Terrestrial ecosystems are increasingly less limited by nitrogen (N) availability. In response to N enrichment, soil microbial communities can shift in composition and function, and consequently affect ecosystem processes such as nitrification and denitrification. Faster microbial N cycling rates can also promote loss of N from the ecosystem through leaching and gaseous emission. If N enrichment is halted, soil microbial recovery could promote ecosystem resilience, but the lag and mechanisms of microbial recovery following cessation of chronic fertilization are not well known. We measured soil microbial recovery for three years following cessation of a 30-y N fertilization experiment at a tallgrass prairie (Konza Prairie) in eastern KS, USA. This 12.5-m-2 plot-scale experiment historically augmented N availability via both fertilization (10 g N m-2 y-1 (NH4NO3)) and the cessation of annual fire. Fertilization was ceased after the 2016 growing season. We predicted that microbial recovery would be quicker in the burned treatment, due to lower historical reliance of soil microbes on supplemental N. To address this, we measured soil nitrification and denitrification potential rates, associated ammonia monooxygenase (amoA) and nitrous oxdide reductase (nosZ) gene abundances, and archaeal and bacterial 16S rRNA community composition monthly during the 2017-2019 growing seasons.
Results/Conclusions
Over the three years following cessation of chronic N enrichment, temporal variability was high, with N-cycling potentials declining in all treatments over the course of the study. The magnitude of recovery of nitrification (NP) and denitrification (DNP) potentials was also different year-to-year (Fertilization×Year interaction, P<0.001). For all three years, annual burning facilitated DNP’s decline (Fire×Year P<0.001), suggesting that microbially-mediated N loss is less of a constraint on ecosystem recovery in the presence of fire. One year after fertilizer cessation, neither the total microbial community composition, nor archaeal nitrifying populations, nor nosZ clade I populations recovered; but bacterial nitrifying populations did decline. This suggests that population and community turnover lag behind functional response in most microbial guilds. Data collection is in progress to assesses population and community recovery within three years. Taken together, sensitivity to changes in N availability may influence microbial resilience to chronic fertilization. Soil organic N buildup from years of fertilizer addition could sustain greater net N mineralization and greater ecosystem N loss from denitrification and plant N uptake despite a lack of supplemental N addition. Microbial N cycling responses vary temporally, but can be affected by legacy and current land management factors like fertilization and fire.