Characteristics of Stemflow Nitrate and Its Isotopic Composition in a Temperate Pine Plantation

Increasing atmospheric nitrate (NO₃⁻) deposition can influence plant growth and productivity and has been studied in forest biogeochemical cycles. However, the transport and fate of atmospheric NO₃⁻ via stemflow (SF) remain poorly understood. We investigated the concentrations and fluxes of SF NO₃⁻ in relation to the influencing factors at both the inter- and intra-event scales, and used the δ¹⁵N, δ¹⁸O, and Δ¹⁷O values to elucidate the transformations and sources of SF NO₃⁻ in a temperate Chinese pine plantation throughout two growing-season periods. SF NO₃⁻ concentrations decreased exponentially, whereas SF NO₃⁻ fluxes linearly increased with increasing inter-event bulk precipitation (BP) volume. Using a linear mixed model, it was determined that SF NO₃⁻ fluxes of individual trees were significantly influenced by tree height, rainfall duration, and rainfall intensity. The intra-event SF NO₃⁻ concentrations exponentially decreased to a steady input, and this trend was significantly influenced by rainfall volume. Furthermore, both the δ¹⁵N and δ¹⁸O values of SF NO₃⁻ were significantly enriched compared to those of BP, indicating that tree trunks played an important role in buffering dry deposition. Based on a mass-balance approach using the Δ¹⁷O values, it was found that SF NO₃⁻ was derived almost entirely from atmospheric deposition, while biological nitrification was rarely detected but occurred in October. This may be related to microbial nitrifying activity on tree trunks. These findings enhance the mechanistic understanding of the translocation and transformation of SF NO₃⁻ and provide insights for determining the loss of nitrogen from soil via leaching and denitrification.