In ‘Biological indicators of soil health’.
88, 119–127.| | Doran JW, Zeiss MR (2000) Soil health and sustainability: managing the biotic component of soil quality.
Soil health: Looking for suitable indicators
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Broos K, Macdonald LM, Warne MSJ, Heemsbergen DA, Barnes MB, Bell M, McLaughlin MJ (2007) Limitations of soil microbial biomass carbon as an indicator of soil pollution in the field.
The soil system is an open one and its health is affected by external environmental and anthropogenic pressures. The reaction of the soil system to these pressures can be described in terms of resistance and resilience (; ). Resistance is denoted by the magnitude of the change in state for a given level of perturbation. It further indicates a change in conversion ratio, for example a reduction in the respiration rate arising from compaction. Resilience describes the capacity of the system to return to its original state following perturbation and reflects the ‘self-healing’ capacity of the soil system, a concept that maps onto that of self-organization. Indeed, resilience may be a way of measuring the capacity for self-organization in soils. Some formally demonstrated examples of soil resilience are where the soil structure rejuvenates following compaction (), microbial biomass reverts to antecedent concentrations following a drying cycle () or decomposition potential is restored following a temperature perturbation (). If the perturbation is within the capacity, the soil system can recover to its original condition, but if not, a permanent loss of soil health is expected. For example, in the latter study, while the grassland soil under study was resilient to a heat perturbation, this was not the case where the soils were subjected to copper ().
Soil health: looking for suitable indicators
The ecosystem services provided by soil are driven by soil biological processes, but our concept of soil health embraces not only the soil biota and the myriad of biotic interactions that occur, but also the soil as a habitat (). The key concept here is that soil provides a living space for the biota, which is defined by the architecture of the pore networks. Indeed, it is the porous nature of soils that governs so much of their function since the physical framework defines the spatial and temporal dynamics of gases, liquids, solutes, particulates and organisms within the matrix, and without such dynamics there would be no function. The walls of soil pore networks provide surfaces for colonization, and their labyrinthine nature defines how, and to large extent where, organisms can move through the total soil volume. The enormous range in pore sizes affords physical protection mechanisms for prey from their larger predators and organic matter from microbial decomposition. Hence the capacity of the soil biota to deliver ecosystem services may be compromised not only by loss of diversity or impairment of function but also by destruction of the habitat via changes in soil structure and physical–chemical properties. Organisms aggregate the solid constituents of soil, and hence generate structure and associated pore networks. These mechanisms occur across orders of magnitude in scale and involve processes of adhesion, coating, enmeshment, particle alignment and gross movement (; ; ). Biotic activity can also degenerate structural integrity, primarily through the decomposition of organic material that, while it may be a binding agent, also represents energy-rich substrate to a predominantly C-limited biota. The community and the habitat therefore have a two-directional interactive relationship, which encompasses both feed-forward and feedback interactions between the biota and architecture of the soil. These mechanisms lead to the concept that soil may be a self-organizing system (). The capacity for self-organization can be recognized as an essential component of soil health, which relies on the presence of appropriate constituents and sources of energy to drive biological processes.
The understanding of the mechanistic basis of soil health would be greatly enhanced by quantifying the flows of energy to and between each of the four aggregate functions and demonstrating how these allocations change under different circumstances, particularly those of agricultural management. The complexity of the functional interactions makes this a major challenge. In order to do this, it would be necessary to build energy flow models for functions such as soil structural dynamics and biological control of soil-borne pests and diseases (as discussed in ) and then map them onto the established models of nutrient cycling and soil organic matter dynamics to identify the overlaps and convergences. This might prove particularly valuable in identifying keystone species or functional groups which may be susceptible to particular types of soil management. This type of argument has been used to identify organisms such as the Collembola as indicators of change in below-ground food webs ().
