Effects of tillage and reseeding on phosphorus transfers from grassland

Pasture tillage and reseeding is part of the normal rotation cycle of grassland systems in the UK and a process that could increase the rate of phosphorus (P) transfer to water, thus potentially contributing to eutrophication. The effect of tillage and reseeding on P transfer was investigated at two scales: from drained and undrained 1 ha hydrologically isolated pasture plots within the Rowden Drainage Experiment in Devon, during a 6-month winter drainage period in 1998-1999, and in replicated soil box experiments during simulated rainfall 'events'. At the plot scale, total P exports of 3.75 kg P ha(-1) were determined over a 16-day period, indicating that soil and P were most vulnerable to detachment and mobilization during rainfall and run-off in this period. Once the sward had developed, and the vulnerability to soil detachment reduced, reseeded swards with pipe drainage transferred less P (approx. 0.3 kg P ha(-1) yr(-1)) to water than is commonly measured on permanent grassland (approx. I kg P ha-1 yr-1). Soil box experiments showed that tilled soil transferred more P > 0.45 mu m but P < 0.45 mu m was retained. Sward cover is critical to reducing detachment and resulting P transfer from surface soil, and therefore careful consideration should be taken for the need to reseed. The effects of tillage and reseeding on phosphorus transfers from grassland can be potentially significant and ought to be mitigated against using low-till practices to reduce potential contribution to water quality.