COS 55-5
Soil microbial enzyme responses to changes in temperature and nutrient additions across Hawaiian gradients in mineralogy and nutrient availability
Microbial enzyme activities are the direct agents of organic matter decomposition, and thus play a crucial role in global carbon (C) cycling. Global change factors like warming and anthropogenic nutrient inputs have the potential to alter the activities of these enzymes, with background site conditions likely driving responses. We hypothesized that enzyme activities in sites with high background nutrient and/or C availability would be less sensitive to nutrient additions than nutrient-poor sites. We also hypothesized that sites poor in nutrients and/or C would show greater sensitivity to changes in temperature because of smaller microbial communities. To test our hypotheses we used laboratory temperature incubations combined with long- and short-term nutrient additions to assess changes in enzyme activities for 8 common soil enzymes that acquire nitrogen (N), phosphorus (P) and C from organic matter. We collected mineral soils (0-10 cm depth) from 8 Hawaiian sites that provided maximum variation in nutrient availability and background soil C. Soils were sieved, pooled by site, and homogenized prior to a laboratory addition of a simple C (sucrose) plus N and/or P in full factorial design. The 8 soils were incubated at 7 temperatures from 4 – 40 ºC.
Results/Conclusions
We found that temperature sensitivities varied significantly among sites, and that the laboratory fertilizations altered enzyme activities. Across the 8 sites, laboratory sucrose+N additions nearly doubled P-acquisition enzyme activity (p < 0.05). Similarly, laboratory sucrose+N and sucrose+NP additions significantly increased N-acquiring enzyme activity (p < 0.05), with the strongest effect, as predicted in nutrient- and C-poor soil. Carbon-acquiring enzyme activities were less responsive, but also increased significantly with additions of sucrose+N and sucrose+NP across sites. Results suggest that C-, N-, and P-acquisition enzyme activities are generally nutrient limited across sites, regardless of background nutrient status. In particular, P-acquisition was broadly N limited even in relatively N-rich soils, whereas C- and N-acquisition activity appeared to be generally colimited by N and P. Overall, enzyme activities responded most strongly to the addition of sucrose+N or sucrose+NP. Temperature sensitivity varied significantly across sites and among enzymes, with greater N- and P-acquisition temperature sensitivities in wetter sites. In contrast to our hypothesis, enzyme temperature responses were strongest for soils from relatively nutrient- and C-rich forests, and lowest for drier sites poorer in soil nutrients and C. Taken together, these results have implications for decomposer activity responses to global warming and altered nutrient availability.