Spatial-temporal patterns of foliar and bulk soil 15N isotopic signatures across a heterogeneous landscape: Linkages to soil N status, nitrate leaching, and N2O fluxes

Abstract

The natural abundance of plant and bulk soil ¹⁵N isotopic signatures provides valuable insights into the magnitude of nitrogen cycling and loss processes within terrestrial ecosystems. However, ¹⁵N isotopic signatures are highly variable in space due to natural and anthropogenic factors affecting N cycling processes and losses. To date, most studies on foliar and bulk soil ¹⁵N isotopic signatures have focused on N-limited forest ecosystems at relatively large spatial scales, while similar studies in N-enriched ecosystems at finer spatial scales are lacking. To address this gap and evaluate links between soil ¹⁵N isotopic signatures and ecosystem N cycling and loss processes (plant N uptake, N leaching, and gaseous loss), this study quantified foliar and bulk soil ¹⁵N isotopic signatures, soil physicochemical parameters, gaseous (N₂O), and hydrological (NO₃) N losses at 80 sites distributed across a heterogeneous landscape (∼5.8 km²). To account for the spatial-temporal heterogeneity, the measurements were performed in four campaigns (March, June, September 2022, and March 2023) at sites that considered different land uses, soil types, and topography. Results indicated that foliar and bulk soil ¹⁵N isotopic signatures were significantly (P < 0.05) more enriched in arable and grassland ecosystems than forests, suggesting a more open N cycle with significant N cycling and losses due to higher N inputs from fertilizers. Similar to soil inorganic N, N₂O fluxes, and NO₃ leaching rates, landscape-scale foliar and soil ¹⁵N isotopic signatures varied widely spatially, particularly at grassland and arable land (−3 to 9.0‰), with bivariate and multivariate analyses also showing significant relationships between landscape-scale soil ¹⁵N isotopic signatures and the aforementioned parameters (r²: 0.29 to 0.82). Based on these relationships, our findings suggested that foliar and bulk ¹⁵N isotopic signatures may capture fine-scale areas with persistently high and low environmental N losses (N₂O fluxes and NO₃ leaching) within a heterogeneous landscape.