The world’s soils are neither barren nor dead, for they constitute the habitats of one trillion times more bacteria than there are stars in the universe. Even a mere teaspoon of soil contains a million bacteria, 120,000 fungi and 25,000 algae. These microscopic organisms perform key material-cycle functions.
“Plants, animals, fungi and microorganisms purify the water and air, and ensure fertile soils. Hence the uncompromised ability of soil and water to carry out self-purification functions is crucial for drinking water abstraction. The natural fertility of the soil ensures a supply of wholesome food. These are not mechanical processes, but instead form part of a complex structure of ecological interactions Ecosystems have a high absorption capacity and ability to regenerate, but they too have their limitations.”
This passage from Germany’s national biodiversity strategy (adopted in 2007) succinctly describes the so called service functions of biodiversity. The strategy contains measures that are intended to, among other things, improve soil habitats and in so doing help to preserve these service functions. Soil is a habitat that is of incalculable importance for biodiversity. But despite years of research, we still know far too little about soil ecology, whose systems are highly complex and defy attempts at direct observation. Thanks to the advent of advanced microbiological, optoelectronic and gene-technology methods, as well as holistic soil-ecosystem approaches, advances have been made in recent years.
A gram of soil contains billions of microorganisms: bacteria, fungi, algae and protozoans. A mere one square meter of soil is home to anywhere from hundreds of thousand to millions of soil fauna such as nematodes, earthworms, mites, woodlice, springtail, and insect larvae. A hectare of soil rooting layers contains around 15 tons of live weight – the equivalent of around 20 cows. In other words, immeasurably more organisms live in the soil than on it.
Soil organism functions
Soil organisms play a highly complex role when it comes to nutrient cycling, pollutant breakdown and soil formation. The following examples illustrate the essential role played by these organisms in the humus and soil formation process:
The straw shredded by soil fauna greatly increases the area available for microbes (this process is known as pellitizing).
The fact that micro- and mesofauna such as collembola selectively feed on certain organisms ensures that they remain in an optimal growth phase.
Soil fauna keep the substrates that are relevant for microbes in a state of constant flux. For example, earthworms and other macrofauna transport nutrient-rich organic substances to lower soil substrates.
The activities of soil fauna suppress bacteriostasis (i.e. an inhibiting effect on microbes) for processes such as ecto- or endosymbiosis.
But above all, microorganisms perform essential functions in soil ecosystems, and in particular have a highly beneficial effect on substances that are key for plant growth, via the following processes:
Plant residue breakdown.
They contribute to the creation of humic substances.
They have a stabilizing effect on soil aggregates, via mucus substances.
Organic-substance mineralization and nutrient release.
They promote chemical weathering.
Fixation and release of atmospheric nitrogen such as rhizobium bacteria.
Breaking up of mineral substances; release of phytoactive substances; root-system enlargement (e.g. mycorrhiza)
Oxidation and reduction of the compounds in myriad elements such as sulfur, manganese, nitrogen and carbon
They break down biocides and other foreign matter.
Soil ecology plays a key role in natural soil functions. For example, the biological processes that unfold in soil ecosystems integrate plant residues into the soil, shred them and ultimately break them down. The upshot of these processes is the release of fixed nutrients as minerals that can be used by plants. At the same time, soil organisms also help to create favourable physical conditions in the soil. The storage and mixing of soil materials (bioturbation) in conjunction with the cementing together of soil particles through mucus secretion (revegetation) makes soil organisms instrumental for the formation of soil pore systems. Soil organisms also form stable clay-humus complexes with high water and nutrient storage capacity, and create a fine-grained, quasi erosion-resistant crumb structure. These organisms can to some extent mitigate the harmful effects of organic substances on soil, groundwater, and the food chain.
Inasmuch as soil biocoenosis involves a complex constellation of soil microorganisms, plus flora, fauna and fungi that together contribute to the performance of habitat functions, it is necessary to study these elements. Soil biocoenosis also plays a role in other soil functions, namely substance cycling and soil fertility. Hence insight into the complex ecological impact of this process can best be gained by expanding the scope of biological observation systems.
Soil organisms – an overview
As the table below shows, soil organisms are classified differently by various scientific disciplines.
These organisms occur in varying densities. However, organism and species counts only provide indirect insight into their importance in substance and energy cycles.
