Extracellular enzyme activity and stoichiometry: The effect of soil microbial element limitation during leaf litter decomposition

To explore carbon (C), nitrogen (N), and phosphorus (P) dynamics during leaf litter decomposition, we investigated the temporal variability of soil microbial biomass and associated soil enzyme activities. Our 342-day leaf litter (Quercus wutaishanica) decomposition experiment at the Loess Plateau (China) sheds light on how soil microorganisms – mainly the soil microbial biomass C, N, P, and soil enzyme activities – maintain element homeostasis of C, N, and P by producing soil enzymes during various phases of litter decomposition. Overall, the highest soil enzyme activities were measured in summer, whereas soil microbial biomass carbon (MBC) and soil microbial biomass nitrogen (MBN) were highest in winter. Soil water content and soil temperature had significant effects on soil enzyme activities and stoichiometry. The results indicate that the stoichiometry of extracellular enzyme activities (lnBG:lnNAG:lnAP) changes significantly between individual stages of litter decomposition. The resources available for microorganisms were restricted by C and P, while P limitation progressively decelerated during decomposition and C limitation was enhanced at the latest stage. Strong positive correlations between ecological indicators of microbial element limitation and soil enzyme activities indicated that microbes allocate C and nutrients towards soil enzyme production to mine for scarce nutrients. This adaptation strategy enabled the soil microorganisms to maintain element homeostasis. Soil MBC, MBN, soil microbial biomass phosphorus (MBP), MBC:MBN, MBC:MBP, and MBN:MBP rarely showed significant correlations with soil or leaf litter C, N, P, C:N, C:P, and N:P. This suggests no resource dependency of microbial element composition but supports the concept of element homeostasis of soil microorganisms. Besides compensating for element imbalances by adjusting soil enzyme production, the microorganisms also adjusted element use efficiencies such as C use efficiency. We conclude that the maintenance of element homeostasis by soil microorganisms induces a tight co-regulation of enzymes involved in covering their C and nutrient supply during successive litter degradation.