cm soil layer were increased by 95.2% and 19.5%, respectively. In the 20-50 cm soil layer, fLFC, oLFC and HFC increased by 16.7%-29.2%, 15.4%-46.2% and 12.0%-20.8%, and EOC, DOC, HAC and FAC increased by 12.0%-18.0%, 119.1%-129.4%, 37.1%-43.8% and 9.7%-13.3%, respectively. In general, chicken manure deep mixed returning (35 cm) was the most effective measure to improve the organic carbon storage and active carbon component content of black soil soil profile, and the influence depth can be below 40 cm soil layer, which would provide a theoretical basis for the improvement of cultivated land fertility and the optimization of carbon sequestration technology in black soil area in Northeast China.