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

Abstract

Abstract To 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 (NO 3 − atm ) and remineralized nitrate produced by microbial nitrification to total nitrate (NO 3 − ) in stream water have been investigated in many studies. However, the dynamics of these two types of NO 3 − from the forest canopy to the soil are not well understood. Therefore, we determined the changes in the NO 3 − flux and the 17 O excess (Δ 17 O) of NO 3 − , a robust tracer of NO 3 − 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 Δ 17 O values in NO 3 − dramatically decreased after passing through the forest floor, indicating that the dominant source of NO 3 − leaching is nitrification in the forest floor. In contrast, a large decrease in NO 3 − 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 NO 3 − atm . The retention of NO 3 − 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 Δ 17 O value of NO 3 − is a promising tool for quantifying the atmospheric nitrate dynamics in complex forest N cycling.