Intrigue at the Dinner Table: Non-Additive Effects in the Microbial Communities of Decaying Leaf Mixtures.

Microbial degradation of organic material is the principal conduit of the energy and carbon harnessed by primary producers into the soil food web. The decomposition trajectory of complex substrates such as leaf material is known to be dependent on litter "quality", the physical and chemical traits that affect the availability of carbon and nutrients to soil animals and microorganisms. The phenomenon of  "non-additive effects" during mixed-litter decomposition—the deviation from expected rates of mass loss based on the mean rates and/or composition of mixture constituents—marks a gap in our fundamental understanding of the mechanisms and determinants of decomposition which form the basis of soil carbon models. Recognizing that non-additive effects arise from microbial interactions associated with litter decay, we asked if positive, non-additive effects (enhanced respiration) correlated with greater microbial functional diversity. Thirty-two treatments consisting of ground foliage from 10 tropical tree species and 21 mixtures of two species added in equal weight, were incorporated into acid-washed quartz sand at 1:50 (w/w) and incubated in the dark at 28°C. Net respired CO₂ and C mineralization rates were followed over 80 days at which the microbial community was extracted from the sand:tissue treatments. Functional diversity of the extracted microbial communities was assessed using Biolog EcoPlates and summarized using the Gini coefficient of substrate utilization over seven days of the 31 carbon sources. Results showed that lower functional diversity positively correlated with cumulative respired CO₂, C mineralization rates and positive non-additive effects. Further, the carbon-utilization profiles of mixtures were also non-additive—the resulting microbial activity of mixtures did not behave as the "sum" of the phyllosphere communities constituting initial microbial conditions. While relaying information only on culturable organisms, this analysis suggests that positive, non-additive effects derive from the emergence of specialist rather than functionally diverse communities.