Biological indicators of soil health
The integrated nature and high diversity of the soil health system may contribute a significant degree of resilience under conditions of disturbance, particularly at lower (largely microbial) trophic levels. Nonetheless, the conversion of natural vegetation to agricultural land results in major changes in both physical organization and community structure in the soil, including species loss and changes in dominance among the surviving biota. This then becomes the resource with which agriculture must work and any targets should realistically be set in relation to the potential equilibria in agricultural systems rather than the natural systems from which they are derived, as has sometimes been advocated. More importantly, it is clear that subsequent agricultural practices may also impair soil health through significant impacts on the composition and structure of the soil biological community and consequently on soil-based ecosystem functions and services. Damage to ecosystem functions can arise both owing to an inadequate supply of resources (carbon, energy, nutrients or water) and through the impact of intensive substitutive practices such as continuous mechanical tillage, the use of pesticides and excessive amounts of fertilizers. These interventions may also impact on soil functions by destroying or changing the habitat of the soil organisms and their capacity to repair it.
An integrative approach is also essential for assessment of soil health. It is not feasible to assess soil health directly on the basis of its delivery of different ecosystem services. Furthermore, soil health is related to functional capacity rather than actual service outputs. As argued above, an effective approach appears to be using a set of diagnostic tests for soil system performance, chosen to be indicative of habitat condition, i.e. physical (e.g. bulk density) and chemical (e.g. pH, salinity), of energetic reservoirs (e.g. soil organic matter content) and key organisms and community structure (e.g. earthworms and phenotypic profiling). Nevertheless, we consider that this essentially reductionist method for diagnosing soil health falls short of that required to properly assess the condition of the integrated and complex soil system. While it offers the only current means to attempt diagnosis, the development of more integrative biological methods is a research priority. It is necessary to assess soil health by comparison of diagnostic test data for relevant populations of soils at the landscape scale, covering combinations of soil type and land management classes (e.g. arable and grassland). At present, there are no agreed distinct thresholds above or below which the soil can be said to be healthy or not in a definitive sense.
--Biological indicators of soil health: synthesis / C.E ..
as interpretable biotic indicators of soil health ..
What should be considered to assess the effects of use and management on soil health
Biological indicators of soil health: synthesis
‘Biological indicators of soil health’.
Role of Enzymes in Maintaining Soil Health (PDF …
Doube BM, Gupta VVSR (1997) Biological indicators of soil health ..
2 Role of Enzymes in Maintaining Soil Health …
Agricultural interventions, such as the use of pesticides, powered tillage and the use of inorganic sources of nutrients, impact upon the biological communities of soil, damage their habitats and disrupt their functions to varying extents. The link between disturbance, targeted biota and effect on function is far from linear owing to the high level of interaction between organisms and functions. The main integrating feature in the soil community is energy flow. The majority of the soil organisms depend directly or indirectly via one or more trophic levels on the processes of organic matter decomposition for their source of energy and carbon. Any disruption of this energy generating system may thus result in changes in the flow of energy and carbon to the different functions. Assessment of the relative energy allocation to different functions remains to be computed but may prove difficult owing to the second integrating feature of the soil health system, that of the probability of participation in more than one function by the same organisms. Although distinct ‘functional assemblages’ of organisms responsible for the different functions have been recognized (), a significant proportion of the soil biota may contribute to more than one function. For example, a substantial proportion of the organisms participating in functions such as nutrient cycling and soil structure maintenance are also primary or secondary agents of decomposition; while earthworms and termites can clearly be identified as major ‘ecosystem engineers’ with respect to their role in soil structure maintenance, they also contribute significantly to nutrient cycling. At the trophic levels of microbivores and predators, the crossover in function is even more evident as is apparent from food web diagrams ( and ). A third major integrative feature is that of the relationship between organism and habitat. The activities of soil organisms are influenced by the condition of their habitat in the soil, but at the same time continuously modify it. Any shift in one function is thus likely to influence others by habitat change.
Bioindicators and Biomarkers in the Assessment of Soil ..
Eti et al. (1995) examined the urinary mercury concentration and NAG excretion in 100 volunteers (18–44 years old) divided into two groups, with (66) or without (34) amalgam fillings. The authors concluded that, although there was a very small difference in urinary NAG, which probably indicates an apparent renal effect from metal absorbed from amalgam fillings, this is insufficient to be a public health hazard for renal injury. A similar study by Herrström et al. (1995) used several proteins, including NAG, as markers of renal damage in 48 Swedish volunteers. These authors also failed to detect any significant indication of renal dysfunction or damage from amalgam.
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