Soils And Gaia Possible Mechanisms For Evolution Of The Fitness Of The Soil Environment

As was mentioned in Chapter 1, there are many positive feedback mechanisms in soils, in which organisms have arisen and/or evolved together. These include roots and arbuscular mycorrhiza (AM), and many of the genera and families of soil fauna. The following discussion is based on the very insightful and stimulating article by van Breemen (1993) entitled "Soils as biotic constructs favouring net primary productivity." Van Breemen asks the central question: Have soils merely been influenced by biota, or have biota created soils as natural bodies with properties favorable for terrestrial life? He presents five hypotheses or postulates related to the overarching theme: (1) there are soil properties "favorable" for terrestrial life in general; (2) biota, including plants and the soil dwelling organisms, are able to affect those soil properties; (3) on a scale of ecosystems and a global ("Gaian") scale, biotic action makes the outermost (1-100 cm) layer of the earth's crust more favorable for terrestrial life in general than it would have been in their absence; (4) at an ecosystem-level scale, biota tend to offset the effects of unfavorable properties of the soil or soil parent material by modifying those soil prop erties; and (5) modification of soil properties may play a role in species competition.

Following from the ideas of Odum and Biever (1984), there should be some positive or donor-recipient controls on interactions between primary producers and other biota, with the AM being a prominent example. As we have noted earlier, feeding on detrital organic matter in the soil is generally the principal energy flow in terrestrial ecosystems. Therefore feedback loops arising in the soil community (such as detrital food webs, see Chapter 6) should have a major effect on net primary productivity. Thus soil-biota interactions may be a most fruitful area to investigate and test hypotheses about positive effects of biota on the environment.

As far as favorable soil properties are concerned, changes and general improvements in soil porosities and aggregation as well as soil organic matter status are prime examples of general improvements in soil characteristics; these changes occur through cumulative interactions of the soil biota. This is not a simple linear progression however; there are examples of surface-feeding earthworms, which remove enough of the surface leaf litter material to cause a greater amount of soil erosion in their presence than in their absence (Johnson, 1990).

In the areas of soil texture and structure, as well as soil chemical properties, there are numerous examples of soil biotic interactions having a generally beneficial effect in the top meter of soil material. One example of this is provided by Gill and Abrol (1986), who described how planting Eucalyptus teretocornis and Acacia nilotica on an alkali soil (pH 10.5) markedly decreased pH and salinity within 3 to 6 years. These changes were probably caused by a suite of factors including increased water permeability, which followed the development of root channels and the accumulation of organic matter in the upper 20-50cm of the soil profile. Other biota, notably termites, can promote higher salt content in soils, as detected by measurements in inhabited and abandoned termite hills compared to the surrounding soil (de Wit, 1978). Many of these processes tend to increase the amounts of heterogeneity within soil profiles, which has been well reviewed recently by Stark (1994).

At both ecosystem and global scales, there are significant effects of biota on rock and soil weathering. The early pioneering researches of Vernadsky (1944, 1998) and Volobuev (1964) in particular originated and made popular the concept of "organic weathering." The able partnership of roots and microbes in mineral translocation is noteworthy, for example, removing the interlayer K from phlogopite (vermiculitization) within the first 2 mm of the rhizosphere. For other references on biological impacts on mineral weathering, see Schlesinger (1996). As noted in Chapter 6, the soil physical effects of earthworms on soil structure, for mation of heterogeneous pores, and high structural stability are hallmarks of soil-by-biota interactions over long time-intervals.

