Spatial heterogeneity provides organic matter refuges for soil microbial activity in the Patagonian steppe, Argentina

GONZALEZ-POLO, M.; AUSTIN, AMY T.

SOIL BIOLOGY AND BIOCHEMISTRY

Elsevier

Lugar: Amsterdam; Año: 2009 vol. 41 p. 1348 - 1351

0038-0717

The spatial heterogeneity of vegetation in arid and semiarid ecosystems has important consequences for ecosystem-level carbon and nutrient pools and their transformations. Although these ecosystems are frequently classified as ‘water-limited’, it is unclear how the spatial and temporal variability of low rainfall ecosystems could affect microbially-mediated processes in the soil. The objective of our study was to determine how aboveground spatial heterogeneity creates characteristic soil conditions that modulate conditions for microbial growth and activity in a semiarid steppe in Patagonia, Argentina. In particular, we explored how variability in microenvironmental and biogeochemical characteristics in microsites generated by the native vegetation could control extracellular enzyme activity in soils. We sampled soils beneath the dominant shrub species (Mulinum spinosum, Adesmia volckmannii, Senecio filaginoides), beneath grasses (Stipa speciosa, Bromus pictus, Poa ligularis), under mosses and in bare soil, at three times during the year. Abiotic characteristics of microsites included soil temperature and gravimetric soil water content, inorganic nitrogen, pH, total C and N. Finally, microbial biomass C and beta-glucosidase activity were assessed for all microsites and seasons. Both life form and season exerted strong and significant controls on all measured abiotic and biotic soil characteristics. Partial correlations between beta-glucosidase activity and the measured variables demonstrated consistently across seasons that extracellular organic C (EOC) was the variable that explained the highest proportion of the variance in microbial activity. At the same time, soil water content did not correlate with soil enzymatic activity. We postulate that labile soil carbon rather than water availability functions as a principal limitation of microbial activity in this semiarid ecosystem, and the distribution of this labile carbon is, in large part, determined by the patchy distribution of the vegetation. As such, changes in the patch size or relative cover of shrubs and grasses or bare soil due to land-use impacts such as grazing could have large impacts on soil microbial activity and ecosystem functioning.