Nitrate isotopic composition reveals nitrogen deposition and transformation dynamics along the canopy–soil continuum of a suburban forest in Japan

RATIONALE Heavy nitrogen (N) deposition often causes high nitrate (NO 3 – ) accumulation in soils in temperate forested ecosystems. To clarify the sources and production pathways of this NO 3 – , we investigated NO 3 – isotope signatures in deposition processes along the canopy–soil continuum of a suburban forest in Japan. METHODS The stable isotopes of N and oxygen (O) were used to trace the source and transformation dynamics of nitrate (NO 3 – ) in two forest stands: a plantation of Cryptomeria japonica (coniferous tree; CJ) and a natural secondary forest of Quercus acutissima (broadleaf, deciduous tree; QA). The NO 3 – and ammonium (NH 4 + ) concentrations were measured, as well as the δ 15 N and δ 18 O values of NO 3 – , in rainfall, throughfall, stem flow, litter layer water, and soil water (10, 30, and 70 cm depths). RESULTS Seasonal variations were observed in the δ 15 N values of throughfall and stem flow NO 3 – at both sites, and in the δ 18 O values of throughfall and stem flow NO 3 – at the QA site. The range in the δ 18 O values of rainfall and throughfall NO 3 – was large (65–70‰) but decreased dramatically to 2–5‰ in soil water at both sites. At the QA site, the δ 18 O values of stem flow NO 3 – decreased to 40‰ during several rain events, especially in the growing season. CONCLUSIONS NO 3 – from atmospheric deposition was replaced by microbially generated NO 3 – mainly in the organic horizon and surface portion of the mineral soil under excess N deposition in this suburban forest. Microbial activity, including both immobilization and nitrification in organic‐rich horizons near the surface, contributed to incorporating atmospheric NO 3 – quickly into the internal microbial N cycle. We also found evidence of microbial nitrification in the canopy of the QA stand during the growing season. Copyright © 2014 John Wiley & Sons, Ltd.