Organic carbon quality, composition of main microbial groups, enzyme activities, and temperature sensitivity of soil respiration of an acid paddy soil treated with biochar

The role of soil organic C (SOC) quality affecting microbial community composition and function under biochar application is poorly understood. We investigated the relationship between the pool size and chemical composition of SOC; composition of main microbial groups; enzyme activities involved in C, N, and P cycling; and soil respiration in a rice paddy amended with biochar for 20months in a laboratory experiment at 15, 25, and 35 degrees C. Soil labile and recalcitrant organic C pools were determined by a two-step sulfuric acid (H2SO4) hydrolysis method. The chemical composition of SOC was determined with C-13-nuclear magnetic resonance spectroscopy. The biochar amendment at 20 and 40tha(-1) significantly decreased the soil labile C pool I (extracted by 5N H2SO4), alkyl, and carbonyl C contents and increased the recalcitrant C pool (acid-resistant) and aromatic C contents and the aromatic C to O-alkyl C ratio. The phospholipid-fatty acid concentrations and soil enzyme activities were unchanged by biochar application at 10 and 20tha(-1), but both were increased at 40tha(-1). Biochar increased the ratio of gram-positive (G+) to gram-negative (G-) bacteria and decreased that of fungi to bacteria. The recalcitrant C pool and aromatic C contents were positively correlated to the G+ bacteria abundance and were important factors in shaping composition of the main microbial groups and improving enzyme activities. Biochar application at 40tha(-1) lowered soil respiration rates at 15 and 25 degrees C by decreasing labile C pool and increasing C recalcitrancy while increased temperature sensitivities of soil respiration at 25/15 degrees C and 35/25 degrees C by stimulating microbial abundance and enzyme activities. Together, our results suggest that biochar soil amendment shifted microbial community composition and function through influencing the composition of SOC.