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

Canopy is a critical interface influencing atmospheric nitrogen (N) inputs into forest soils. Ammonium (NH₄⁺) is a major chemical form of N deposition. However, the fluxes of precipitation-canopy NH₄⁺ 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 (δ¹⁵N) data, we quantified the actual input of NH₄⁺ by comparing the differences in bulk precipitation and throughfall NH₄⁺ fluxes. We then constructed a novel model based on δ¹⁵N to identify the sources contributing to elevated throughfall NH₄⁺ fluxes and to elucidate the NH₄⁺ interactions between atmosphere and canopy. Results showed that throughfall had generally higher NH₄⁺ fluxes (by 26% and 74%) and δ¹⁵N values than bulk precipitation. Throughfall gained total 67% of NH₄⁺ (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 NH₄⁺ (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 NH₄⁺ exchange processes increased with NH₄⁺ deposition levels. These results revealed that dry deposition and microbial mineralization in canopy are important NH₄⁺ replenishment to throughfall and atmospheric NH₄⁺ loading would stimulate precipitation-canopy NH₄⁺ exchanges. Our findings enrich the knowledge of forest N biogeochemistry and are useful for evaluating forest N deposition effects.