Tracing agricultural nitrogen policy impacts on groundwater quality through an integrated modeling approach

Assessing the effectiveness of agricultural nitrogen policies requires capturing the interactions between socio-economic decision-making, crop production, and catchment hydrology. Our study looks at the impact of uniform per hectare payments to farmers for reducing nitrogen (N) input to soil in German agriculture on groundwater quality in Rhine and Elbe catchments. To evaluate nitrogen surplus reductions and resultant nitrate leaching at the catchment scale under different subsidy schemes and climatic scenarios, we use an integrated modeling framework linking an agro-economic model (SNAg) – based on simulated yield data from the dynamic global vegetation, hydrology and crop model „Lund-Potsdam-Jena managed Land (LPJmL)“ - with a process-based water quality model (mQM). Our approach captures farmer responses to policy instruments and spatial heterogeneity in productivity, nitrogen use efficiency and hydrological processes.

Specifically, we compare a moderate subsidy scheme characterized by high participation and moderate nitrogen surplus reductions with a tighter scheme featuring lower uptake but stronger reductions. This allows us to examine how different spatial distributions of nitrogen reductions affect nitrate leaching and exceedances of groundwater quality thresholds. In the modeled scenarios, the share of catchments exceeding a nitrate leaching limit of 12 mg L⁻¹ in 2030 is reduced by 48% under the moderate subsidy scheme and by 59% under the tight scheme, relative to the no-policy baseline.

Our results further show that spatial heterogeneity in catchment structure plays a stronger role than interannual climatic variability in shaping nitrate export. Consequently, policy instruments calibrated solely on agricultural N input risk producing outcomes that are misaligned with hydrological processes. For instance, not taking spatial differences in nitrogen retention into account can lead to misleading assessments of policy effectiveness.

This work highlights the importance of integrating agro-economic, biogeophysical and hydrological properties in models to inform policy design capable of accommodating both spatial variability in agricultural systems and the biogeochemical complexity of catchments. By explicitly modeling outcomes across a range of scenarios and including farmers decision-making, we provide insight into how agricultural policy effectiveness can be evaluated and enhanced under global change conditions.