Dafydd writes...

There was a very telling paper recently published in the Journal, Science in the Total Environment, where Aberystwyth University scientists warned that flooding could worsen unless liming practices on uplands are maintained.

In the second half of the 20th century an estimated 200,000ha of the Welsh uplands were converted to more productive pasture to increase livestock production and support rural communities, encouraged by government incentives and indeed grassland advisors like me at the time, largely based on work at Pwllperian Experimental Husbandry Farm in Cwm Ystwyth.

Pasture improvement

A key component of pasture improvement was raising soil pH by liming, through the application of ground limestone. Withdrawal of support for maintenance liming and economic/practical constraints have led to reductions in the lime applications needed to maintain soil pH, for the upland soils. Liming of acid soil increases earthworm activity and therefore soil macro-porosity and temporary water storage. Earthworm burrows, and the vertical systems of anecic species (earthworms that make vertical burrows) increase surface infiltration and reduce surface runoff.

Acid sensitive improved pastures are present disproportionally. It was hypothesised that, in the absence of maintenance liming, upland soils would acidify over time, anecic earthworm abundance would decline and surface water infiltration would decrease. If these responses to soil acidification were to occur over extensive areas within a catchment, there was a potential for accelerated surface runoff into river systems, especially where slope gradients are steeper.

Acidification of improved grasslands was associated with a very marked reduction in infiltration rates which in turn were closely related to declines in earthworm abundance. In addition to the role in surface water infiltration of their burrows, earthworms affect soil aggregation, porosity, and hydrology in general. Maximising earthworm activity increases soil water holding capacity.

Flood risk

The flood risk and more general hydrological implications associated with soil acidification have received little attention. The work demonstrated that reduced liming of improved upland grasslands may contribute to enhanced flooding risk by limiting rainwater capture and storage. As these grasslands cover almost 20% of upland Wales and that overall, a majority are on steeper sloping land and although there are some variations between different catchments, this is an increased flood risk factor.

Modelling of Welsh catchments under a severe climate change scenario suggest a strong seasonal change in river flows, with increased spring and reduced autumn flows. If current trends
for reduced lime applications continue, this acidification of previously limed soils may further exacerbate flood risks.

Report highlights

The report does highlight differences between the four catchments studied but if acidification proves to affect flood risk, targeted agri-environment support for liming of previously improved grassland will mitigate this risk. On top of this it would promote wider ecosystem services, including collateral water quality improvements and increased food resources for farmland birds, both especially important considerations. What this report shows is a need for a more nuanced debate on the role of well managed upland pastoral agriculture in the delivery of environmental services.