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This study on humic acids (HAs) of podzol horizons from the southeastern region of Brazil investigated the accumulation and degradation of soil organic matter (SOM) in warm-climate podzols. Humic acids from sandy coastal Spodosols (Histic Alaquod and Arenic Alorthod) from Cardoso Island in Sao Paulo State were characterized by pyrolysis in combination with gas chromatography and mass spectrometry (Py-GC/MS). The soils developed on quartziticsandynutrient-poor parent materials under restinga forest in a coastal area. The Py-GC/MS results show that the chemical composition of HAs in B horizons in one out of three profiles was influenced by both dissolved organic C (DOC) and C from in situ decaying roots. Carbon derived from DOC dominated in the deepest part of the B horizons. In the Arenic Alorthod (H9) profileSOM from the B horizon seemed to be largely derived from roots. The DOC-derived SOM was characterized by large contributions of phenolmethylphenolsand degraded polysaccharide productswhile root contributions were mainly characterized by lignin products. Aliphatics were most abundant in A horizons (E and EB horizons were not analyzed). Both DOC-derived and lignin-derived SOM showed distinct degradation patterns. The Py-GC/MS results corroborate those obtained by 13C variable amplitude cross polarization magic angle spinning nuclear magnetic resonance spectroscopy and Fourier-transform infrared spectroscopy. Molecular chemistry showed that podzol B horizons are variable in compositionand local influencessuch as drainage and vertical or lateral water transportand SOM decomposition play an important role in podzolization processes in warm climates.

Soil organic matter (SOM) in Alu-andic Andosols and Alu-humic Umbrisols is believed to accumulate because of the protection caused by binding to aluminium (Al). We investigated soils that differed in the abundance of organo-Al complexes to determine the effect of such binding on SOM chemistry. For this, the surface horizons of three types of acid soils in the Basque Country (northern Spain) under forest stands were studied: (i) Alu-andic Andosols (AND soils) on basalts and trachytes, (ii) Umbrisols or so-called 'aluminic'(ALU) soils also on basalts and trachytes and (iii) soils with a podzolizing trend (POD), on quartzites. Values of Al extractable with sodium pyrophosphate (Alp) in the surface horizons of these soils ranged between 8.5 and 13.1, 1.9 and 9.3, and 0.8 and 3.7 g kg-1 dry weight, for the AND, ALU and POD soils respectively. For POD and ALU soils, surface horizons were sampled at two depths, 0-5 and 5-20 cm, whereas the AND soils were sampled at different depths down to the B horizon. NaOH-extractable SOM from three AND soils, 12 ALU soils and 12 POD soils was studied by pyrolysis-gas chromatography/mass spectrometry. The POD soils had the largest loads of plant-derived markers (lignin, long-chain alkanes and alkenes, methyl ketones, fatty acids); SOM of the AND soils had the smallest amounts of plant-derived SOM and the largest amounts of microbial products (microbial sugars and N-compounds) of the soils studied. ALU soils had an intermediate pattern, as expected. The results indicate that the SOM of Alu-andic Andosols, developed from basalt and trachyte rocks, is essentially dissimilar to that of soils derived from quartz-rich parent material, under the same climate conditions and similar forest stands. The dominance of secondary (microbial-derived) SOM in Alu-andic Andosols, also observed in previous research on Sil-andic Andosols (these are dominated by short-range ordered Si compounds in contrast to the dominance of organo-Al complexes in Alu-andic Andosols), reveals the small contribution of primary (plant-derived) material to SOM in soils with andic properties. © 2011 The Authors. Journal compilation © 2011 British Society of Soil Science., M.S.A.’s period at Wageningen University was funded by INIA, Instituto Nacional de Investigación y Tecnología Agraria y Alimentación (proyecto SUM2006-0013.00.00). N.G.B. was funded by the same Project.

