Scientists have warned that flooding could worsen unless liming practices on uplands are maintained, and agri-environment support could help mitigate the risk.
New research shows that if steep improved pastures are allowed to become acidic, earthworm populations will decline, so reducing the capacity of the land to infiltrate and store rainfall.
The report suggests that targeted agri-environment support for liming of improved grassland could mitigate the flooding risk and promote wider benefits, including better water quality.
In the second half of the 20th century large areas of the UK uplands were converted to more productive pasture to increase livestock production, encouraged by government incentives.
This practice was particularly extensive in Wales where just over 200,000 hectares, almost 20% of the uplands, is classed as improved pasture.
A key component of pasture improvement was making soil less acidic, commonly through the application of ground limestone.
Liming of acid soils increases earthworm activity, which means the land can soak up more rainwater.
Withdrawal of support for maintenance liming, along with economic and practical constraints, has led to less liming, which is needed to counter soil acidity in the UK, particularly in upland soils.
Dr John Scullion, from Aberystwyth University, led the new study in collaboration with the Environment Systems Ltd Consultancy.
He said: “We are seeing more frequent intense rainfall events due to climate change with a resulting increase in flood risk.
"Previous research into land use impacts on flood risk has focussed on soil compaction and afforestation. Here we demonstrate that soil acidification may represent a further risk factor.”
Dr Scullion added that in the past, farmers received payments for regularly liming of their land to improve its agronomic potential.
The cessation of such grants and pressure on farming economics meant that liming became less prevalent, he said, particularly on the steeper slopes in the Welsh uplands, but also elsewhere in the UK.
"Some fields used in the present study had not been limed for 30 years with limited earthworm populations and poor water infiltration," Dr Scullion said.
“Our research highlights that on previously improved grasslands, targeted liming to promote healthy earthworm populations may be a practical tool that farmers can use to contribute towards this important flooding ecosystem service.
"We need to better understand the role of steep sloping grasslands in upland catchments and their management in mitigating flood risks."
Responding to the research, NFU Cymru President Aled Jones said the union had long argued that there should be an element of public support for liming.
"The benefits of liming to soil health and productivity are well known but this very important study highlights yet another benefit of major consequence as we strive to design policies that address flood alleviation."
