Soil dissolved organic matter quality and bacterial community composition regulate the substrate-binding affinity of hydrolytic enzymes under short-term nitrogen addition

Soil enzymes play important roles in soil C and nutrient cycling. However, the effects of N addition on soil enzyme kinetics and the underlying mechanisms remain unclear. Thus, we aimed to determine the effects of short-term N addition on the soil properties, microbial properties, maximum reaction rate (Vm, which is attained at saturating substrate concentrations), and Michaelis constant (Km, where a high Km indicates low substrate affinity) of microbial C- (β-1,4-glucosidase and cellobiohydrolase), N- (β-1,4-N-acetylglucosaminidase and L-leucine aminopeptidase), and P-degrading (acid phosphatase and alkaline phosphatase) enzymes in subtropical coniferous (Pinus taiwanensis) and broadleaf (Castanopsis faberi) forests. In the broadleaf forest, N addition increased the Vm and substrate-binding affinities (decline in Km) of C- and P-degrading enzymes by triggering a P deficiency response in microorganisms (i.e., increased microbial biomass N:P ratio). These findings indicate that the soil enzyme kinetics followed the optimal foraging strategy in response to N addition. Moreover, N addition reduced the proportion of complex organic molecules in dissolved organic matter (DOM; e.g., reduced abundance of humic-like fluorophores and humification index), suggesting that N addition increased soil DOM quality and thus increased the affinities of C-degrading enzymes. N addition increased the abundance of Acidobacteria and Chloroflexi but reduced the abundance of Proteobacteria and Rhizobiales, indicating a shift in microbial community toward efficient P acquisition. N addition affected bacterial composition and thus indirectly influenced N- and P-degrading enzymes. In the coniferous forest, N addition significantly increased the Vm of C-degrading enzymes but did not change other enzyme kinetics, which could be partly attributed to the unchanged N availability and microbial properties. Collectively, our findings provide insights into the relationship between enzyme kinetics, DOM quality, and microbial properties, which are important for predicting soil nutrient cycling and parameterizing models of C cycling under N deposition.