Removal of nitrate and phosphate from runoff by near-stream soils under future climate scenarios

Abstract

Near-stream soils are a critical buffering zone for protecting water quality by removing excess nutrients from agricultural runoff. However, projected increases in drought frequency, punctuated by intense flash floods, may alter these zones’ capacity to mitigate eutrophication. We conducted a laboratory experiment with soil cores of two contrasting textures (sandy loam vs. silty clay loam) subjected to different antecedent moisture conditions (drought vs. field capacity) and water application methods (flooding vs. capillary rise). We regularly sampled porewater for nitrate (NO₃⁻), ammonium (NH₄⁺), and phosphate (PO₄³⁻), and measured O₂ consumption to assess microbial activity. Results showed that drought antecedent moisture combined with flash flooding significantly elevated porewater NO₃⁻ concentrations, suggesting reduced nitrogen removal. In contrast, PO₄³⁻ concentrations were lower in sandy soils and under drought conditions, yet flash floods still mobilized phosphorus. These findings indicate that changing precipitation patterns under climate change can impair the nutrient-buffering function of near-stream soils and contribute to greater nutrient loads in surface waters. Management strategies and policies that maintain the resilience of these buffering zones are essential for protecting freshwater ecosystems against accelerated eutrophication.