Nitrogen cycling in soil and groundwater, localphenomena on a global scale

Introduction

Agriculture is a major source of nitrate (NO3) through leaching, flow of NO¬3 with water that infiltrates to deeper layers, to groundwater, which acts as storage and sink. This leads to accumulation of NO3 in groundwater and associated water quality changes impact human health through use of groundwater as drinking water and to ecosystems by discharge to the surface. Secondly, through denitrification, a redox reaction that occurs under anoxic conditions, the greenhouse gas N2O is produced from NO3. Because of long transport times through groundwater and because of denitrification, direct effects of NO3 in groundwater are attenuated, but legacy effects are carried over into the future (Beusen et al. 2015, Ascott et al. 2017). This global role of groundwater in the N cycle is poorly quantified. Current conceptual modelling approaches lack explicit spatial variable representation of globally relevant hydrological and geochemical processes; importantly regional groundwater flow, dynamic soil-crop cycling and redox state control on denitrification. It is however known from field and aquifers studies that this spatial variability is very important (Henri & Harter, 2022; Wood et al. 2022). This means that current models have errors, that could be improved with novel approaches.

Project description

This research integrates an innovative global geohydrological model VIC-WUR/MODFLOW, into the conceptual large-scale nutrient balance model IMAGE-GNM. The project defines NO3 sinks in groundwater on newly-generated global hydrogeochemical maps. This integrated approach quantifies past-current NO3 groundwater cycling and environmental effects by combining soil column, crop and groundwater numerical modelling. It is expected that this model will better characterize groundwater in the nitrogen cycle and its continuing legacy even if drastic reduction of nitrogen losses is achieved. The developed integrated model is furthermore used to study impacts on the nitrogen cycle by climate change with standardized SSP-RCP scenarios. The results of nitrogen cycling under these scenarios provide critical information for tailoring future agricultural and groundwater management that optimizes food production, relative to climate, health and ecosystems goals on the large-scale.

Publications

Ascott, M. J., Gooddy, D. C., Wang, L., Stuart, M. E., Lewis, M. A., Ward, R. S., & Binley, A. M. (2017). Global patterns of nitrate storage in the vadose zone. Nature Communications, 8(1), 1416.

Beusen, A. H. W., Van Beek, L. P. H., Bouwman, A. F., Mogollón, J. M., & Middelburg, J. J. (2015). Coupling global models for hydrology and nutrient loading to simulate nitrogen and phosphorus retention in surface water–description of IMAGE–GNM and analysis of performance. Geoscientific model development, 8(12), 4045-4067.

Henri, C. V., & Harter, T. (2022). Denitrification in heterogeneous aquifers: Relevance of spatial variability and performance of homogenized parameters. Advances in Water Resources, 164, 104168.

Wood, W. W., Smedley, P. L., Lindsey, B. D., Wood, W. T., Kirchheim, R. E., & Cherry, J. A. (2022). Global groundwater solute com