Typologies of Nitrogen Surplus Across Continental US: Shifting Hotspots and Dominant Controls

Flows of reactive nitrogen (N) have significantly increased over the last century, corresponding to increases in the global population. The pressures on the N cycle include human waste, fossil fuel combustion as well as increasing food production (i.e., increasing fertilizer consumption, biological N fixation, and livestock manure production). The result is humans causing a 10-fold increase in the flow of reactive N globally. The influx of anthropogenic N into aquatic environments degrades water quality, alters fresh and saline ecosystem productivity, and poses an increasing threat to drinking water sources. In the U.S., decades of persistent hypoxic zones, created by elevated concentrations of nitrate from the landscape, have altered ecosystem trophic structure and productivity. Additionally, increasing N contamination of groundwater aquifers places over 20% of the U.S. population at increased risk of diseases and cancers. Despite billions of dollars of investment in watershed conservation measures, we have not seen proportional improvements in water quality. It has been argued that delayed improvements in water quality can be attributed to legacy stores of N, which has accumulated in the landscape over many decades. There is considerable uncertainty associated with the fate of N in the landscape; however, studies quantified increasing stores of N in the subsurface, suggesting increasing stores of N in groundwater aquifers, in soil organic nitrogen pools, and the unsaturated zone. Nevertheless, the spatial distribution of legacy N across the conterminous U.S. is poorly quantified. Here, we have synthesized population, agricultural, and atmospheric deposition data to develop a comprehensive, 88-year (1930 to 2017) dataset of county-scale N surplus trajectories for the U.S. N surplus, defined as the difference between N inputs and usable N outputs (crop harvest), provides insight into the trends and spatial distribution of excess N in the landscape and an upper bound on the magnitude of legacy N accumulation. Our results show that the spatial pattern of N surplus has changed drastically over the 88-year study period. In the 1930s, the N inputs were more or less uniformly distributed across the U.S., resulting in a few hotspots of N surplus. The following decades had sharp increases in N surplus, driven by the exponential use of fertilizer and combustion of fossil fuels. Contemporary N surplus distribution resembles a mosaic of varying degrees of excess, concentrated in the heavily cultivated areas. To understand dominant modes of behavior, we used a machine learning algorithm to characterize N surplus trajectories as a function of both surplus magnitudes and the dominant N inputs. We find ten primary clusters, three in crop dominated landscapes, four in livestock dominated landscapes, two in urban dominated landscapes, and one in areas minimally impacted by humans. Using the typologies generated can facilitate nutrient management decisions. For example, watersheds containing urban clusters would benefit from wastewater treatment plant upgrades. In contrast, those dominated by livestock clusters would have more success in managing nutrients by implementing manure management programs. The estimates of cumulative agricultural N surplus in the landscape highlights agronomic regions that are at risk of large stores of legacy N, possibly leading to groundwater and surface water contamination. In these agronomic regions, the average cumulative N surplus exceeds 1200 kg-N/ha by 2017. Despite having minimal agricultural activity in urban areas, urban fertilizer use has led to an average cumulative N surplus of over 900 kg-N/ha. While our estimates are an upper bound to legacy stores, significant uncertainty remains regarding the magnitude of the estimate of N accumulation. However, our results suggest that legacy N is at varying degrees, impacting most counties in the U.S. The significant investment and corresponding lack of returns can lead to disillusionment in farmers, watershed managers, and the general public. Developing such N surplus typologies helps improve understanding of long-term N dynamics. Beyond refining the supporting science, appropriately communicating uncertainties and limitations of water quality improvements to the stakeholders, authorities, and policymakers are essential to continuing efforts to improve national water quality

Legacy Phosphorus Across Canada: Insights from a 60-Year Dataset

Human activities over decades of agriculture and urbanization have altered phosphorus (P) cycling, posing a threat to water quality and ecosystem function. Algal blooms have become a pervasive problem in both small and large waterbodies across Canada. Despite concerted efforts to reduce P loading to surface waters, there has yet to be a noticeable improvement in water quality. This can be attributed to the accumulation of legacy P in the landscape as a result of excessive use of synthetic fertilizers and the production of livestock manure. These legacy P can reach the waterbodies decades after implementing P management practices. Therefore, to better understand long-term P dynamics and their drivers, it is crucial to develop long-term datasets of P inputs and outputs. We developed a 60-year (1961–2021), 250-meter grid resolution data of P components and P surplus across Canada. P surplus is the difference between P inputs (fertilizer inputs, livestock manure, detergent, and human waste) and non-hydrological P output (crop uptake). Our result shows the different drivers of P surplus across Canada. In Ontario and Quebec, the P surplus decreased from nutrient regulation programs in 1981 and subsequently rebounded in 2006 due to an increase in P fertilizer use. In prairie provinces, low P inputs and increasing crop yields have led to the mining of the P stores in the soils. This new, longer dataset will improve our understanding of long-term P dynamics and allow for explicit consideration of the impacts of legacy P on environmental outcomes.This research was undertaken thanks, in part, with support from the Global Water Futures Program funded by the Canada First Research Excellence Fund (CFREF)

