Pyrolysis temperature of biochar affects ecoenzymatic stoichiometry and microbial nutrient-use efficiency in a bamboo forest soil

Abstract

A microcosm study was performed to investigate changes in soil enzyme activities and microbial C- and N-use efficiencies (CUE and NUE) with amendment of biochars prepared at three pyrolysis temperatures (350, 500 and 700 °C) in an acid bamboo (Phyllostachys praecox) forest soil. The results showed that, compared to the non-amended control, biochars produced at 500 and 700 °C significantly (P < 0.05) increased soil pH, total N, and dissolved N (DN) concentrations, whereas significantly decreased dissolved organic C (DOC) and exchangeable acidity concentrations after three months. The microbial biomass C (MBC) and N (MBN) and the ratio of fungi: bacteria (F:B) were only significantly increased under 350 °C biochar. The ratios of both soil C:N and DOC:DN to MBC:MBN were reduced under 500 and 700 °C biochars, suggesting a lower C:N imbalance between resources and microorganism. The ratio of C- to N-acquiring enzyme activities increased gradually under biochars with increasing temperature. Moreover, microbial CUE increased whereas NUE declined under biochars at 500 and 700 °C, and the threshold elemental ratio (TER) revealed that the microbial nutrient metabolisms were limited by N in soils amended with residue, but were limited by C under biochars at 500 and 700 °C. Structural equation modeling indicated that the C:N imbalance had a great impact on microbial CUE, while changes in F:B ratio and soil pH were closely associated with NUE. This study suggests that changes in microbial nutrient-use efficiency and ecoenzymatic stoichiometry reveals a clear C-limitation, but a N-availability under short-term amendment of biochar produced at a high pyrolysis temperature.