Phosphorus content of ditch sediments as indicator of critical source areas

An inexpensive method for identifying high phosphorus (P) loads on ditches carrying water from critical source areas ("hot spots", i.e. manure-affected areas) was tested. Water flowing from the hotspot areas is rich in dissolved nutrients (e.g. P) and nutrient-rich eroded material. In this study the water from hot spots contained an average of 2.2 mg/l dissolved reactive P but field ditch water 0.14 mg/l. We selected 62 ditches with varying P-load levels for sediment analysis. Five ponds and 16 lakes were also studied. The sediment samples were analysed by the AAAc-extraction (0.5 M ammonium acetate, 0.5 M acetic acid, pH 4.65) method used for routine soil testing in Finland. The analysis of sediment AAAc-P proved to be a useful method for identifying ditches carrying high P waters from hot spots. Half of the ditch sediments in the hot spot ditches showed high or excessive P concentrations. AAAc extraction of the sediment is less expensive than water analysis and the sampling can be extended to periods when no water flows in the ditches.