Bioavailable Phosphorus in Fine‐Sized Sediments Transported from Agricultural Fields

Abstract

Sediments transported from agricultural fields in surface and subsurface waters contain particulate P (PP) that could be partitioned into two pools—one available for aquatic organisms including cyanobacteria, namely bioavailable particulate P (BAPP, determined by 0.1 mol L−1 NaOH extraction), and the remainder not bioavailable (non‐BAPP). This study aimed to quantify the PP and BAPP concentrations in surface runoff and tile drainage water from eight agricultural fields with clay and sandy soils in the Missisquoi Bay region of Quebec, Canada. Particulate P in surface and drainage water varied spatially (among fields) and temporally, with concentrations as high as 3181 μg P L−1 in surface runoff and 1346 μg P L−1 in tile drainage. About 30% of PP was BAPP regardless of the drainage pathway. The PP and BAPP concentrations were related linearly (R2 = 0.86) to total suspended solids (TSS) in fine (0.05–1‐μm) and coarse (1–100‐μm) fractions. About 68% of the PP in clay soils and 50% of the PP in sandy soils were associated with the 0.05‐ to 1‐μm size particles, which had more BAPP, on average 0.46 g P kg−1, than the coarser 1‐ to 100‐μm fraction (0.22 g P kg−1). Soil parameters such as Mehlich‐3 extractable (M3) P, M3Fe, and the degree of soil P saturation were related to the PP and BAPP concentrations in TSS and particle size fractions. We concluded that sediments with particle size <1 μm contained more BAPP and their loss from agricultural fields could contribute to the eutrophication downstream.