Catabolic profiles of soil fungal communities along a geographic climatic gradient in Israel

The great distances that typically separate different climatic zones have previously restricted the investigation of climatic controls over soil microbial function to only short-scale gradients, or alternatively, to the investigation of extreme instead of real gradients. In this study, we comparatively assessed sole carbon source utilization profiles and resultant catabolic diversity levels of soil fungal communities along a geographically continuous steep climatic gradient, stretching from the Upper Galilee in the north of Israel to the Negev Desert in the south. Four sites along the gradient were studied, representing humid-Mediterranean, Mediterranean, semi-arid and arid climate types, and they were characterized by a mean annual rainfall of 780, 537, 300, and 90mm, respectively. The relationships between abiotic soil characteristics and fungal activity parameters (basal respiration, biomass, and the metabolic quotient qCO2) as well as fungal substrate utilization profiles and functional diversity were explored in the context of climate and seasonality (rainy versus dry seasons). To make an ecologically relevant assessment of fungal activity in soil, a MicroResp™ microrespiratory system capable of analyzing whole-soil samples was modified to allow, for the first time, the testing of fungal utilization of low-solubility carbon substrates, alongside other major plant-derived substrates, reflective of the soil environment. The accurate profiling of substrate utilization in the calcareous soils comprising this Mediterranean gradient could not have been accomplished without the correction of measured CO2 evolution data, which were biased due to CaCO3–CO2–H2O equilibria reactions. The correction of the data was conducted according to a protocol presented in the preceding article of this series. The utilization potential of all 14 substrates tested was higher during the dry than during the rainy season, particularly in the Mediterranean sites, with polymeric substrates being utilized preferentially in the arid sites in comparison with the Mediterranean sites. The distinctive approach exercised in this study enabled us to make a relevant ecological interpretation, particularly in face of the prospect of fungi being responsible for the bulk of recalcitrant organic matter decomposition in soil.