Soil nitrogen responses to an ice storm manipulation experiment in a northern temperate forest

Background/Question/Methods

Temperate forest ecosystems are increasingly impacted by human-induced changes in climate, which have the ability to alter the prevalence, severity, and extent of extreme weather events. Ice storms, an example of such extreme events, tend to be rare and often occur as localized events, making them difficult to predict. As such, their impacts on ecosystem structure and functioning are poorly understood. We utilized a field manipulation experiment that effectively simulated natural ice storms of varying intensities to mechanistically understand the short-term nitrogen (N) responses to such extreme weather events. Potential net N mineralization and nitrification were quantified for both the organic and mineral soil horizons via 30-day in situ incubations of intact soil cores, as well as via 10-day laboratory incubations of disturbed soil cores. In situ denitrification rates were quantified by directly measuring dinitrogen (N₂) and nitrous oxide (N₂O) production in intact soil cores using the Nitrogen-Free Air Recirculation Method (N-FARM) and denitrification enzyme activity (DEA) was measured in disturbed soil cores as an index of potential denitrification activity. These microbial transformation measurements were carried out in the fall of the pre-treatment year (2015), and the spring and fall of the post-treatment years (2016 and 2017).

Results/Conclusions

We found that over the short-term there was little N cycle response to the field-simulated ice storm events. Specifically, the availability of inorganic N to the microbial community did not significantly change over the 2-year sampling period for any of the ice treatment amounts. None of the rapid, dynamic response variables of the N cycle – i.e. NO₃^- (nitrate) pools, net nitrification rates, and N₂O production rates responded to the treatments. We hypothesize that the forested ecosystem is now in a state of N oligotrophy, and thus less likely to show any N response to disturbance in the short-term. This suggests that recovery of the forest over the long-term may be slower than that observed following a natural ice storm event that took place in 1998 in the same forest. During this natural event, N loss processes markedly increased due to declines in plant uptake, but recovery was rapid. Now both response and recovery processes appear to have been altered by changes in climate and atmospheric chemistry that have induced N oligotrophication.