What earthworm would live in a burrow like this?

That earthworms are good for soil is well known: by digesting organic material they add nutrients to soil and by burrowing they aerate it. But there are many different species of earthworm that behave in different ways and so have different effects on the soil. For example, endogeic species live in the topsoil and feed solely on soil, while anecic species liver deeper in the soil but come up to the surface to feed on plant material such as dead leaves.

So to get a full understanding of how earthworms benefit the soil, scientists really need to understand what species are actively burrowing through the soil. A European team of scientists led by Anne Zangerlé at the Centre d’Ecologie Fonctionnelle et Evolutive in Montpellier, France, have now investigated whether NIR spectroscopy can help in this regard, by determining what species of earthworm produced a specific burrow.

Because different species of earthworm have different diets, the walls of their burrows contain a different mix of organic matter. Furthermore, earthworm burrows are quickly colonised by various species of microbe, with the precise combination depending on the mix of organic matter, which is then further modified by the microbes. Thus, studies have shown that the burrows produced by specific species of earthworm possess a characteristic mix of organic matter.

Zangerlé and her team decided to see whether this characteristic mix could be detected by NIR spectroscopy, allowing it to be used to identify the earthworm species that produced a specific burrow. To do this, they collected samples of three different types of soil from fields in Luxembourg and extracted earthworms from these soils. Next, they watched as these different species of earthworms formed burrows in the soils and analysed the organic matters in the burrows with NIR spectroscopy. Using partial least squares discriminant analysis, they then developed models for predicting the earthworm species that produced a specific burrow from this NIR data.

As reported in Soil & Tillage Research, they found that these models could distinguish the burrows produced by endogeic and anecic species, and those produced by specific anecic species, but not those produced by specific endogeic species. Furthermore, they needed to develop different models for each sample of soil, in order to take account of the physical differences between the different samples. Nevertheless, they conclude that NIR spectroscopy could still make an effective tool for improving scientists’ understanding of earthworm burrowing behaviour and its effects on soil.

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