Landscape pattern exhibits threshold-driven effect on nitrogen export of typical land use in subtropical hilly watershed under specific hydrological regimes

Landscape pattern can affect the export of non-point source nitrogen (N) in watersheds by regulating hydrological and biogeochemical processes. However, due to the inherent spatiotemporal variability of N exports under different land uses and hydrological regimes, it is essential to quantitatively evaluate the impact of landscape pattern on total N export coefficients (TNECs) and identify the threshold effect of TNECs between key landscape metrics. Based on river monitoring data for eight years in nine catchments of a typical agricultural watershed in the central subtropical region of China, we estimated the spatiotemporal variability and uncertainty of typical land-use TNECs (cropland, forest, and tea gardens) by combining an improved pollutant export coefficient model and Bayesian statistics. Subsequently, the contribution of landscape pattern to the variability in land use TNECs under different hydrological regimes was quantified, and the abrupt change points of TNECs along with the gradient of key landscape metrics were further determined. The results revealed that landscape pattern only had a significant effect on TNECs under medium-flow (representing discharge values within the 30–70th percentile range) and low-flow hydrological regimes (70–100th percentile range), explaining 48.1–54.7% and 55.2–69.3% of the variability, respectively. Threshold effects were observed between key landscape metrics representing different landscape categories (area edge, shape, and aggregation) and TNECs for each land use, and the threshold intervals corresponding to the TNEC abrupt change points were relatively consistent under medium- and low-flow regimes. These results serve as a critical reference for land-use planning and management to improve water quality, the approaches developed can be broadly extended to other regions with diverse land-use types and pollutant sources.