Ecosystem Functions and Soil

Biotic control over the functioning of soil ecosystems also follows the model described by Chapin et al. (1997) for universal ecosystem. Types of soil biota and their composition are likely to alter soil ecosystem processes through changes in the functional traits (Fig. 10.1). It can alter ecosystem processes, such as phosphorous transformation and availability by plants, which in turn modifies community

Land use vt

Biotic >Species composition introductions and diversity

Atmospheric composition

Biotic -afunctional -traits

Ecosystem processes

^ Regional processes

Ecosystem

Fig. 10.1 Model representing components of soil ecosystem processes (Modified from Chapin et al. 1997). It depicts biotic control over the functioning of soil ecosystem. Biotic introductions, land use and atmospheric compositions influences species composition and diversity, which controls the biotic functional traits and ecosystem processes processes. Changes in ecosystem processes can also alter regional processes such as methane emissions from rice field or gaseous nutrient transfers to aboveground ecosystems, extending the impacts beyond the original zone (Fig. 10.1). Some alteration in ecosystem diversity and regional processes change ecosystem services to plants, such as unavailability of nutrients (Nannipieri et al. 2003). Biodiversity can influence these same processes, for two reasons. First, the number of species in a community is a measure of the probability of the presence of species with particularly important traits; second, greater diversity allows a greater range of traits to be represented in the ecosystem, providing opportunities for more efficient resource use in a variable environment.

Changes in species composition and diversity will affect the functioning of ecosystems most strongly when species differ in their effects on ecosystem processes or in their response to environmental changes.

Soil is a heterogeneous mass and it acts as a microhabitat. Soils contain an intricate network of plants and microbes in a heterogeneous solid medium in which chemical and physical conditions vary at the scale of the molecule and the cell. Soil represents the largest carbon pool on the Earth's surface (2157-2293 Pg), the amount of this element being twice as high in soil as in the atmosphere and two or three times larger than the amount in all living matter (Bajtes 1996; Prentice et al. 2001). Because of the large quantity of C stored in soils, small modifications in soil C status may have a significant effect on the global C balance and therefore on climate change (Gonzalez-Perez et al. 2004). The living population inhabiting soil includes macrofauna, mesofauna, microfauna and microflora. Soil comprises a variety of microhabitats with different physicochemical gradients and discontinuous environmental conditions. Microorganisms adapt to microhabitats and live together in consortia with more or less sharp boundaries, interacting with each other and with other parts of the soil biota. A number of investigations emphasize the impact of soil structure and spatial isolation on microbial diversity and community structure (Tiedje et al. 2001; Sessitsch et al. 2001; Ranjard and Richaume 2001). Analysis of the spatial distribution of bacteria at microhabitat levels showed that more than 80% of the bacteria were located in micropores of stable soil microaggregates (2-20 mm) (Ranjard and Richaume 2001). Such microhabitats offer the most favorable conditions for microbial growth with respect to water and substrate availability, gas diffusion and protection against predation.

In their role as "engineer" of soil ecosystem processes, soil organisms perform a number of vital functions. Out of those functions, most of the activities are beneficial to plants and in turn to humans as ecosystem services. Decomposition of organic sources with high C/N ratio, availability of plant nutrients, biological N2 fixation, plant growth promotion, biocontrol and biodegradation are the few beneficial functions of microorganisms. Some of the important interactions that take place in this ecosystem are suppressing soil-borne pathogen through antagonism, synthesizing enzymes, vitamins, hormones, vital chelators, and allelochemicals that regulate population and processes, direct interaction with plants through mutualism, com-mensalisms, competition, and pathogenesis.

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