Carbon dioxide (CO2) in the atmosphere is fixed by plants (or autotrophic microorganisms) and added to soil through processes such as (1) root exudation of low-molecular weight simple carbon compounds, or deposition of leaf and root litter leading to accumulation of complex plant polysaccharides. (2) Through these processes, carbon is made bioavailable to the microbial metabolic “factory” and subsequently is either (3) respired to the atmosphere or (4) enters the stable carbon pool as microbial necromass. The exact balance of carbon efflux versus persistence is a function of several factors, including aboveground plant community composition and root exudate profiles, environmental variables, and collective microbial phenotypes (i.e., the metaphenome). Figure inspired by Bonkowski, 2004.

• Bonkowski M.(2004) "Protozoa and plant growth: the microbial loop in soil revisited". New Phytol., 162(3): 617–31.