Scale of measurement effects on phosphorus in runoff from cropland

Phosphorus (P) risk loss assessment tools such as P indices are usually developed from small-plot scale data showing the relationships between various site and management variables and runoff P losses. Little information is available on how small-plot runoff composition compares with field-scale measurements. This study was conducted to compare runoff volume and composition measurements at the field scale with those obtained from natural runoff at the small-plot (1 m2 [10.8 ft2]) scale. Sediment, dissolved P, and total P in natural runoff from small plots located in two fields (7.2 and 12 ha [17.8 and 29.6 ac]) were compared with similar measurements from the fields over an 18-month period. Runoff from small-plot rainfall simulations in both fields was also analyzed for P and sediment. The fields, cropped with either corn (Zea mays L.) or alfalfa (Medicago sativa L) interseeded with bromegrass (Bromus inermis), were located on a Tama silt loam soil (fine-silty, mixed, superactive, mesic Typic Argiudoll) in southwest Wisconsin (42°42? N, 90°22?W). Statistical analysis using repeated measures showed no significant differences between the two scales of measurements for dissolved P concentrations in runoff. Total P concentrations in small-plot runoff were greater than those in field runoff. Runoff volume and dissolved P concentrations were greater in winter than in summer, but summer runoff had higher sediment concentrations. Small plots had greater cumulative runoff volumes per unit area in both seasons compared to the fields. The dissolved P concentration relationships between the two scales of measurement for individual runoff events were very good in the corn field (r2 = 0.90) but not in the alfalfa field (r2 = 0.09). Sediment P enrichment ratios varied by crop and were similar in the small-plot and field runoff. Cumulative runoff-dissolved P concentrations were strongly related (r2 = 0.95) to average soil-test P at both scales. The agreement of small-plot and field runoff-dissolved P concentrations during the 18-month measurement period supports use of small-plot data in P loss risk assessment tools.