Understanding the relationship between agricultural nutrients NPS, land management, and climate using distributed hydrologic models
The use of chemical fertilizer has enhanced agricultural production that helped provide food to seven billion people worldwide. Particularly, the use of nitrogen (N) and phosphorus (P) fertilizers, which increased more than nine and four times, respectively, during the last five decades, played significant roles in improving soil fertility, crop health, and yield performance. However, the massive use of N and P fertilizer has led to severe environmental issues such as harmful algal bloom (HAB), a decline in biodiversity, and contaminated drinking water which can lead to serious threats to public health and critical stress to the agroecosystem because of the high fertilizer loss. To mitigate the excessive nutrient loading in our water bodies, different land management schemes have been developed and implemented to reduce N and P NPS in the agroecosystem, but the effectiveness of these land management practices needs evaluation. Due to the complexity of the watershed response to natural and anthropogenic stressors like changing climate and changes in land management scenarios, a modeling framework that allows the conceptualization of physically-based hydrologic and transport processes should be adopted to provide reliable predictions and a better understanding of the systemic responses. This study was geared at understanding how agricultural nutrients NPS were impacted by natural and anthropogenic changes such as climate and land management practices, respectively. Modeling frameworks with field-level spatial resolution were developed to evaluate the effectiveness of NPS reduction practices when detailed and field-level specific land management practices were implemented under extreme climate projections. First, the impacts of different land management practices and climate on surface runoff linked N load at the field-level scale were evaluated. Second, different types of N NPS patterns in the Upper Sangamon River Basin (USRB), a highly cultivated watershed, under wet and dry future climate projections were compared when implementing different land management practices at the field-level scale. Finally, the risks of HAB occurrence and the effectiveness of different land management practices in mitigating this risk under extreme climate projections were assessed. Our results showed that fertilizer application rate was the most critical factor in determining both the amount and the probability of high N load. If 50% less fertilizer was applied in a highly cultivated watershed focusing on growing corn and soybean, the nitrate concentrations were expected to remain lower than the United States Environmental Protection Agency (EPA) and World Health Organization (WHO) limit even under extreme climate projections. Other possible land management practices that could also reduce N concentration and load included the target application management approach, crop rotation using alfalfa, and cover cropping with winter wheat. Ammonia and nitrite transformed from nitrate fertilizer, however, were not expected to become an environmental concern in the USRB under climate projections. Furthermore, P was found to be the limiting nutrient despite an important P point-source in the USRB. In cases like this, land management practices focusing on P reduction should be adopted when HAB or GAB occurrences are a concern and N reduction practices should be applied to the USRB if downstream HAB or GAB occurrence risks were high. With the improved understanding of the relationship between agricultural nutrients NPS and natural and anthropogenic stressors, more specific and effective land management practices targeting the reduction of agricultural nutrients NPS could be applied based on different climate and natural environment characteristics. Additionally, our projects also provided two modeling frameworks focusing on simulating the different agricultural nutrients NPS considering the field level processes. These frameworks can provide a better platform in evaluating the effectiveness of different land management practices in reducing agricultural nutrients NPS. Overall, my research enabled other researchers and decision-makers to assess environmental issues risks and compare the tradeoffs between the welfare of agro-ecosystems and economic benefits using models that are closer to reality than empirical models that have been widely used currently.
Agricultural nutrients, NPS, land management practices, modelling
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign