Land-use change and agricultural intensification have increased food production but at the cost of polluting surface and groundwater. Best management practices implemented to improve water quality have met with limited success. Such lack of success is increasingly attributed to legacy nutrient stores in the subsurface that may act as sources after reduction of external inputs. These legacy stores have built up over decades of fertilizer application and contribute to time lags between the implementation of best management practices and water quality improvement. However, current water quality models lack a framework to capture these legacy effects and corresponding lag times. The overall goal of this thesis is to use a combination of data synthesis and modeling to quantify legacy stores and time lags in intensively managed agricultural landscapes in the Midwestern US. The specific goals are to (1) quantify legacy nitrogen accumulation using a mass balance approach from 1949 - 2012 (2) develop a SWAT model for the basin and demonstrate the value of using crop yield information to increase model robustness (3) modify the SWAT (Soil Water Assessment Tool) model to capture the effect of nitrogen (N) legacies on water quality under multiple land-management scenarios, and (4) use a field-scale carbon-nitrogen cycling model (CENTURY) to quantify the role of climate and soil type on legacy accumulation and water quality. For objectives 1 and 2, the analysis was performed in the Iowa Cedar Basin (ICB), a 32,660 km2 watershed in Eastern Iowa, while for objective 3, the focus has been on the South Fork Iowa River Watershed (SFIRW), a 502 km2 sub-watershed of the ICB, and for objective 4 the focus was at the field scale. For the first objective, a nitrogen mass balance analysis was performed across the ICB to understand whether legacy N was accumulating in this watershed and if so, the magnitude of accumulation. The magnitude of N inputs, outputs, and storage in the watershed was quantified over 64 years (1949 - 2012) using the Net Anthropogenic Nitrogen Inputs (NANI) framework. The primary inputs to the system were atmospheric N deposition (9.2 ± 0.35 kg/ha/yr), fertilizer N application (48 ± 2 kg/ha/yr) and biological N fixation (49 ± 3 kg/ha/yr) and while the primary outputs from the system was net food and feed that was estimated as 42 ± 4.5 kg/ha/yr. The Net Anthropogenic Nitrogen Input (NANI) to the system was estimated to be 64 ± 6 kg/ha/yr. Finally, an estimated denitrification rate constant of 12.7 kg/ha/yr was used to estimate the subsurface legacy nitrogen storage as 33.3 kg/ha/yr. This is a significant component of the overall mass budget and represents 48% of the NANI and 31% of the fertilizer added to the watershed every year. For the second objective, the effect of crop yield calibration in increasing the robustness of the hydrologic model was analyzed. Using a 32,660 km2 agricultural watershed in Iowa as a case study, a stepwise model refinement was performed to show how the consideration of additional data sources can increase model consistency. As a first step, a hydrologic model was developed using the Soil and Water Assessment Tool (SWAT) that provided excellent monthly streamflow statistics at eight stations within the watershed. However, comparing spatially distributed crop yield measurements with modeled results revealed a strong underestimation in model estimates (PBIAS Corn = 26%, PBIAS soybean = 61%). To address this, the model was refined by first adding crop yield as an additional calibration target and then changing the potential evapotranspiration estimation method -- this significantly improved model predictions of crop yield (PBIAS Corn = 3%, PBIAS soybean = 4%), while only slightly improving streamflow statistics. As a final step, for better representation of tile flow, the flow partitioning method was modified. The final model was also able to (i) better capture variations in nitrate loads at the catchment outlet with no calibration and (ii) reduce parameter uncertainty, model prediction uncertainty, and equifinality. The findings highlight that using additional data sources to improve hydrological consistency of distributed models increases their robustness and predictive ability. For the third objective, the SWAT model was modified to capture the effects of nitrogen (N) legacies on water quality under multiple land-management scenarios. My new SWAT-LAG model includes (1) a modified carbon-nitrogen cycling module to capture the dynamics of soil N accumulation, and (2) a groundwater travel time distribution module to capture a range of subsurface travel times. Using a 502 km2 SFIR watershed as a case study, it was estimated that, between 1950 and 2016, 25% of the total watershed N surplus (N Deposition + Fertilizer + Manure + N Fixation - Crop N uptake) had accumulated within the root zone, 14% had accumulated in groundwater, while 27% was lost as riverine output, and 34% was denitrified. In future scenarios, a 100% reduction in fertilizer application led to a 79% reduction in stream N load, but the SWAT-LAG results suggest that it would take 84 years to achieve this reduction, in contrast to the two years predicted in the original SWAT model. The framework proposed here constitutes a first step towards modifying a widely used modeling approach to assess the effects of legacy N on time required to achieve water quality goals. The above research highlighted significant uncertainty in the prediction of biogeochemical legacies -- to address this uncertainty in the last objective the field scale CENTURY model was used to quantify SON accumulation and depletion trends using climate and soil type gradients characteristic of the Mississippi River Basin. The model was validated using field-scale data, from field sites in north-central Illinois that had SON data over 140 years (1875-2014). The study revealed that across the climate gradient typical of the Mississippi River Basin, SON accumulation was greater in warmer areas due to greater crop yield with an increase in temperature. The accumulation was also higher in drier areas due to less N lost by leaching. Finally, the analysis revealed an interesting hysteretic pattern, where the same levels of SON in the 1930s contributed to a lower mineralization flux compared to current.

