Reactive nitrogen oxides (NOy; NOy= NO + NO2+ HONO) decrease air quality and impact radiative forcing, yet the factors responsible for their emission from nonpoint sources (i.e., soils) remain poorly understood. We investigated the factors that control the production of aerobic NOyin forest soils using molecular techniques, process-based assays, and inhibitor experiments. We subsequently used these data to identify hotspots for gas emissions across forests of the eastern United States. Here, we show that nitrogen oxide soil emissions are mediated by microbial community structure (e.g., ammonium oxidizer abundances), soil chemical characteristics (pH and C:N), and nitrogen (N) transformation rates (net nitrification). We find that, while nitrification rates are controlled primarily by chemoautotrophic ammonia-oxidizing archaea (AOA), the production of NOyis mediated in large part by chemoautotrophic ammonia-oxidizing bacteria (AOB). Variation in nitrification rates and nitrogen oxide emissions tracked variation in forest communities, as stands dominated by arbuscular mycorrhizal (AM) trees had greater N transformation rates and NOyfluxes than stands dominated by ectomycorrhizal (ECM) trees. Given mapped distributions of AM and ECM trees from 78,000 forest inventory plots, we estimate that broadleaf forests of the Midwest and the eastern United States as well as the Mississippi River corridor may be considered hotspots of biogenic NOyemissions. Together, our results greatly improve our understanding of NOyfluxes from forests, which should lead to improved predictions about the atmospheric consequences of tree species shifts owing to land management and climate change.
Forest stands dominated by ectomycorrhizal (ECM) associated trees often have more closed nitrogen (N) cycling than stands dominated by arbuscular mycorrhizal (AM) associated trees, with slower N mineralization in ECM stands thought to suppress inorganic N cycling. However, most estimates of N mineralization come from measurements of net processes, which can lead to an incomplete view of ecosystem N retention and loss. To explore the mechanisms driving mycorrhizal N cycling syndromes, we measured gross N production and assimilation rates and net and potential N flux rates in paired N addition (from NH4SO4and NaNO3) and control plots within ECM and AM-dominated stands. We observed greater gross N mineralization and microbial ammonium assimilation in ECM compared to AM stands, suggesting that increased microbial N demand drove lower net N mineralization rates in ECM stands. We found lower nitrification rates in ECM compared to AM stands and no effect of N addition on nitrification in ECM stands. Therefore, the low soil pH or high C:N ratios found in those stands, not limited ammonium supply, may have suppressed nitrification. Finally, potential denitrification rates and nitrous oxide fluxes were lower in ECM compared to AM stands with no effect of N addition, suggesting that denitrification is controlled by the endogenous supply of nitrate from nitrification, not exogenous nitrate inputs. Overall, we conclude that N mineralization may not play a central role in forming mycorrhizal nutrient syndromes, and that acidic conditions in ECM stands may ultimately control nitrification and the potential for ecosystem N loss.
more » « less- NSF-PAR ID:
- 10411852
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Biogeochemistry
- Volume:
- 164
- Issue:
- 3
- ISSN:
- 0168-2563
- Page Range / eLocation ID:
- p. 473-487
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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