Conservation practices modify soil phosphorus sorption properties and the composition of dissolved phosphorus losses during runoff

International audienceIt is well established that shallow tillage (ST) and no-tillage (NT) of cultivated soils can have higher risk of dissolved phosphorus (P) loss during runoff than conventional tillage (CT). However, quantitative descriptors of how differences in tillage practices influence soil P sorption properties and how these properties in turn influence dissolved P loss remain lacking. Moreover, the influence of tillage practices on the forms of dissolved P, particularly molybdate unreactive dissolved P (MUDP), has rarely been investigated. Thus, we simulated runoff using pilot soils that had experienced three tillage practices (CT, ST, and NT) and two types of fertilization (pig and poultry manure) for 20 years. The results indicated that shifting from CT to ST or NT changed the soil’s capacity to bind P and increased Olsen-P, Dyer-P, equilibrium P concentration (EPC0), water soluble P (WSP), and the degree of P saturation (DPSOX). The experiment confirmed that switching from CT to ST or NT could increase P loss during runoff. The increase was evident for the two fertilizations (pig manure and poultry manure) with an amplified effect for the poultry manure modality. Switching from CT to ST or NT increased the loss of MUDP, which again was higher for poultry manure fertilization, for which the percentage of MUDP increased to more than 40% of TDP. Correlations between the risk of dissolved P loss and risk assessment indicators were positive, but DPSOX was opposite predicted results to other indicators, indicating that DPSOX was not suitable for predicting the risk of dissolved P loss. Our results suggest that MUDP should be considered when assessing the risk of dissolved P loss when adopting ST and NT, since MUDP is bioavailable and thus can contribute to the eutrophication of surface water