Global population has seen a more than threefold increase over the last 100 years, accompanied by rapid changes in land use and a dramatic intensification of agriculture. Such changes have been driven by a great acceleration of the global nitrogen (N) cycle, with N fertilizer use now estimated to be 100 Tg/year globally. Excess N commonly finds its way into both groundwater and surface water, leading to long-term problems of hypoxia, aquatic toxicity and drinking water contamination. Despite ongoing efforts to improve water quality in agroecosystems, results have often been disappointing, with significant lag times between adoption of accepted best management practices (BMPs) and measurable improvements in water quality. It has been hypothesized that such time lags are a result of the buildup of legacy N within the landscape over decades of fertilizer application and agricultural intensification. The central theme of my research has been an exploration of this N legacy, including (1) an investigation of the form, locations and magnitudes of legacy N stores within intensively managed catchments; (2) development of a parsimonious, process-based modeling framework for quantifying catchment-scale time lags based on both soil nutrient accumulations (biogeochemical legacy) and groundwater travel time distributions (hydrologic legacy); and (3) use of a statistical approach to both quantifying N-related time lags at the watershed scale, and identifying the primary physical and management controls on these lags. As a result of these explorations I am able to provide the first direct, large-scale evidence of N accumulation in the root zones of agricultural soils, accumulation that may account for much of the 'missing N' identified in mass balance studies of heavily impacted watersheds. My analysis of long-term soil data (1957-2010) from 206 sites throughout the Mississippi River Basin (MRB) revealed N accumulation in cropland of 25-70 kg ha-1 y-1, a total of 3.8 ± 1.8 Mt y-1 at the watershed scale. A simple modeling framework was then used to show that the observed accumulation of soil organic N (SON) in the MRB over a 30-year period (142 Tg N) would lead to a biogeochemical lag time of 35 years for 99% of legacy SON, even with a complete cessation of fertilizer application. A parsimonious, process-based model, ELEMeNT (Exploration of Long-tErM Nutrient Trajectories), was then developed to quantify catchment-scale time lags based on both soil N accumulation (biogeochemical legacy) and groundwater travel time distributions (hydrologic legacy). The model allowed me to predict the time lags observed in a 10 km2 Iowa watershed that had undergone a 41% conversion of area from row crop to native prairie. The model results showed that concentration reduction benefits are a function of the spatial pattern of implementation of conservation measures, with preferential conversion of land parcels having the shortest catchment-scale travel times providing greater concentration reductions as well as faster response times. This modeling framework allows for the quantification of tradeoffs between costs associated with implementation of conservation measures and the time needed to see the desired concentration reductions, making it of great value to decision makers regarding optimal implementation of watershed conservation measures. To better our understanding of long-term N dynamics, I expanded the ELEMeNT modeling framework described above to accommodate long-term N input trajectories and their impact on N loading at the catchment scale. In this work, I synthesized data from a range of sources to develop a comprehensive, 214-year (1800-2104) trajectory of N inputs to the land surface of the continental United States. The ELEMeNT model was used to reconstruct historic nutrient yields at the outlets of two major U.S. watersheds, the Mississippi River and Susquehanna River Basins, which are the sources of significant nutrient contamination to the Gulf of Mexico and Chesapeake Bay, respectively. My results show significant N loading above baseline levels in both watersheds before the widespread use of commercial N fertilizers, largely due to 19th-century conversion of natural forest and grassland areas to row-crop agriculture. The model results also allowed me to quantify the magnitudes of legacy N in soil and groundwater pools, thus highlighting the dominance of soil N legacies in the MRB and groundwater legacies in the SRB. It was found that approximately 85% of the annual N load in the MRB can be linked to inputs from previous years, while only 47% of SRB N loading is associated with “older” N. In addition, it was found that the dominant sources of current N load in the MRB are fertilizer, atmospheric deposition, and biological N fixation, while manure and atmospheric deposition account for approximately 64% of the current loads in the SRB. Finally, long-term N surplus trajectories were paired with long-term flow-averaged nitrate concentration data to as means of quantifying N-related lag times across an intensively managed watershed in Southern Ontario. In this analysis, we found a significant linear relationship between current flow-averaged concentrations and current N surplus values across the study watersheds. Temporal analysis, however, showed significant nonlinearity between N inputs and outputs, with a strong hysteresis effect indicative of decadal-scale lag times between changes in N surplus values and subsequent changes in flow-averaged nitrate concentrations. Annual lag times across the study watersheds ranged from 15-33 years, with a mean lag of 24.5 years. A seasonal analysis showed a distribution of lag times across the year, with fall lags being the shortest and summer lags the longest, likely due to differences in N delivery pathways. Multiple linear regression analysis of dominant controls showed tile drainage to be a strong determinant of differences in lag times across watersheds in both fall and spring, with a watershed's fractional area under tile drainage being significantly linked to shorter lag times. In summer, tile drainage was found to be an insignificant factor in driving lag times, while a significant relationship was found between the percent soil organic matter and longer N-related lag times. By moving beyond the traditional focus on nutrient concentrations and fluxes, and instead working towards quantification of the spatio-temporal dynamics of non-point source nutrient legacies and their current and future impacts on water quality, we make a significant contribution to the science of managing human impacted landscapes. Due to the strong impacts of nutrient legacies on the time scales for recovery in at-risk landscapes, my work will enable a more accurate assessment of the outcomes of alternative management approaches in terms of both short- and long-term costs and benefits, and the evaluation of temporal uncertainties associated with different intervention strategies.

