Associate Professor Division of Biology - Kansas State University Manhattan, Kansas, United States
Background/Question/Methods
Global change drivers risk tipping terrestrial ecosystems from being net retainers of nitrogen (N) to net exporters. N eutrophication from atmospheric deposition or fertilization increases N-cycling activities and causes plant and microbial community shifts that may feed negative consequences to ecosystems. Fire cessation can increase ecosystem N-cycling rates, but net N retention could exist still in unburned prairies if microbially-mediated N loss is less than fire-mediated N combustion. Since N deposition is declining, we need to study how these drivers interact and to assess how annual burning influences N dynamics. Because burning combusts litter-N and subsequently limits soil available N, we hypothesized that fire would decrease ecosystem sensitivity to supplemental N, and increase resilience to chronic fertilization. To test our hypothesis, we used a long-term fertilization experiment at Konza Prairie (Manhattan, KS, USA), where burned and unburned prairies were either unfertilized (control) or fertilized with N (10 g N m-2 y-1 as NH4NO3) for thirty years until 2017 when fertilization ceased. In 2017-2021, we quantified plant- and microbe-available N, nitrification potentials, and denitrification potentials in long-term control and previously-fertilized plots, and continually fertilized sub-plots. Moreover, we estimated ecosystem N stocks five years after ceasing N.
Results/Conclusions
Total available inorganic N, measured using resin bags, declined sharply in previously-fertilized soils, and unburned prairies had more soil available N than burned prairies (P=0.004); however, available N remained higher in previously-fertilized prairies than control prairies except for 2020 (Fertilization×Year: P<0.001). Nitrification potentials for most years were not different between fertilization treatments in unburned prairies, but nitrification potentials were lower in previously-fertilized, burned prairies than continuously fertilized, burned prairies in 2019 and 2020 (Fire×Fertilization×Year: P<0.001), showing greater resilience of nitrification following relief of chronic fertilization in burned prairies. Denitrification potentials were similar between control and previously-fertilized soils by 2021 (Fertilization×Year: P=0.002) in both burned and unburned prairies. In 2021, total belowground N stocks were highest in unburned prairies (P<0.001), largely because unburned prairies had higher soil N stocks, and fertilizer-derived N was retained slightly more in unburned prairies than burned prairies (64 vs. 58%). Overall, we find support that burned prairies are less sensitive to supplemental N than unburned prairies, and some support that burning increases resilience. Additionally, unburned prairies support higher N-cycling rates, and accrued more fertilizer-N, thus may contribute more to N pollution via leaching and nitrous oxide emission from denitrification.
