Evaluating the roles of microbial functional breadth and home-field advantage in leaf litter decomposition

Soil biota are increasingly recognized as a primary control on litter decomposition at both local and regional scales, but the precise mechanisms by which biota influence litter decomposition have yet to be identified. There are multiple hypothesized mechanisms by which biotic communities may influence litter decomposition-for example, decomposer communities may be specially adapted to local litter inputs and therefore decompose litter from their home ecosystem at elevated rates. This mechanism is known as the home-field advantage (HFA) hypothesis. Alternatively, litter decomposition rates may simply depend upon the range of metabolic functions present within a decomposer community. This mechanism is known as the functional breadth (FB) hypothesis. However, the relative importance of HFA and FB in litter decomposition is unknown, as are the microbial community drivers of HFA and FB. Potential relationships/trade-offs between microbial HFA and FB are also unknown. To investigate the roles of HFA and FB in litter decomposition, we collected litter and soil from six different ecosystems across the continental US and conducted a full factorial litter x soil inoculum experiment. We measured litter decomposition (i.e. cumulative CO2-C respired) over 150 days and used an analytical model to calculate the HFA and FB of each microbial decomposer community. Our results indicated clear functional differences among decomposer communities, that is, litter sources were decomposed differently by different decomposer communities. These differences were primarily due to differences in FB between different communities, while HFA effects were less evident. We observed a positive relationship between HFA and the disturbance-sensitive bacterial phylum Verruomicrobia, suggesting that HFA may be an important mechanism in undisturbed environments. We also observed a negative relationship between bacterial r versus K strategists and FB, suggesting an important link between microbial life-history strategies and litter decomposition functions. Microbial FB and HFA exhibited a strong unimodal relationship, where high HFA was observed at intermediate FB values, while low HFA was associated with both low and high FB. This suggests that adaptation of decomposers to local plant inputs (i.e. high HFA) constrains FB, which requires broad rather than specialized functionality. Furthermore, this relationship suggests that HFA effects will not be apparent when communities exhibit high FB and therefore decompose all litters well and also when FB is low and communities decompose all litters poorly. Overall, our study provides new insights into the mechanisms by which microbial communities influence the decomposition of leaf litter. Read the free Plain Language Summary for this article on the Journal blog.