Using Spatially Explicit Soil Mapping and Modelling to Understand and Mitigate Nitrate Leaching in an Agricultural Catchment

Abstract

Diffuse water pollution is a significant environmental management problem associated with nitrate movement from agricultural catchments to groundwater sources. Several factors including agricultural management practices, soil texture and soil depth to the bedrock are responsible for this pollution risk. The Fell Sandstone aquifer which extends across a large part of Northumberland is the lone source for drinking water supply in Berwick upon Tweed and is showing deterioration that is believed to be due to diffuse pollution. Environment Agency monitoring boreholes in the catchment area indicate that nitrate levels in the Fell Sandstone aquifer may exceed the allowable limit for nitrate in drinking water in the next 5 to 15 years. Land use in the catchment is mostly agricultural and fertilizer and manure management accompanied by other agricultural activities are believed to be major contributors to groundwater contamination. In this thesis, high-resolution soil sensing was used to design a better monitoring system for soil nitrate concentration and leaching events in the Fell Sandstone catchment study area. The porous ceramic cups technique was used to extract soil solution below the root zone for three drainage seasons (between autumn 2017 and spring 2020) to monitor nitrate leaching from various soil types, crop rotations and conventional vs organically managed fields. The digital soil mapping approach (DSM) was used to understand soil texture and depth to the bedrock variability within the study area. In the results based on the predicted soil texture components, soil texture (clay and sand %) varies within a field in a distance of 300m in the top layer of soil. Soil depth to the bedrock also varies from very shallow (30 cm) to deep (> 120 cm) within the study area. Due to the role of soil texture and depth to the bedrock in nitrate movement, the locations of sandy shallow soil profiles might be hotspots for nitrate leaching. The results further emphasised that the time and amount of drainage volume varied with soil type and soil profile depth. Effect of crop rotations, fertilizer amount and impact of a wet and dry year in terms of drainage was studied. Nitrate leaching from the field after winter wheat followed by potatoes during the first drainage season 2017/2018 was much higher than any other crop rotation in conventionally managed fields and from organically managed winter wheat grown after two years of clover was highest among organic fields. Whereas lowest leaching was recorded from grass fields regardless of the management system. The results reveal no notable difference in leaching from organic and conventionally managed fields. Several strategies are outlined in literature to mitigate the losses of nitrogen from agricultural land. Innovative approaches like nitrification inhibitors and slow-releasing N fertilizer were investigated along with tillage management in field trials in a long term organic and conventional crop rotation and fertility management trial at Nafferton Farm, England. The role of a nitrification inhibitor in reducing nitrate leaching was demonstrated in the field trial but no apparent effect of slow-releasing fertiliser iii was recorded on leaching but, fertiliser use was improved with slow-releasing fertiliser which could result in less surplus N at the end of the season. A mechanistic nitrogen dynamics model was calibrated and validated with observed soil mineral nitrogen and nitrate leaching data to assess the efficacy of the model in simulating nitrate leaching and to simulate the impact of management practices. This study demonstrated the importance of spatial variability of soil properties, particularly soil texture and soil depth along with other factors, on nitrate leaching. The results can be used to support decisions about management of spatially variable zones within a field for the purpose of controlling nitrate leaching by implementing proven strategies without compromising economic loss. Eventually the outcomes of this thesis can add to the knowledge of understanding the factors causing diffuse nitrate pollution and innovative mitigation strategies to minimise these losses.