Temperature sensitivity of ecoenzyme kinetics driving litter decomposition: The effects of nitrogen enrichment, litter chemistry, and decomposer community

Current mechanistic models for soil organic matter (SOM) that explicitly incorporate ecoenzyme kinetics and thermodynamics have been shown to be superior in predicting SOM dynamics. However, how kinetic parameters (Vmax and Km) and their temperature sensitivity (Q10) are affected by environmental changes (e.g., nitrogen [N] deposition) and substrate chemistry remains a major uncertainty. In this study, we conducted a litter bag decomposition experiment under simulated N depositions in a subtropical broadleaf forest and measured the Q10 of Vmax and Km for six hydrolases and two oxidases. We also investigated decomposer community composition according to phospholipid fatty acid analysis and high-throughput sequencing. We found that the mean Q10-Vmax value for the six hydrolases (2.25) was significantly higher than that for the two oxidases (1.46) and that the mean Q10-Km of all hydrolases (1.13) was lower than the mean values of Q10-Vmax. The mean Q10-Vmax and Q10-Km values of all ecoenzymes were higher for the relatively high-quality Castanopsis chinensis (CC) litter (lignin/N of 1.64) than those for the low-quality Schima superba (SS) litter with (lignin/N of 3.01). With the exception of the Q10-Vmax values of 1,4-β-xylosidase and 1,4-β-N-acetylglucosaminidase and the Q10-Km value of acid phosphomonoesterase (ACP), N addition generally had non-significant effects on Q10. Q10-Vmax and Q10-Km varied significantly with decomposition time in either a linear or parabolic manner, as did litter chemistry and decomposer biomass and community composition. The relative abundance of Basidomycota fungi increased whereas that of Ascomycota decreased toward the end of the 18-month decomposition. We detected notable increases in the abundance of three lignolytic fungal groups (Hymenochaetales, Agaricales, and Xylariales) in CC litter, which indicated that these fungi might be responsible for the more rapid decay of CC litter relative to the SS litter. Our variation partitioning analysis revealed that the phosphorus (P) content of microbial biomass and decomposer community composition were the dominant factors in shaping the Q10 of ecoenzyme kinetics. These results indicate that specific temperature response functions should be derived for hydrolase and oxidase and key functional microbial groups should be incorporated into the current process-based enzyme kinetics-driven SOM models in order to achieve a more mechanistic representation of the structural and functional interactions within a decomposer community.