Effect of agronomic management on risk of suspended solids and phosphorus losses from soil to waters

Purpose Fertilisation may cause an accumulation of phosphorus in soil, which may increase risk of P transfer to waters both in colloidal and dissolved forms. This study evaluated the effect of agronomic management on the potential risk of P losses from soil to water bodies in a long-term experimental platform (NW Italy) subjected for 15 years to different maize-based crops and mineral or organic fertilisation based on nitrogen crop requirements. Materials and methods The field experiment is based on maize for grain and maize for silage cropping systems fertilised with nitrogen–phosphorus–potassium and phosphorus–potassium or with manure and slurry. Soil and P losses were estimated using a simple dispersion test and the amount of suspended solids, total P, colloidal and soluble P determined. Suspended solids were also characterised for their Fe, Al, Ca, C and N contents. Results and discussion Fertilisation management, mineral or organic, strongly modified the processes which drive soil P buildup and its mobilisation rather than the cropping system type. Manure and slurry application for 15 years has led to a larger P soil surplus compared to mineral fertilisation. However, the contrasting effect of added organic material and P on aggregation/dispersion processes resulted in minor losses of dispersed solids and colloidal P than those expected from the P increase and high Olsen P. Potentially mobilised P was mostly found in suspended solids (>92%); however, a consistent amount of dissolved P was present in the manured soils, indicating that in soils which have reached excessive P levels and are close to P saturation, mobilisation of dissolved P becomes relevant. The amount of dissolved P was well related with Olsen P ( r 2 = 0.92, on average), whilst, surprisingly, particulate P was not related, indicating that particulate P losses cannot be estimated only considering the soil P status. Mineral or organic fertilisation caused also a preferential selection of the suspended solids which showed a different composition, highlighting that the type of fertilisation may affect not only the amount but also the type of P and suspended solids which are mobilised. Conclusions These findings show that fertilisation, mineral and organic, rather than cropping system strongly affects the amount and the type of suspended solids and P which are mobilised. This may have important implications on understanding the factors and mechanisms that control P transfer and its release from particles into waters.