Rethinking soil erosion, phosphorus and colloid transfers from intensive grasslands

The general consensus from the relatively small amount of existing research into soil erosion and nutrient export from intensive grasslands suggests that they are not a significant source of either sediment or phosphorus (P) (Bilotta et al., 2006). Authors have often attributed the low rates of erosion on intensive grasslands to the effect of the high surface cover which acts to intercept raindrops and retard runoff, resulting in limited detachment and transport of soil particles and so low sediment and particulate-phosphorus losses from these environments. Consequently, researchers have proposed that the phosphorus transfer from intensive grasslands is dominated by soluble forms of phosphorus, arbitrarily defined as anything that passes through a 0.45 μm filter (Sharpley et al., 2000). Nevertheless, researchers studying surface water quality issues in areas that are dominated by intensive grasslands are finding problems associated with the delivery of sediment and sediment-bound contaminants (Heathwaite and Dils, 2000). Clearly there are some inconsistencies within the literature which need to be re-examined. Work presented here aims to question why these inconsistencies exist and presents results from a field experiment designed to further understanding of the response of grasslands in terms of sediment and phosphorus delivery from intensively managed grasslands. The field-based experiment comprises paired lysimeter studies, run as part of the Rowden Experimental Research Platform (RERP) at the Institute of Grassland and Environmental Research, Devon, UK. Six one-hectare Lysimeters are used to observe response to natural rainfall events in terms of overland flow, subsurface (or drained) flow, sediment flux, Total Phosphorus (TP) flux, conductivity, temperature and turbidity. Plots are equipped to monitor surface flow and drained flow in isolation, so that the effect of land drainage, common to intensively grazed land on heavy soils, can be assessed alongside undrained plots. Results are presented as time series at each plot as well as event, monthly, seasonal and annual budgets. Results indicate that significant, event-based yields of both sediments (1.18 kg ha-1 from drained plots and 3.09 kg ha-1 from undrained plots) and nutrients (3.9 g ha-1 of TP from drained plots and 9.86 g ha-1 from undrained plots) are forthcoming, as the artificial drainage influences both the hydrograph response and form to natural rainfall events. In addition, significant yields of fine particles (<0.45ìm), of both organic and inorganic origin (colloids) are observed, from both drained and undrained plots, associated with high levels of particulate phosphorus. Such results suggest that though grasslands may not suffer from `classical' erosion processes, such as rilling and gullying, they can still contribute significant levels of fine sediments and nutrients, whether they are drained or not, which in turn will make a detrimental impact on the quality of adjacent surface waters. Bilotta, G. S., and Brazier, R.E. (in press). Rethinking soil erosion and sediment and colloid transfers from intensive grasslands. Hydrological Processes. Heathwaite, A.L. and Dils, R.M. (2000) Characterising phosphorus loss in surface and subsurface hydrological pathways. The Science Of The Total Environment, 251/252: 523-538. Sharpley, A.N., Foy, R.H. and Withers, P.J.A. (2000) Practical and innovative measures for the control of agricultural phosphorus losses to water. An Overview. Journal of Environmental Quality, 29: 1-9.