The quality of soil organism habitats is determined by numerous factors, which broadly speaking fall into two categories;
Local natural factors such as the following:
Microclimate (soil moisture and temperature)
Interactions within a given biotic community
The impact of human activities such as the following:
Contamination and unbalanced nutrient and energy inputs
Disruption of ecosystem communities
Habitat destruction resulting from urban sprawl and the like
Soil habitats and substance cycling functions can be characterized using various soil investigation methods, which also allow for the detection of ecological damage. These methods include collecting soil samples from open land for which the subsequent investigations mainly focus on bacteria and fungi that break down substances. The key parameters for soil microbiology are microbial biomass and soil respiration as a unit of measure for soil metabolism rates. Soil biologists also investigate the key phases of substance cycles that mainly soil microorganisms are involved in, including nitrogen mineralization and various enzyme activities. The characterization of soil ecology and its typical communities is based on individual density, biomass, and biodiversity.
Soil investigations, which are a key component of Germany’s agricultural research programs, are carried out at regular intervals for soils that are monitored over extended periods. Soil ecology characteristics can serve as an early warning system that allows for the detection of deleterious changes in the soil (Barth et al. 2000). They also help scientists to determine whether the tenets of good farming practice are being adhered to in terms of conserving or promoting biological activity. And lastly, soil ecology characteristics provide a basis for the assessment of threshold reviews for pollutants such as heavy metals and harmful organic substances, concerning soil and soil organism exposure pathways.
Associative microorganisms and symbioses
The interactions among soils, plants and soil organisms are particularly extensive in the rhizosphere (plant roots). Here, communities form that display associative interactions and symbioses. Azotobacter, clostridium, pseudomonas and other bacteria fix atmospheric nitrogen and make it available to plants as a solute. This process takes on an even more complex and specialized form with rhizobium bacteria in legumes.
Other microorganisms produce phytoeffective substances that promote plant growth and health. Fungi form a very close symbiosis with plants, in a process known as mycorrhiza – and in the case of forest trees, as ectomycorrhiza. This is because the fungal hyphae on the root surfaces take the form of what are known as Hartig nets. Fruit bodies comprise the familiar button mushrooms. Orchids and ericaceous plants produce specialized mycorrhiza fungi, which do not form a fruit body. Many herbs form, in conjunction with a fungi genus known as zygomycetes (zygote fungi), an endomycorrhiza that is not host-specific. These fungi form intra-root vesicles that act as storage organs for fungi and tree-like structures (arbuscules) and facilitate substance interchange. Fungal hyphae support the plant’s water and mineral supply more efficiently than root hairs that are only 0.1 millimeter thick. Inoculating crop plants with these fungi allows for higher crop yields and heightened plant vitality (Glante 1988).
The government’s 2007 national biodiversity strategy fails to give soil ecology its due in that for the various spheres of action, it limits itself to setting goals aimed at protecting soil organisms indirectly – one such goal being to reduce substance inputs. But the fact of the matter is that nature and species conservation, as well as the envisaged contribution to extensivation made by land use and forestry management, would also serve the cause of soil organism protection. Further research on surface (epigeal) organisms is needed in order to better describe their interactions with subsurface (endogeal) organisms. For example, spiders are the main invertebrate predators in farm areas. However, too little is known about the impact of these insects on other biocoenosis compartments.
Soil protection laws
The natural soil functions whose protection is mandated by German law are often characterized using abiotic and/or soil resource parameters. However, valid conclusions concerning the suitability of any given type of soil as a soil organism habitat can only be reached using soil ecology parameters as a basis. And even if all of the factors that determine the potential propagation of a given biocoenosis are identified, they still don’t tell us whether the biocoenosis also occurs at the site in question (Römbke et al. 2000). The protection of natural soil functions also extends to characteristic soil organisms. In determining the precautionary values for averting harmful changes in the soil, soil/soil organism exposure pathways are taken into account. Unlike the values for testing and protective measures, these precautionary values apply to all types of soil and all conservation objectives. In order to adequately protect natural soil functions, it is essential that soil organisms be factored into the equation. However, further research is needed concerning the determination of soil ecology indicators for purposes of characterizing the good ecological status of soil.
