Loss of soil phosphorus by tile drains during storm events

•Storm event in summer resulted in loss of P rich ochre flocs from tile drains.•Floating fungal mycelia were identified as carrier for Fe and P.•Flocky nature of ochre suggested high temporal bioavailability of P in water.•Retention of P at drain outlet is recommended.

Phosphorus losses from agricultural soils are usually predominately caused by surface-runoff and only rarely by tile drain discharge. However, storm events can drastically increase P transfer to water bodies. In 2011 summer storm events caused 3.2 fold higher precipitation compared to the long-term average in Northern Germany. During these storm events excessive yellow-brown ochre flocs were observed in drain discharges and supposed to be potential P carrier. Therefore, water samples with ochre flocs were taken and analyzed by light and electron microscopy and for total elemental concentrations of P and its common binding partners (Al, Fe, Mn). Additionally, sandy sediment samples were taken in winter and analyzed for total and oxalate-extractable amounts of these elements. Water samples with ochre flocs revealed total P concentrations of approximately 27 to 141 mg l−1. During the high discharge period a cumulative discharge of 124.6 mm from a drain plot of 4.2 ha was recorded, which resulted in a calculated minimum loss of 34 kg Pt ha−1. This was assumed to be promoted by the excessive occurrence of ochre flocs, which were mainly formed by fungal mycelia with precipitated Fe- and Mn-(hydr)oxides at the surface. The poor crystallinity of the Fe-oxides from the discharge of tile drains was confirmed by an amount of up to 94% of active Fe-oxides in the sandy sediment. The extremely high discharge from tile drains, the flocky nature of the ochre along with a poor crystallinity of Fe-oxides and high P concentrations possess the risk of increased input and bioavailability of P in water bodies. Therefore, discharge and P forms from tile drained areas should be monitored more intensively and retention of P on tile drain outlets should be considered to prevent water bodies from eutrophication.