Vertical Changes in the Flux of Atmospheric Nitrate From a Forest Canopy to the Surface Soil Based on Δ17O Values

Abstract

AbstractTo better understand the factors that control N retention and N export in forested watersheds, it is necessary to understand the relationships among atmospheric nitrogen (N) deposition, internal N cycling within plant‐soil systems, and N leaching. The relative contributions of atmospheric nitrate (NO3−atm) and remineralized nitrate produced by microbial nitrification to total nitrate (NO3−) in stream water have been investigated in many studies. However, the dynamics of these two types of NO3− from the forest canopy to the soil are not well understood. Therefore, we determined the changes in the NO3− flux and the 17O excess (Δ17O) of NO3−, a robust tracer of NO3−atm, from bulk deposition to the soil water beneath oak and spruce trees as well as dwarf bamboo‐dominated canopy gaps in a natural coniferous‐broadleaved mixed forest in northern Japan. The Δ17O values in NO3− dramatically decreased after passing through the forest floor, indicating that the dominant source of NO3− leaching is nitrification in the forest floor. In contrast, a large decrease in NO3−atm flux was observed between bulk deposition and throughfall, especially for oak and spruce, suggesting that the forest canopy is an important sink for deposited NO3−atm. The retention of NO3−atm by the canopy was higher for oak (86.3 ± 10.1%) and spruce (87.7 ± 8.8%) than for Sasa in the canopy gap (49.9 ± 26.6%). Our study demonstrated that the Δ17O value of NO3− is a promising tool for quantifying the atmospheric nitrate dynamics in complex forest N cycling.