Getting soil nitrogen to growing roots cuts farm pollution

Farmers could significantly reduce agricultural emissions by getting the soil's nitrogen-cycling processes to work together more closely, new research shows.

A new study published in Nature Communications, co-authored by researchers at the University of Reading, found that replacing synthetic fertiliser with organic sources such as manure, crop residues and biochar cut nitrous oxide emissions by an average of 21.8% and  ammonia emissions by 40.6%.

The international team analysed 2,235 comparisons from 210 experimental sites worldwide. They found that the size of the benefit had little to do with individual processes speeding up or slowing down. What mattered far more was how closely the different stages of the nitrogen cycle –  how nitrogen is released from organic matter, taken up by soil microbes, converted between different forms in the soil, and ultimately returned to the atmosphere. These processes are known as mineralisation, immobilisation, nitrification and denitrification.

The researchers adapted a concept called "synchrony", borrowed from community ecology, to measure this coordination. When nitrogen processes fall out of step, intermediate forms of nitrogen build up in the soil and escape into the atmosphere as nitrous oxide, a potent greenhouse gas, or ammonia, a major driver of air pollution.

Professor Zhaolei Li (Northwest A&F University, China), senior and corresponding author of the study, said: “The key conceptual advance is to treat the nitrogen cycle as an interconnected system rather than a collection of separate processes.

“We adapted the idea of ecological synchrony to quantify that coordination, and found that it gives us substantially more information about reactive nitrogen losses. This could provide a new way both to improve models of agricultural emissions and to evaluate management practices.”

'Nitrogen production line'

Professor Simon Willcock, co-author of the study from the University of Reading, said: "This changes the question we need to ask. Rather than just asking how quickly each individual nitrogen process is happening, we need to ask how well the whole system is working together.

"You can think of the soil nitrogen cycle like a production line. If one stage runs much faster than the next, material starts to accumulate, and that excess nitrogen can be lost to the atmosphere.

"For agriculture, the message is not simply 'replace synthetic fertiliser with organic fertiliser'. It's that we should manage nitrogen in ways that help the whole soil system work better together."

Modelling based on global climate, soil and management data suggested the biggest gains from improved synchrony would be seen in European croplands, where nitrous oxide emissions could fall by around 40.7%, and in African croplands, where ammonia emissions could fall by around 46.1%.

The team say the findings could help improve models used to predict agricultural emissions, and support more targeted, locally tailored approaches to nitrogen management.