Emissions from soil of the potent greenhouse gas (GHG) nitrous oxide (N2O) can be drastically reduced by adding carbon-rich manure, new research shows.
Doing so essentially ‘locks-in’ nitrogen (N) as it is no longer turned into as much N20 gas by microbes such as fungi and bacteria.
N2O is roughly 300 times more potent that carbon dioxide (CO2) as a GHG, and fertilised soils are its major source.
The gas also contributes to ozone depletion, and human-induced emissions of the gas have increased worldwide by 30% over the last 40 years – mainly through an increased use of chemical fertilisers.
Rothamsted Research
According to this latest Rothamsted-led study, arable soils receiving such inorganic fertilisers were found to retain only half the amount of N compared to soils receiving farmyard manure, with the losses mainly in the form of N20.
And the data strongly suggests this is because carbon and N are inexorably linked within soils – and to a much greater extent than ever realised.
Previous Rothamsted-led research showed how carbon plays a key role in determining the very structure of soil, and subsequently, how it functions.
Prof. Andrew Neal and colleagues found that increasing the organic matter in soils leads microbes to secrete sticky polymers that produce a well-connected network of relatively small pores.
They have now shown that it is this structural feature of soil that also determines the fate of soil nitrogen.
Prof. Neal said: “Our latest findings shows that its organic matters impact on soil structure that influences how nitrogen is metabolised in soils.
“Greater pore connectivity in carbon-rich soil allows air to circulate and means microbes are metabolising nitrogen in such a way as to reduce nitrous oxide emissions.
“This means wider application of organic matter in arable systems has the potential to reduce nitrous oxide emissions and agriculture’s contribution to climate change,” he added.
Soil nitrogen and emissions
Until now, this interplay between carbon and nitrogen in soils was not well known.
Using archived samples and new data collected from the 179-year-old Broadbalk wheat experiment, Prof. Neal’s team compared a variety of soils, including those that had received N fertiliser inputs ranging from none up to 240kg/ha/yr, and those getting only farmyard manure.
They also looked at a soil from woodland created in 1882, and a soil from another experiment under a mown pasture.
After N enters farmland soil, it ultimately ends up in one of three places – it either remains in the soil, is taken up by crops (meaning it is removed at harvest), or it is ‘lost’ from the system – such as in N20 gas or as nitrate dissolved in groundwater.
In soils with limited or no organic matter inputs, the arrangement of small, poorly interconnected pores forceS microbes such as fungi and bacteria to switch to what is called anoxic – or oxygen free – metabolism.
As a result, they produce much greater quantities of N2O rather than biomass in the form of proteins produced under aerobic, or oxygenated, conditions.
The legacy of the fertilisation regime can also be seen when comparing the microbial genomes across the different soil treatments.
Looking at the soil microbial genes involved in N metabolism, the team found two distinct groupings.
One was associated with woodland and grassland soils and comprised genes involved in helping microbes absorb N as a nutrient to build biomass. Microbial genomes from soils receiving manure were most like this grouping.
The other cluster was associated with low carbon, inorganically fertilised soils, and were mostly genes responsible for the breakdown of N compounds solely to produce ‘energy’ – which results in N20 emissions.
