Frequent burning of a tallgrass prairie increases soil C and N storage as pyrogenic organic matter, while reducing N availability

Litter decomposition is the main process that returns C and nutrients assimilated by plants during net primary production back to the atmosphere and soil from where they came. Grasslands store an estimated 30% of the total world's soil C, and are often managed by frequent burning. In frequently burned grasslands, fire stimulates grass productivity but also removes the aboveground litter layer that would otherwise decompose and contribute to SOM formation, while returning thermally altered pyrogenic organic matter (py-OM) to the soil surface. We hypothesized that the removal of litter and input of py-OM from frequent burning directly alters the SOM formation process in ways that impact C and N cycling. Here, we will present the results of a study where we incubated 13C and 15N labeled litter and labeled py-OM in the field at both an annually burned and an infrequently burned tallgrass prairie site to examine these effects of fire on SOM formation. Over the course of one year, we traced C and N from the decomposing litter and py-OM into CO2 fluxes, the bulk soil down to 40 cm, and microbial phospholipid fatty acids (PLFAs) and SOM fractions in the top 5 cm of the soil. We found little evidence of biological decomposition of py-OM, and the py-OM was recovered mainly in the light fraction of the surface soils untransformed by soil microbes. In contrast, the litter had lost almost 50% of its mass, with significant losses to CO2 and microbial PLFA uptake. PLFA uptake as well as sand, silt and light fraction SOM incorporation of litter C was significantly higher in the annually burned site as compared to the infrequently burned site, likely due to N limitation created by a long history of altered SOM formation due to burning. Although the annually burned site showed biological evidence for N limitation, it also had a higher amount of overall C and N in the soils. These results imply that although py-OM accumulation can add to long term C and N storage in frequently burned soils, the chronic removal of aboveground litter by burning limits the recycling of nutrients provided by the litter decomposition process, leading to N limitation. These results provide insight into the mechanisms underlying SOM formation in frequently burned grasslands, which will be critical to modeling SOM dynamics under future scenarios of increased fire frequency.