Inter-annual variation in soil extra-cellular enzyme activity in response to simulated global change and fire disturbance

The interactive effects of global change drivers (e.g. climate change, nitrogen deposition, and elevated CO2) on soil microbial activity have important implications for ecosystem carbon and nutrient cycling. However, these interactions have primarily been explored in single-year, single-factor studies despite the reality of multiple simultaneous global changes. We measured the activity of six enzymes (β-glucosidase, α-glucosidase, cellobiohydrolase, xylosidase, acid phosphatase, and N-acetylglucosaminidase) for 3 years (2004–2006) following wildfire (2003) at the Jasper Ridge Global Change Experiment (JRGCE), a long-term multi-factor global change experiment in a California annual grassland. JRGCE manipulations include climate change (elevated temperature and precipitation), nitrogen addition, and elevated CO2. In general we found only minimal interactions among treatments and found the strongest main effects from nitrogen addition and burning. Enzyme activities increased under elevated nitrogen, a response that persisted throughout the 3 years of the study. Wildfire was slightly related to decreased enzyme activities (by 10–20%) in 2004, with a larger decrease (by 25–50%) in 2005. The response was gone by 2006, suggesting that the microbial community was able to recover by 3 years following wildfire. Finally, enzyme responses to treatments, even where statistically significant, were of smaller magnitude than annual variation in activity. We propose that overall decreases in enzyme activity from 2004 to 2006 were due to decreased temperature and increased precipitation in 2005 and 2006 relative to 2004. Our results suggest that while it is important to assess response to specific global change treatment and treatment interactions, these responses (A) may change over time and (B) should be characterized within the context of inter-annual fluctuation in microbial community function.