Hydraulic conductivity reduction can be decoupled from plant available water and can influence nitrogen leaching in amended sandy soils

Improvements in sandy-soil hydraulic behavior are often inferred from reduced permeability rather than from direct gains in plant-available water (AW). This study quantified the extent to which amendment-induced reductions in saturated hydraulic conductivity (K s ) correspond to changes in AW and how these responses influence nitrogen (N) leaching. Controlled soil-column experiments evaluated the effects of clay powder, compost, and a biochar-based biofertilizer (biofertilizer-char) on soil water retention characteristics, K s , water discharge, and N-losses. Clay powder significantly reduced K s but did not significantly increase field capacity, permanent wilting point, or AW, demonstrating that restricted drainage does not necessarily improve water availability. Compost increased saturation percentage (18.6–20.8%), but higher application rates reduced AW (10.4–7.6%) and increased N-leaching. Combined compost and biofertilizer-char treatments maintained AW (~ 10.3%) while reducing water discharge and moderating N-losses. Nitrogen leaching varied among amendment treatments. High compost rates produced the greatest losses (0.63 mg column −1 , 15.75%), while moderate compost and compost + biofertilizer-char treatments reduced losses to 0.09–0.20 mg column −1 (3.25–8.67%). Leached-N increased with initial total N-content (R 2 = 0.378) and decreased with leachate volume (R 2 = 0.253). These findings demonstrate that reduction in K s can be decoupled from gains in AW and that N-leaching is jointly regulated by nitrogen availability and amendment-induced modifications of soil water transport.