Soil moisture deficit drives proteolytic enzyme activity in a consistent way at the site-level and across a network of global change experiments

Background/Question/Methods

Nitrogen (N) availability regulates the Earth's climate system by constraining the terrestrial sink for atmospheric CO₂.  Proteolytic enzymes control the rate at which N becomes available for plants and soil microbes, yet there is little to no understanding of the response of these enzymes to global change.  We examined the sensitivity of proteolytic enzymes to warming and altered soil moisture by using a single standardized methodology to quantify enzyme activities in 1) multiple ecosystems exposed to experimental climate manipulations and 2) a series of forest plots that are subject to large inter-annual variation in climate. Soils were collected from 16 global change experiments from across the US (including tundra, grassland, and forest ecosystems), and across 45 plots in the central hardwood forests of southern Indiana that span a gradient in soil organic matter (SOM) chemistry and fertility.   

Results/Conclusions

Regardless of geographical location or experimental manipulation (i.e., temperature, precipitation, or both), the response of proteolytic enzymes to global change was highly correlated with shifts in soil moisture deficit (i.e., defined as the difference between precipitation and potential evapotranspiration; R²=0.47; p<0.001).  In particular, there was a relatively narrow range of soil moisture deficit (−200 mm to −100 mm) above which proteolytic rates tended to increase and below which they tended to decline.  Similar to the cross-ecosystem results, proteolytic activity at the site-level was highly sensitive to seasonal shifts in soil moisture deficit with rates increasing during warm and wet conditions and declining during periods of drought.  However, the sensitivity of proteolytic enzyme activity to shifts in soil moisture deficit increased as the C-to-N ratio of SOM decreased suggesting that the baseline availability of protein substrates controls the amplitude of proteolytic responses.  Collectively, our results highlight the power of coupling comparative approaches with detailed site-level observations and suggest that there is an important interaction between SOM chemistry and water availability that controls N-cycling responses to global change.