The current draft of the Soil Framework Directive gives short shrift to soil biodiversity. The directive’s framers simply take it for granted that the recommended measures (protection against soil compacting, erosion, salination, and acidification; reducing pollutant inputs) will also be beneficial for soil biodiversity, will help to meet the goals of the Convention on Biological Diversity and will prevent species loss.
The European Commission’s Thematic Strategy for Soil Protection (European Commission 2006) refers to the loss of soil biodiversity as a soil hazard. But the document also states that further research is needed in this sphere, via projects in the seventh framework program. To this end, some soil biodiversity projects and activities such as the European Atlas of Soil Biodiversity have been carried out in the EU.
Raising soil consciousness
Experience has shown that our environmental dilemma cannot be solved through laws and administrative measures alone; for it is also important to involve the general public. One good example of this is the Museum of Natural History in the UK sponsoring the first-ever nationwide census of earthworms. This census was conducted using a standardized questionnaire and a robust method for driving the worms into the open using mustard oil. The worms were then identified and counted and the results were sent to the museum. A high participation rate on the part of a random sample using a method such as this one yields a highly satisfactory outcome. Biodiversity projects based on similar methodology also exist in Germany, e.g. the annual national biodiversity day and four-day nationwide bird watching event. Other initiatives that help to raise public awareness of soil organisms include a walking path punctuated by exhibits that focus on “the soil habitats under our feet,” sponsored by the Staatliches Museum für Naturkunde Görlitz; and a soil exhibit titled Unter.Welten (Under.Worlds) at Museum am Schölerberg in Osnabruck. In the interest of raising children’s’ awareness of soil ecology, the UBA has issued a pamphlet titled “Die abenteuerliche Reise von Fridolin dem Regenwurm” [The adventures of Fridolin the earthworm], which describes soil hazards from the vantage point of an earthworm.
The Ministry of the Environment’s scientific advisory panel for soil conservation, which met from 1998 to 2003, found that the following issues need to be addressed in the field of soil ecology:
Soil organisms play an essential role when it comes to soil breakdown, formation and cycling – and are thus equally important for substance cycles and indirectly for the water cycle. Hence protecting soil ecology and assessing the damage sustained by it are inextricably tied up with the overall natural functions of the soil (as well as some of its beneficial functions); such damage should be avoided as much as humanly possible. We therefore feel that it is urgent to elaborate requirements for protecting the habitat functions of soil.
We need a paradigm for the assessment of habitat functions that incorporates not only the ecological impact of substances, but also and above all the findings of studies concerning biocoenosis distribution in the soil. To this end, reference soils should be defined whose ecological quality is monitored regularly as part of an official monitoring program.
The currently available data strikes us as being inadequate for assessing soil habitat functions for broad expanses of land. Hence in our view the next step should be to expand the scope of the existing data through, for example, collaborations between the operators of areas where soil is continuously monitored, and the Federal Institute for Geosciences and Natural Resources (Bundesanstalt für Geowissenschaften und Rohstoffe).
We furthermore propose that recommended parameters be developed for the assessment of natural soil functions and farming related soil utility functions, and that such indicators also factor in (a) the relevant physical effects (such as those attributable to major changes in the water table and soil atmosphere); and (b) the impact of nutrients and other substance flows.
UBA efforts in this domain (elaborating soil quality categories and conducting soil ecology workshops) in no way detract from the topicality of the panel’s recommendations – which should be followed up through research. One such project evaluated extensive data from the long-term soil monitoring program and zoological collections of German museums. This research allowed for the identification of characteristic occurrence patterns for some species and species groups.
The program for long-term soil monitoring does not cover the organisms normally found in soil such as beetles, spiders and mites. Recently developed methods such as DNA screening have yet to be incorporated into monitoring processes. Association of German Engineers (VDI) working groups now use soil biology parameters and standardized bioindication methods to study the impact of air pollution on soil and the impact of genetically modified organisms on soil organisms. Hence it is essential that currently available scientific data be consolidated and that the available data be evaluated with the goal of reaching sound conclusions concerning soil biology status, so as to allow for the following within the foreseeable future:
Optimization of the soil monitoring (BDF) program
Ecological assessment of soils
Recommending measures aimed at strengthening soil protection
The impact of climate change will be felt more strongly in the future – and in Germany too. This is the conclusion reached in what is called the vulnerability analysis, a comprehensive study on Germany's vulnerability to climate change.