Potential of different buffer zones as nature-based solutions to mitigate agricultural runoff nutrients in the subtropics

Agriculture is a major driver of land-use change and nutrient leaching worldwide, promoting eutrophication of surface water bodies. A frequent strategy to reduce nutrient external loads is the maintenance or re-establishment of riparian zones. We conducted a year-long, in situ monitoring of surface and subsurface water in three buffer zones (grassland, shrubland, forest) and adjacent croplands around the major water reservoir in Uruguay to assess runoff dynamics and nutrient potential reduction across different precipitation levels. All three buffer zones delayed surface runoff by twofold, yielding lower runoff than croplands. Also, they effectively retained phosphate (P-PO₄) loads in both surface and subsurface runoff but were less effective in reducing their concentrations. The forest achieved the highest surface water P-reduction (80%). The effect was variable for nitrate (N-NO₃), with buffers acting as either nutrient sinks or sources depending on the vegetation and runoff layer. Surface N-NO₃ loads were lower in the buffers, with a maximum reduction in grassland (∼50%), when compared to crops. In the subsurface layer, a reduction was only observed for N-NO₃ concentration in grassland (30%). Surface TP and P-PO₄ loads increased linearly with runoff rate only in the buffers, while both N-NO₃ and ammonium (N-NH₄) loads increased with runoff in both crops and buffers. Our results may indicate that riparian buffers comprised of herbaceous and woody vegetation have high phosphorus and nitrogen reduction rates, emphasizing their potential as nature-based solutions for nutrient mitigation and water storage. Future increased precipitation may, however, challenge buffer effectiveness.