Limited Ecosystem Recovery from Simulated Chronic Nitrogen Deposition

Abstract

The realization that anthropogenic nitrogen (N) deposition is causing significant environmental change in many ecosystems has led to lower emissions of reactive N and deposition rates in many regions. However, the impacts of N deposition on terrestrial ecosystems can be long lasting, with significant inertia in the return of the biota and biogeochemical processes to baseline levels. To better understand patterns of recovery and the factors that may contribute to slow or no responses following declines in N deposition, we followed plant species composition, microbial abundance, N cycling rates, soil pH, and pools of NO 3 À and extractable cations in an impacted alpine ecosystem following cessation of 12-yr experiment increasing N deposition rates by 0, 20, 40, and 60 kg NÁha À1 Áyr À1 . Simulated N deposition had resulted in a tripling in the cover of the nitrophilic species Carex rupestris, while the dominant sedge Kobresia myosuroides had decreased by more than half at the highest N input level. In addition, nitrification rates were elevated, soil extractable magnesium (Mg 2+ ) and pH decreased, and aluminum (Al 3+ ) and manganese (Mn 2+ ) were elevated at the highest N treatment inputs. Over the nine years following cessation of N additions to the impacted plots, only the cover of the nitrophilic C. rupestris showed any recovery to prior levels. Abundances of both bacteria and fungi were lower with N addition in both treatment and recovery plots. Rates of nitrification and pools of NO 3 À remained elevated in the recovery plots, likely contributing to the lack of biotic response to the cessation of N inputs. In addition, nutrient base cations (Ca 2+ and Mg 2+ ) and soil pH remained depressed, and the toxic metal cations (Al 3+ and Mn 2+ ) remained elevated in recovery plots, also potentially influencing biotic recovery. These results emphasize the importance of considering long-term environmental impacts of N deposition associated with legacy effects, such as elevated N cycling and losses of base cations, in determining environmental standards such as the metrics used for critical loads.

Plant–bacteria–soil response to frequency of simulated nitrogen deposition has implications for global ecosystem change

“…Moreover, N amendment frequency may affect ecosystem recovery rates after amendments cease. For example, following cessation of long‐term N additions, higher frequency inputs (weekly throughout a year) show rapid recovery in soil pH (Power, Green, Barker, Bell, & Ashmore, ), whereas infrequent amendments (three times per year) show persistent negative effects on soil pH (Bowman et al, ). Our study demonstrated that N‐induced effects on soil pH were smaller in the more frequent 12X relative to less frequent 2X‐N amendments (Figure S3).…”

Legacy effects of experimental environmental change on soil micro‐arthropod communities

“…, Bowman et al. ), so the availability of food to fungivorous micro‐arthropods would have to be tested directly. Second, an increase in the abundance of predators could have controlled Collembola abundance.…”

“…If this effect was reduced shortly after cessation of the treatments, there may not have been sufficient food available to sustain high Collembola abundances. However, the effects of nitrogen addition are reported to be long-lasting and recovery is often incomplete (Street et al 2015, Bowman et al 2018, so the availability of food to fungivorous micro-arthropods would have to be tested directly. Second, an increase in the abundance of predators could have controlled Collembola abundance.…”

A new sampler for the collection and retrieval of dry dust deposition

“…Other constituents relevant to the NADP network include acid anions and base cations. The dissolution of calcium carbonate and other dust minerals either in precipitation or in depositional water bodies can provide readily available Ca 2+ and acid neutralizing capacity (ANC) to remote mountain lakes and soils 1,38,40,[57][58][59][60][61] . For regions with catchments having low natural calcium abundances, this contribution may be a critical source of calcium to aquatic organisms with relatively high calcium requirements, e.g., Daphnia.…”