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This discussion looks at the relationship of the soil to biodiversity, at some aspects of the soil that can be managed in relation to biodiversity, and raises some catchment management considerations. Contents 1 Soil and biodiversity 2 Soil management and biodiversity 2.1 Soil process impacts 2.2 Catchment scale impacts 3 Conclusion 4 See also 5 References Soil and biodiversity Biodiversity is “the variety of life: the different plants, animals and micro-organisms, their genes and the ecosystems of which they are a part” (Department of the Environment and Water Resources, 2007). Biodiversity and soil are strongly linked – soil is the medium for a large variety of organisms and interacts closely with the wider biosphere; conversely, biological activity is a primary factor in the physical and chemical formation of soils (Bardgett, 2005). Soil provides a vital habitat – primarily for microbes including bacteria and fungi; also for microfauna such as aprotozoa and nematodes; mesofauna such as microarthropods and enchtraeids; and macrofauna such as earthworms, termites and millipedes (Bardgett, 2005). The primary role of soil biota is to recycle organic matter that is derived from the “above-ground plant-based food web”. Soil is in close cooperation with the wider biosphere - the maintenance of fertile soil is “one of the most vital ecological services the living world performs”; the “mineral and organic contents of soil must be replenished constantly as plants consume soil elements and pass them up the food chain” (Baskin, 1997). The correlation of soil and biodiversity can be observed spatially – for example, both natural and agricultural vegetation boundaries correspond closely to soil boundaries, even at continental and global scales (Young & Young, 2001). Soil management and biodiversity A “subtle synchrony” is how Baskin (1997) describes the relationship that exists between the soil and the diversity of life, above and below the ground. It is not surprising that soil management has a direct impact on biodiversity – including practices that influence soil volume, structure, biological and chemical characteristics, and whether soil exhibits adverse effects such as reduced fertility, soil acidification or salinisation. This section touches on selected soil factors that may be affected by soil management, and the according impact they can have on biodiversity. Soil process impacts Soil acidification Soil acidity (or alkalinity) refers to the concentration of hydrogen ions (H+) in the soil. Measured on the pH scale, soil acidity is an invisible condition that directly affects soil fertility and toxicity by determining which elements in the soil are available for absorption by plants. Soil acidity increases (soil acidification) by: removal of agricultural product from the paddock, leaching of nitrogen as nitrate below the root zone; inappropriate use of nitrogenous fertilizers and build up of organic matter (Slattery & Hollier, 2002). Many of Victoria’s soils are naturally acidic, however about 30,000 square kilometres or 23% of Victoria’s agricultural soils suffer reduced productivity due to increased acidity (Slattery & Hollier 2002). Soil acidification has an impact on of soil biodiversity. It reduces the numbers of most macrofauna including, for example, earthworm numbers (important in maintaining structural quality of the topsoil for plant growth). Also affected is rhizobium survival and persistence. Decomposition and nitrogen fixation may be reduced which affects the survival of native vegetation; biodiversity may further decline as certain weeds proliferate under declining native vegetation (Slattery & Hollier 2002; Hollier & Reid 2005). In strongly acid soils the associated toxicity may lead to decreased plant cover, leaving the soil susceptible to erosion by water and wind. Extremely low pH soils may suffer from structural decline as a result of reduced microrganisms and organic matter – this brings a susceptibility to erosion under high rainfall events, drought and agricultural disturbance (Slattery & Hollier, 2002) Soil structure decline Well-developed, healthy soils are complex systems in which physical soil structure is as important as chemical content. Soil pores, which are maximised in a well-structured soil, allow oxygen and moisture to infiltrate to depths and plant roots to penetrate to obtain moisture and nutrients (Aplin 1998). Biological activity helps in the maintenance of relatively open soil structure as well as facilitating decomposition and the transportation and transformation of soil nutrients. Changes in soil structure can lead to reduced accessibility by plants to necessary substances. Traditional agricultural practices have generally caused declining soil structure: cultivation, for example, causes the mechanical mixing of the soil, compacting and sheering of aggregates and filling of pore spaces - organic matter is also exposed to a greater rate of decay and oxidation (Young & Young, 2001). Soil structure is essential to soil health and fertility – soil structure decline has a direct impact on soil and surface food chain and biodiversity as a consequence. Soil sodicity Soil sodicity refers the content of sodium in the soil compared to other cations, for example calcium. Sodium, compared to other cations, tends to cause soil particles to repel rather than attract each other – soils of high sodium content are said to be sodic to be of high sodicity. Sodicity may increase if additional sodium is introduced, such as under irrigation – increasing sodicity leads to soil structure decline, hard-setting of soils in dry conditions, erosion; all conditions that are detrimental to soil fertility, health and biodiversity. Soil salinisation Soil salinisation is the concentration of salt within the soil profile or on the soil surface. Excessive salt directly affects the composition of plants and animals due to varying salt tolerance – along with various physical and chemical changes to the soil including structural decline and in the extreme, denudation, exposure to soil erosion and export of salts to waterways. Soil salinity has localised and regional effects on biodiversity, ranging for example, from changes in plant composition and survival at a local discharge site through to regional changes in water quality and aquatic life. Soil erosion Soil erosion leads to a loss of topsoil, organic matter and nutrients; it breaks down soil structure and decreases water storage capacity, in turn reducing fertility and the availability of water to plant roots. Soil erosion is therefore a major threat to biodiversity (NSW Government, 2006) Catchment scale impacts Biological systems, both natural and artificial, depend heavily on healthy soils – it is the maintenance of soil health and fertility in all of its dimensions that sustains life. The interconnection spans vast spatial and temporal scales; the major degradation issues of salinity and soil erosion, for instance, can have anywhere from local to regional effects – it may take decades for the consequences of management actions affecting soil to be realised in terms of biodiversity impact. Maintaining soil health is a regional or catchment-scale issue; because soils are a dispersed asset the only effective way is to ensure soil health generally is to encourage a broad, consistent and economically appealing approach. Examples of such approaches, as applied to an agricultural setting include the application of lime (calcium carbonate) to reduce acidity so to increase soil health and production, and the transition from conventional farming practices that employ cultivation to limited or no-till systems, which has had a positive impact on improving soil structure. Conclusion The relationship of soils to biodiversity is intimate and complex; it spans vast spatial and temporal scales and is essential to life. Soil is an asset as is biodiversity – the two should not be considered separately when it comes to protecting one or the other. Soil can be managed to optimise its fertility and health under natural and agricultural land uses, so as to benefit biodiversity. Due to the dispersed nature of the soil asset, a broad but consistent and economically appealing approach to its protection is needed. See also soil health Soil structure soil carbon References Aplin, G (1998). Australians and Their Environment: An Introduction to Environmental Studies. Oxford University Press, Melbourne. Bardgett, RD 2005, The biology of soil: a community and ecosystem approach, Oxford University Press Inc, New York. Baskin, Y 1997, The work of nature, The Scientific Community on Problems of the Environment (SCOPE), Island Press, Washington, DC Department of Environment and Water Resources, 2007, viewed June 2007, [1] NSW Government, 2006, New South Wales State of the Environment 2006, Chapter 4: Land, viewed July 2007, [2] Slattery, B and Hollier, C (2002). Impacts of Acid Soils in Victoria, A report for Department of Natural Resources and Environment, Goulburn Broken Catchment Management Authority and North East Catchment Management Authority Hollier, C and Reid, M (2005). Acid Soils. DPI AgNote April 2005. Young, A & Young R 2001, Soils in the Australian landscape, Oxford University Press, Melbourne.