Understanding the role of biodiversity in our soils
By: Rachel Creamer (Professor, Wageningen University, Netherlands), Dorothy Stone (Leeds University) and Paul Massey.
This article was originally published when all three authors were working at Teagasc, Johnstown Castle, Wexford, Ireland. The article was published in Organic Matters Magazine in 2013.
Soil Biodiversity encompasses a huge array of life on the planet. In some cases, 5 tonnes of animal life can live in one hectare of soil (5). The variety of soil biodiversity is also quite astounding ranging from bacteria, which are from 1-100 μm in size (i.e. completely invisible to the eye) through to the macrofauna which are on average 2 mm or larger in size and can be easily seen, such as earthworms, ants, woodlice, centipedes etc. The size of an organism is extremely important as this controls its life cycle and its impact on the soil functions. While an individual bacterium is tiny, it fits into minute spaces and there can be 3,000,000 to 500,000,000 bacteria present in 1 g of soil. The role of soil biota in the soil is essential for everyday functions and ecosystems services to take place such as water filtration, nutrient cycling, organic matter breakdown, development of soil structure, plant growth and pollination.
In terms of agricultural production, the intensification of management systems has led in some cases to reduced soil biodiversity due to increased mechanisation, addition of chemical based fertilisers and application of mono-culture systems. The organic farming approach acknowledges the key role of soil biodiversity in the production of food and fibre. However, in order to maintain yields in low input organic systems, every single addition needs to be used as efficiently as possible. As little of the nitrogen from a green manure, or carbon from the ploughing back in of stubble, should be lost. To ensure that none of these additions are lost, it is essential to achieve as diverse a below-ground community as possible. The more diverse the biological community i.e. more species, sizes of organisms, feeding habits, life cycles etc the more potential to capture and cycle nutrients within farm and therefore provide an added source of nutrition to the organic production system.
There are some key groups of biology which are important in the delivery of these soil functions these include; bacteria and fungi (due to the large number present in soil and their role in decomposition of organic matter), nematodes (regulation of nutrient cycling) earthworms which are the “engineers” of the soil and are responsible for the large scale movement of soil particles and organic matter in the soil that define soil structure.
The biological community structure in soils is quite similar to how all animals live on the planet. Different species live in different parts of the soil, some in water films, others in air spaces and the feeding strategies (known as a food web) are very complex. The food webs that exist below ground (a chaotic interlinked tangle of organisms that eat dead organic material and the organisms that in turn eat them), are more efficient and resilient to stresses like drought or disturbance if they have a complex structure, with many pathways for nutrients to move along as they are transformed from waste material to a form that crops can use. Within these foodwebs certain species are quite important as they feed upon other species or are fed upon by others. These species are vital for the flow of nutrients through the system. The presence of these keystone species helps us assess potential problems in the soil. In this article, we will discuss the role of nematodes and earthworms as case studies as they are considered a good bioindicators (show changes in soil health) due to their central role in foodwebs and soil structure.
Nematodes are aquatic animals and within soil (“free-living” nematodes as opposed to “marine” nematodes or “plant parasitic” nematodes), live in the thin water films around soil particles and within soil pores (1). It is estimated that there are between 40,000 and 10,000,000 species of nematodes (2) and there can be up to a million individuals per m2 in soil (3). Sometimes called “roundworms”, these small invertebrates (0.2 – 2.5 mm long) are impossible to see in soil with the naked eye, though they are visible when they are extracted into water, where they look like short thin white hairs. Nematodes are too small to affect the structure of soil in the way that earthworms can (4), but they have the potential to contribute massively to the efficiency of nutrient cycling and thus the amount of carbon, nitrogen and other important elements available for crop growth. When nematodes are present in soil at a high level of biodiversity, they contribute to the complexity of these nutrient cycling food webs. They are able to do this because they feed on a wide variety of different sources within the soil. Each species of nematode has specialised mouth parts, therefore some nematodes can eat bacteria, others feed on fungi, there are some nematode species that only eat plants and others that have the ability to eat all of these things (see diagram below for the different mouth parts associated with the different feeding types). There are also predator nematodes that prey on other nematodes and sometimes even other soil fauna such as enchytraeid worms. This ability to provide so many varied and different connections between the pathways along which nutrients are cycled mean that nematodes are an intrinsic and important group of species for maintaining soil fertility.
Earthworms are considered the farmer’s friend, as they are essential for the maintenance of soil structure. As I explained in my last article maintenance of soil structure should be one of the main considerations for any organic farmer, as it influences so many other functions in the soil such as nutrient availability, seed propagation, rooting, drainage etc. There are three main trophic groups of earthworms;
Surface living (epigeic) worms – which feed on litter and manure and break these down on or near the surface of the soil;
Night feeding (anecic) worms which have vertical burrows which come up to the surface from 30- 50 cm down, these include the well known Lumbricus terrestris species and they feed on the surface decomposing plant material which they pull down into the permanent vertical burrows;
Soil-eating (endogeic) worms – these worms make horizontal burrows by eating the soil as they move through the soil and excreting it to fill the space as they move through. The bacteria living in the guts of these worms transform the nutrients in the soil material making it more available on excretion.
While a lot is known about the role of nematodes and earthworms in our soils, much of the soil biology is not yet understood for it’s role in soil functions. We also have very little understanding about which species are present where. Therefore, there is a great need to further our knowledge on this and to understand what baseline is required for a healthy biodiversity in soils of different land use types to deliver the functions we require as a society. Teagasc is involved in a large scale European project (Ecofinders) which is looking into this and will report on the variety of soil biology found in different land-uses across Europe and how some of these species are important for the delivery of soil functions as listed above.
1: Ecology of Plant and Free-Living Nematodes in Natural and Agricultural Soil, Neher D.A., Annu. Rev. Phytopathol, 2010, 48: 371-394
2: The Role of Nematodes in Ecosystems, Yeates G.W., Ferris H., Moens T., Van Der Putten W.H., In Nematodes as Environmental Indicators, Ed Wilson J.W., Kakouli-Duarte T., 2009, CAB International, London UK.
3: SSU Ribosomal DNA-Based Monitoring of Nematode Assemblages Reveals Distinct Seasonal Fluctuations Within Evolutionary Heterogeneous Feeding Guilds, Vervoort et al., PlosOne, 2012, 7:10
4: Ecology of Plant and Free-Living Nematodes in Natural and Agricultural Soil, Neher D.A., Annu. Rev. Phytopathol, 2010, 48: 371-394
5: European Atlas of Soil Biodiversity (available in English & French)