Humus composition of Amazonian Dark Earths in the Middle Amazon, Brazil

Abstract

The process of humus accumulation has been one of the most important aspects in the pedogenesis of Amazonian Dark Earths (ADE) and Terra Mulata (TM). ADE has been characterized by higher contents of P, Ca and humus than adjacent Yellow Latosol (YL) under anthropogenic effects, and TM has been characterized by slightly less dark topsoil than ADE. The objective of this study was to analyze the composition of humus in ADE using the Kumada's method. A total of 20 soil samples of the A horizons and the transitional AB and BA horizons from three soil profiles of ADE (12 samples), adjacent two YL (4 samples) and a TM (4 samples) were subjected to a humus composition analysis, in which successive extraction with NaOH and Na 4 P 2 O 7 was conducted. The amounts of the humic acid fractions in NaOH and Na 4 P 2 O 7 extracts from ADE were distinctly higher than those from the adjacent YL. The RF values, the absorbance at 600 nm per unit concentration of humic acids in 0.1 mol L −1 NaOH of ADE, were not necessarily the highest in the surface horizons, but the darkest humic acids often existed in the transitional AB or BA horizons. The humic acids extracted with NaOH from seven of 16 samples of ADE and TM were classified into Type A, the most humified group, while the humic acids extracted with Na 4 P 2 O 7 were Type A in 11 samples. Moreover, the amounts of humic acid fractions and RF values of humic acids were positively correlated with the Al + Fe contents in humic acid fractions, especially those extracted with Na 4 P 2 O 7 . Thus, it is possible that humic substances in ADE are stabilized by forming complexes with Al and Fe.

Humus composition and humification degree of humic acids of alpine meadow soils in the northeastern part of the Qinghai–Tibet Plateau

“…This suggests that the presence of HAs associated with Ca was larger than with Fe and Al. HAs extracted with Na 4 P 2 O 7 revealed a stronger association with Fe and Al than with Ca (e.g., Nakamura et al 2007). As described in Section 3.1, the soils in this study were Ca-enriched.…”

“…Type A-HA is commonly observed in the A horizon of Japanese Andosols derived from volcanic ash and charred materials could be an important source (Tate et al 1990;Shindo et al 2004;Nishimura et al 2009). However, in the A horizon of Chernozemic soils, charred organic C was also observed in Europe (Schmidt et al 1999), which contributes to C storage and the formation of Type A-HA, similar to the humus accumulation in Japanese Andosols (Nishimura et al 2009).…”

“…The humification degree of HA Cl was higher than that of HA OH and was always classified as Type A from the deeper horizons (Table 3). The lower humification degree of HAs for the surface horizon was probably due to mixing of fresh plant residue from vegetation (Nakamura et al 2007). The Δlog K-A 600 /C plots of HA OH and HA PP extracted by the OH-PP method in each horizon are shown in Fig.…”

The important characteristics of soil organic matter affecting 2,4-dichlorophenoxyacetic acid sorption along a catenary sequence

“…[52,53] It is also possible that HAC-P (com- bined form C) adsorbed less 2,4-D than HAC-OH (free form C) because combined form C is in closer contact with clay minerals than free form C. [8,9] For example, a previous study reported that the presence of highly (negatively) charged clay minerals reduces the importance of metalhumate complexes as sorbents of 2,4-D. [44] Several researchers have suggested that sorptionpartition coefficients such as Koc can be calculated using the structural characteristics of SOM. [54,55] In the current study, Koc was significantly positively correlated with the ε 280 of HA-OH, suggesting that the capacity of SOM to retain 2,4-D increased with increasing HA-OH aromaticity [9,56] and hence the abundances of conjugated C=C bonds. [7] The CPMAS 13 C-NMR on whole soils indicated that the O-alkyl C region dominated the C pool in all soil samples, which is in agreement with that observed for a wide range of soils in Australia and Pakistan.…”

“…[5,6] HS can be fractionated by sequential extractions with sodium hydroxide (NaOH) followed by sodium pyrophosphate (Na 4 P 2 O 7 ). [7] HAs and FAs extracted by NaOH represent the fractions present as free form C. [7] HAs and FAs that are not extractable by NaOH but can be extracted by Na 4 P 2 O 7 are considered to be the combined form C fractions [7] and in closer contact with clay minerals than free form C. [8,9] Piccolo et al [10] demonstrated that NaOH-extracted HS have a greater sorption capacity for atrazine (a weakly-basic herbicide) than Na 4 P 2 O 7 -extracted HS.…”