A Theoretical Model of Litter Decay and Microbial Interaction

Despite the central role of microorganisms in the decomposition of dead organic matter, few models have integrated the dynamics of litter chemistry with microbial interactions. Here we propose a functional resolution of the microbial community that parallels the commonly used chemical characterization of plant litter, i.e., a guild of opportunist microorganisms that grows quickly and has high affinity for soluble substrates, a guild of decomposer specialists that grows more slowly and has high affinity for holocellulose substrates, and a guild of miners that grows very slowly and is specialized for degrading lignin. This guild-based decomposition model (GDM) includes the interactions of holocellulose and lignin, manifest as mutual feedback controls on microbial-based activities. It also includes N limitations on early stages of litter decay resulting from nutritional demands of microorganisms and N inhibition on late stages of litter decay resulting from reduced lignin degradation. Competitive interactions between microbial guilds result from different growth rates and substrate affinities, given limits on microbial colonization of litter. Simulations are consistent with commonly reported and proposed patterns of microbial community succession during litter decay, changes in and controls imposed by litter chemistry, and system responses to N availability. Modest impacts of litter chemistry and N effects on patterns of decay can yield substantial impacts on the relative amount of litter remaining through time, the time required to stabilize litter carbon (i.e., as the lignin content approaches Ìf70% of the total litter carbon), the relative contributions of different guilds to decay, and the net amount of microbial production. Moreover, seemingly inconsistent patterns in system responses to N regimes can be explained by interactions between litter chemistry and microbial guilds. A validation exercise demonstrated general correspondence of model behavior to field observations. However, relationships among mass loss, litter chemistry, and N availability were more variable in field studies than in simulations. Also, observed changes in litter quality indicated the progressive accumulation of microbial products. Hence, field studies suggest expanding GDM to include dynamics of microbial products and also suggest the utility of GDM in exploring site effects on decomposition as a result of differences in microbial community composition.