Land-to-Water Linkages: Nutrients Legacies and Water Quality Across Anthropogenic Landscapes

Abstract

An increasing population and the intensification of agriculture has driven rapid changes in land use and increases in excess nutrients in the environment. Globally, excess nutrients in inland and coastal waters have led to persistent issues of eutrophication, ecosystem degradation, hypoxia, and drinking water toxicity. Over the past few decades, we have seen policies set to mitigate the degradation in water quality. The existing paradigm of water quality management is based on decades of research finding a linear relationship between the net nitrogen inputs to the landscape and stream nitrogen exports. For instance, in the U.S., in response to these nutrient problems, working groups have spent approximately a trillion dollars to improve water quality by upgrading wastewater treatment plants and implementing nutrient management plans to decrease watershed nitrogen and phosphorus inputs. Despite concerted efforts, in many cases we have not seen marked improvements in water quality. In cases where water quality has improved, it is frequently after decades of nutrient management. The lack of or delayed water quality improvement suggests the importance of other drivers in modulating the relationship between nutrient inputs and watershed exports. Indeed, watershed nutrient loads are not just a function of current-year nitrogen inputs but can also depend on the history of inputs to the watershed. However, we still have little understanding of the relationship between nutrient inputs related to exports and the extent that accumulated stores of nitrogen and phosphorus influence this relationship. The central theme of my research has been an exploration of the history of anthropogenic nutrient use and the relationship between nutrient inputs and the response in water quality. Specifically, I have focused on the role of current nutrient inputs versus historical nutrient use in impacting water quality at the watershed scale, as well as the various landscape and climate controls that can mediate responses to changes in management. My research objectives will be to (1) develop a multi-decadal mass balance of nitrogen and phosphorus at the sub-watershed scale across the contiguous U.S. in order to investigate (2) the relationship between watershed nitrogen inputs and export and the drivers of changes in watershed nitrogen export, (3) the magnitude, spatial distribution, and drivers of nitrogen retention and legacy stores, and (4) the use and management of phosphorus in agricultural landscapes in the context of both food security and environmental health. I began by developing county-scale nitrogen and phosphorus surplus datasets, TREND-N and TREND-P, for the contiguous U.S.—with surplus defined as the difference between anthropogenic inputs (fertilizer, manure, domestic inputs, biological nitrogen fixation, and atmospheric deposition) and non-hydrological export (crop and pasture uptake). In Chapter 2, I present the updates to a previously published TREND-N county-scale nitrogen mass balance dataset, improving crop and pasture uptake and livestock excretion methods. In Chapter 3, I develope new county-scale phosphorus surplus dataset, using similar methods. These datasets were then downscaled to a 250 m gridded-scale dataset, known as gTREND-Nitrogen and gTREND-Phosphorus, a step led by my collaborator Shuyu Chang. These novel datasets serve as the foundational data for the subsequent chapters. Next, in Chapter 4, I explored the relationship between net nitrogen inputs and nitrogen export for over 400 watersheds across the U.S. I used the newly developed nitrogen surplus dataset to understand how watershed-scale nitrogen surplus magnitudes and exports change over time and examine how the relationships are influenced by both natural and anthropogenic controls within watersheds. To achieve this, I used a set of 492 watersheds with nitrogen input and export data spanning from 1990 to 2017. We found that 284 watersheds had a significant (p<0.1) increasing or decreasing trend in both nitrogen surplus and nitrogen load. Of these watersheds, we identified 62 where both nitrogen surplus and export have been significantly increasing over the last two decades. These input-driven watersheds are characterized by high livestock density, agricultural area, and tile drainage. In contrast, nitrogen surplus and export have been decreasing in 127 "bright spot" watersheds, characterized by high population density and urban land use. Nitrogen surplus is also decreasing in 60 "transitioning" watersheds, but export is increasing as nitrogen surplus decreases. We argue that these watersheds are transitioning from agriculture to more urban areas, such that fertilizer inputs have decreased, but the higher