Cutting out soil disturbance is probably the first principle many people consider when they think of regenerative arable farming.
And thanks to the longevity of Rothamsted Research’s Broadbalk and other experiments, researchers have been able to explore the true extent of the impact that management practices, including cultivations, are having on soil biological communities.
Prof Andrew Neal, microbial genetics expert at Rothamsted, says: “Tillage and inorganic fertiliser usage is affecting microbial communities at a variety of different levels.
Damage “It’s well known there’s physical damage that happens to soil when it’s ploughed because tillage breaks up mycorrhizal fungal networks. The communities of different species also change and so do the functions of the whole community.”
However, Prof Neal’s studies have determined the ‘diet’ that is being fed to the soil in arable monocultures is promoting a different kind of bacterial and fungal communities.
“A pasture or unmanaged system will have a very rich, diverse diet of inputs – lots of different plant species and organic chemical inputs into those soils.
In a pasture with that diverse diet, most fungi make a living off the dead plant material – organisms we call saprotrophs – or ectomycorrhizal associations with living plants.
When you start to reduce that diversity to a continuous diet of winter wheat, we see that because of the nutritional monotony, pathogenic fungi that survive by attacking insects, plants and lichen become more abundant.
“Actively building a system that encourages pathogens because of a poor diet is not a particularly sensible way to go when you’re trying to grow crops. The more diverse the diet, or organic matter, the more balanced and benign the fungal communities are in the soils.”
This is why broad and diverse rotations, cover crops and organic manures play such a key role in building soils with beneficial biological activity.
It is also well known that tillage results in heightened carbon losses from soil.
The extent to which organic carbon helps create structure and control soil function has recently been explored by a collaboration of researchers from Rothamsted and the University of Nottingham’s Sutton Bonington campus who used non-destructive 3D imaging of soil by X-ray computed tomography to help deter mine the influence of organic carbon on soil structure and microbial communities.
It revealed that low carbon soils with a resultant poor structure, including conventionally managed arable soils, are associated with a greater number of genes associated with emission of the greenhouse gas nitrous oxide (N2O) and other biological processes which represent inefficient nutrient use.
Separately, the Sutton Bonington team has also studied the potential benefits to soil structure and the reduction in greenhouse gas emissions from no-till.
Their studies took place on 22 farms in the East Midlands with a range of soil textures where the soils managed under no-till practices had a higher bulk density, soil shear strength and moisture content.
Moisture The X-ray CT-measured porosity was higher in the tilled soils, as was pore size and the surface area of the soil pore system.
The no-till-managed soils contained higher levels of soil organic matter and more microbial biomass.
These factors influenced the potential CO2 and methane flux, which were both higher in tilled soils.
In contrast, N2O levels were higher from the no-till-managed soils.
As a result, the net global warming potential was significantly higher, by 31% on an area basis and 26% on a weight basis, from tilled soils than the no-till-managed soils.
The researchers say this work indicates that no-till practices could play a significant part in reducing greenhouse gas emissions and contribute to efforts to mitigate climate change.
The results also showed the length of the period under no-till management (between five and 10 years) did not significantly affect the net emissions of greenhouse gases from the no-tilled soils.
However, the reasons behind these greater carbon losses in cultivated soils are only just being understood at a fundamental level.
Prof Neal says: “We think carbon is effectively sequestered in the soil once that rich diet is broken down into very small molecules.
Those become associated with clay particles, acting as glue to stick soil together.
This is the creation of that lovely soil tilth and structure we’re all conscious of as ‘good’ soil.
“Once that structure is formed, microbes are unable to get access to the sequestered carbon and rather than being mineralised and lost to the atmosphere, it becomes locked away so microbes can’t get at it anymore.
When you till, you break down that structure and a lot of that hidden carbon becomes available again to microbial activity. Gradually, over long periods of time, organic carbon becomes less abundant in the soil, unless you act to replenish it.”
The challenge here is that organic carbon in soil is quick to lose, but slow to get back.
More than 170 years ago, as part of the Broadbalk experiment, soil previously managed as permanent wheat was effectively abandoned and allowed to revert to grazed pasture.
In the first 20 years, soil carbon accumulated relatively rapidly after ploughing was stopped.
However, as time went on, the rate of increase has gradually slowed.
“To some extent the time frame that is required to go from a highly managed, low carbon soil to one that is rich in carbon can take well over 100 years,” Prof Neal says.
Structure “It’s also likely that the microbial community and larger organisms in soil respond on similar time frames and you only get the final community structure once organic carbon has reached its maximum levels in those soils.
“Yes, in the first few years there will be a rapid change, but when we think now about regenerative agriculture, that total regeneration takes much longer than those first 20 years.
“In a farmer’s lifetime there will be appreciable changes and improvements, but really what they’re doing now will benefit whoever inherits the land after them and future society as much as it benefits them today.”