To gain understanding about the process of global warming, it is essential to study the global C cycle. In the global C cycle, soil organic matter (SOM) is a major source and sink of atmospheric C. Turnover times of C in these soil organic compounds vary from hours to thousands of years. Clay minerals can stabilize SOM through the formation of organo-mineral bonds. The aim of this research was first, to determine the mean residence time (MRT) of organic matter that is bound to different clay mineral surfaces, and second, to explain the variance in the measured MRTs using multilinear regression. We especially studied organic matter that is bound to kaolinite or smectite. We analyzed the 14C activity of organic matter in the whole and heavy clay-size fraction of kaolinite- and smectite-dominated soils from N'Ropa, in northern Mozambique. The soils originated from natural savanna systems and bamboo forest. We assumed that C inputs and outputs are in equilibrium in such soils, so that the 14C age equals the MRT of the organic C. For both kaolinite- and smectite-dominated soils, the organic matter in the whole and heavy clay-size fraction and extracts had a fast turnover (400?500 yr on average). The MRT of kaolinite-bound organic matter did not differ significantly from that of smectite-bound organic matter. Multiple linear regression indicates that the effective cation-exchange capacity (ECEC) is the main factor to explain variance in the MRT of the extracted SOM. These results agree with previously found trends in organic matter turnover of kaolinite and smectite-associated clay.

To gain insight into the effect of clay mineralogy on the turnover of organic matter, we analysed the C-14 activity of soil organic matter associated with clay in soils dominated by kaolinite and smectite in natural savanna systems in seven countries. Assuming that carbon inputs and outputs are in equilibrium in such soils, we took the C-14 age as mean residence time of the organic matter. We corrected the C-14 activity for the Suess effect, Bomb effect and difference between date of sampling and date of C-14 measurement. Organic matter associated with kaolinite turned over fast (360 years on average). Organic matter associated with smectite turned over relatively slowly, with an average mean residence time for the whole clay-size fraction of 1100 years. Multiple linear regression indicates that clay mineralogy is the main factor explaining differences in the mean residence time of the organic matter extracted.

Summary Ultrasonic energy has been widely used to disrupt soil aggregates before fractionating soil physically when studying soil organic matter (SOM). Nevertheless, there is no consensus about the optimum energy desirable to disrupt the soil. We therefore aimed (i) to quantify the effect of varied ultrasonic energies on the recovery of each particle-size fraction and their C, N and δ13C distribution, and (ii) to determine an ideal energy to fractionate SOM of a specific soil. Our results show that the 2000–100 μm particle-size fraction was composed mainly of unstable aggregates and the 100–2 μm fraction of stable aggregates. Energies of 260–275 J ml−1 were sufficient to disrupt most of the unstable aggregates and leave stable aggregates. The use of this threshold energy combined with particle-size fractionation was not satisfactory for all purposes, since litter-like material and relatively recalcitrant organic carbon present in stable aggregates > 100 μm were recovered in the same pool. An ultrasonic energy of 825 J ml−1 was not sufficient to stabilize the redistribution of soil mass and organic matter among particle-size fractions, but at energies exceeding 260–275 J ml−1 relatively stable aggregates would fall apart and cause a mixture of carbon with varied nature in the clay fraction.

Summary To elucidate the mechanism of podzolization in its first stages we studied the fate of the water-soluble organic matter (WSOM) in incipient podzols in sandy soils by comparing the composition of the WSOM from L, F and H horizons with that in the bulk of the Bh horizons and fibres of three profiles. The WSOM appeared to consist significantly of ligno-cellulose and proteins, but these biopolymers were hardly present in the Bh horizons. The material of the fibres, however, greatly resembled the WSOM composition, thereby suggesting that in these soils most of the WSOM is transported through the B horizon and accumulates hardly changed in thin bands where the water stops moving. This implies that in the early steps of podzolization, accumulation of organic matter in the B horizon is not likely to be caused by water-soluble material.