nitrogen export is driven by legacy nitrogen stores. Finally, we found 35 watersheds demonstrating a delayed response, with nitrogen export decreasing despite an increase in nitrogen surplus. Climate appears to be the driver of response in these watersheds, with aridity likely driving lower nitrogen export, despite increasing inputs. The four typologies of nitrogen inputs and export relationships suggest that watersheds can act as filters and modulate the movement of nitrogen. Our results provide insights into the complex dynamics of nitrogen surplus and export relationships, as well as how the landscape, climate, and legacy nitrogen can influence these relationships. In Chapter 4, I analyzed relationships between changes in nitrogen inputs and export, to understand what drives changes in watershed export, finding that legacy stores may be modulating the watershed response to changing net nitrogen inputs. However, we have limited knowledge of the magnitude and spatial distribution of legacy stores across North America. Therefore, in Chapter 5, we quantified how much nitrogen retention, which is the mass of nitrogen stored in legacy pools and nitrogen lost to denitrification, has accumulated in watersheds, and where it can be accumulating. To achieve this, we used existing datasets and machine learning algorithms to calculate the mass of ‘retained’ nitrogen in the landscape—defined as the nitrogen stored in the soil organic nitrogen pool, the groundwater pool, or lost through denitrification. Specifically, we built a random forest modeling framework trained on the watersheds’ nitrogen surplus and components, loads, and characteristics to predict nitrogen loads at the HUC8 scale across the U.S. We calculated retention for HUC8, which is the difference between nitrogen surplus and predicted loads, and found that nitrogen retention is highest in the Midwestern and Eastern U.S. because of low exports in regions with high agricultural inputs or high population density. Next, we used a data-driven approach to estimate legacy stores by allocating retained nitrogen mass into their legacy pools. We partition nitrogen retention in the Upper Mississippi region HUC8 watersheds into the mass stored in the groundwater pool, soil organic nitrogen pools, and mass lost to denitrification. We found that, on average, 42% of the mass is stored in the soil organic nitrogen pool, 16.5% is stored in the groundwater pool, and 40% is lost to denitrification. While these two chapters focused on nitrogen, in my final chapter we shifted to explore phosphorus use in agricultural systems. In my final chapter, we used the new gridded phosphorus surplus and components dataset to explore current and historical agricultural phosphorus use and management in landscapes within the context of both food security and environmental health. To characterize the extent of phosphorus depletion and excess, we employed indicators such as annual and cumulative phosphorus surplus and phosphorus use efficiency (PUE). We found that the evolution of agricultural phosphorus management is shaped by changing fertilizer management, the proliferation of concentrated animal operations, climate, and the landscape's memory of past phosphorus use. We further integrated both cumulative phosphorus surplus and PUE into a framework to quantify phosphorus sustainability in intensively managed landscapes. We found that in the 1980s, much of the agricultural land was undergoing ‘intensification,’ with positive and increasing cumulative stores because phosphorus inputs exceeded crop uptake (PUE < 1). By 2017, 29.5% of the agricultural land was undergoing ‘recovery’ and had positive cumulative phosphorus stores that were being depleted through improved phosphorus management (PUE > 1). However, 70% of the agricultural area in the U.S. is still undergoing ‘intensification,’ particularly in areas with more of their inputs from livestock manure, pointing to the need to treat manure as a resource instead of the current approach of treating it as a waste product. By using novel datasets, we have been able to explore nutrient use across space and time and its impact on food security and environmental outcomes. I have made significant contributions towards expanding the discussion of nutrient us and fate, understanding the magnitude and distribution of cumulative net nutrient inputs stores in the landscape, as well as the ways in which intrinsic watershed properties, climate, land management, and historical nutrient use can modulate the relationship between inputs and export. Overall, my findings underscore the importance of nuanced, place-based, and context-dependent nutrient management strategies, with a focus on manure management, to address the diverse challenges of different agricultural systems and prevent unintended environmental consequences.