Accurate source identification is critical for optimizing water pollution control strategies. Although the dual stable isotope (15N-NO3-/18O-NO3-) approach has been widely applied for differentiating riverine nitrogen (N) sources, the relatively short-term (<1 yr) 15N-NO3-/18O-NO3- records typically used in previous studies often hinders rigorous assessment due to high temporal variability associated with watershed N dynamics. Estimated contributions of legacy N sources in soils and groundwater to riverine N export by modeling approaches in many previous studies also lack validation from complementary information, such as multiple stable isotopes. This study integrated three years of multiple stable isotope (15N-NO3-/18O-NO3- and 2H-H2O/18O-H2O) and hydrochemistry measurements for river water, groundwater and rainfall to elucidate N dynamics and sources in the Yongan watershed (2474 km2) of eastern China. Nonpoint source N pollution dominated and displayed considerable seasonal and spatial variability in N forms and concentrations. Information from δ15N-NO3- and δ18O-NO3- indicated that riverine N dynamics were regulated by contributing sources, nitrification and denitrification, as well as hydrological processes. For the three examined catchments and entire watershed, slow subsurface and groundwater flows accounted for >75% of river discharge and were likely the major hydrological pathways for N delivery to the river. Riverine NO3- sources varied with dominant land use (p < 0.001), with the highest contributions of groundwater (60%), wastewater (35%), and soil (50%) occurring in agricultural, residential and forest catchments, respectively. For the entire watershed, groundwater (∼50%) and soil N (>30%) were the dominant riverine NO3- sources, implying considerable potential for N pollution legacy effects. Results were consistent with observed nitrous oxide dynamics and N sources identified in previous modeling studies. As the first attempt to apply multiple isotope tracers for exploring and quantifying N transformation and transport pathways, this study provides an integrated approach for verifying and understanding the N pollution legacy effects observed in many watersheds worldwide. This study highlights that river N pollution control in many watersheds requires particular attention to groundwater restoration and soil N management in addition to N input control strategies.

Legacy nutrient accumulation from excess anthropogenic inputs has become a serious environmental issue in many watersheds worldwide. In this review, we provide a systematic overview of sources, legacy nutrient pools, watershed-scale water quality models, and agronomic and environmental implications of legacy nitrogen and phosphorus pools. Hydrological, biogeochemical, and anthropogenic factors exert interacting controls on legacy nutrient dynamics in soils, sediments, and vadose zone/groundwater. Most current watershed models do not effectively incorporate legacy nutrient dynamics, while models that consider legacy effect often have high uncertainty in their treatment of legacy nutrient dynamics. In many intensively managed watersheds, legacy nutrients are a dominant and long-term (> 10 years) source of nutrients to receiving waters, as well as a potentially important nutrient source for crop production. Many existing beneficial management measures have limitations for reducing legacy nutrient losses to surface waters due to appreciable differences in legacy nutrient forms, watershed storage locations, and temporal dynamics compared to those of contemporary nutrient inputs. Recognizing the importance of legacy nutrients is necessary for developing sustainable watershed nutrient management plans for future food, bioenergy, and water security. These plans require strategies to maximize use of legacy nutrient resources to minimize their loss to the atmosphere (e.g., N2O emissions) and surface waters. Finally, this synthesis identified future research needs for improving the understanding, utilization, and mitigation of watershed legacy nutrient pools.

