Lake sedimentary evidence of phosphorus, iron and manganese mobilisation from intensively fertilised soils

A study of historical P inputs to Friary Lough, Co. Tyrone, Northern Ireland used a multi-sediment core approach. One of the sediment cores taken from the littoral zone at 2.5 m water depth showed exceptionally high P, Fe and Mn concentrations below 20 cm sediment depth. Concentrations increased to 14 mg P g^-1, 238 mg Fe g^-1 and 35 mg Mn g^-1 in the sediment profile and compared with deep basin maxima of 7 mg P g^-1, 70 mg Fe g^-1 and 2 mg Mn g^-1 in surface sediments at 8.5 m water depth. It is proposed that the high concentrations in the littoral zone core are due to post-depositional intrusion of chemicalrich local groundwater from soils in adjacent fields that are excessively fertilised with organic slurry. Soil analyses showed Olsen-P concentrations in these fields up to 125 mg kg^-1 at the soil surface (0–7.5 cm) and 39 mg kg^-1 in the sub-soil at 20 cm depth. We suggest that the mobilisation of P, Fe and Mn is due to leaching following P saturation and/or the loss of P absorbing chemicals due to prolonged reduction and complexing in wet soils. Further work will explore this relationship and the nature of the hydrological pathways through soil.