Characteristics and distributions of humic acids in two soil profiles of the southwest China Karst area

Characteristics and distributions of humic acid (HA) and soil organic matter (SOM) in a yellow soil profile and a limestone soil profile of the southwest China Karst area were systematically investigated to reveal their evolutions in different soils of the study area. The results showed that characteristics and distribution of SOM along the two soil profiles were notably different. Total organic carbon (TOC) contents of soil samples decreased just slightly along the limestone soil profile but sharply along the yellow soil profile. TOCs of the limestone soils were significantly higher than those of the corresponding yellow soils, and C/N ratios of SOMs showed a similar variation trend to that of TOCs, indicating that SOM can be better conserved in the limestone soil than in the yellow soil. The soil humic acids were exhaustively extracted and further fractionated according to their apparent molecular weights using ultrafiltration techniques to explore underlying conservation mechanisms. The result showed that C/N ratios of HAs from different limestone soil layers were relatively stable and that large molecular HA fractions predominated the bulk HA of the top soil, indicating that HA in the limestone profile was protected while bio and chemical degradations were retarded. Combined with organic elements contents and mineral contents of two soils, we concluded that high calcium contents in limestone soils may play a key role in SOM conservation by forming complexation compounds with HAs or/and enclosing SOMs with hypergene CaCO₃ precipitation.     

Rayleigh distillation and the depth profile of C/C ratios of soil organic carbon from soils of disparate texture in Iron Range National Park, Far North Queensland, Australia

A depth- and particle size-specific analysis of soil organic carbon (SOC) and its isotopic composition was undertaken to investigate the effects of soil texture (or particle size) on the depth profile of stable carbon isotopic composition of SOC (δ¹³C_SOC) in two tropical soils. Depth-specific samples from two soil profiles of markedly different texture (coarse grained and fine grained) were separated into particle size classes and analyzed for the (mass/mass) concentration of SOC (C) and δ¹³C_SOC. Within 1 m of the soil surface, δ¹³C_SOC in the coarse-textured soil increases by 1.3 to 1.6‰, while δ¹³C_SOC from the fine-textured soil increase by as much as 3.8 to 5.5‰. This increasing depth trend in the coarse-textured soil is approximately linear with respect to normalized C, while the increase in the fine-textured soil follows a logarithmic function with respect to normalized C. A model of Rayleigh distillation describing isotope fractionation during decomposition of soil organic matter (SOM) accounts for the depth profile of δ¹³C_SOC in the fine-textured soil, but does not account for the depth profile observed in the coarse-textured soil despite their similar climate, vegetation, and topographic position. These results suggest that kinetic fractionation during humification of SOM leads to preferential accumulation of ¹³C in association with fine mineral particles, or aggregates of fine mineral particles in fine-textured soils. In contrast, the coarse-textured soil shows very little applicability of the Rayleigh distillation model. Rather, the depth profile of δ¹³C_SOC in the coarse-textured soil can be accounted for by mixing of soil carbon with different isotopic ratios.     

Oxidation of humic acids from an agricultural soil and a lignite deposit: Analysis of lipophilic and hydrophilic products

The composition of humic acids (HAs) isolated from an agricultural soil and a lignite deposit was examined via H₂O₂ and RuO₄ oxidation. The oxidation digests were separated into lipophilic and hydrophilic components. Information with regard to the source, degree of humification and preservation of easily degradable constituents of the HAs was obtained and results were compared with those obtained earlier for base hydrolysates of solvent-extracted fractions.H₂O₂ oxidation of both HAs afforded lipophilic fractions containing high molecular weight compounds. The composition of the base hydrolysates of the lipophilic fractions strongly differed with the origin of the HA. The lipophilic components of the soil HA derived mainly from the higher plant polyesters cutin and suberin. The lipophilic components of the lignite HA predominantly comprised long chain alkanoic acids and alkanols. The patterns for the hydrophilic components released upon H₂O₂ oxidation were found to be identical irrespective of the origin of the HA. The hydrophilic fractions comprised aliphatic (poly)carboxylic acids related to carbohydrate moieties and benzene polycarboxylic acids. The relative abundance of benzene polycarboxylic acids increased with the degree of humification.For both HAs, RuO₄ oxidation resulted in a lipophilic fraction containing low molecular weight products identical to those found in the base hydrolysate of the lipophilic fraction released upon H₂O₂ oxidation. The hydrophilic components released upon RuO₄ oxidation were independent of the HA origin and consisted mainly of monosaccharides and disubstituted aromatic compounds. In agreement with the greater aromaticity of lignite HA, the aromatic compound/carbohydrate ratio was higher for lignite HA than soil HA. The results show that the fused aromatic structures had a small size and that carbohydrates could escape degradation during the humification process.     

