Rainfall-driven pathway switching reorganizes nitrate transport and transformation in a terrace-ditch-pond system

Agricultural terrace-ditch-pond (TDP) systems are common connected drainage units in hilly agricultural regions, where terraces, ditches, and ponds jointly regulate runoff and nutrient transport. Although nitrate sources and catchment-scale runoff export have been widely studied, the relative contributions of direct terrace drainage and indirect ditch drainage to pond nitrate remain insufficiently quantified in TDP systems. To address this gap, a representative TDP system in Guang'an, Sichuan Province, China, was investigated from April to July 2023, and a pathway-level TDP framework was established. Hydrochemical analyses, a Bayesian stable isotope mixing model (MixSIAR), and structural equation modeling (SEM) were integrated to examine coupled changes in nitrate source apportionment, pathway contributions (including concentration-based and load-based contributions), and transformation under contrasting rainfall conditions. During low-flow (LF) conditions, direct terrace drainage was the dominant concentration-based pathway contributing to pond nitrate (42.3%), with fertilizer as the primary nitrate source (49.0%). During high-flow (HF) conditions, indirect ditch drainage became the dominant concentration-based pathway (53.2%), accompanied by increased contributions from manure/sewage and soil nitrogen. This pathway shift was also consistent with CB-SEM results and with load-based results showing higher TN and NO 3 ⁻-N loss loads along the direct terrace drainage during LF but along the ditch pathway during HF. PLS-SEM indicated coordinated responses among precipitation intensity, DIN, and nitrate transport and transformation indicators. Integrated isotope, hydrochemical, and SEM evidence further suggested that increased rainfall may enhance ditch nitrate transport while also being associated with localized transformation signals. This study offers a pathway-level framework for interpreting nitrate source-pathway relationships in TDP systems and may help inform rainfall-responsive management of agricultural drainage networks.