Humification and Humic Acid Composition of Suspended Soil in Oligotrophous Environments in South Vietnam

Abstract

Humification is considered to be a global process that is implemented in soils and organic sediments and also in natural water and air. e term "suspended soils" has become increasingly common in recent years. Suspended soils are defined as the part of the organic matter that has not undergone the full decomposition process and has not turned into the humus of terrestrial soils. Suspended soils were shown to contain higher total nitrogen, phosphorus, and potassium contents than the forest soil, but the moisture content in suspended soils was significantly lower. Our study of the structural composition of humic substances in suspended soils was conducted with an aim of evaluating the humification rates and structural composition of humic acids in the suspended soil in tropical forests of South Vietnam. Soil samples from three selected areas were investigated: the soil under phorophytes (mineral soil presented by samples of topsoil of the typical dry savanna landscape) and two soils from epiphytous formations. Samples were collected from savanna-type sparse communities, located on oligotrophous plains in Phú Quốc Island (South Vietnam) in 2015. General properties of the soil and the elemental composition of suspended soils were determined, and the humic substance chemical composition was evaluated using solid state 13 C-NMR. Data obtained showed that the pH of the soils under phorophytes was higher than in the suspended soils; basal respiration did not tend to change indices between soils under phorophytes and suspended soils, but the suspended soil was less enriched by nitrogen than the soil under phorophytes.is can be related to the total amount of organic matter exposed to humification in various soils and to the presence of an essential portion of mineral particles in the soil under phorophytes. Data on elemental composition of the humic acids (HAs) indicated that one method of humification is implemented in all three soils that were investigated. e humic acids of the phorophyte soil showed the same content of aromatic fraction as the suspended soil. e most comparable soil type in terms of humic substance composition is Cambisols from humid forests of subboreal and subtropical zones. e humification process implemented in suspended soils showed the absence of mineral compounds or mineral fine earth, which indicated that humification in conditions of pure organic substrates can result in formation of deep humified organic matter, as shown by humic acids with an essential aromatic fraction content.

Assessments of Organic Carbon Stabilization Using the Spectroscopic Characteristics of Humic Acids Separated from Soils of the Lena River Delta

“…This was due to the erosion of carbonate rocks in the middle course of the Lena River; the presence of carbonates was also noted in the ridges adjacent to the delta [43]. In the studied soils, relatively little SOM accumulated, with the highest content noted in the wind shelter site, which was associated with more favorable conditions for humification [44]. The enrichment of organic matter with nitrogen is low, according to Orlov [45].…”

“…From the data obtained, we noted that the most humified remains were formed in buried organic remains (numbers 6, 2, 8). The increased proportion of aromatic substances was associated with local position in the relief, the duration of the humification process, wind conditions, hydromorphism of the territory and the precursors of humification (vegetation) [44]. According to degradation hypotheses, stable plant and microbial biopolymers, during transformation, form aromatic benzene rings and labile aliphatic fragments are almost completely mineralized.…”

Microbiomes in Suspended Soils of Vascular Epiphytes Differ from Terrestrial Soil Microbiomes and from Each Other

“…These findings indicate that the crown nutrition cycle and epiphytic nutrition are largely separate from wood litter [ 4 ]. By contrast, our previous studies have suggested that complete humification processes do occur in suspended soils [ 11 , 12 ]. Therefore, the edaphic role of epiphytic substrates remains underestimated.…”

The influence of organic and inorganic nutrient inputs on soil organic carbon functional groups content and maize yields

“…On degradation, the lignin-derived aromatic C contributes to a humic fraction of SOM (humification), which is core to soil fertility ( Fuchs et al., 2011 ). Abakumov et al. (2018) suggest that humification processes supplemented by organic inputs application boosts SOM.…”