Print PageFire plays an important role in the productivity and biogeochemistry of grassland ecosystems. Fire frees nitrogenous compounds from biomass creating a pulse of available ammonium and nitrate. Semiarid grasslands are characterized by patchy vegetation that creates grass-based resource islands for plant and microbial communities. By transforming biomass nitrogen into more mobile compounds fire speeds nitrogen cycling and may cause a redistribution of nutrients in this spatially structured nutrient landscape. Such a redistribution would allow for the restructuring of plant communities as well as the communities of ammonia-oxidizing microbes (i.e., nitrifiers). Variations in available inorganic nitrogen should cause changes in nitrifier communities more quickly than plant communities, especially in semiarid grasslands where growth of many organisms is retarded due to moisture availability. Environments enriched in ammonium have been observed to cause increases in the ratio of bacterial to archaeal nitrifiers (AOB:AOA). Furthermore, fire has been found to cause dramatic changes in the structure of bacterial nitrifiers in forest ecosystems, however no data has been published that describes effects of fire on the recently discovered archaeal nitrifiers.
Results/Conclusions
Working on established burn plots at the Shortgrass Steppe LTER in Colorado we measured the concentration of inorganic nitrogen, the ratio of nitrifier communities, and the structure of each domain’s community. We analyzed soils of vegetated and unvegetated samples taken from plots left unburned or burned at one of two fire frequencies. Burned plots contained ~30% higher concentrations in total inorganic nitrogen and vegetated soils had higher ammonium and lower nitrate than bare soils (all p<0.001). The inorganic nitrogen levels of burned soils were elevated uniformly across vegetated and bare soils, supporting the hypothesis that burning allows for redistribution of resources. The ratio of nitrifiers (AOB:AOA) varied by two orders of magnitude between vegetated and bare soils with more bacterial sequences detected in vegetated soils. Smaller variations in this ratio were related to burn treatments. The community structure of bacterial communities also varied strongly between bare and vegetated soil, though the archaeal communities remained statistically similar. The bacterial nitrifier structure in vegetated soils was similar to communities found in ammonium addition studies. In summary, shortgrass steppe nitrifier communities and nitrogen resources are strongly controlled by vegetation and fire in this ecosystem can redistribute resources.
