The Mediterranean climate has two clear seasons—the cool wet winter growing season, and the hot dry summer, which routinely experiences 6 months or more without rain and is routine in southern California. Microbes survive and biochemical processes continue even during the driest parts of the long summer. Biofilms, or extracellular polymeric substances (EPS), are thought to be an important means for microbes to survive through physically stressful times, (i.e. drought). Do EPS concentrations increase with the length of the dry season? Do EPS concentrations vary with different levels of carbon (C) inputs? We hypothesize that drier soils will have greater microbial EPS due to the amplified need for survival; additionally, soils with higher C inputs will have more C to allocate to EPS production, but may be dominated by plant produced EPS.
To answer these questions, we manipulated plant cover and dry season length and measured EPS in seasonally dry grassland soils and evaluated pools of total EPS in the soils as well as the mix of sugars making up EPS. Soil cores were collected monthly from our research plots to capture the transition from the dry dormant summer to the wet winter growing season, from July 2014 to February 2015. Because EPS are largely made up of sugars, we used extractable sugar residues as a proxy for EPS and we analyzed them using Gas Chromatography coupled with Mass Spectroscopy (GC-MS).
The GC-MS data shows a significant decrease in sugar concentrations with increased moisture across all sample dates. Drier soils show greater accumulation or production of EPS, which supports our hypothesis. Plant removal does lessen EPS accumulation or increase consumption and drive overall concentrations down slightly.
We conclude that after subjecting the soils to a range of dry season length treatments, there were reductions in EPS production with moist conditions. However, these changes were not as drastic as we expected thus suggesting that other microbial survival mechanisms may be involved.