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The term humus form is not the same as the term humus. Forest humus form describes the various arrangement of organic and soil horizon at the top of soil profiles. It can be composed entirely of organic horizons, meaning an absence of any mineral horizons. Experts worldwide have developed different types of classifications over time, and humus forms are mainly categorized into mull, mor humus, and moder humus orders in the ecosystems of British Columbia. Mull humus form is distinguishable from the other two forms in formation, nutrient cycling, productivity, etc.
Earthworms are the dominant fauna group in the mull order. They break down the decomposed material into small pieces and mix those residues into the soil through their activity, having significant roles in decomposition. Thus, conducting abundant clay-mineral complexes in the macro-structured A horizon. The functions of the have a decisive influence on the control of soil organic matter levels. Other agents may also contribute to incorporating soil organic matter into the mineral soil, such as white rot fungi and belonging to the microbial group. They act as essential decomposers, facilitating the breakdown process of organic residues. Potential horizons included in the mull order are L, F, H, and Ah horizons, with no consideration of B or C horizons in humus form.
The intensive bioturbation processes that create mull humus forms occur primarily within the soil biomantle—the upper organic-rich zone where biological mixing dominates soil formation. This biomantle represents the active interface between organic inputs and mineral soil, where earthworms and other soil fauna continuously restructure the soil matrix through biomixing and biotransfers. The resulting intimate association of organic matter with mineral particles creates the characteristic granular structure and high fertility associated with mull systems.
A rapid nutrient cycling can further contribute to soil fertility and enrich aboveground and belowground biodiversity, indicating a high level of biodiversity and productivity. Optimal plant growth depends on the degree of litter decomposition because litter provides most nutrients required by plants. Associating with soil organisms, a positive feedback loop will be formed: the higher the litter quality, the faster organic matter decomposition, the faster nutrient cycling, and the faster vegetation growth. More plants cohabit in mull humus, and the plant biodiversity is highly related to nutrient availability and the impacts of the soil acidity. Too much nutrient availability may negatively impact plant growth, but the high competition between soil organisms can deal with the concern. The effects of mull humus on the vegetation growth are apparent.
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