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 (V-max and K-m) and their temperature sensitivity (Q(10)) 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 Q(10) of V(max )and K-m 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 Q(10)-V-max value for the six hydrolases (2.25) was significantly higher than that for the two oxidases (1.46) and that the mean Q(10)-K-m of all hydrolases (1.13) was lower than the mean values of Q(10)-V-max. The mean Q(10)-V-max and Q(10)-K-m 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 Q(10)-V-max values of 1,4-beta-xylosidase and 1,4-beta-N-acetylglucosaminidase and the Q(10)-K-m value of acid phosphomonoesterase (ACP), N addition generally had non-significant effects on Q(10). Q(10) V-max and Q(10)-K-m 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 Q(10) 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.