Biochar interaction with chemical fertilizer regulates soil organic carbon mineralization and the abundance of key C-cycling-related bacteria in rhizosphere soil

Biochar amendment in soils has been proposed as an important strategy to improve soil C retention. However, while the exact roles of different biochar types on soil organic carbon (SOC) dynamics are not clear, the effect of their interactions with chemical fertilization on SOC, the dynamics of C-cycling enzyme activities and key C-cycling-related bacteria in rhizosphere soil remain unexplored. Therefore, biochars derived from rice straw, bamboo, cow, and pig manure were applied with/without chemical fertilizer to study soil C dynamics, the activities, diversity, and abundance of different C-cycling-related bacteria in the plant-soil system. Biochar + fertilizer (BF), irrespective of biochar source, induced an initial increase in dissolved organic and inorganic C (DOC and DIC) that were subsequently reduced compared to biochar over 15-weeks. Higher intensities of C-containing functional groups rather than the total C content of biochar induced the mineralization of SOC. Also, higher nutrient uptake, biomass, and CO2 concentration in plants under BF had an insignificant influence on SOC. Similarly, the reduction in mineral-associated organic C, DOC, DIC, and invertase activity in BF soils was associated with a decrease in the relative abundance of some key C-cycling related bacterial orders: Gemmmatimonadales, Myxococcales, Nitrosomonadales, and Acidimicrobiales, while the hydrocarbon-degrading taxa (Sphingmonadales and Xanthomonadales), and N-cycling bacteria were stimulated. Hence, BF amendment could exert varied influences on C-cycling-related bacteria and on global C-cycling. The use of BF limited C-cycling and its loss by providing more N-substrates for N-cycling organisms, which also competed with most C-cycling-related bacteria for organic C to regulate C mineralization and its associated loss.