Stable carbon isotopic composition of soil organic matter in the karst areas of Southwest China

This study dealt with the distribution characteristics of soil organic carbon （SOC） and the variation of stable carbon isotopic composition （δ^13C values） with depth in six soil profiles, including two soil types and three vegetation forms in the karst areas of Southwest China. The δ^13C values of plant-dominant species, leaf litter and soils were measured using the sealed-tube high-temperature combustion method. Soil organic carbon contents of the limestone soil profiles are all above 11.4 g/kg, with the highest value of 71.1 g/kg in the surface soil. However, the contents vary between 2.9 g/kg and 46.0 g/kg in three yellow soil profiles. The difference between the maximum and minimum δ^13C values of soil organic matter （SOM） changes from 2.2‰ to 2.9‰ for the three yellow soil profiles. But it changes from 0.8‰ to 1.6‰ for the limestone soil profiles. The contrast research indicated that there existed significant difference in vertical pattems of organic carbon and δ^13C values of SOM between yellow soil and limestone soil. This difference may reflect site-specific factors, such as soil type, vegetation form, soil pH value, and clay content, etc., which control the contents of different organic components comprising SOM and soil carbon turnover rates in the profiles. The vertical variation patterns of stable carbon isotope in SOM have a distinct regional character in the karst areas.     

土壤腐殖物质特征及其对有机碳长时间尺度稳定性的指示：以陕西洛川黄土剖面为例

在长时间尺度上研究土壤中腐殖物质的组成和腐殖化特征对于正确评价土壤有机质的稳定性具有重要意义。文中对洛川黄土/古土壤剖面S8以上的17个黄土和古土壤样品中的胡敏酸（HA）、富里酸（FA）和胡敏素（HM）进行定量研究,并对HA和FA的E465、E665值等进行了测定。结合该课题组对矿物组成、总有机碳（TOC）及其各组分含...     

Comparison of the structural,spectroscopic and phenanthrene binding characteristics of humic acids from soils and lake sediments

The structural, spectroscopic and phenanthrene binding characteristics were compared for humic acids (HA) extracted from two different sources: eight soils and six lake sediments. The elemental analysis revealed that HA from sediments had higher H/C, N/C, and (N + O)/C ratios compared to HA from soils, reflecting a lower degree of humification and more autochthonous organic input upon the formation of the HA for lake sediments versus soil environments. HA from soils exhibited a higher content of aromatic carbon structures than the sediment HA based on ¹³C NMR results. The source of HA was easily distinguished by comparing the synchronous fluorescence spectra of each HA group. The presence of a protein like fluorescence was prominent for the HA from sediment while it was minor for the HA from soil. Irrespective of the HA source, however, humification index (HIX) exhibited a common positive correlation with the aromatic content, and a negative correlation with O-alkyl carbon structures of the HA. The correlations were consistent with the general structural trends of humification processes, suggesting that HIX may serve as a source independent predictor to describe the structural information and humification degree of terrigenous HA. Aromatic carbon structures did contribute to enhancing the phenanthrene binding for both sources of HA. However, the primary structures associated with non-ideal phenanthrene binding (i.e., non-linear sorption isotherm) appear to differ by the HA source because the opposed correlations were obtained between aromaticity and the isotherm linearity for the two HA groups. Our results suggest that the HA structural function associated with specific non-linear sorption for hydrophobic organic contaminants (HOCs) may be more strongly governed by the HA source than by the apparent physico-chemical properties.     

Effect of N content and soil texture on the decomposition of organic matter in forest soils as revealed by solid-state CPMAS NMR spectroscopy

N has a controlling effect on litter biodegradation in the forest floor, while stabilization of organic matter in the mineral soil may be influenced by physical parameters related to soil texture. In this study, in order to understand the processes involved in soil organic matter (SOM) formation, the chemical composition of SOM was followed and evaluated with regards to N contents and soil texture. Samples were taken on sites covered with Norway spruce and displaying contrasting values of C/N ratios in the forest floor. The chemical structure of OM was characterized using solid-state CPMAS ¹³C and ¹⁵N nuclear magnetic resonance (NMR) spectroscopy, along with Proton Spin Relaxation Editing (PSRE) sequences. Four groups of sampling sites were defined based on the NMR spectra of Oh and A horizons. In each group displaying similar NMR characteristics, N content and soil texture could be highly different among sites. Some Oh horizons with similar NMR spectra had very different N contents. Highly humified OM in Oh horizons were observed mainly on sites with low N contents. Some A horizons with different soil texture displayed similar OM chemical structure. High contents of O-alkyl C in some A horizons could originate from higher fresh root material input.     

Vertical patterns of stable carbon isotope in soils and particle-size fractions of karst areas, Southwest China

Taking limestone soil and yellow soil, the two major soil types in karst areas as examples, analyzing stable carbon isotope composition (δ¹³C value) of soil organic matter (SOM) in bulk soils and particle-size fractions of four soil profiles under three vegetable forms, the following results are reached: in the limestone soil profile, soil organic carbon contents are all above 1.0%, the highest value is 7.1% in the surface soil; however, they are between 0.3% and 4.6% in the three yellow soil profiles. From the surface to the bottom of the soil profiles, the variation of δ¹³C value of soil organic carbon for limestone soil profile is only between −24.1‰ and −23.0‰, however, it’s between −24.8‰ and −21.1‰ for yellow soil profiles. The variation range of δ¹³C value of soil organic carbon associated with particle-size separates is slight for limestone soil but is considerable for yellow soil. The contrast research indicates that the changes between the contents and the δ¹³C value of soil organic carbon with depth are complex. The vertical patterns of stable carbon isotope in soil organic matter have a distinct regional characteristic in karst areas.     

