Potential effect of cheating for substrate by microbial guilds during extracellular enzyme-catalyzed decomposition of plant litter
Discrepancies between empirical decomposition data and model predictions are often attributed to poorly understood or inadequately modeled phenomena related to microbial physiology and community ecology. Guild-based carbon decomposition models incorporate an important assumption: organisms producing extracellular enzymes (“investors”) take up all of the simple compounds released by extracellular enzyme activity. In contrast to this assumption, taking up degradation products without contributing to the extracellular enzyme pool (“cheating”) has been shown to be a successful strategy in experiments and models of microbial populations. We modified the Guild Decomposition Model to incorporate the impact of microbial cheating on leaf litter decomposition under varying conditions of resource limitation. The basic GDM includes competition of three microbial guilds (functional groups) competing for three classes of plant biochemicals, ranging from labile to recalcitrant and energy-inefficient. In our simulations we also included no cheating, obligate cheating, and opportunistic cheating as additional features of the microbial guilds. These strategies were simulated during growth on leaf litter with low and high lignin and nitrogen contents.
Simulations run over a 100-year time frame illustrated that cheating strategies had variable impacts on the biomass of microbial decomposer guilds and carbon decomposition. The fate of lignin carbon pools was closely linked to the Guild 3 (“miner”) decomposition guild. A general trend regarding nitrogen was a decrease in miner biomass associated with high N availability, and an increase associated with low N availability. Guild 3 biomass was greater under conditions of high LCI than conditions of low LCI. However, under both LCI levels, as the amount of carbon consumed by opportunistic cheating increased, there was a consistent and large suppression of the miner biomass guild. This was accompanied by stabilization of the Guild 1 biomass that included the opportunistic cheaters. In contrast, when Guild 1 included obligate cheaters, Guild 3 was stabilized under all conditions except high N. The total proportion of carbon remaining after 100 years was greater with decreased levels of Guild 3 miner biomass (associated with increased levels of cheating by Guild 1). This indicates that cheating may be an important factor in longterm soil carbon dynamics that has been overlooked despite increasing emphasis on enzymatic drivers of decomposition in soil carbon models.