In addition to root deposits, soil is another source of electron
donors, if it is used as the support matrix. Inorganic elements
in soil can generate electrons by anaerobic respiration
or chemical processes. The reactions that generate electric
power include i) the chemical oxidation of microbially produced
reductants, such as humic acids, iron(II), and sulfur compounds,
ii) the microbial oxidation of sulfur to sulfate,[21] iii) the
oxidation of ammonia to nitrite or nitrate by ammonia-oxidizing
bacteria,[22] iv) the conversion by self-supporting bacteria of
carbonate into organic carbon, which is further degraded to
generate electrons.