Here’s a great article on the Chemical and Engineering News site about worms; the positive effects they have on soil structure and plant growth and the negative effects they seem to be having on greenhouse gas emissions (edit. since this article was researched it has been put behind a pay wall).
Soil Science Professor Josef H. Gorres from the University of Vermont points out some of the positive effects the digestive process of worms have on soil, including that they:
- Speed up the decomposition of and release of nutrients from wood and leaves;
- Excrete waste rich in organic material (worm castings), similar to compost;
- Castings provide bio-available Nitrogen, Phosphorous and Potassium at rates of roughly 3%, 1% and 1% of weight respectively; and
- Aerate the soil, improving soil structure.
Seeing worms working their way through your soil should make you feel pretty good about the health of your soil. A study cited in the article found that, in general, crop yield increased with increased earthworm activity.
However, worms do seem to have a hidden dark side, uncovered in the article with reference to a simulation study published in Nature. The study looked at no-till agriculture systems, generally considered to be a means of reducing greenhouse gas emissions in agriculture, and looked at the effect of the increase in earthworm activity that results from soil not being disrupted under such a system.
The researchers found that the addition of earthworms to a no till model increased greenhouse gas emissions by 31-42% compared to a no-till model with no worms. In comparison, worms increased emissions by only 7-16% when added to conventional tilling models.
Figure Global Warming Potential (GWP) under no till (NT) and conventional tillage (CT) without worms (eg NT0), after the addition of L. rubellus (eg NTr); A. caliginosa (eg NTc) and both types (eg NTrc) worms after 750 days.
The discussion in the article points out that a study like this shows that although no till systems demonstrate reduced greenhouse gas emissions compared to conventional tilling, the resultant increase in earthworm presence in no till systems significantly reduces the size of that benefit.
Further, the researchers state that when this finding is coupled with reduced crop yield from no till farming, the nett outcome may be equivalent global warming potential for equivalent crop yield under either system.
The assertion about crop yield is referenced to a letter in the journal Nature. In an earlier Legume Laboratory article I wrote about a Journal of Agricultural Science paper on the same issue which found that while on average yields from no till systems were less than conventional tillage, the difference lessened the longer the no till system was in practice. It also found that the yield effect differed for different crops. The Nature letter made similar findings and commented further that if no till practices are combined with crop residue retention and crop rotation practices in dry climates, crop yield can be increased compared to conventional tillage.
Therefore, although worm activity may increase greenhouse gas emissions and no till systems increase the number of worms living in the soil, it may be that for some crops or in some climate conditions the long term effect of no till farming is more advantageous than conventional cropping.
It seems that a similarly conducted study (or studies) comparing worm activity and greenhouse gas emissions under the two cropping systems but also:
- under different climate conditions; or
- with different crops; or
- with different crops in different climates
could provide greater clarity on whether no till farming has the ability match or better yield in some circumstances while also decreasing greenhouse gas emissions.