Increasing Nutrient Circularity and Reducing Water Pollution Through Anaerobic Digesters

Abstract

While the intensification of agricultural practices over the last few decades has increased livestock and crop production, it has also led to unintended environmental consequences such as harmful algal blooms, drinking water contamination, and increased emissions of greenhouse gasses. Much of the increase in crop and livestock production can be attributed to a shift towards specialized agriculture which has resulted in the decoupling and spatial separation of livestock and crop systems. The spatial separation of the two systems has disrupted the circular flow of nutrients in agricultural systems. Relinking the livestock-nutrient economy has been identified as a strategy to reduce the overall environmental burden of the sector. The use of anaerobic digesters to manage livestock manure presents a promising pathway towards the recoupling of crop and livestock systems.

Anaerobic digesters, or also referred to as biodigesters, utilize anaerobic decomposition to transform organic wastes into valuable by-products. During the digestion process, methane – a potent greenhouse gas emitted in traditional manure management – is captured to produce biogas which is a source of renewable energy. The process also produces digestate which is a nutrient rich effluent that can be applied to cropland as a fertilizer source. The nutrient dense nature of digestate, and the potential revenue from biogas production enable it to be economically transported over a greater distance than untreated manure – thereby providing a pathway to enhance the nutrient circularity in spatially separated livestock and crop systems. However, there is concern that digestate use can result in greater nitrogen leaching losses than manure.

The work presented in this thesis estimates the nitrogen leaching losses from the corn and soybean cropland across 263 regions in Ontario and assesses the water quantity implications of manure and digestate land-application. To do this, a DeNitrification-DeComposition (DNDC) model was developed for each region. The models were calibrated individually to observed crop yields from 2011 and 2021. The calibrated models were able to capture the general magnitude and annual variation in reported corn and soybean yields across the study region. The median error between simulated and observed crop yields across all regions was 5.8% (mean absolute percent error). Corn crops were provided with synthetic fertilizer at an optimal rate, as determined by calibration. The results of the calibration showed that observed crop yields across the study region could be met through the application of 69% of the nitrogen fertilizer purchased in Ontario in 2021. This finding suggests corn nitrogen requirements are met through the application of purchased synthetic fertilizer while manure is applied to cropland in addition of crop needs. Next, I used livestock population data to estimate the quantity of manure nitrogen produced in each region. Using the calibrated DNDC models, I simulated a number of scenarios which explored various manure and digestate distribution configurations across the landscape.

The results of this work show that when digestate was substituted for manure and subject to the same transportation constraints, the amount of nitrogen lost by leaching across the study region increased by 6% (from 46.77 to 49.42 kt N/yr). However, when the digestate distribution configuration was altered to reflect re-distribution from a centralized biodigester and its ability to be transported over a greater distance, the amount of nitrogen lost through leaching across the study region was reduced by 7% (43.42 kt N/yr). These findings show that when digestate was used as a direct substitute for manure and applied at equal rates based on total nitrogen content, it contributed to increased nitrogen leaching losses. However, when the distribution of the digestate was considered at a regional scale and the system dynamics of the biodigester were accounted for, the use of digestate reduced the total nitrogen leaching losses across the study region. This research shows that biodigesters can provide benefit to water quality when considered at a regional scale.