Microflora Activities and Decomposition 295 



tions that are readily leachable, and 2) the more recalcitrant compounds, 

 such as lignin, cellulose, hemicellulose, pectin and starch. The first, more 

 soluble group contains approximately 25<^o of aboveground plant pro- 

 ducts, exits as leachate from moribund tissues and contains the bulk (> 

 80<^o) of plant leaf nitrogen, phosphorus and potassium. The second 

 group represents the bulk of the available organic substrate and is rela- 

 tively poor in nitrogen, phosphorus and potassium (Flanagan and Veum 

 1974, Van Cleve 1974). Substrates of the more soluble group are fre- 

 quently utilized by organisms decomposing the more resistant group. 



Microflora in all Biomes display a broad diversity of enzymatic po- 

 tential to decompose the various organic substrates. Tundra microflora 

 share this capacity, with the restriction that the cold-dominated environ- 

 ment has selected for taxa or strains that utilize these substrates under 

 cooler conditions. The restricted number of taxa may or may not reduce 

 the potential to complete a given phase of decomposition such as conver- 

 sion of cellulose to carbon dioxide, but it does reduce the number of 

 modes that such a reaction can follow in the ecosystem. 



As in many temperate zone habitats, bacteria in tundra are often 

 weak competitors with the fungi for those substrates that both groups 

 have the enzymatic potential to metabolize. The competitive difference is 

 especially obvious in such habitats as standing dead and litter, and in 

 drier surface soils. Conversely, in wet habitats bacteria play a propor- 

 tionately greater role in decomposition. In extremely wet or anaerobic 

 habitats, such as sediments of tundra ponds or soils at depth, bacteria are 

 the dominant group of decomposers. The potentials to utilize specific 

 forms of nitrogen and phosphorus are addressed in Chapters 7 and 12; 

 here we consider only carbon. 



Bacteria 



The sources of carbon most commonly exploited by soil bacteria are 

 of intermediate molecular size (Table 9-1). Large molecules such as pec- 

 tin and cellulose, which form major structural entities of plant cells, can 

 be decomposed by relatively few of the plateable bacteria. Cytophaga ap- 

 pears important among the cellulose-decomposing bacteria since it fre- 

 quently occurs on plate isolations. Enrichment studies of "most prob- 

 able number" show Cytophaga populations varying from 10' (gdw soil)"' 

 after thaw to 10* (gdw soil)"' at mid-season. No cultures of indigenous 

 aerobic bacteria that could decompose humic substances were obtained. 



The relative ability to utilize humic substances is one of the major 

 enzymatic differences that separate bacteria from fungi (cf. Table 9-1 

 and Figure 9-1). Widden (1977) has documented similar differences be- 

 tween bacteria and fungi at Devon Island. Enzymes that cleave the aro- 



