Topographic differences constrain event-scale soil nitrogen and N2O dynamics on a subtropical karst hillslope: Insights from isotopic signatures

Under the East Asian monsoon, Karst forest hillslopes in SW China experience strong soil erosion and nutrient runoff, yet the influence of rainfall event on soil nitrogen (N) transformation pathways and nitrous oxide (N₂O) emission from these fragile soil systems remains poorly constrained. We conducted in situ measurements of soil N₂O fluxes along a forested hillslope at the Yaji karst experimental site (Guilin, SW China), covering two contrasting topographic positions with replicated plots sampled across five summer rainfall events. We quantified soil moisture and extractable substrates (mineral N and dissolved organic carbon), measured nitrate dual isotopes (δ¹⁵N and δ¹⁸O), the abundance of denitrification-related functional genes and N₂O isotopocule site preference (SP). The FRactionation And Mixing Evaluation (FRAME) model was used to partition pathway contributions and to estimate the fraction of N₂O reduction.

Our results demonstrated that the topographic position significantly shapes the spatial distribution of soil moisture and inorganic N substrate, leading to divergent N₂O emission patterns. Soils at the footslope functioned as a biogeochemical hotspot characterized by the preferential accumulation of nitrate (NO₃^-) with elevated water-filled pore space (WFPS), which resulted in two-fold higher N₂O fluxes on average compared to the upper hillslope. Superimposed on this spatial contrast, fluxes varied strongly among events. During intensive rainfall, near-saturated conditions led to a strong dampening of the N₂O flux (r = -0.75). FRAME results indicate that rainfall shifts the balance between nitrification-associated and denitrification-associated N₂O production, with the direction and magnitude varying by topographic position. FRAME further suggests that the fraction of N₂O reduction to N₂ tends to increase under rainfall-influenced conditions at the footslope but decrease at the upper slope.

These findings highlight that hillslope topography acts as a key landscape variable in explaining spatial heterogeneity of water and N substrate balance as well as N₂O emission patterns. Our study underscores the importance of integrating topographic driven resource redistribution into greenhouse gas models for subtropical Karst landscapes.

Keywords: Karst hillslope; N₂O dynamics; Denitrification; Isotopic signature