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

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. 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. 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. 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.