Phosphorus (P) pollution of surface waters remains a challenge for protecting and improving water quality. Central to the challenge is understanding what regulates P concentrations in streams. This quantitative review synthesizes the literature on a major control of P concentrations in streams at baseflow-the sediment P bufferto better understand streamwater-sediment P interactions. We conducted a global meta-analysis of sediment equilibrium phosphate concentrations at net zero sorption (EPC 0), which is the dissolved reactive P (DRP) concentration toward which sediments buffer solution DRP. Our analysis of 45 studies and >900 paired observations of DRP and EPC 0 showed that sediments often have potential to remove or release P to the streamwater (83% of observations), meaning that "equilibrium" between sediment and streamwater is rare. This potential for P exchange is moderated by sediment and stream characteristics, including sorption affinity, stream pH, exchangeable P concentration, and particle sizes. The potential for sediments to modify streamwater DRP concentrations is often not realized owing to other factors (e.g., hydrologic interactions). Sediment surface chemistry, hyporheic exchange, and biota can also influence the potential exchange of P between sediments and the streamwater. Methodological choices significantly influenced EPC 0 determination and thus the estimated potential for P exchange; we therefore discuss how to measure and report EPC 0 to best suit research objectives and aid in interstudy comparison. Our results enhance understanding of the sediment P buffer and inform how EPC 0 can be effectively applied to improve management of aquatic P pollution and eutrophication.

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Understanding the lag time between land management and impacts on riverine nitrate–nitrogen (N) loads is critical to understand when action to mitigate nitrate–N leaching losses from the soil profile may start improving water quality. These lags occur due to leaching of nitrate–N through the subsurface (soil and groundwater). Actions to mitigate nitrate–N losses have been mandated in New Zealand policy to start showing improvements in water quality within five years. We estimated annual rates of nitrate–N leaching and annual nitrate–N loads for 77 river catchments from 1990 to 2018. Lag times between these losses and riverine loads were determined for 34 catchments but could not be determined in other catchments because they exhibited little change in nitrate–N leaching losses or loads. Lag times varied from 1 to 12 years according to factors like catchment size (Strahler stream order and altitude) and slope. For eight catchments where additional isotope and modelling data were available, the mean transit time for surface water at baseflow to pass through the catchment was on average 2.1 years less than, and never greater than, the mean lag time for nitrate–N, inferring our lag time estimates were robust. The median lag time for nitrate–N across the 34 catchments was 4.5 years, meaning that nearly half of these catchments wouldn’t exhibit decreases in nitrate–N because of practice change within the five years outlined in policy.

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Agricultural losses of dissolved reactive phosphorus (DRP) emanate from both historic P applications (i.e., “old P”) and recently applied fertilizer (i.e., “new P”). Understanding the relative contributions of these sources is important for mitigating DRP losses from agriculture. This study provides a proof‐of‐concept for resolving new P vs. old P source contributions to DRP losses in subsurface tile drainage using edge‐of‐field water quality data and management records from eight fields in Ohio. Weighted regressions on discharge and season (WRDS) were fitted using data from periods without P fertilizer applications and then used to predict DRP losses in tile drainage during new P loss risk periods (default length, 90 d) after fertilizer applications. Differences between observed and predicted DRP concentrations during the new P loss risk period were attributed to the new P source. Remaining losses were attributed to the old soil P source. The WRDS model performance was modest (modified Kling–Gupta efficiency ranged from −0.074 to 0.484). New P sources contributed between 0 and 17% of overall DRP losses (average, 7%), with old soil P contributing 83–100%. Individual P fertilizer applications were associated with new DRP losses up to 192 g P ha−1. Increasing the length of the risk period for new P losses up to 180 d after fertilizer application marginally increased the estimated contribution of the new P source. The WRDS‐based analysis provides a novel approach for resolving the contributions of new and old sources to edge‐of‐field DRP losses.

<p><strong>Abstract.</strong> Streams can attenuate inputs of phosphorus (P) and, therefore the likelihood of ecosystem eutrophication. This attenuation is, however, poorly understood, particularly in reference to the geochemical mechanisms involved. In our study, we measured P attenuation mechanisms in the form of (1) mineral (co-)precipitation from the water-column and (2) P sorption with benthic sediments. We hypothesized that both mechanisms would vary with catchment geology and, further, that P sorption would depend on reactive Fe content in sediments. We sampled 31 streams at baseflow conditions, covering a gradient of P inputs (via land use), hydrological characteristics, and catchment geologies. Geochemical equilibria in the water-column were measured and benthic sediments (< 2 mm) were analyzed for sorption properties and P and iron (Fe) fractions. Neither P-containing minerals (e.g., hydroxylapatite) nor calcite-phosphate co-precipitation had the potential to occur. In contrast, in-stream dissolved reactive P (DRP) correlated with labile sediment P (water-soluble and easily reduced Fe-P), but only for streams where hyporheic exchange between the water-column and the coarse sediment porewaters was likely sufficient. The non-labile P fractions contained most of sediment P (generally > 90 %) and varied with parent geology. Similarly, most sediment Fe was in a recalcitrant form (generally > 90–95 %). However, despite its small contribution to total sediment Fe, the pool of surface-reactive Fe was a strong predictor for sediment P sorption potential. Our results suggest that, in these streams, it is the combination of biogeochemical Fe and P cycles and the exchange with the hyporheic zone that attenuates DRP in baseflow. Such combinations are likely to vary spatiotemporally within a catchment and must be considered alongside inputs of P and sediment if the P concentrations at baseflow – and eutrophication risk – are to be well managed.</p>