SUMMARY

1. We use a historical context to examine the accomplishments of soil biodiversity and ecosystem research. These accomplishments provide a framework for future research, for enhancing and driving ecological theory, and for incorporating knowledge into sustainable management of soils and ecosystems.

2. A soil ecologist's view of the world differs from that of a terrestrial ecologist who focuses research primarily on above-ground organisms. We offer ‘ten tenets of soil ecology’ that illustrate the perspectives of a soil ecologist.

3. Challenges for the future are many and never has research in soil ecology been more exciting or more relevant. We highlight our view of ‘challenges in soil ecology’, in the hope of intensifying interactions among ecologists and other scientists, and stimulating the integration of soils research into the science of terrestrial ecology.

4. We conclude with the vision that healthy soils are the basis of global sustainability. As scientists, we cannot achieve our future goals of ecological sustainability without placing emphasis on the role of soil in terrestrial ecology.

Introduction

Despite the visionary appeals of an earlier generation of soil scientists, soil biologists and others (Jacks & Whyte 1939; Hyams 1952), above-ground ecologists have hitherto shown insufficient awareness of the significance and fragility of soils and the need to understand how life in soils relates to sustaining our global environment. However, many scientists, including microbial ecologists, atmospheric scientists, biogeochemists and agronomists, as well as economists and policy makers, are now starting to take heed of the multiple issues involving soils and their biota, on both local and global scales.

Soil has generally been treated as something of a ‘black box’ by ecologists. It provides the physical support for plants, and both the living and non-living components contribute to a variety of important environmental functions. These include decomposition and the recycling of nutrients, which are both key functions in terrestrial ecosystems. Other roles, such as the breakdown of pollutants and the storage of bioelements, have immense applied significance in a changing environment. Soil provides a habitat for many species of bacteria, fungi, protists and animals; it is generally recognised as a habitat that is species rich. But many questions about the ecological significance of the soil's biological diversity, and in particular how it affects ecosystem function, have never been asked. Until fairly recently this has been because the linkages between above-ground ecology, which is rich in ecological theory, and below-ground ecology, where investigation has been restricted by methodological difficulties, have not been made. It is now time to open the ‘black box’ and to start to understand how it works.

At the end of the twentieth century and with the start of the twenty-first century, efforts have been going on around the world to gain a greater understanding of the diversity of life in the soil and of the functions that these many species perform. In the UK there have been two major programmes of research on biological diversity and the function of soil ecosystems.

SUMMARY

1. The mechanistic origins and functional consequences of soil biodiversity, in terms of general principles and across a broad context, are reviewed.

2. The origins of below-ground biodiversity are discussed in terms of the spatial isolation that soil structure imparts, substrate diversity, competition and environmental fluctuation. Community structure is governed by many factors.

3. The consequences of soil biodiversity are explored in relation to the functional repertoire that the biota carries, the potential and realised interactions between components, and functional redundancy.

4. A wide variety of relationships are expressed between soil biodiversity and function. These are discussed in relation to resilience, the impact of biodiversity upon individual organisms, complexity, and above- and below-ground linkages.

5. Biodiversity per se, particularly in terms of species richness that prevails in most soils, is apparently of little functional consequence. The functional repertoire of the soil biota is considerably more pertinent.

6. Improved understanding of the relationships between soil community structure and function underpins the effective and sustainable management of ecosystems in an agricultural, forestry, conservation or restoration context. Knowledge is burgeoning and an improved understanding is following, but a unifying framework is currently elusive. Soil architecture may be the key.

Introduction

The aims here are to take an ‘underview’ of soil biodiversity within the broad context of the preceding 19 chapters, the presentations and discussions that ensued at the symposium on which this volume is based, and to discuss some additional concepts and issues that received less emphasis.

SUMMARY

1. Empirical and theoretical evidence suggests that the rate and magnitude of below-ground ecosystem processes depend on the architecture of the detrital food web. Although some species have an indisputable keystone role in determining soil processes, there is little evidence suggesting that species diversity per se has any major influence at a system level.

