Eleven years of runoff and phosphorus losses from two fields with and without manure application, Iowa, USA

Monitoring runoff at field edges can show how cropping systems and conservation practices affect runoff hydrology and water quality. Multi-year records are needed to characterize these effects, because of the variable, ephemeral nature of rainfall-runoff events. This study compared runoff and phosphorus (P) losses from two fields in central Iowa from 2000 through 2010. Both fields were managed in the same three-year, corn (Zea mays L.)-corn-soybean (Glycine max (L.) Merr.) rotation, but one field received applications of swine manure for each year of corn. Results comprised 116 events at the manured site and 94 events at the non-manured site, with 74 events common to both locations. Rainfall-runoff relationships for the two fields were similar; annual runoff averaged 54 mm from the non-manured field and 47 mm from the manured field. Large storms (>60 mm rainfall) comprised about 10% of the runoff events in both watersheds, producing 12-16% of the total P loads. Moderate storms (30-60 mm rainfall) generated most (65-70%) of the P load from both watersheds. Losses of P averaged 1.8 kg P ha−1 year−1 from the manured watershed and 1.05 kg P ha−1 year−1 from the non-manured watershed. Relationships between runoff-volumes and P-loads differed between the two watersheds (p < 0.05). Results highlight the challenge of maintaining adequate soil P levels while minimizing runoff P losses under a corn-soybean rotation, but indicate conservation practices that can limit runoff from storms of 30-60 mm of rainfall can help producers meet that challenge.