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 C (C) cycling. Global change factors like anthropogenic nutrient inputs and warming 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 soil 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. To test our hypotheses we used long- and short-term nutrient additions combined with laboratory temperature incubations 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), N, and/or P in full factorial design. The 8 soils also were incubated at 7 temperatures from 4 - 40 ºC.
We found that the laboratory fertilizations altered enzyme activities, and that temperature sensitivities varied significantly among sites. Across the 8 sites, laboratory sucrose+N and sucrose+NP additions increased C-, N-, and P- acquiring enzymes activities (p < 0.05), with the strongest effect, as predicted in nutrient and C-poor soil. Phosphorus-acquiring enzymes were the most sensitive to these additions, while C-acquiring enzyme activities were less responsive. Results suggest that C-, N-, and P-acquisition enzyme activities respond positively to added nutrients across sites, regardless of background nutrient status. In particular, P-acquisition was broadly sensitive to N addition even in relatively N-rich soils, whereas C- and N-acquisition activity appeared to be generally sensitive to 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 temperature sensitivities for enzymes that acquire N and P in wetter sites than drier sites. In contrast to our hypothesis, enzyme temperature sensitivities were strongest for soils from relatively nutrient- and C-rich forests, than for drier sites poorer in soil nutrients and C. Enzyme Q10 were measured using the Arrhenius equation to determine temperature dependency of reaction rates. Enzyme Q10 values followed a log linear relationship and were most sensitive to changes from the 10 °C to 20 °C range as opposed to warmer temperatures (p < 0.05). Enzyme responses to changes in climate such as nutrient additions and warming will likely have negative implications for soil C storage as enzymes generally responded positively to warming and nutrient additions.