There are several examples of transformations that counteract unfavorable soil properties. These arise principally from the influence of the biota on translocation and concentration of nutrients in the upper 1 meter of the earth's mantle, the living soil. In general, biota tend to invest more in increasing nutrient supply under nutrient-poor than in nutrient-rich conditions. Root production and activity, as a fraction of total net primary production, tends to be higher in the nutrient-poor conditions (Odum, 1971). This should be considered against a background of the generally slow growth rates and nutrient fluxes that occur in many wild plants on low-nutrient soils (Chapin, 1980). There is also an intriguing nutrient conservation process that occurs in many low-nutrient ecosystems. Development of mor humus types, characterized by thick organic horizons, is typical for "poor" (low productivity) sites, and may represent nutrient conservation brought about as a result of the slow-to-decompose litter formed in the surface layers (Vos and Stortelder, 1988). This in turn may lead to further inhibition of decomposition and net primary production, so is an example of a positive feedback effect, which may require occasional fires or other disturbances to act as a suitable "reset" over millennial time spans. Soil phosphorus, in its various inorganic and organic forms, is perhaps the most limiting element in terrestrial ecosystems (van Breemen, 1993). Storage of phosphorus by secondary iron and aluminum phases is partly under biotic control and may be regarded as part of tight biotic cycling of phosphorus for three reasons: (1) secondary iron and aluminum oxides result from biologically mediated weathering of primary minerals; (2) the oxides are often precipitated under the influence of iron oxidizing bacteria; and (3) the oxides can be kept in a mostly amorphous form by interaction with humic substances, from which phosphorus can be extracted by plants more efficiently than from crystalline oxides.

A further development in assessment of soil genesis and ecosystem condition is a quantitative assessment of forest humus forms, on a scale ranging from 1 (Eumull) to 7 (Dysmoder), which is called the humus index (Ponge et al., 2002).

In the 72 sites studied, the humus forms were arranged as follows:

1. Eumull (crumby A horizon, Oi horizon absent, Oe horizon absent, Oa horizon absent)

2. Mesomull (crumby A horizon, Oi horizon present, Oe and Oa horizons absent)

3. Oligomull (crumby A horizon, Oi horizon present, Oe horizon 0.5 cm thick, Oa horizon absent)

4. Dysmull (crumby Ahorizon, Oi horizon present, Oe horizon 1 cm or more thick, Oa horizon absent)

5(a). Amphimull (crumby A horizon; Oi, Oe, and Oa horizons present)

5(b). Hemimoder (compact Ahorizon, Oi and Oe horizons present, Oa horizon absent)

6. Eumoder (compact A horizon, Oi and Oe horizons present, Oa horizon 0.5 or 1 cm thick)

7. Dysmoder (compact Ahorizon, Oi and Oe horizons present, Oa horizon more than 1 cm thick)

This index is well correlated with several morphological and chemical variables describing forest floors and topsoil profiles: thickness of the Oe horizon, depth of the crumby mineral horizon, Munsell hue, pHKCl and pHH2O, H and Al exchangeable acidity, percentage base saturation, cation-exchange capacity, exchangeable bases, carbon and nitrogen content, and available phosphorus of the Ahorizon (Fig. 8.9) (Ponge et al., 2002). Used in concert with the Ponge (2003) concept of humus forms as a framework of soil biodiversity, this approach should provide a more general comparative tool for assessing the chemical and biological conditions of a wide range of soil systems worldwide.

All of the foregoing perhaps raises more questions than answers. However, the general trend is for the number of species and individuals with positive effects to increase, both in successional sequences and over evolutionary time. In essence, the property of an individual that improves the environment for that individual, or increases its reproductive success, will benefit both it and its competitors as well. The selective advantage for such a trait(s) is probably small, viewed in a classical Darwinian context. If viewed in more general contexts such as enhancement of site qualities, then this can be considered a more general application of community and ecosystem development. Van Breemen (1992) notes that development of a trait in an earthworm allowing it to better control the moisture content and CO2/O2 balance of its immediate surroundings would redound to the benefit of other organisms and site properties. If requirements of plants or a plant species happen to match those of the earthworm, then coevolution of the plant and worm might be possible too. Wilson (1980) suggested that one might envision further development and evolution of a community of microbes, which could coevolve with the earthworm, to better enhance nutrient cycling processes. This is an evolutionary example of significant processes at "hot spots," as noted in Chapter 6. The scenario is speculative, but serves as an example of where we may be expecting to see additional breakthroughs occurring in the cryptic and fascinating world of soil ecology.

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