Scientists from Ulster University are to re-examine the way phosphorus (P) loads are measured in NI rivers.
The latest figures, which were published in 2020, suggest agriculture is the biggest contributor to the main water quality problem in NI.
The study, known as RePhoKUs, found agriculture was responsible for 62% of excess P in NI waterways, with 24% coming from wastewater treatment works and 12% from septic tanks.
“I have worked in all three of those areas over the past 30 years.
“I know where the assumptions are,” said Professor Phil Jordan from Ulster University.
Jordan said various assumptions are used in the calculations, including with overall P levels in waterways, known as the total P load.
Speaking to the NI Institute of Agricultural Science (NIIAS), he explained that P contributions from all other sources are calculated before agriculture.
“We add them up and compare it to the total load to get the difference.
“We think there is only one thing left over so it must be agriculture.
“The issue is agriculture is so wide and diffuse, it is difficult to measure any one particular point for its P contribution,” he said.
Jordan also pointed to a study which was carried out 20 years ago and used a different method for assessing P sources in the rivers that feed into Lough Neagh.
The study used “a balance sheet approach” for calculating P contributions and found that 59% of excess P in NI waterways came from agriculture.
“There is not much difference in terms of the agricultural contribution,” Jordan acknowledged.
However, he said the method used to estimate total P load “hasn’t changed too much” in recent decades and his team are aiming to use “more precise methods” in a new project. Jordan explained that if the current way of calculating total P load calculation is found to be “imprecise”, then the contribution of excess P that comes from agriculture will need to be reassessed.
“Of course, this could go either way” he said.
The Ulster University academic also presented an analysis of numerous studies from different countries including Ireland which found P levels in agricultural soils are linked to water quality.
“It looks like we have got a very good relationship between soil test P and dissolved P [in waterways],” he said.
He also highlighted a study from the Agri Food and Biosciences Institute (AFBI) which found that soil test P increased as P balances increase. It found that a farm P balance of 5.5kg/ha/year was optimum for good soil nutrient management and also water quality.
P balances are calculated by subtracting the amount of P that leaves a farm through the likes of milk and livestock sales from P imports such as feed and fertiliser purchases.
However, whilst previous studies have linked farm P balances to P levels in soils and waterways, Jordan said there is a clear need for more research.
“The P balance study is good science at farm scale, but it has not been repeated in nearly 10 years and there are very few of these studies on the island of Ireland,” he said.
“There are knowledge gaps on farmgate P balances. The next level of research needs to look at how does it relate to soil pH, or in different production systems. There are questions to be asked, and it’s good to challenge accepted rules” Jordan said.
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