Functional Consequences of Solving Elemental Imbalances

Currently, most microbially-explicit biogeochemical models use flexible carbon-use efficiency (i.e., overflow respiration) to balance the mismatch between microbial biomass and litter stoichiometry (e.g. carbon : nitrogen, C:N). However, other known mechanisms might lead to different biogeochemical outcomes. Here we perform a rigorous test of the functional consequences of several mechanisms that aid in solving this mismatch. We used an individual-based, trait-based leaf litter decomposition model that represents microbial functional groups by uptake and extracellular enzyme genes. The original model incorporates overflow respiration and flexible biomass stoichiometry as mechanisms to solve elemental imbalance. We further introduce a novel mechanism of enzyme allocation. We established 4 simulation treatments: overflow, overflow + flexible stoichiometry, overflow + enzyme allocation, and overflow + flexible stoichiometry + enzyme allocation. In each treatment we manipulate initial litter C:N from 10 to 90. We also manipulate the initial community to yield scenarios with high and low functional redundancy based on the number of polymers each “taxon” can degrade. We found that biomass production was greatest when all mechanisms were in operation, followed by enzyme allocation, flexible stoichiometry, and overflow being the lowest. This pattern inverted in the low redundancy scenario. Total respiration decreased with higher litter C:N but was greater for flexible stoichiometry and lowest for enzyme allocation. When enzyme allocation was present, mass loss and nutrient mineralization were consistently decreased. As suggested by other studies, carbon-use efficiency remained high when having alternatives to overflow. This, however, occurs only in the low redundancy scenario. We conclude that current microbially-explicit biogeochemical models might be overestimating carbon losses for high C:N substrates due to an unrealistic increase in respiration rates by overflow. We urge for the quantification of these mechanisms in natural systems.