The impact of tillage farming on soil ecosystems has been likened to creating a ‘gangsters’ paradise’ by one of the authors of new research aimed at helping damaged soils recover. Based on a unique 50-year field experiment, the study shows that common farming practices such as ploughing, fertilising and adding pesticide to fields, result in a chaotic new (microbial) world order, where nitrogen-stealing archaea and killer fungi have muscled their way in at the expense of many plant beneficial fungi.

The research also found the more ‘nutritionally monotonous’ tillage soil environment has led many bacteria to ‘reduce their running costs’ by jettisoning more than 600 of the genes usually needed when faced with the diverse range of food sources found in grasslands or pastures.

This means that typical measures of soil biodiversity aren’t adequately explaining what is going on in soil - as farming not only changes the number and relatedness of the species present, but also the genetic complement of the community as a whole.

Soil sampling

Using soil samples from the long running Highfield Ley experiment, the team from Rothamsted Research in the UK and the US Department of Energy’s Pacific Northwest National Laboratory, compared arable soils with their original grassland state, as well as bare soils that have been left fallow for over 50 years.

According to lead author, Prof. Andy Neal, when grassland is converted to arable, the richness of species doesn’t change very much, but new species move in, and they don’t necessarily fill the same ecological roles.

He explained: “Following ploughing, the combination of species floating around as potential immigrants, combined with the influence of existing community members upon these newcomers, means that there is an increased randomness to the community assembly.
“And the communities are less resistant to the establishment of pathogens as a result. It’s the microbial equivalent of a gangsters’ paradise.”

Fungi types

Compared to their original grassland state, arable soils had fewer, but more varied, species of fungi.

Mycorrhizal fungi - those that form mutual beneficial associations with plants and play important roles in plant nutrition - are reduced in favour of pathogenic fungi that survive by attacking insects, plants and lichen.

The researchers also saw a greater variety of bacteria in arable soil, while the total number of species of archaea - a group of single cell organisms (members of which generate the greenhouse gas (GHG) nitrous oxide as a by-product of ammonia oxidation) – also increased in response to fertilisation. The results also show that the responses of these three different types of organisms varied markedly, depending on the physical and chemical challenge presented by farmers.

Effect of ploughing

According to Prof. Neal, ploughing seems to increase the randomness in which communities of bacteria and archaea assemble within soil - whereas applying fungicide leads to more consistent assembly of fewer, presumably resistant, fungal species. He commented:
“Farming practices cause physical disruption and alter the nutritional inputs, which means less diverse plant materials and more readily available nitrogen to soil.
“As a result, some species lose out, allowing new ones, often with very different ways of making a living, to thrive. “Even those that survive have had to change the way they live their lives," he concluded.