Soil Organic Matter Formation

The decomposition activities and life strategies of soil microorganisms and fauna act as the "waste management" crew of an ecosystem and play an important role in maintaining the global C budget by balancing the CO2 fixed through photosynthesis and releasing CO2 back to the atmosphere by decomposition. In temperate ecosystems, a small fraction of the C in NPP and NSP is preserved because it either becomes metabolized to a recalcitrant state, such as humic substances, or is protected physically through association with secondary minerals and soil aggregates. In boreal ecosystems, cold temperatures and water logging (peat soils) impede decomposer activity, leading to the accumulation of soil C as plant residues or particulate organic matter (POM). Permafrost areas such as tundra accumulate significant soil C and POM because low temperatures decrease microbial activity.

Soil organic matter consists of unrecognizable partially decayed plant residues, soil microorganisms, soil fauna, and the by-products of decomposition that lead to the production of humic substances. This process is called humification. The formation of humic substances results from many events of oxidation and hydrolysis that create materials with increased C and H and lower O content compared to the original animal, microbial, and plant tissue (Fig. 12.15). During the repeated degradation, N compounds react through free-radical coupling and increase the

Plant labile proteins, lipids etc.

Decomposition intermediates melins, sugars, cellular metabolites etc.

Nutrient availability soil/plant source

Cell wall cellulose & lignin condensation Humic precursors

Humic substances

Microbial 'production

Organic N amino acids amino sugars etc.

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