A new approach to identifying the farmland most likely to leach nitrogen to water could fast-track progress on reducing nitrate levels in rivers and estuaries, delegates at the EPA conference heard last week.

The Local Authority Water Programme (LAWPRO) is piloting a new method of classifying the waterbodies with the highest nitrogen load, and using that information to quickly implement on-farm actions that could reduce the level of water nitrates.

Eoin McAleer, catchments manager in LAWPRO’s midlands and east region, outlined how there is a predictable correlation between the amount of nitrogen loading in the soil in the October period when grass growth slows, soil moisture deficits fall and rainfall increases, with the nitrates levels found in the river the following spring. “So if we reduce the surplus nitrogen in our soil in October, we should see a net improvement [in water nitrate levels] the following spring,” he explained.

Breaking a river catchment into smaller subcatchments and then ranking each area by the nitrogen load could speed up the process of identifying the highest-priority areas for action.

One of the trials under way is on the Greese catchment, a tributary of the river Barrow.

Having measured nitrates levels in the Barrow estuary, the LAWPRO team then measured the nitrates loading in each of its tributaries.

They found nitrates levels in the Barrow were equivalent to about 5,000t of nitrogen being lost to the estuary. Within that, the Greese tributary accounted for 517t of nitrogen loss or around 10% of the Barrow’s total nitrogen. Within the Greese catchment, there are 10 smaller waterbodies.

The team set a target of reaching 2.6mg NO3/l nitrate, which would mean taking out 280t of nitrogen from the catchment, or the equivalent of 20kg N/ha.

Their analysis found a wide range of nitrates levels in the 10 waterbodies, ranging from 2.9mg NO3/l to 6.6mg NO3/l – all above the target of 2.6mg but some much higher than others.

On average, there was 38kg N/ha being lost from each of the 10 areas, ranging from 2.4N/ha to 70kg N/ha.

Using the EPA’s pollution impact potential (PIP) maps, they found that there were 340 farms with some land in the highest-risk category.

They then took out all the farms with less than 1ha of highest-risk land, leaving 237 farms to be addressed.

“Referring 237 farms for an ASSAP adviser to visit is too high a number,” said McAleer. “The adviser might get to 80 farms per year, and that’s a really significant number when you factor in revisits. So if I refer on 237 farms, I’m giving four years of work to an adviser.”

Narrowed down

The team then decided to narrow down their selection to farms with the highest number of hectares in the highest-risk category for nitrogen leaching on the PIP maps, categorising them according to how many hectares of high-risk land they had.

“In Greese category one, there are 12 farms and, between them, they make up 24% of all of the highest-risk land in the catchment,” he explained, compared to Greece category four, which had 125 farms making up 14% of the high-risk land for N losses.”

“Now we can focus on the farms with the highest amount of highest-risk land for N losses,” McAleer said, adding that there is a “much bigger bang for your buck” in visits from ASSAP advisers to the 12 category one farms than there is in visits to 125 lower-risk farms.