Summary Podzols tend to form on drift sands in The Netherlands given sufficient time. Differences in organic matter (OM) composition relate to vegetation succession on drift sand that evolves eventually into a micropodzol. We examined the relation between the water-soluble organic matter (WSOM) and the total OM composition in the Ah horizons of the first vegetation stages, under grass (Corynephorus canescens), moss (Polytrichum piliferum), heather (Calluna vulgaris) and pine (Pinus sylvestris), as determined by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and analysis of polysaccharides after acid hydrolysis. Polysacharides dominate the OM in the Ah horizons under grass and moss. Lignin and aliphatic compounds are minor compounds in these profiles, but are more important under heather and pine. The Py-GC traces of the WSOM under grass and moss contain abundant products derived from polysaccharides. The water-soluble fraction is enriched in phenolic and aliphatic compounds in comparison with their contents in pyrolysates of the total OM. In the WSOM fractions under heather and pine, phenols dominate the pyrolysates, whereas lignin-derived methoxyphenols are rare. Remarkably large amounts of alkenes and alkanes are present in the pyrolysates of these water-soluble fractions, most probably derived from aliphatic biopolymers. Their presence in water might be explained through the formation of colloids, which are stabilized by micelle-like arrangements.

The occurrence of 2:1 phyllosilicates in Andisols is variously ascribed to in situ pedogenic origin, aeolian addition, or the presence of hydrothermally altered rock fragments. We studied the origin of 2:1 phyllosilicates that occur in Holocene Hapludands on andesitic, sandy beach ridges in Costa Rica by micromorphological, mineralogical, and submicroscopical techniques. The 2:1 phyllosilicates also occur as pseudomorphs after primary minerals in fresh rock of the inland volcanoes, from which the parent material of the beach ridges was mainly derived. Hydrothermal processes are most likely responsible for the formation of such pseudomorphs. Rock weathering produces sand-sized rock fragments with clay pseudomorphs and also liberates individual pseudomorphs. Subsequent erosion and alluvial transport affect their shape, but not their internal fabric. In the beach ridges, clay pseudomorphs appear as individual, sand-sized clay bodies, and inside sand-sized andesitic rock fragments. Submicroscopical analyses A.G. Jongmans, A. Nieuwenhuyse, P. Buurman, and J.D.J. van Doesburg, Dep. of Soil Science and Geology, Agricultural Univ. P.O. Box 37, 6700 AA Wageningen, the Netherlands; F. van Oort, INRA, Station Agropedoclimatique, Guadeloupe, France; A.M. Jaunet, INRA, Station de Science du Sol, Versailles, France. Received 31 Aug. 1992. *Corresponding author. Published in Soil Sci. Soc. Am. J. 58:494-501 (1994). of these individual clay bodies and andesitic rock fragments with clay pseudomorphs indicate a predominance of 2:1 phyllosilicates. This implies that they are inherited from the parent material and are not due to postdepositional soil formation in the beach ridges. Weathering and biological activity affect the clay bodies and rock fragments with clay pseudomorphs, leading to the formation of clay-sized particles consisting of 2:1 phyllosilicates. Toward the soil surface, these particles are incorporated into the allophanic groundmass resulting from actual soil formation. The geographically extensive occurrence of 2:1 phyllosilicates in Andisols suggests that the genetic processes described here may have more than regional validity. A and 1:1 phyllosilicates are the most common secondary minerals in soils formed in pyroclastic deposits in humid tropical areas without a distinct dry season (Mizota and van Reewijk, 1989; Parfitt and Kimble, 1989; Quantin et al., 1990). The occurrence of 2:1 phyllosilicates has often been reported in such soils Abbreviations: EDS, electron diffraction spectometer; EG, ethylene glycol; SSXRD, step-scan x-ray diffraction; TEM, transmission electron microscope; XRD, x-ray diffraction. JONGMANS ET AL.: INHERITANCE OF 2:1 PHYLLOSILICATES 495 (Wada, 1980; Pevear et al., 