Frequent burning is a common land practice in many grasslands worldwide, and this land use strategy has large impacts on a wide variety of ecosystem functions and services. Fire in tallgrass prairie, in the absence of grazing, alters plant community composition, decreases plant richness, and increases plant production. Proposed mechanisms for the changes in community composition and function are that fire decreases N availability (through volatilization) and removes litter (thereby increasing light availability and decreasing soil moisture). Additionally, the effects of fire do not stop at the plant community, impacting other trophic levels and below-ground processes. While the responses to fire of these different trophic levels are frequently studied independently, rarely are they examined together as interacting components of a tightly coupled system. At the Konza Prairie Biological Research Station (Konza), fire frequency has been manipulated since the 1970s, creating the ideal backdrop to study fire frequency effects on multi-trophic community dynamics and ecosystem functioning. In 2014, we collected data on plants (above- and below-ground), soil microbes, mycorrhizal fungi, insects (aboveground only), and environmental data from the same permanently marked plots in unplowed, native, tallgrass prairie that is either annually burned or burned every 20 years.
Results/Conclusions
Long-term fire frequency manipulations strongly impacted almost all variables measured. While light at the soil surface was lower for the 20-yr burn return interval as expected, surprisingly soil N was not significantly different between the two fire return intervals. Plant community composition was significantly different between the two burn return intervals, and ANPP was significantly lower in annually burned grassland but BNPP was significantly higher. Typically, annually burned grassland has higher ANPP and BNPP than a less frequently burned grassland; however, 2014 was a relatively dry year, and we hypothesize that the litter in the 20-yr insulated the soil keeping moisture levels higher and thus stimulating ANPP. Microbial activity of the C:N acquiring enzymes was also lower in annually burned grassland reflecting N limitation. This is surprising given that we did not find lower levels of soil N. Additionally, insect biomass and composition differed significantly between the two burn return intervals. Overall, we saw strong differences in both community and ecosystem functions across all trophic levels. Interestingly, we found that above- and below-ground components were more tightly linked in the unburned grassland, potentially leading to increased stability of ecosystem function through time.