Whenever agricultural greenhouse gas emissions are discussed, the focus tends to be on cattle and their prodigious methane production. However, our soil is also a very significant source of greenhouse gas emissions, and a new study from Teagasc has revealed that these could be reduced quite dramatically through a quite simple measure.
The paper, from researchers at the Teagasc Environment, Soils and Land-Use Research Department in Johnstown Castle, Co Wexford, has just been published in Nature’s Scientific Reports journal. It concludes that increasing soil phosphorus to the recommended optimum level for grass productivity reduces fertiliser-derived nitrous oxide (N2O) emissions in intensively managed temperate grasslands.
The department’s head, Dr Karl Richards, explains the significance of N2O as a greenhouse gas. “Nitrous oxide is laughing gas, but it really is no laughing matter in this context,” he says. “Approximately 30 to 35 per cent of agricultural emissions are a result of nitrogen fertilisers applied to the soil. The other 65 per cent or so is made up of methane which comes directly from animals and the manure they produce. The N2O released from the soil is 295 times more powerful than CO2 as a greenhouse gas. A small amount of it does a lot of damage and it lasts in the atmosphere for centuries.”
That 35 per cent figure is arrived at by taking the N2O emissions volume and multiplying it by 295 to arrive at its CO2 equivalent.
Chemical products
The emissions chiefly arise as a result of the fertilisers – which can include chemical products as well as animal manures and compost – interacting with the soil biology. Bacteria and fungi in the soil break down the fertiliser to consume its oxygen and that leads to the release of nitrogen in the form of N2O.
“When I think of nitrogen fertiliser, I think of fossil fuel in a bag,” adds Teagasc research officer David Wall. “About 1 per cent of all energy consumed on earth is used to produce nitrogen fertiliser. But it produces a lot of food.”
This latest piece of research draws on other studies which have been ongoing for decades, according to Wall. “In postwar Ireland there was a big push to drain soils and apply phosphorous and lime to them to improve grass growth,” he says. “They wanted to optimise beef production on marginal soils, and phosphorous is a key element of that. An experiment started in 1965 to look at the effects of long-term application of phosphorous to the soil. That study continues and we were able to use it as a window into the past to help us to forecast the future.”
When soils become depleted of phosphorous things slow down a bit. Applying phosphorous can increase the productivity of soils
The study also uses an existing intensive grassland phosphorus fertiliser trial established in 1995 to investigate the effect of long-term phosphorus management on N2O emissions. “These long-term studies which were set up with the aim of increasing productivity in food production have come of age now,” Wall adds.
Richards explains the overall importance of phosphorous: “It is essential to all biology. It is the power reactor in all living cells.”
Key component
It is a key component of adenosine triphosphate (ATP), a molecule found in the cells of living organisms which transports the energy necessary for all cellular metabolic activities. “When soils become depleted of phosphorous things slow down a bit. Applying phosphorous can increase the productivity of soils.”
But phosphorous is not a naturally occurring element in Irish soils. “One thing Europe generally doesn’t have in its soils is a source of phosphorous,” Wall points out. “The soil needs an exogenous source of phosphorous and it is usually imported from Africa, Russia and the Baltic region.”
The question is how much to apply and what benefits it can deliver in terms of productivity and emissions reductions. The 1965 and 1995 studies applied different rates of phosphorous, from zero to 45kg per hectare, which is regarded as the optimum level.
“Our study shows that there are reduced N2O emissions from fertiliser applied to soils where they have the recommended agronomic optimum soil phosphorus levels,” says Richards. “Farmers who improve soil fertility for agronomic benefits can also reduce N2O emissions. This represents a win-win for the farmer and the environment.”
The results showed that applying 45kg of phosphorous per hectare per year significantly reduced N2O emissions by 42 per cent. “The study highlights that achieving agronomic soil fertility targets can reduce greenhouse gas emissions and maintain agricultural production,” Wall points out. “It is further evidence that maintaining long-term soil fertility will help to deliver on the sustainability objectives for the agriculture sector.”