Increased fire incidence in California annual grasslands can alter soil microbial responses to multi-factor global change

Background/Question/Methods

Wildfire in California annual grasslands is an important ecological disturbance and ecosystem control. Regional and global climate changes that affect aboveground biomass will alter fire-related nutrient loading and promote increased frequency and severity of fire in these systems. This can have long-term impacts on soil microbial dynamics and nutrient cycling, particularly in N-limited systems such as annual grasslands.  Using the Jasper Ridge Global Climate Change Experiment (JRGCE) system, we examined the effects of a low-severity fire and nitrogen deposition (N-dep) on soil microbial biomass, microbial biolipid markers and ammonia oxidizing Bacterial communities in a time series following a summer grassland prescribed burn.  Samples were collected before the July 2011 fire, immediately after the fire, 4 months post-fire (December 2011) and in the following growing season (April 2012).   We examined single and multi-factor effects of fire and N-dep to soil microbial communities.

Results/Conclusions

Results suggest that fire increases the amount of soil ammonium concentrations in similar quantities to the N-dep treatment at the JRGCE.  In N-dep treatments, a significant relationship between ammonium concentrations and total ammonia-oxidizer abundance ammonia oxidizing community structure is present, suggesting that N-dep significantly alters nitrifying microbial communities and processes.  However, in fire treatments this significant relationship is not seen, despite similar increases in ammonium concentrations.  This difference in response to N-dep may involve a mediation of P-limitation and increased soil water content as a result of fire, enhanced by increased plant competition and arbuscular mycorrhizal fungi-plant associations after fire.