By Olaf Schmidt (University College Dublin, Ireland) and Maria J. I. Briones (University of Vigo, Spain)
“Let us beat our swords into ploughshares” is an evocative slogan used by peace builders around the world. However, when it comes to earthworms, ploughs are swords that can kill you and destroy your homes.
We have known for a long time that tillage operations impact large soil macrofauna such as earthworms, directly by mechanical injury and indirectly by destroying their channels and burying surface plant residues. For example, a study from Ireland showed that very intensive soil cultivation for potato production (including grubbing, destoning and ridging) can virtually eliminate earthworm populations. Many such individual studies exist around the world. For the first time, scientists have assembled all available primary research results from individual field experiments from the five continents and analysed them together in a meta-analysis, a statistical tool that allows us to look for (and quantify) common effects or trends across many independent studies.
The scientists from the University of Vigo, Spain, and University College Dublin, Ireland, extracted data from 165 publications, from across 40 countries, published between 1950 and 2016. Each of the studies investigated earthworm populations under conventional tillage (inversion tillage such as mouldboard ploughing to 25 cm depth) and other forms of reduced tillage (such as soil loosening up to 25 cm depth and no tillage).
The findings published in the scientific journal Global Change Biology show a systematic decline in earthworm populations in soils that are ploughed every year. The deeper the soil is turned, the more harmful it is for the earthworms.
Results show convincingly that most forms of reduced tillage will increase earthworm numbers and biomass. Among the five forms of reduced tillage analysed separately, the most positive effects were seen in no-tillage (direct drilling) and also superficial tillage or soil loosening <15 cm (non-inversion tillage). Another form known internationally as Conservation Agriculture (which involves retention of at least 30% of organic residues or mulching) also prompted a significant increase in earthworm populations (see figure). These reduced tillage practices are increasingly being adopted world-wide due to their environmental benefits in terms of erosion control and soil protection. They are economically attractive because not having to plough brings savings in cost, labour and fuel.
The study also analysed ecological groups of earthworms (namely epigeics, anecics and endogeic) and the most common species separately. According to the findings, the earthworm species most vulnerable to tillage are the larger ‘anecic’ earthworms that create permanent vertical burrows and feed on soil surface residues. Of all species included in the study, the nightcrawler (Lumbricus terrestris) suffered most under conventional ploughing. The small ‘epigeic’ earthworms that live in the organic litter layers of soil and convert debris to topsoil were also found to be highly susceptible.
These findings can be translated into advice for farmers in different parts of the world. Switching to reduced tillage practices is a win-win situation for farmers because they save costs and, in return, larger earthworm populations help in soil structure maintenance and nutrient cycling. The larger the populations of these beneficial soil organisms, the more of these beneficial functions a farmer will get – for free. Earthworms are also good indicators of soil quality and soil health, it is easy to check for a farmer of his/her soil is in good status by just digging up a bit of soil and checking worm numbers (there are simple guides available that show how many worms you should expect in a spade-full of soil). We know of course that there is much more life in the soil (bacteria, fungi, mites, springtails, nematode worms etc), but to study them is difficult for non-specialists. Earthworms are so useful because everybody can look for them easily.
Coming back to our opening slogan, when we speak about earthworm populations and soil protection, perhaps we should say “Let us beat our swords into ploughshares… but use them less often”. Reduced tillage practices will restore productive earthworm populations and help maintain soil structure, nutrient recycling and other biological soil functions.
Read the original manuscript here:
Briones MJI, Schmidt O (in press) Conventional tillage decreases the abundance and biomass of earthworms and alters their community structure in a global meta-analysis. Global Change Biology DOI:10.1111/gcb.13744