Fire plays an important role in the productivity and biogeochemistry of many temperate ecosystems. Some effects of fire are physical (e.g. soil moisture) or chemical (e.g. nutrient availability) while others are mediated by soil microbes (e.g. nitrification). However the degree to which microbial activity is modified by fire regimes in grassland systems has received little attention. Fire has been shown to affect nitrogen cycling significantly in tallgrass prairie ecosystems, but the effect on less productive grasslands (e.g. shortgrass prairie) is not known. Nor is it known through which microbial mechanism or set of mechanisms fire affects nitrification, a key transformation in the soil nitrogen cycle. Fire may also affect soil methane flux in grassland ecosystems. Although methane-oxidizing microbes in upland soils contribute the primary organic sink to this powerful greenhouse gas the effect of fire on methane-oxidizing microbes has not been studied. Rates of soil methane uptake are often closely related to soil moisture due to gas diffusion and microbial desiccation, and bacterial methane-oxidation can be inhibited by high levels of ammonium. Both soil moisture and nitrification are expected to be modified by fire, potentially causing indirect effects of fire on soil methane flux.
We measured methane uptake, soil inorganic nitrogen pools, net ecosystem CO2 exchange, and abiotic and biotic factors known to influence microbial activity. Our goal was to determine the degree to which these abiotic and biotic factors were influenced by fire regime and how those effects transferred to biogeochemical effects via the soil microbial community.
In this poster we will present findings from field research conducted over the spring and summer of 2009 on two burn regimes at the Shortgrass Steppe LTER: an annual spring burn and a 4-year spring burn, both of which burned in Spring 2009. We will discuss the seasonal changes and treatment effects in soil moisture, inorganic nitrogen, pH, and temperature and how these factors relate to ecosystem productivity, biogeochemistry, and the microbial community.