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 rn 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 rn water depth. It is proposed that the high concentrations in the littoral zone core are due to post-depositional intrusion of chemical-rich 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. (C) 2003 Elsevier Science Ltd. All rights reserved.