NWFP Blog on Nitrous Oxide Jun 20

Using the North Wyke Farm Platform LTE in Devon, Rothamsted Research scientists have demonstrated that nitrous oxide emissions from cattle urine deposited on soils may be lower than previously thought on certain pasture types. As Graham McAuliffe and Laura Cardenas explain, this has implications for our detailed understanding of land use and its link with climate change.

When we think of livestock’s contribution to climate change, the first image that may spring to mind is a cow belching methane. This is understandable, since methane which is produced by bacteria in a cow’s first stomach, or rumen, is indeed a huge challenge for global warming. However, there is another powerful greenhouse gas that is also associated with livestock but garners relatively little attention in the media - nitrous oxide.

Nitrous oxide is perhaps better-known as “laughing-gas”, but it is also 265 times more effective at trapping heat than carbon dioxide. For comparison, methane is 28 times more potent than carbon dioxide. Further to nitrous oxide’s significant greenhouse effect, it can also be responsible for over 40% of beef supply-chains’ carbon footprints, and it is incredibly spatially variable and difficult to measure representatively. This may go some way to explaining why it receives less attention than methane. Conversely to methane, nitrous oxide is not produced directly by livestock. It is instead produced by microbes in soil which convert nitrogen - generally applied to land as organic (e.g. manure) or inorganic (i.e. manufactured) fertiliser - into nitrous oxide through two processes known as nitrification and denitrification. These usually occur under aerobic and anaerobic conditions respectively. Whilst we understand the processes which produce the gas quite well, few studies have examined how different pasture-types may affect nitrous oxide generation.

A team of scientists from Rothamsted Research therefore set about improving our understanding of how the most commonly adopted pastures in the UK differ in terms of soil-based gaseous losses, a critical piece of information required as the country strives to achieve cross-sector net-zero carbon emissions by mid-century.

The study was carried out on Rothamsted’s North Wyke Farm Platform (NWFP), a BBSRC National Capability located near the town of Okehampton in Devon, which is also part of the ECT’s national network of long-term ecological field experiments. The NWFP is a unique grassland trial where three distinct pasture-types maintain 30 finishing cattle and 75 ewes and their offspring each year. The three farms, known colloquially as “farmlets”, are as follows:

• permanent pasture (PP);

• white clover and high sugar perennial ryegrass mixed sward (WC);

• high sugar perennial ryegrass monoculture (HS).

On each of the farmlets, plot experiments were established to allow the Rothamsted team to measure nitrous oxide emissions resulting from soil amendments of cattle urine + nitrogen fertiliser, cattle dung + nitrogen fertiliser and inorganic nitrogen fertiliser applied on its own. The dung and the urine applied to each of the pastures were collected from animals fed the same pasture, maintaining the nutrients within each system. As a legume, white clover fixes (extracts) nitrogen from the atmosphere, making it available for plants, thus allowing farmers to reduce or eliminate nitrogen fertiliser applications. High sugar grasses, on the other hand, replace a portion of protein in forage with water soluble carbohydrates, thereby lowering concentrations of nitrogen in livestock excreta and improving the balance between nitrogen and energy.

Whilst the team did indeed find that cattle on the HS system excreted less nitrogen than their PP and WC counterparts, this reduction did not translate into lower emissions. Across most treatments examined, the HS system generated the largest cumulative nitrous oxide emissions. It is possible that the higher carbon, sourced from higher levels of carbohydrates, in the HS farmlet provided the soil microbes with substrate to increase the production of nitrous oxide (despite the fact that HS animals had the lowest levels of nitrogen in their excreta). We found that microbial genes associated with nitrous oxide production were indeed in highest abundances in the soils under HS compared to PP and WC. Nevertheless, there was an observed complementarity between white clover and high sugar grass that merits further investigation, given the multifaceted benefits of both forages combined.

What do these findings mean for agriculture in general? We summarised the implications of our findings for farmers in a recent press release: “Although white clover is unlikely to be a 'silver bullet' for agriculture’s net-zero ambitions on its own, adopting combinations of multiple emissions-abatement interventions, such as increasing legume-inclusion in pasture compositions and utilisation of ‘low-carbon’ fertilisers, will be essential to maximise farming’s national and international contribution to a cooler planet.”

Additional Information

Further detail on the history and progress of the experiment is available on ECT’s North Wyke Farm Platform webpage.