Microbial Response to the Addition of Glucose and 14C-[U]Glucose in Western Australian Soils.
F.C. Hoyle1, D.V. Murphy1, and P.C. Brookes2. (1) Univ of Western Australia, 35 Stirling Highway, Mailbag M087, Crawley, Australia, (2) Rothamsted Research, Harpenden, Hertfordshire, United Kingdom
Soil microbial activity is often limited by the absence of readily available carbon (C) based substrates. Addition of a range of soluble organic substrates to soil has been shown to either accelerate or constrain the rate of CO2-C evolution. This has previously been attributed to either (i) a priming effect resulting in altered turnover of the microbial biomass and/or non-living components of the soil organic matter, or (ii) the activation of a component of the microbial population where endocellular-C reserves are utilized to maintain a state of metabolic alertness in anticipation of a forthcoming food event. Rain-fed grain production systems in Western Australian are typified by 7 months of active crop growth (average growing season rainfall 200-400 mm) and 5 months summer fallow (0-100 mm rainfall). Annual organic matter inputs are small due to relatively low production levels (e.g. average wheat production 2 t ha-1) and comprise largely of plant roots/exudates and standing stubble after harvest. Under these conditions, C and water availability are often limited and the microbial community remains dormant for extended periods. The aim of this study was to investigate the capacity of the microbial population to become activated in response to small additions of glucose-C (10-50 µg C g-1 soil) in three arable soils, either amended or not with cellulose. An immediate CO2-C release between 0 and 69 h (equivalent to a maximum 59% of glucose-C applied) was observed and was attributed to microbial decomposition of applied glucose-C. A subsequent experiment using 0.01 µg 14C-[U]glucose g-1 soil also demonstrated a transitory peak in activity (14CO2-C evolved) within 43 h of application which appeared consistent with non-labeled glucose-C responses. However, only half of the total additional CO2-C respired after the application of 14C-[U]glucose was recoverable as 14C-CO2. We found no evidence of an immediate release of ‘extra' C on application of glucose-C to soils that could be attributed to a physiological response by microorganisms in readiness for a forthcoming food event. However, the subsequent pattern of CO2 release from soil did indicate the utilization of an alternate C source as would be consistent with a priming effect. Two further phases of microbial activity observed in cellulose-amended soils were attributed to either the activation of different microbial populations in cellulose amended soils on addition of glucose-C or the end-product inhibition of cellulase activity. A rapid decline in the rate of 14C-CO2 evolved after 43 h resulted in the mineralisation of approximately 59% of the applied 14C-[U]glucose after 2500 h. At the end of the incubation (2500 h), 2.7% of the remaining 14C-[U]glucose was contained in the microbial biomass and 3.1% was present as 0.5M K2SO4 extractable C. Thus 35% of the 14C contained in the original glucose solution was estimated to have been assimilated into the stable Soil Organic Matter (SOM) pool. Consideration of the impact of applying trigger molecules to soil needs to be assessed with respect to potential priming of soil organic matter decomposition in soil systems where C sequestration is desirable.