Humus form
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Mull humus is a type of humus that occurs in deciduous woodlands or grasslands, usually associated with a milder climate in the ecosystem.[1] It is one of three classifications of forest humus based on the morphology and arrangement of organic horizons.[2] The other two types are Mor and Moder, and three forest humus types are distinguishable in formation, nutrient cycling, productivity, etc.
Formation
Humus is the brown pigment that remains after the decomposition of litter from dead parts of plants and animals, and this part of the soil is enriched in organic matter.[3] The intimate mixing of soil organic matter with mineral matter in the A horizon forms the mull humus, contributed by the activity of soil organisms.[4] Only a few proportions of soil organisms are functional in disintegrating the decomposed material that is nitrogen sequestered and releasing essential plant nutrients.[4] Earthworms are the dominant fauna group in the mull humus.[1] The roles of soil-dwelling earthworms in mixing activity conduct abundant clay-mineral complexes in the macro-structured A horizon, decisive influence on the control of soil organic matter levels.[5] Other agents may also contribute to incorporating soil organic matter into the mineral soil, such as white rot fungi and bacterias belonging to the microbial group.[3]
Characteristics
Mull humus has a more rapid and complete decomposition. Due to the presence of soil organisms and high biological activity, the disappearance of plant litter is speedy, and there are no apparent distinct layers because thick organic horizons do not accumulate.[4] Conversely, the Ah horizon is well developed, resulting from the assemblage of organic matter with mineral particles. Broadleaf tree species, the significant components in the deciduous ecosystem aligned with mull humus, appear to be effective in building soil organic matter levels.[6] Also, plants under the mull humus produce litters that are easier decomposable with a low C:N ratio, allowing nutrient release, preventing immobilization, and encouraging high bioturbation.[6] Moreover, a relatively complete decomposition relates to more completely oxidized organic acids, promoting a more alkaline soil.[2]
Biodiversity and fertility
Significantly, mull humus is characterized by tight plant and soil interactions, particularly its positive feedback relationship between litter quality, soil nutrient availability, and organism activity.[3] Mull humus has high biomass and species richness of soil fauna, ranging from megafauna to microfauna.[7] Those soil organisms have high nutrient requirements due to their high energy costs for capturing spaces and nutrients at a high level of competition, explaining the fast use of nutrients.[8] Also, the variety of organisms reflects nutrient availability, the prerequisite for building up mull humus. Consequently, the high nutrient availability and fast use of nutrients allow rapid cycling of nutrients.[1]
A rapid nutrient cycling can further contribute to soil fertility and enrich aboveground and belowground biodiversity, indicating a high level of biodiversity and productivity.[1] Adequate plant growth depends on the degree of litter decomposition because litter provides most nutrients required by plants.[9] The contribution of soil organisms completes the positive feedback loop: the higher the litter quality, the faster organic matter decomposition, the faster nutrient cycling, and the faster vegetation growth.[3] 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.[10] The effects of mull humus on the vegetation growth are apparent.
Stability and implications
The enriched species abundance and richness of soil organisms create more heterogeneity, allowing the maintenance of high biodiversity, which ensures the functionality of soil fauna.[11] The high biodiversity, both aboveground and belowground, help generate the conditions of stability. For instance, soil animals in humidified organic matter have significant importance in sustaining moisture and nutrients.[5] Thus, mull ecosystems are more stable, providing more resistance to the changing environment, such as pollution and climate change.[1] Furthermore, the mull humus system also has a greater capacity for recovery, often related to the early development stages in the forests, which can be applied for forest regeneration or rehabilitation after the disturbance.
References
- ^ a b c d e Ponge, J (2003). "Humus forms in terrestrial ecosystems: A framework to biodiversity". Soil Biology & Biochemistry. 35 (7): 935–945.
- ^ a b Briggs, R.D. (2004). "Soil development and properties: The forest floor". Encyclopedia of Forest Sciences: 1223–1227.
- ^ a b c d Ponge, J (2013). "Plant–soil feedbacks mediated by humus forms: A review". Soil Biology & Biochemistry. 57: 1048–1060.
- ^ a b c Brethes, A; Brun, J.J; Jabiol, B; Ponge, J; Toutain, F (1995). "Classification of forest humus forms: A french proposal". Annales Des Sciences Forestières. 52 (6): 535–546.
- ^ a b Wolters, V (2000). "Invertebrate control of soil organic matter stability". Biology and Fertility of Soils. 31 (1): 1–19.
- ^ a b Prescott, C.E.; Frouz, J; Grayston, S.J; Quideau, S.A.; Straker, J. "Rehabilitating forest soils after disturbance". Developments in soil science: 309–343.
- ^ Petersen, H; Luxton, M (1982). "A comparative analysis of soil fauna populations and their role in decomposition processes". Oikos. 39 (2): 288–388.
- ^ Vitousek, P (1982). "Nutrient cycling and nutrient use efficiency". The American Naturalist. 119 (4): 553–572.
- ^ Vinton, M.A.; Goergen, E.M. (2006). "Plant-soil feedbacks contribute to the persistence of bromus inermis in tallgrass prairie". Ecosystems (New York). 9 (6): 967–976.
- ^ Tilman, D (1999). "The ecological consequences of changes in biodiversity: A search for general principles". Ecology (Durham). 80 (5): 1455.
- ^ Hansen, R.A. (2000). "Effects of habitat complexity and composition on a diverse litter microarthropod assemblage". Ecology (Durham). 81 (4): 1120.