Objectives We examined the dynamic changes of microbial carbon fractions, and their contribution to soil organic carbon in the black soil farmland after the input of different types and amounts of organic materials, aiming to deeply understand the carbon sequestration mechanism dominated by microbial.

Methods Indoor incubation method was used for the research. The test black soil was from a typical black soil farmland in Keshan Experimental Station, Heilongjiang Province. And the test organic materials were corn stover (S), biochar (B), and dissolved organic fertilizer (D), with three application levels (low, medium and high) for each organic material. NPK fertilizer (F) and no fertilizer (CK) were used as the controls. Sampling was carried out at the 5, 15, 30, 60 and 105 days of incubation for analysis of soil organic carbon content (SOC), microbial biomass carbon content (MBC), bacterial biomass (Ba), fungal biomass (Fu), and microbial residue carbon content (MRC). The biomass of both bacterial and fungal taxa was characterized by their characteristic phospholipid fatty acid content, and the microbial residual carbon was calculated by converting the amount of bacterial and fungal residues characterized by different soil amino sugar contents.

Results Compared with the F and CK treatments, all three organic materials increased SOC, MBC, and MRC contents, and elevated the contribution of MRC to SOC (MRC/SOC). Increase of straw and biochar application rate elevated soil MBC and MRC contents, while the increase of dissolved organic fertilizers declined them. During the 105 days of incubation, all the organic treatments were recorded the peaks of MBC at 30 days, and MRC at 60 days; S3, B3 and D1 exhibited the highest promotion effects. MBC increased by 39.1%, 8.2% and 28.9% and fungal residue carbon (FRC) content by 47.9%, 43.5% and 58.1% in S3, B3 and D1 treatments, and bacterial residue carbon (BRC) content by 18.5% and 16.5% in S3 and D1 treatments, respectively, at 105 days of incubation as compared to CK treatment. Regression analysis of carbon to nitrogen ratio (C/N) and total nitrogen (TN) content of the nine organic materials with microbial carbon fractions at 105 days of incubation revealed that MBC and BRC were positively correlated (P<0.05) with organic material C/N below 29.89 and 28.57, respectively, otherwise would be negatively correlated (P<0.05). And the TN of the organic materials was negatively correlated with MBC and FRC when the TN was below 0.18 g/kg and 0.11 g/kg, respectively, likely the correlation became negative when the TN exceeded the two thresholds. The BRC was always positively correlated (P<0.05) with organic matter TN. For all the organic treatments, MBC/SOC ranged from 3.8% to 9.4%, and the MRC/SOC ranged from 24.1% to 35.3%. Compared with the F and CK treatments, biochar contributed to the increase of SOC, but not on MBC/SOC; straw and dissolved organic fertilizer significantly enhanced MBC/SOC and MRC/SOC.

Conclusions Straw with higher nitrogen content and lower C/N ratio could increase soil microbial biomass carbon and microbial residual carbon contents and their contribution to total organic carbon, and higher application rates strengthen the effect; biochar is low in nitrogen and C/N radio, its application is in favor of soil organic carbon, fungal biomass and microbial residual carbon accumulation, but not on the contribution of microbial biomass carbon to organic carbon; dissolved organic fertilizers contain high nitrogen, so are favorable to enhance microbial biomass carbon and residual carbon content and their contribution to organic carbon at low application rates, while the opposite at high application rates. Therefore, straw and biochar are encouraged to apply in high rate, while the application rate of dissolved organic fertilizers should be controlled for the accumulation of microbial carbon fractions and the fixation of soil organic carbon.