Proteolytic enzymes depolymerize proteins in soil organic matter into amino acids that are used as a source of nitrogen (N) by plants and microbes. Proteolytic enzymes are a principal driver of the within-system cycle of soil N, yet the mechanisms controlling their activity remain largely unknown, including their response to global change. I performed a series of integrated observations and experiments to investigate a conceptual model whereby proteolytic activity is a function of soil temperature and moisture, protein substrate availability, and the amount of enzyme. I used temperate forest tree species that differ in the enzymatic capabilities of their fungal symbionts as a model system. These species supported either ectomycorrhizal or arbuscular mycorrhizal fungi. To examine the potential effects of global change on activity, I measured the response of proteolytic activity to experimental manipulations of temperature and precipitation across biomes.

In temperate forest soils, I found that proteolytic activity was more limited by substrate than by temperature, with declines in both limitations as soil temperature increased over the growing season. Likewise, proteolytic activity in an empirical model was more responsive to simulated increases in substrate than temperature alone, although there was an interactive effect of elevated temperature and substrate on activity. Fine roots stimulated proteolytic activity in the rhizosphere, the zone immediately around the root, likely by enhancing microbial enzyme production. Ectomycorrhizal roots stimulated activity more than arbuscular mycorrhizal roots. In soils from widely varying ecosystems—warm, dry grasslands to boreal forests and arctic tundra—I found a threshold in soil moisture deficit above which proteolytic rates increased and below which they declined in response to temperature and precipitation manipulations.

The results of this dissertation suggest that climate warming in the absence of increases in substrate availability will have a modest effect on proteolytic activity in temperate forests. Further, global changes that alter belowground carbon allocation by trees are likely to have a larger impact on N cycling in ectomycorrhizal stands than in arbuscular mycorrhizal stands. Across biomes, the diffusional limits on enzymes and substrates set by large soil moisture deficits may constrain the response of proteolytic enzymes to warming.