1982; Shoji et al., 1985, 1987; van Oort, 1988), but there are different concepts concerning their genesis (Lowe, 1986). Shoji and Yamada (1981) and Shoji et al. (1982) favored an in situ pedogenic origin, and suggested that 2:1 phyllosilicates are formed by weathering of volcanic glass. Other workers such as Mokma et al. (1972) and Mizota and Takahashi (1982) attributed the presence of 2:1 phyllosilicates in soils on tephra deposits to later aeolian additions. Dudas and Harward (1975), Pevear et al. (1982), and Ping et al. (1988) ascribed the occurrence of 2:l-type phylJosilicates in air fall tephra deposits to hydrothermally altered rock fragments that were ejected during eruptions. Hydrothermal alteration in volcanic rocks transforms primary minerals such as olivine, pyroxenes, and plagioclase into clay pseudomorphs (Kristmannsdottir, 1979; Fan, 1979). Dudas and Harward (1975) demonstrated that 2:1 phyllosilicates in some volcanic ash horizons could be relicts of an underlying montmorillonitecontaining paleosol. Micromorphology and related in situ submicroscopy were not carried out in the cited studies, except for Pevear et al. (1982). Consequently, the properties of the individual soil constituents in terms of size, shape, arrangement, and composition were not studied. Among other factors, these properties influence the occurrence of clay minerals in soils formed on volcanic deposits (Nahon, 1991; Jongmans et al., 1994), so they need to be examined in studies concerning mineral weathering and neoformation. In a chronosequence of Andisols and younger Tropopsamments on Holocene andesitic beach ridges of Costa Rica, Nieuwenhuyse etal. (1994) reported the occurrence of 2:1 phyllosilicates. These soils were formed under humid tropical conditions where soil weathering (supergene alteration, Delvigne, 1990) has led to formation of short-range order material. The goal of our study was to determine the origin and genesis of the observed 2:1 phyllosilicates in the Andisols of the Costa Rican beach ridge sequence by means of micromorphological, mineralogical, and in situ submicroscopical techniques of undisturbed samples. MATERIALS AND METHODS Five Andisols (2000-5000 yr old) and three Tropopsamments ( 150 cm) of Hapludand AT7 were studied by XRD. The fractions were prepared after removal of organic matter by buffered (pH 5.5) H2O2 treatment, repeated washing with distilled water (5-10 times) and addition of 0.005 M NaCl solution. The XRD was performed after grinding of the separated particle size fractions. In order to improve the quality of the XRD diagrams, part of the ground material was ultrasonically dispersed and sequentially extracted in darkness with ammonium oxalate at pH 3.3 (Tamm, 1922; Robert and Tessier, 1974), then with sodium citrate at pH 7.3 (Tamura, 1957). The XRD patterns were obtained from Mg-saturated, oriented pastes before and after EG solvation and from K-saturated, oriented pastes after heating. Finally, fresh rock collected on the Irazu volcano was ground and ultrasonically dispersed. The XRD was performed on the clay fraction after Mg and K saturation. The allophane content was calculated by the method proposed by Parfitt and Wilson (1985). From four Hapludands, microquantities of sand-sized, anisotropic, clay bodies from uncovered thin sections were isolated with a microscope-mounted drill (Verschure, 1978). The material was transferred to glass or Al slides according to the method of Beauford et al. (1983). The XRD analyses of the microquantities were obtained by SSXRD, (Meunier and Velde, 1982) with steps of 0.05° 26 and counting times of 80 s per step. Two sand-sized, anisotropic clay bodies from Hapludand AT7 were selected optically and removed from uncovered thin sections without disturbing them according to the method proposed in van Oort et al. (1990). After reimpregnation of these clay bodies, undisturbed sections of 50-nm thickness Table 1. Selected properties and classification of five Andisols from Costa Rica. Particle-size distribution Chemical and physical properties B horizon C horizon A horizon B horizon Profile Classification A horizon Clay Sand Clay Sand Clay Sand Organic matter pH (H2O) Bulk density Allophanet