Modelling Legacy Nitrogen Dynamics in the Transboundary Lake Erie Watershed

Lake Erie is a source of drinking water, recreation, and commercial opportunity for both the United States and Canada, making the protection of its water quality essential. In the past decades, Lake Erie's ecosystems have been adversely impacted by recurring toxic algal blooms. These algal blooms are attributed to nitrogen (N) and phosphorus pollution from agricultural runoff. Despite recent efforts to reduce N application in the Lake Erie basin, high levels of N concentration persist in surface and groundwater systems. One of the reasons for this apparent stasis in N concentrations is legacy stores of N in landscapes that contribute to lag times in water quality response, even after inputs have ceased. Legacy N is stored in the soil and slow-moving groundwater and makes up a large portion of current N contamination. Here, we aim to quantify N legacies across the entire Lake Erie basin to predict time lags in water quality improvements in surface and groundwater. We use a process-based model, ELEMeNT, to quantify legacy N stores and watershed-scale N dynamics over the past century across the basin. Such models inform nutrient management practices across the Lake Erie basin by explicitly incorporating legacy dynamics. Our study shows that N surplus (the difference between N inputs and non-hydrological N outputs) has been rising across most Lake Erie sub-watersheds since 1950 and has only started to plateau or decrease around 2000. Agricultural inputs from manure, fertilizer, and biological fixation were the lead contributors to N surplus in agricultural sub-watersheds, and domestic N was the lead N contributor in urban sub-watersheds. Since 1950, between 4% and 44% of N has been stored as legacy N (23% median). On average, 92% of this N legacy is retained in the soil and 8% is in the groundwater. Through correlation analysis, we have found that higher fractions of groundwater N and SON legacy accumulation are correlated with slower travel times and lower tile drainage, while wastewater denitrification emerged as the dominant component in urban sub-watersheds. These results provide insight into drivers of legacy N and N release in sub-watersheds, which could aid in targeted nutrient management across the watershed.This research was undertaken thanks, in part, with support from the Global Water Futures Program funded by the Canada First Research Excellence Fund (CFREF

Trajectories Nutrient Dataset for Nitrogen (TREND-nitrogen)

In the present work, we have synthesized agricultural, population, and atmospheric deposition data across the contiguous U.S. for the period 1930-2017 to develop the Trajectories Nutrient Dataset for Nitrogen (TREND-nitrogen). The data is provided at U.S. county scale at an annual time step, and the following components of the N mass balance are included: atmospheric N deposition, N fertilizer, biological N fixation, manure N, crop N uptake, human N waste, and the overall N surplus. The dataset was constructed using information from a variety of sources, including the U.S. Census of Agriculture, the National Atmospheric Deposition Program, the United States Geological Survey, and the U.S. Census Bureau. Gap-filling and data-fusion techniques were utilized to provide annual-scale coverage and to harmonize data obtained from disparate sources over different time periods and varying spatial scales. The dataset will facilitate investigations of spatial and temporal changes in anthropogenic N dynamics at multiple scales across the U.S. in the context of historical changes in population, land use, and management

Memory and Management - Nitrogen Dataset

The dataset includes underlying nitrate concentration and load data for over 400 monitored U.S. watersheds. Watershed characteristic data is also included. Additional details regarding the data and the subsequent analysis are available in the paper: "Memory and Management: Competing Controls on Long-Term Nitrate Trajectories in U.S. Rivers." </p

Managing nitrogen legacies to accelerate water quality improvement

Increasing incidences of eutrophication and groundwater quality impairment from agricultural nitrogen pollution are threatening humans and ecosystem health. Minimal improvements in water quality have been achieved despite billions of dollars invested in conservation measures worldwide. Such apparent failures can be attributed in part to legacy nitrogen that has accumulated over decades of agricultural intensification and that can lead to time lags in water quality improvement. Here, we identify the key knowledge gaps related to landscape nitrogen legacies and propose approaches to manage and improve water quality, given the presence of these legacies