Trade-off in key carbon-degrading enzyme activities limits long-term soil carbon sequestration with biochar addition

Amending soil with biochar is a promising agricultural approach to abate
climate change by sequestering carbon (C) into soil. Impacts of biochar addition on microbial mediated decomposition process could influence soil C sequestration, but the underlying mechanisms are uncertain. Here, we conducted a meta-analysis of 130 studies and 563 paired observations worldwide, to investigate the effects of biochar addition on key extracellular enzyme activities that constitute the rate-limiting steps of microbial decomposition. Our results showed that biochar addition significantly
increased soil ligninase activity targeting complex phenolic macromolecules, but
suppressed cellulase activity degrading ordered polysaccharides with simpler structure. This trade-off in C-degrading enzyme activity explained more variation in soil C content response to biochar addition than a wide range of other climatic, edaphic and microbial attributes. Specifically, soil ligninase: cellulase ratio increased with time after biochar addition, and was negatively related to changing soil C content with biochar addition. These results indicate that despite the overall promotion of soil C content by reducing cellulase activity, the greater ligninase activity and increased ligninase:cellulase ratio may contribute to the declining effects of biochar amendment on soil C sequestration over time. Our results suggest that, through shift in key extracellular enzyme activities, physiological acclimation of soil microbial metabolic activity limit the long-term consequence of biochar addition on soil C sequestration.