Throughfall Gained More Ammonium From Particulates and Mineralization Than Precipitation Ammonium Retention in Forest Canopy

Abstract Canopy is a critical interface influencing atmospheric nitrogen (N) inputs into forest soils. Ammonium (NH 4 + ) is a major chemical form of N deposition. However, the fluxes of precipitation‐canopy NH 4 + exchange processes and their environmental responses remain unclear, which impairs the assessment of N deposition effects on forest biodiversity and functions. By collecting and synthesizing the flux and N isotope ( δ 15 N) data, we quantified the actual input of NH 4 + by comparing the differences in bulk precipitation and throughfall NH 4 + fluxes. We then constructed a novel model based on δ 15 N to identify the sources contributing to elevated throughfall NH 4 + fluxes and to elucidate the NH 4 + interactions between atmosphere and canopy. Results showed that throughfall had generally higher NH 4 + fluxes (by 26% and 74%) and δ 15 N values than bulk precipitation. Throughfall gained total 67% of NH 4 + (5.6 ± 6.5 and 6.0 ± 8.4 kg‐N/ha/yr) from canopy particulates and mineralization, while canopy retained 59% and 41% of precipitation NH 4 + (4.1 ± 4.4 and 2.2 ± 4.0 kg‐N/ha/yr) in broadleaved and coniferous forests, respectively. In addition, we identified that the fluxes of precipitation‐canopy NH 4 + exchange processes increased with NH 4 + deposition levels. These results revealed that dry deposition and microbial mineralization in canopy are important NH 4 + replenishment to throughfall and atmospheric NH 4 + loading would stimulate precipitation‐canopy NH 4 + exchanges. Our findings enrich the knowledge of forest N biogeochemistry and are useful for evaluating forest N deposition effects.