Identify the correct timeframes for soil organic matter decomposition

Question

BlackTom AI · Accepted Answer

The question asks us to identify the correct timeframes for soil organic matter (SOM) decomposition, based on the illustrated Timeline of decomposition.
Option A: Days. This corresponds to the initial phase where fresh residues are converted into biomass relatively quickly. In many soils, the rapid leaching and microbial processing of fresh plant material happens over days to a few weeks, so labeling this stage as occurring over days is reasonable for the first step.
Option B: Weeks to Months. After the initial rapid decomposition, the material transitions from biomass to stabilized organic matter (humus). This transformation is slower than the initial breakdown and typically occurs over weeks to months as microbes mineralize and reorganize the material into more stable humus components.
Option C: Years to Decades. Once stabilized SOM forms, its further decomposition to more recalcitrant forms or microbially resistant residues can proceed over longer timescales, often spanning years to decades. This aligns with how humus persists in soils before much of it is considered inert or very slowly decomposing.
Option D: Centuries to Millennia. The final, most recalcitrant pool, inert organic matter (O.M.), accumulates very slowly and can persist for centuries to millennia. This pool is decomposed at exceedingly slow rates due to strong stabilization and protection within the soil matrix.
In summary, the sequence from fresh residues to biomass, to stabilized OM (humus), to inert OM corresponds to progressively longer timescales: days, weeks to months, years to decades, and centuries to millennia. Each option maps to the respective stage in the decomposition timeline, reflecting the typical order and duration of processes in soil organic matter cycling.

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Identify the correct timeframes for soil organic matter decomposition

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