UC Davis

Bacteria and fungi, representing two major soil microorganism groups, play an important role in the global carbon (C) cycle. Despite the critical role of fungi and bacteria in C cycling, our understanding of their roles in terrestrial C cycling was still unclear. In this dissertation, I investigated the biogeography of fungi and bacteria using a synthesized global dataset of fungal (FBC) and bacterial (BBC) biomass C of 0-30 cm. We observed clear distribution patterns of FBC, BBC, and FBC:BBC (F:B) ratio along latitude and environmental gradients including mean annual temperature, mean annual precipitation, net primary productivity, root C density, soil temperature, soil moisture, and edaphic factors. Fungal and bacterial biomass C and their ratio were dominated by different factors, with FBC and BBC predominated by edaphic properties and F:B ratio determined by climates. Combining the empirical model developed for F:B ratio with a global dataset of soil microbial biomass C, we estimated global stocks of living microbial biomass C as 12.6 (6.6~16.4) PgC for FBC and 4.3 (0.5~10.3) PgC for BBC in topsoil. To mechanistically understand microbial role in terrestrial C cycling, I first parametrized the CLM-Microbe model, a microbial-explicit model with fungal and bacterial regulatory role on soil processes represented, using the compiled time-series data of FBC and BBC from nine natural terrestrial biomes. The parameterization suggested the reasonable performance of the CLM-Microbe model in capturing the seasonal dynamics of FBC and BBC across biomes. On overage, the CLM-Microbe model explained 70% of the variation in FBC (P<0.001) and 26% of the variation in BBC (P<0.05) across biomes. Sensitivity analysis showed that microbial turnover rates, biomass carbon-to-nitrogen ratio, and assimilation efficiency were the most important parameters regulating FBC and BBC dynamics. Then, we applied the CLM-Microbe model and investigated the impacts of microbial seasonality on soil C cycling in terrestrial ecosystems. Removing soil microbial seasonality reduced model performance in simulating microbial respiration and soil respiration but led to slight differences in simulating root respiration. Removing soil microbial seasonality underestimated annual averages of soil C emission (by 0.6%-32% for microbial respiration and by 0.4%-29% for soil respiration) and overestimated soil organic C content in the top 1 m (by 0.2%-7%) in natural biomes. Finally, we investigated the historical dynamics of terrestrial C fluxes and pools during 1901-2016 using the CLM-Microbe model. The CLM-Microbe model can reproduce the global distribution of gross (GPP; R2=0.78) and net (NPP; R2=0.63) primary productivity, heterotrophic (HR; R2=0.23) and soil (SR; R2=0.26) respiration, microbial (MBC), the sum of fungal (FBC) and bacterial (BBC), biomass C in the top 30 cm (R2=0.22 for FBC and R2=0.32 for BBC) and 1 m (R2=0.21 for MBC), and dissolved (DOC; R2=0.2 for 0-30 cm and R2=0.22 for 0-1 m) and soil (SOC; R2=0.36 for 0-30 cm and R2=0.26 for 0-1 m) organic C in the top 30 cm and 1 m. The C fluxes and pool sizes increased by about 30 PgC yr-1 for GPP, 15 PgC yr-1 for NPP, 12 PgC yr-1 for HR, 25 PgC yr-1 for SR, 1.0 PgC for FBC and 0.4 PgC for BBC in 0-30 cm, 1.5 PgC for FBC, 0.8 PgC for BBC, 2.5 PgC for DOC, 40 PgC for SOC, and 5 PgC for litter C (LitC) in 0-1 m, and 40 PgC for vegetation C (VegC) from 1901 to 2016. Except for DOC in the top 1 m (increased most in Asia and North America), the absolute increases of C fluxes and pools were the largest in Asia and South America, particularly in east Asia and central and northern South America. Relative changes of C fluxes and pools exhibited different spatial patterns: the relative increase was the largest in Asia and Europe, particularly in east Asia and southern and central Europe, for GPP, NPP, HR, and SR, in South America (central and east coast of South America in particular) for FBC (0-30 cm and 0-1 m), in Europe (central and northern Europe in particular) for BBC (0-30 cm and 0-1 m), in Europe (central and northern Europe in particular) and South America (east coast of South America in particular) for DOC (0-1 m), in Arica (central and southern Africa in particular) for SOC (0-1 m), and in Europe (southern and central Europe in particular) for VegC and LitC (0-1 m). Increases in GPP, NPP, and VegC were closely related to warming and climbing precipitation, while soil C fluxes and litter, microbial, and soil C pools were jointly governed by vegetation C input and soil temperature and moisture. This study advanced our understanding of fungal and bacterial biogeography and assisted our mechanistic understanding of the microbial role in the terrestrial C cycle, providing valuable insights into the C cycle research under a changing climate.