Earthworm Engineers

Written by Eliza Jenkins Community Development at Sectormentor for Soils. Eliza helps to nurture the community at
Sectormentor for Soils – a system that enables you to monitor and assess soil health on your farm.

Earthworm Engineers #1: Ecosystem Services

Many fascinating papers on earthworms have recently been open access to online readers of the European Journal of Soil Science, so we chose four of our favourites to summarise into a series on the on-farm heroism of earthworms.

#1: A review of earthworm impact on soil function and ecosystem services

First, this comprehensive review reminds us of the many reasons why earthworms are farmers’ best friends. We can separate earthworm species into three categories: surfacedwelling worms (epigeic), deep-burrowing worms (anecic), and network-creating worms (endogeic). All three of these worm types play an important role. In their soils, earthworms are considered ‘ecosystem engineers’, and they earn this title for several reasons…

First, earthworms actually create soil! Worms feed on leaf litter on the soil surface then bury the organic matter into the soil, allowing it to be mixed and decomposed, and eventually incorporated as soil organic carbon within soil aggregates. This same process also allows for nutrient cycling in the soil, which is helped by the soils’ increased surface area due to the networks of earthworm channels. In eating soil and moving it around, worms have even been shown to heal soils that are polluted, by breaking down the contamination.

The presence of earthworms improves the soil structure, as the pore network created allows for a higher ‘bulk density’ of stable aggregates. This pore network can also improve plant root penetration, and the water infiltration ability of the soil, by creating space for the water. The increased drainage and the creation of water-stable soil aggregates can also reduce runoff on farms, as well as soil erosion by up to 50%. As earthworms burrow into the soil and bury organic carbon, they also help the process of carbon sequestration – the locking up of of CO2 from the air into soil organic carbon (SOC) in the soil.

But this soil carbon can be re-released again as greenhouse gases, especially when the soil is disturbed during ploughing. The process of building up carbon in the soil is complex, and varies depending on how much organic matter is available to the worms on the soil surface. Considering all of these earthworm endeavours going on beneath our feet, it’s unsurprising that this paper finishes by reporting that the presence of earthworms has been widely shown to improve the growth of plants above ground. Now it’s clear why they’re considered the engineers of their ecosystem!

Earthworm Engineers #2: Arable Farming and Earthworm Populations

#2: Effects on populations of earthworms of different methods of cultivation and direct drilling, and disposal of straw residues

Our second installment comes is a paper that makes some really interesting conclusions about the effects of cultivation on earthworms in topsoil. They tested the number of earthworms over four years on direct-drilled fields that were sprayed with herbicide before planting, and ploughed fields (of varying soil types). They found earthworm populations were consistently greater in the direct-drilled soils compared with ploughed soils, although deep-burrowing species were affected similarly in both treatments. They also test the effect of spreading mulch on the fields compared to burning straw residue, and find (unsurprisingly) that earthworm populations were greater in fields where straw residue was spread rather than burned, particularly in surface feeding species. This surface debris becomes an important food source for the worms, and makes their diet more stable.

It is also suggested that the extra earthworm channels created under no-till soils may help to reduce any compaction in the soil, as well as distributing organic matter and facilitating drainage. The reduction in compaction is also likely due to increased plant root penetration within earthworm channels.

The three key tenets of regenerative agriculture are maintaining soil cover, minimising soil disturbance, and diversifying crop rotations. This paper presents clear scientific evidence of the positive influence of minimal cultivation, and soil cover (as mulch) on the earthworm community, and as we learnt in the first edition of our series, earthworms have a significant influence on the health of soils. All the more reason to farm with a regenerative approach and encourage our earthworm friends.

Earthworm Engineers #3: Organic vs Conventional Symptoms

#3: The impact of soil carbon management on soil macropore structure: a comparison of two apple orchard systems in New Zealand

When testing for earthworm populations, the researchers consistently found more earthworms in the organic soil compared with the conventional soil. They also reconstructed the 3D ‘macroporosity’ structure of both soils using X-rays, and again found greater macroporosity within the organic soil compared to the conventional soil. This isn’t a coincidence! Macroporosity is defined as the network of pores with a diameter of over 0.3 mm in the soil, and earthworms are known to create these kinds of channels. This increased macroporosity is important for several reasons. First, it is known to increase the rate that CO2 in the atmosphere is locked up as soil organic carbon (SOC), which both increases soil fertility and also has potential to reduce the rate of climate change.

As expected, this study then found that the organic orchard had a 32% greater SOC content than the conventional soils. Increased macroporosity also improves the soil structure, as the stability of soil aggregates is increased, which allows more microbes to live in the soil. Denitrification rates are known to increase in anoxic, water-logged soils, which leads to increased emissions of N20, a gas that contributes to climate change. As a result, increased macroporosity reduces denitrification in the soil, by allowing oxygen to penetrate into the topsoil, and reducing the chances of water logging. It’s amazing to see evidence of how organic techniques allow our earthworm friends to flourish, and how positive their presence is orchard soils.

Earthworm Engineers #4: Manure and Earthworm Populations

#4: Quantifying dung carbon incorporation by earthworms in pasture soils

This study looks at the effect of different earthworm communities on the amount of soil carbon (within dung applications) shifted into the soil. They tracked this process by labelling the carbon with isotope tracing, which is a clever technique that gives a really specific picture of where exactly the carbon is moving to. The three main earthworm types were tested in different treatments: surface-dwelling worms (epigeic), deep-burrowing worms (anecic), and networkcreating worms (endogeic).

First, the researchers found that with increasing inputs of dung, the abundance of earthworms tested also increased, presumably because the worms had a more consistent food source in these pots and could flourish.

Most of the tracked carbon was found in the soils top layer (0-75mm), although when the earthworm population included deep burrowing (anecic) earthworms, carbon from dung was often found at depths of up to 300mm, which shows just how effective these worms are at burrowing materials from the soil surface into its lower levels. The most successful treatments (with the greatest flow of dung shifted into soil organic carbon (SOC)) were those with all three types of earthworms present (epigeic, anecic and endogeic). So, a diverse population of worms is necessary for optimal dung break down into soils. In pasture soils, dung left by livestock can therefore contribute to increased earthworm populations, as well as increasing SOC.

This is important for the soils nutrient supply, and also helps to reduce CO2 levels in the atmosphere, which has potential to reduce the effects of climate change. In conventionally grazed systems, the quantity of dung deposited per hectare are less than the amounts used in this study, but it’s interesting to think about how this research adds to the evidence supporting mob-grazing systems, where livestock graze fields more intensively, and more manure is deposited per hectare as the stock moves through. With more manure available, there is potential for enhanced earthworm populations, and increased SOC content as more organic matter can be pulled into the soil.