Physicochemical mechanisms underlying soil and organic amendment effects on runoff P losses

Phosphorus depletion from cultivated lands by runoff is a significant contributor to soil chemical degradation. Our objective was to elucidate the mechanisms through which soil and organic matter amendments affect rain-induced runoff P losses in Mediterranean soils. Clay, loam, and loamy sand mixed with noncomposted activated sludge (AS), manure compost (MC), industrial humic acid (HA), orthophosphate (Pi), or inositol hexaphosphate (IHP) were subjected to six consecutive artificial rainstorms. Adding amendments significantly increased runoff available Pi loss (the sum of solution and sediments bicarbonate extractable Pi) for all soils. The order of loss (Pi, mg m(-2)) was soil dependent; greatest and lowest in the loam and loamy sand, respectively (loam: Pi[270] > IHP[128] > AS[108] > MC[97] > HA[49] > Control[33], and loamy sand: AS[42] > HA[23] > Pi[22] > IHP[18] > MC[13] > Control[4]), although order of runoff and soil loss were clay > loam > loamy sand. Treating with IHP and Pi at a similar total P level led to comparable cumulative total-P runoff losses, but runoff available Pi loss was much greater in the Pi treatment of soils with higher clay content (clay > loam > loamy sand). A derived isotherm relating sediment Olsen Pi to dissolved reactive Pi (DRP) concentration in runoff was found useful for estimating runoff Pi losses from measured DRP, runoff volume, and sediment concentration. The results could be explained by the impact of the amendments on soil structure stability, sealing, runoff, and erosion levels associated with soil physical and chemical degradation.