New York City's municipal water supply system provides about 1 billion gallons of drinking water a day to over 8.5 million people in New York City and about 1 million people living in nearby Westchester, Putnam, Ulster, and Orange counties. The combined water supply system includes 19 reservoirs and three controlled lakes with a total storage capacity of approximately 580 billion gallons. The city's Watershed Protection Program is intended to maintain and enhance the high quality of these surface water sources. Review of the New York City Watershed Protection Program assesses the efficacy and future of New York City's watershed management activities. The report identifies program areas that may require future change or action, including continued efforts to address turbidity and responding to changes in reservoir water quality as a result of climate change.

Stream Ecosystems in a Changing Environment synthesizes the current understanding of stream ecosystem ecology, emphasizing nutrient cycling and carbon dynamics, and providing a forward-looking perspective regarding the response of stream ecosystems to environmental change. Each chapter includes a section focusing on anticipated and ongoing dynamics in stream ecosystems in a changing environment, along with hypotheses regarding controls on stream ecosystem functioning. The book, with its innovative sections, provides a bridge between papers published in peer-reviewed scientific journals and the findings of researchers in new areas of study. - Presents a forward-looking perspective regarding the response of stream ecosystems to environmental change - Provides a synthesis of the latest findings on stream ecosystems ecology in one concise volume - Includes thought exercises and discussion activities throughout, providing valuable tools for learning - Offers conceptual models and hypotheses to stimulate conversation and advance research

Finalist for the 2021 PROSE Award for Environmental Science! An integrated approach to understanding and mitigating the problem of excess nitrogen Human activities generate large amounts of excess nitrogen, which has dramatically altered the nitrogen cycle. Reactive forms of nitrogen, especially nitrate and ammonia, are particularly detrimental. Given the magnitude of the problem, there is an urgent need for information on reactive nitrogen and its effective management. Nitrogen Overload: Environmental Degradation, Ramifications, and Economic Costs presents an integrated, multidisciplinary review of alterations to the nitrogen cycle over the past century and the wide-ranging consequences of nitrogen-based pollution, especially to aquatic ecosystems and human health. Volume highlights include: Comprehensive background information on the nitrogen cycle Detailed description of anthropogenic nitrogen sources Review of the environmental, economic, and health impacts of nitrogen pollution Recommendations and strategies for reducing humanity's nitrogen footprint Discussion of national nitrogen footprints and worldwide examples of mitigation policies The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals. Read the Editors' Vox: https://eos.org/editors-vox/exploring-the-widespread-impacts-of-ongoing-nitrogen-pollution

Advances in Agronomy, Volume 149, the latest release in the series, continues to be recognized as a leading reference and first-rate source for the latest research in agronomy. Each volume contains an eclectic group of reviews by leading scientists throughout the world. As always, the subjects covered are rich, varied and exemplary of the abundant subject matter addressed by this long-running serial. - Includes numerous, timely, state-of-the-art reviews on the latest advancements in agronomy - Features distinguished, well recognized authors from around the world - Builds upon this venerable and iconic review series - Covers the extensive variety and breadth of subject matter in the crop and soil sciences