Phosphorus losses in runoff from manured grassland of different soil P status at two rainfall intensities

In many areas, excessive manure application on agricultural land has led to a substantial build-up of soil phosphorus (P) stocks, increasing the risks for diffuse pollution of surface waters. Recent studies highlight the need to differentiate between runoff types for better prediction of these risks. In a factorial field-plot experiment we investigated the role of soil-P status, band-applied manure and rainfall intensity on P losses from two Swiss grassland sites. Artificial rainfall was applied on each plot first at medium intensity using a sprinkler and then at high intensity using a watering can, simulating two different runoff conditions. Under both conditions, dissolved reactive P (DRP) in runoff increased linearly with water soluble P in the soil (WSP), but the extraction coefficients differed substantially between the two phases of runoff generation. It was 0.017 kg L−1 for runoff generated with the watering can (WCR), and 0.085 kg L−1 for runoff generated with the sprinkler (SR). Manure application increased DRP losses, but did not override the effect of soil P status. Phosphorus losses with runoff were more sensitive to soil P status for SR than for WCR. Reducing soil-P is therefore crucial to reduce runoff-P.