Size distribution of colloidal molybdate reactive phosphorus in river waters and soil solution

Two river waters (a lowland chalk stream and an upland river containing humic materials) and two soil waters (freely drained leachate to 135 cm depth and storm surface runoff, both from grassland soils) were fractionated into unfiltered <0.45, <0.22, <0.1 and <0.025 μm using composite cellulose nitrate/acetate (CNA) membranes, under vacuum at 80 kPa. Fractions less than 10,000 and 1000 molecular weight (MW) size were separated by ultrafiltration at 276 kPa. Molybdate reactive phosphorus (MRP) and dissolved organic carbon (DOC) were measured using the filtrates in each size range. In three of the four waters, the concentration of MRP in the portion <1000 MW was significantly smaller (P < 0.05) than that determined in the fraction <0.45 μm. In the soil surface runoff water, which was dominated by eroded mineral particles and colloidal organic material, the <1000 MW fraction represented 14% of the total reactive P (TRP), and most of the reactive P was associated with the unfiltered (>0.45 μm) fraction, being 71% of the TRP. In contrast, 87% of the TRP in soil leachate and 55% of the TRP in the chalk stream water was <1000 MW, with significantly different but smaller portions in the range <0.45 μm to >1000 MW. Particle and colloidal material plays an important role in the transformations of P. These findings are particularly novel for soils, where implications for mobility within the soil solution need to be considered.