Varying pyrolysis temperature impacts application effects of biochar on soil labile organic carbon and humic fractions

Limited information is available to understand the effect of biochar (BC) on soil labile organic carbon (C) and humic fractions. A laboratory incubation experiment was conducted with Lou soil (Earth-cumuli-Orthic Anthrosol) to study the influence of BCs produced at 300, 400, 500 and 600 °C (BC300, BC400, BC500 and BC600) on soil labile organic C and humic fractions, respectively. BC products were added at the rate of 2% and incubated at 25 °C over 360 days. Non-amended soil was used as the control (CK). Compared with CK, BCs amendment significantly increased the contents of soil organic C and humin by 167.51%-255.52% and 288.42%-618.11%, respectively. Microbial biomass C (MBC), dissolved organic carbon (DOC), humic acid (HA) and fulvic acid (FA) were lower in BC500 and BC600 amendment after the initial stage of incubation, while MBC, DOC and HA were higher in BC300 and BC400 amendment during the whole incubation in comparison with CK. The color tone coefficients of HA and FA increased by 6.74%, 30.87% in BC300 amendment and by 2.57%, 9.27% in BC400 amendment, while decreased by 0.32%, 5.98% in BC500 amendment and by 3.19%, 12.19% in BC600 amendment at the end of incubation as compared with CK, respectively. Compared with CK, BC300 and BC400 amendment significantly increased the ratios of HA/FA, while BC500 and BC600 amendment showed no significant difference in comparison with CK. Thus, the properties of BC should be individually characterized and the application of BC should be carefully monitored due to the positive and negative effect on soil labile organic C and humic fractions after BCs amendment.