Legacy phosphorus (P) accumulated in watersheds from excessive historical P inputs is recognized as an important component of water pollution control and sustainable P management in watersheds worldwide. However, little is known about how watershed P buffering capacity responds to legacy P pressures over time and how long it takes for riverine P concentrations to recover to a target level, especially in developing countries. This study examined long-term (1960–2010) accumulated legacy P stock, P buffering capacity and riverine TP flux dynamics to predict riverine P-reduction recovery times in the Yongan watershed of eastern China. Due to a growing legacy P stock coupled with changes in land use and climate, estimated short- and long-term buffering metrics (i.e., watershed ability to retain current year and historically accumulated surplus P, respectively) decreased by 65% and 36%, respectively, resulting in a 15-fold increase of riverine P flux between 1980 and 2010. An empirical model incorporating accumulated legacy P stock and annual precipitation was developed (R² = 0.99) and used to estimate a critical legacy P stock of 22.2 ton P km⁻² (95% CI 19.4–25.3 ton P km⁻²) that would prevent exceedance of a target riverine TP concentration of 0.05 mg P L⁻¹. Using an exponential decay model, the recovery time for depleting the estimated legacy P stock in 2010 (29.3 ton P km⁻²) to the critical level (22.2 ton P km⁻²) via riverine flux was 456 years (95% CI 353–560 years), 159 years (95% CI 57–262 years) and 318 years (95% CI 238–400 years) under scenarios of a 4% reduction in annual P inputs, total cessation of P inputs, and 4% reduction of annual P inputs with a 10% increase in average annual precipitation, respectively. Given the lower P buffering capacity and lengthening recovery time, strategies to reduce P inputs and utilize soil legacy P for crop production are necessary to effectively control riverine P pollution and conserve global rock P resources. A long-term perspective that incorporates both contemporary and historical information is required for developing sustainable P management strategies to optimize both agronomic and environmental benefits at the watershed scale.

Quantitative information on long-term net anthropogenic phosphorus inputs (NAPI) and its relationship with riverine phosphorus (P) export are critical for developing sustainable and efficient watershed P management strategies. This is the first study to address long-term (1980-2015) NAPI and riverine P flux dynamics for the Yangtze River basin (YRB), the largest watershed in China. Over the 36-year study period, estimated NAPI to the YRB progressively increased by ∼1.4 times, with NAPIA (chemical fertilizer input + atmospheric deposition + seed input) and NAPIB (net food/feed imports + non-food input) contributing 65% and 35%, respectively. Higher population, livestock density and agricultural land area were the main drivers of increasing NAPI. Riverine total phosphorus (TP), particulate phosphorus (PP) and suspended sediment (SS) export at Datong hydrological station (downstream station) decreased by 52%, 75% and 75% during 1980-2015, respectively. In contrast, dissolved phosphorus (DP) showed an increase in both concentration (∼7-fold) and its contribution to TP flux (∼16-fold). Different trends in riverine P forms were mainly due to increasing dam/reservoir construction and changes in vegetation/land use and NAPI components. Multiple regression models incorporating NAPIA, NAPIB, dam/reservoir storage capacity and water discharge explained 84% and 92% of the temporal variability in riverine DP and PP fluxes, respectively. Riverine TP flux estimated as the sum of DP and PP fluxes showed high agreement with measured values (R2 = 0.87, NSE = 0.84), indicating strong efficacy for the developed models. The model forecasted an increase of 50% and 7% and a decrease of 15% and 22% in riverine DP flux from 2015 to 2045 under developing, dam building, NAPIA and NAPIB reduction scenarios, respectively. This study highlights the importance of including enhanced P transformation from particulate to bioavailable forms due to river regulation and changes in land-use, input sources and legacy P pools in development of P pollution control strategies.

Streamflow components (e.g., young water vs old water) and hydrological transit times play an important role in water resource and water quality management. We collected a four-year record of stable isotopes of oxygen and hydrogen (δ18O and δ2H) in precipitation, groundwater and stream water for six catchments in the Yongan watershed of eastern China. The stable isotope records were used to identify spatio-temporal variations in the young water fraction (Fyw, defined as the proportion of the transit-time distribution younger than a threshold age) and mean transit time (MTT) based on sine-wave fitting and convolution integral methods, respectively. The Fyw ranged from 14 to 35% in the Yongan watershed. Cumulative transit time showed contrasting distributions, suggesting considerable heterogeneity and a complex interplay between catchment characteristics. Estimated MTTs ranged from 3.2 to 6.3 years and may be explained by catchment characteristics (e.g., elevation and topographic gradient). Observed spatial trends in MTTs likely result from contrasting contributions of different-aged subsurface water flows across the six catchments. The use of Fyw constraints in estimating MTTs reduced uncertainty in some catchments, suggesting the potential benefits of combining multiple approaches (e.g., Fyw) to optimize the results of traditional calibration methods. The relatively low Fyw and long MTTs highlight the importance of groundwater contributions to streamflow generation and imply a considerable lag time in river water quantity and quality responses to catchment-scale water resource management. Coupling multiple metrics (e.g., Fyw and MTT) and isotope models enhances our understanding of watershed-scale hydrologic processes and hydrograph separation.