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Moore‚ 1986; Swift et al.‚ 1981; Garay et al.‚ 1986a‚ Andren et al.‚ 1993). Meentemeyer
(1978) and Berg et al. (1993) found a significant relationship between the actual evapo-
transpiration (AET) and decomposition constants (k) applicable across a climatic range
extending from arctic to tropical areas. Within individual climatic areas‚ however‚ this
relationship was only significant in the Mediterranean region (Aerts‚ 1997). In a Scots
pine ( Pinus sylvestris ) forest in central Sweden‚ the combined effects of different tem-
perature and moisture regimes explain 95-99 % of the variation in decomposition rates
(Jansson and Berg‚ 1985). This is mainly due to the inactivation of micro-organisms‚
although they may be active under much more extreme conditions than are plants and
invertebrates (Dommergues and Mangenot‚ 1970; 1980; Billès et al.‚ 1975; Legay and
Schaefer‚ Singh and Shekhar‚ 1986).
In deserts‚ decomposition processes may be severely drought limited. In an arid coastal
desert in Chile‚ Cepeda-Pizarro (1993) estimated annual biomass losses of Atriplex
(Chenopodiaceae) litter at 11 to 18 %‚ the lowest values ever recorded in litter bag
experiments.
Although temperature and moisture conditions are suitable much of the time for
near-optimal biological activity in the humid tropics‚ locations differ in the length and
intensity of their dry seasons. Furthermore‚ under comparable conditions of temperature
and rainfall‚ the type of vegetation present influences the soil moisture regime; savanna
soils are less prone to desiccation than those of forests‚ because grasslands transpire at
lower rates than forests (Anderson and Swift.‚ 1983; Lavelle‚ 1983b; Vitousek and
Sanford‚ 1986).
The effects of clay minerals and the ionic environment
Decomposition processes may be inhibited simply by the physical inaccessibility
of resources to decomposers within soil structural aggregates. This occurs (1) when
micro-organisms are included within micro-aggregates and have no contact with decom-
posing substrates and (2) when micro-organisms or organic substrates are coated with
clay minerals thereby limiting access to adjacent substrates and organisms‚ respectively.
Vitousek and Sanford (1986) showed a clear relationship between patterns of
nutrient cycling in tropical forests and soil type‚ and this ultimately depends on the
amounts and types of the clay and other minerals present. Clay minerals may directly
limit decomposition by forming coatings on organic substrates and micro-organisms.
This may also occur through the adsorption of organic molecules ( e.g.‚ on allophanes)
or perhaps by the sequestering of organic matter between the layers of phyllosilicate
clays‚ particularly the smectites and vermiculites‚ or within tactoids. This results in
the interposition of further barriers between micro-organisms and potential substrates
(Figures I.11 and IV.8a.). Consequently‚ clear inverse relationships have been
established between the abundances of certain clay minerals (of an homogeneous nature)
and mineralisation rates in soils (see‚ for example‚ Kobus and Pacewiczowa‚ 1966;
Stotsky and Rem‚ 1966; Chaussod et al.‚ 1986; Darici et al .‚ 1986; Schäfer et al .‚ 1993).
In some environments‚ clay minerals may inhibit microbial activity‚ and therefore
decomposition‚ by adsorbing enzymes (McLaren‚ 1975; Haider and Martin‚ 1980;
Sarkar et al.‚ 1989; Burns‚ 1990). Since this effect depends on a number of other factors
(Burns‚ 1990) its expression is variable and‚ in some cases‚ enzymes immobilised on
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