Particulate Phosphorus and Sediment in Surface Runoff and Drainflow from Clayey Soils

Recent work has shown that a significant portion of the total loss of phosphorus (P) from agricultural soils may occur via subsurface drainflow. The aim of this study was to compare the concentrations of different P forms in surface and subsurface runoff, and to assess the potential algal availability of particulate phosphorus (PP) in runoff waters. The material consisted of 91 water‐sample pairs (surface runoff vs. subsurface drainage waters) from two artificially drained clayey soils (a Typic Cryaquept and an Aeric Cryaquept) and was analyzed for total suspended solids (TSS), total phosphorus (TP), dissolved molybdate‐reactive phophorus (DRP), and anion exchange resin–extractable phosphorus (AER‐P). On the basis of these determinations, we calculated the concentrations of PP, desorbable particulate phosphorus (PPi), and particulate unavailable (nondesorbable) phosphorus (PUP). Some water samples and the soils were also analyzed for 137Cs activity and particle‐size distribution. The major P fraction in the waters studied was PP and, on average, only 7% of it was desorbable by AER. However, a mean of 47% of potentially bioavailable P (AER‐P) consisted of PPi. The suspended soil material carried by drainflow contained as much PPi (47–79 mg kg−1) as did the surface runoff sediment (45–82 mg kg−1). The runoff sediments were enriched in clay‐sized particles and 137Cs by a factor of about two relative to the surface soils. Our results show that desorbable PP derived from topsoil may be as important a contributor to potentially algal‐available P as DRP in both surface and subsurface runoff from clayey soils.