Among the remaining key gaps in soil biogeochemistry modeling are the (i) functional partitioning of soil organic carbon (SOC) pools, (ii) representation of microbial biomass and diversity, and (iii) mechanistic coupling of carbon and nutrient cycles. Few existing test cases are typically limited to soil column scale or to effects on global soil organic carbon. We report a new soil biogeochemistry module linked with a well-tested land-surface and terrestrial biosphere model (T&C). The soil biogeochemistry module explicitly separates different litter pools and distinguishes SOC in particulate, dissolved, and mineral fractions. Extracellular enzymes and microbial pools are differentiated based on the functional roles of bacteria, saprotrophic, and mycorrhizal fungi. Soil macrofauna is also modeled. The model considers the cycles of nitrogen, phosphorous and potassium. The model is challenged to reproduce (i) global patterns of biomass resolved among belowground communities, (ii) the responses to bare fallow and litter manipulation experiments, (iii) ecosystem response to nitrogen addition, and (iv) response to burning treatments. Furthermore, the study offers new insights into the relative magnitudes of often poorly constrained quantities such as partitioning of soil respiration components among fungi, bacteria, roots, and macrofauna, and estimates of root exudation and carbon export to mycorrhizal.
Model simulations in 20 sites compared favorably with global patterns of microbial and macrofaunal biomass relations with soil organic carbon, soil respiration, and NPP. Long-term responses to bare fallow experiments and burning treatments were also in agreement with observations. Differences are appreciable for the relative abundance and productivity of bacteria and fungi in a litter manipulation experiment. Despite non-negligible site-to-site variability, fine root, bacteria, fungal and macrofaunal respiration account on average for 33%, 39%, 25% and 3% of total belowground respiration, respectively. On average, root exudation and carbon export to mycorrhizal fungi represent about 12% of plant NPP. These results offer mechanistic and general estimates of microbial biomass and its contribution to respiration fluxes and to soil organic matter dynamics. Ability to conduct virtual experiments that systematically vary effects of environmental variables on soil microbial dynamics, carbon storage, plant growth, and nutrient leaching can greatly expand understanding of belowground soil biogeochemical responses to environmental changes critical for a wide range of ecosystem services.