Limitations of microbial activity in different soil microsites in the Patagonian steppe, Argentina

Background/Question/Methods In arid and semiarid ecosystems, an indirect effect of low rainfall for microorganisms is the small amount of organic matter entering the soil.  As soil microorganisms obtain C and energy from plant debris by enzymatic degradation, these reactions could be limited by substrate or enzyme, and regulated by environmental conditions. We conducted a laboratory experiment with soils from the shrub-grass steppe in Patagonia, Argentina, examining the effects of the vegetation patches on the cellulose enzymatic degradation. Under optimal assays conditions and with an excess of either substrate or enzyme, it is possible to disentangle the relative importance of these two factors on enzymatic activity. We collected soils from below the 2 dominant species of shrubs (Mulinum spinosum and Adesmia volckmannii) and soils from microsites experimentally created by either the removal of these shrubs or the addition of a dead shrub to a bare soil area.  We determined the comparative response of soil amendments in these soils with: 1) buffer+carboximethyl-cellulose (CMC) 2) buffer+cellulase (Sigma- Trichoderma viridae) and 3) buffer (control). 

Results/Conclusions We found that the addition of available substrate (CMC), but not enzymes alone, significantly increased cellulase activity, and the magnitude of the increase depended on the microsite.  For instance, soil under Adesmia increased their activity 800% with respect to control, but soil from bare soil microsites did not respond to CMC additions.  Soils from beneath the Adesmia removal demonstrated significantly increased activity but not in soils from Mulinum removal microsites. These results suggest that cellulase activity in these semiarid soils are primarily limited by C availability rather than enzyme concentration in soil under shrubs.  On the contrary, in bare soil microsites, cellulase activity appears to be limited by both C and enzyme availability.  Results from the generated microsite suggests  that the Mulinum ‘footrprint’  was erased 3 years after removal, being more similar to bare soil, but the legacy of the Adesmia microsite still remained