# Organic Matter Structural Composition of Vascular Epiphytic Suspended Soils of South Vietnam
## 1 Introduction  
Soil organic matter is the most important storage for carbon on the planet. It plays very important role in climate mitigation and the stabilization of biogeochemical processes in a whole biosphere. There are numerous works dealing with the characterization of organic matter in \"normal\" soils, formed on mineral parent materials [1, 2]. At the same time, soil formation occurs not only under the folic material of a forest floor located on the surface of mineral soil forming substrata but also in epiphytic organic and organo-mineral formations, typical for tropical wet forests [3]. In these communities, soil formation is stratified in a vertical scale. The first is a \"classical\" soil [4, 5, 6] formed as result of interaction of the mineral part with incoming organic matter; these soils are not rich in organic matter due to the high mineralization rate. One can see that in tropical forest there are no continuous forest floors and the existing folic material on the soil surface is sporadic, loose and shallow. The second type of soil is so-called suspended soils. The main difference between suspended soils and classical mineral soils is that suspended soils have no direct connection withthe mineral part of the landscape, including soil parent materials. The organic mineral nutrition of suspended soils is carried out at the expense of atmospheric precipitation and a partial return of biogenic elements. Thus, suspended soils have both the attributes of soils proper and the properties of soil-substrate organogenic bodies. In the most primitive case (in temperate ecosystems), they are formed from the litter stuck in the forks of skeletal branches [7]. In tropical ecosystems, well-structured communities of vascular epiphytes in tree crowns form the basis of suspended soil clusters. With the help of various adaptations (a special type of roots [8], leaves forming funnels, clumps of outgrowths of epiphytes), they are able to massively trap falling organic material, retain water and provide shelter for settling consort organisms--from fungi to ants and vertebrates [9]. Together with the living biomass of epiphytye, suspended soil forms epiphytye material (EM) [10]. The rate of epiphytye organic matter deposition is impressive: the maximum recorded estimate is 44 t ha\({}^{-1}\)[10]. This also leads to a large-scale accumulation of mineral nutrients in epiphytye matter. One of the few estimates described provides: N = 37.9 \(\pm\) 9.0; P = 1.97 \(\pm\) 0.47; K = 9.6 \(\pm\) 2.3; Ca = 9.6 \(\pm\) 2.3; Mg = 2.64 \(\pm\) 0.63; Na = 0.25 \(\pm\) 0.06 kg ha\({}^{-1}\) for a total EM biomass of 2261 \(\pm\) 537 kg ha\({}^{-1}\)[11]. Moreover, recent studies have shown a wide diversity of tropical suspended soil metabolomes [12]. It appears that suspended soils have strong buffer properties and are much less dependent on chemical interference from the outside than terrestrial soils [13]. This makes suspended soils an essential functional unit of the tropical ecosystem, which appears to replace the tropical forest's absent ground litter [14]. Organic matter accumulates in suspended forms, with organic residues intensively transformed; there is evidence of humification, which takes place under conditions of very low mineral particles [15]. Due to the low content of mineral particles of atmospheric origin, only the ash components may play an essential role in binding of humification products and stabilization of organic matter. As was mentioned by numerous researchers [16; 17], soil organic matter humification degree depends essentially on quality and composition of organic precursors of humification. Although the end products of humification have been investigated previously [9; 15], particular interest is given to the investigation of the quality of the organic precursors of humification. Thus, the aim of this work was to characterize for the first time the structural and component composition of various plant materials in the suspended soils of South Vietnam.