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How do soils store and recycle carbon and nitrogen?

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Zenaida macroura: mouring doves — ecology Mourning doves (*Zenaida macroura*) on their nest protected within the prickly branches of a cactus in the Sonoran Desert, Arizona, U.S.

Soils act as dynamic reservoirs that store and recycle carbon and nitrogen through complex biological and chemical processes mediated by microorganisms.12

Carbon Storage and Cycling

Soils store carbon primarily as organic matter, such as humus, and as inorganic compounds like calcium carbonate in arid regions. Carbon enters the soil through plant biomass and is recycled when microorganisms decompose this organic matter. Aerobic microbes convert carbon into carbon dioxide (CO $_{2}$ ) through soil respiration, while anaerobic conditions can lead to the production of methane. This CO $_{2}$ is then available for plants and photosynthetic microorganisms to create new biomass, restarting the cycle.12

Nitrogen Storage and Cycling

Nitrogen enters the biosphere from the atmosphere and is deposited into the soil. Microorganisms play a critical role in transforming this nitrogen:1
* Mineralization: Microbes break down proteins in dead biomass into ammonium ions (NH $_{4}^{+}$ ).2
* Nitrification: Bacteria such as Nitrosomonas and Nitrobacter convert ammonium into nitrate (NO $_{3}^{-}$ ), a mobile form easily absorbed by plants.12
* Fixation: Nitrogen fixation by bacteria like Rhizobium and cyanobacteria converts atmospheric nitrogen into biomass, replenishing the soil supply.2