2. We review studies that shed light on the degree of functional redundancy in decomposer food webs – from microbes to soil fauna. As well as emphasising the need to define accurately functional redundancy (using both ‘Hutchinsonian ecological niche’ and ‘functional niche’ concepts), we also focus on features specific to soils and their communities that may affect the levels at which functional redundancy exists in detrital food webs.

3. We also explore the levels of ecological hierarchy (from species to trophic levels) at which diversity differences manifest themselves as altered ecosystem-level processes.

4. We conclude that the high degree of generalism – even omnivory – in resource-use among decomposer organisms, and the highly heterogeneous environment of soil organisms (reducing competition between species, thus allowing taxa with similar feeding preferences/environmental tolerances to co-exist), play major roles in explaining the high degree of functional complementarity found in decomposer communities.

Introduction

The accelerating loss of biodiversity in various global ecosystems (Lawton & May 1995; Lawton 2000; Schmid et al. 2002) and recent findings emphasising the close linkage between the above- and below-ground components of ecosystems (Wardle 2002) have led many ecologists to direct their interest to soil ecology and processes.

SUMMARY

1. The issue of how plant community composition affects decomposer community composition and function is considered, by reviewing recent literature and through the use of two examples.

2. It is apparent from the available literature that plant species identity exerts important effects on soil food webs, and that specific attributes such as the body size distribution of soil animals, and the relative importance of bacterial-based vs. fungal-based energy channels, respond to plant species identity. This has important implications for ecosystem functioning.

3. The first example involves below-ground effects of changes in plant community composition, such as might occur during C4 grass invasion resulting from global warming, in a perennial pasture in New Zealand. The second involves below-ground consequences of changes in vegetation community structure caused by introduced browsing mammals in New Zealand rainforest. Both examples point to above-ground, human-induced changes affecting the composition of the soil food web across several trophic levels, and key ecosystem functions carried out by the soil biota.

4. The issues of how above-ground biodiversity affects below-ground biodiversity, and the nature of reciprocal feedbacks between the above-ground and below-ground biota, are discussed. It is concluded that understanding the nature of above-ground–below-ground feedbacks may offer opportunities for better understanding how ecosystems function and the ecological consequences of global change phenomena.

Introduction

All functional ecosystems consist of explicit producer and decomposer subsystems. Producers fix atmospheric carbon, which is utilised by the decomposer organisms, and the decomposers in turn break down organic matter, which regulates the availability and supply of nutrients required for plant growth.

SUMMARY

1. In many ecological studies, soil carbon is regarded as a barely differentiated whole with little attention paid to its underlying characteristics.

2. Although it is widely appreciated that decomposer organisms are nearly infallible as degraders of organic molecules, there are marked differences in the utilisation of different components of organic matter by organisms depending on chemical and physical characteristics, location and availability in time in soil.

3. We discuss the characteristics of soil carbon as a substrate and emphasise a ‘soil metabolomic’ approach for characterising the range of molecules in complex, composite substrates, and the potential that stable isotope probing offers for linking organisms to their substrates via enrichment of their biomolecules as they exploit isotopically enriched substrates.

4. Using selected examples, we examine the influence of the chemical characteristics/quality, quantity, location and timing of supply of organic matter on the amount, activity and, where possible, the diversity of soil organisms.

5. We are some way from unifying relationships between the quality, quantity, location and timing of delivery or availability of soil carbon on the size, activity and diversity of soil organisms. However, we point ways forward in which the information on the physics, chemistry and management are linked to the biology of soils.

Introduction

Currency of soil carbon

Humans view soil carbon in various physical (e.g. aggregates, density fractions), chemical (e.g. carbohydrates, aromatic compounds), biological (e.g. microbial biomass) and even economic (e.g. dollars per tonne or carbon credits) ways which are not usually ecological.