Well-Aerated Southern Appalachian Forest Soils Demonstrate Significant Potential for Gaseous Nitrogen Loss

Peter Baas,Jennifer D Knoepp,Jacqueline E Mohan

doi:10.3390/f10121155

Abstract

Understanding the dominant soil nitrogen (N) cycling processes in southern Appalachian forests is crucial for predicting ecosystem responses to changing N deposition and climate. The role of anaerobic nitrogen cycling processes in well-aerated soils has long been questioned, and recent N cycling research suggests it needs to be re-evaluated. We assessed gross and potential rates of soil N cycling processes, including mineralization, nitrification, denitrification, nitrifier denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) in sites representing a vegetation and elevation gradient in the U.S. Department of Agriculture (USDA) Forest Service Experimental Forest, Coweeta Hydrologic Laboratory in southwestern North Carolina, USA. N cycling processes varied among sites, with gross mineralization and nitrification being greatest in high-elevation northern hardwood forests. Gaseous N losses via nitrifier denitrification were common in all ecosystems but were greatest in northern hardwood. Ecosystem N retention via DNRA (nitrification-produced NO3 reduced to NH4) ranged from 2% to 20% of the total nitrification and was highest in the mixed-oak forest. Our results suggest the potential for gaseous N losses through anaerobic processes (nitrifier denitrification) are prevalent in well-aerated forest soils and may play a key role in ecosystem N cycling.

Highlights

Changing patterns of nitrogen (N) deposition and climate are expected to dramatically alter ecosystem N and carbon (C) cycling
dissimilatory nitrate reduction to ammonium (DNRA) rates were greater in northern hardwood compared with mixed oak–pine (F3,8 = 19.2, p < 0.001)
The percent of nitrate produced from nitrification that was reduced by DNRA was greater in mixed oak (20%) compared with northern hardwood (2%) (F3,7 = 3.8, p = 0.07)

Summary

Introduction

Changing patterns of nitrogen (N) deposition and climate are expected to dramatically alter ecosystem N and carbon (C) cycling. Increased N availability and higher temperatures could result in greater forest productivity and increase the storage of atmospheric C in soil [6,7]. Greater reductions in moisture conditions in this ecosystem could affect forest productivity [8] These gains could be reversed if the addition of labile C as leaf and root material results in enhanced respiration [9] and higher emissions of nitrous oxide, a potent greenhouse gas that is almost 300 times more effective than carbon dioxide at heating the atmosphere [10]. Recent studies have demonstrated the importance of anaerobic processes (e.g., denitrification and dissimilatory nitrate reduction to ammonium (DNRA)) in generally well-aerated soils [11], which are soils that are low in soil moisture and have no evidence of anaerobic conditions such as sulfate reduction, suggesting that to fully elucidate the effects of changing N deposition and temperature

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