The Biology of DesertsSecond Edition by David Ward

Author:David Ward [Ward, David]
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2016-06-07T09:06:00+00:00

8.4.4 Decomposition

Many scientists have assumed that water is the primary factor that controls soil processes in arid regions, starting probably with Noy-Meir’s (1973) review. The relationship between water availability and other ecosystem processes (as well as in particular soil processes) is much less clear. For example, microbial biomass and activity in deserts is much lower on average than in mesic or humid ecosystems (Fierer et al. 2009a). However, at least two studies have demonstrated that litter decomposition (Martinez-Yrizar et al. 2007; Vanderbilt et al. 2008) does not correlate with seasonal or annual precipitation in deserts, although it is correlated with actual evapotranspiration (Meentemeyer 1978). In addition, in hyper-arid environments, carbon turnover can continue in the absence of rainfall for extended periods of time (Ewing et al. 2008), or during rainless seasons (Dirks et al. 2010). Negative or neutral responses to rainfall inputs suggests that belowground processes have unique controls which are not directly linked to the positive relationship between precipitation and aboveground net primary production (ANPP; Austin 2011).

A major component of decomposition of surface litter in deserts is by the action of termites (Holm and Scholtz 1980; Whitford 2002). Termites may consume up to half the total surface litter and most animal faeces in some deserts. Most materials consumed by termites are completely converted into CO2 and water because of the action of the symbiotic gut microflora of termites. However, there is a strong negative (not positive) correlation between termite abundance and soil organic matter (and soil nutrients) (r = −0.97) in spite of the fact that soil organic matter and nitrogen are usually positively correlated (Whitford 2002). Thus, little of the litter is converted to soil organic matter, and minerals in litter are returned to the soil in a mineralized state through animal faeces of species that prey on termites, such as lizards and spiders (Whitford 2002).

The process of decomposition of buried litter and dead roots may be very different from that of surface litter (Whitford 2002). Buried litter accumulates moisture, even in relatively dry soils. This consequently lowers temperatures and increases moisture contents, allowing microflora and microfauna to grow. Increasing populations of microflora attract grazers such as protozoans, nematodes, and microarthropods, which attract predators such as predatory nematodes and nematophagous mites (Acari). Complex food webs may be established around moist roots and buried litter. The extracellular enzymes of the microflora rapidly decompose compounds such as sugars, fats, starches, celluloses, and waxes. More complex acid-soluble compounds are slowly attacked by a small subset of microbial heterotrophs (Whitford 2002).

It is important to note the faunal:microbial biomass ratio was far lower in desert soils (faunal biomass carbon is <0.02% of microbial biomass carbon) than in the other six biomes (Fig. 8.10), where faunal biomass ranged from 1.5 to 3.6% of microbial biomass (reviewed by Fierer et al. 2009b). This may suggest that the desert biome represents a (relatively) more inhospitable environment for soil fauna than for microbes, given that a larger portion of the belowground biomass is microbial. However, a more parsimonious explanation (Fierer et al.

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