The coupled release of REE and Pb to the soil labile pool with time by weathering of accessory phases, Wind River Mountains, WY

Rare earth element (REE) distributions and Pb isotope compositions were explored in soils varying in age from ca. 0.4 to ?300 ka, developed on moraines in the Wind River Mountains, Wyoming. Soil extracts (0.6 M HCl) were used to examine the soil labile pool while the major element distribution in soil profiles was used to determine the extent of weathering at different soil depths. The results show that the chondrite-normalized REE patterns of the deepest bulk soil within each profile reflects the composition of the moraine till, except for the oldest soil. Up to ca. 12 ka, the soil extract fraction is enriched in light REE, indicating early release of light REE to the soil labile pool while that of the two oldest soils are relatively enriched in heavy REE. In the soil extracts the La/Sm ratio normalized to the deepest soil (La_D/Sm_D) decreases systematically with soil age. Similarly, the Eu-anomaly in the deepest soil from each profile (Eu_D/Eu_D*) decreases slightly with soil age in the three young soils; however, Eu_D/Eu_D* increases with soil age in the older soils. The systematic trends of these two ratios indicate the depletion of light REE in young soils and the enrichment of Eu and heavy REE in the older soils. Based on the Pb isotope ratios, the relative contribution of Pb to the soil labile pool via mineral weathering of U- or Th-rich phases was assessed for the different stages of weathering. The whole-soil profile ²⁰⁸Pb/²⁰⁴Pb ratio was found to decrease with soil age and with La_D/Sm_D, whereas it increased with the Eu_D/Eu_D* ratio. In each horizon, Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb) ratio generally decrease with soil age. In order to overcome possible effects from parent material heterogeneity, the amount of radiogenic Pb as compared to the whole-soil composition was calculated and this was found to decrease systematically with soil age.     

Sr and Nd isotopes as tracers in pedogenic studies: Evidence for Saharan dust contribution to the soils of Muravera (Sardinia,Italy)

Sr and Nd isotopes were applied to 5 soil profiles from the Muravera area, in south-eastern Sardinia.All the soils, which have developed during the Quaternary on the Lower Paleozoic metamorphic basement except for one on Eocene carbonates, are located far from major sources of pollution. Therefore, they are suitable for testing pedogenic processes and geochemical evolution to benefit for environmental studies.The Sr isotopic ratios range largely (δ⁸⁷Sr = 1.7–65.9‰), even in each soil profile. In particular, the observed increase of δ⁸⁷Sr with depth in the most of the metamorphic rock-based soils can be accounted for by the downward decrease of Sr contributions from organic matter and Saharan dust, both displaying lower isotopic ratios than the soil bedrocks. The carbonate rock-based soil exhibits δ⁸⁷Sr higher (1.7–18.1‰) than the bedrock, indicating a significant contribution of radiogenic Sr from the siliciclastic fraction of the soil, and probably from dust input. The Nd isotopic ratios are slightly variable through the profiles (ɛ_Nd from −7.8 to −14.5), confirming little mobility of Nd and Sm during the pedogenesis. Among the minerals present in the soils, phosphates, albite, and calcite are those important in providing low radiogenic Sr and Nd to organic matter of the soils.Lastly, this isotopic study has in particular allowed for evaluating the potential proportion of contribution of Saharan dust to south-eastern Sardinia, thus corroborating the findings of other studies related to soils from the central-western Mediterranean.     

OSL ages and possible bioturbation signals at the Upper Paleolithic site of Lagoa do Bordoal,Algarve, Portugal

Three sediment samples were collected from a soil profile at the Lagoa do Bordoal site, Algarve, Portugal. Quartz extracted from soil horizons within the profile, was optically dated using the single-aliquot regenerative-dose (SAR) protocol outlined by Murray and Wintle (Radiation Measurements 5 (2000) 523). The optically stimulated luminescence (OSL) emitted by quartz within the 90–150 μm size range was measured using three aliquot sizes. The equivalent doses (D_E) that were obtained show an asymmetric distribution. As the numbers of grains in each aliquot are reduced, the distribution of doses is clearly shifted to lower D_E values including zero doses on single grain aliquots. This shift is accompanied by an increase in the asymmetry of the distribution. These patterns indicate that as the aliquot size decreases, the distributions are increasingly characterized by aliquots with lower D_E's. With respect to the present day modern surface, this shift is most evident in the sample collected from a depth of 37 cm, within an ancient B-horizon. It is least evident in the sample located only 10 cm below it, within a sand unit. An asymmetric distribution of D_E's can be a result of a large number of grains that were well bleached at burial being mixed with a small number of grains that were partially bleached at that time (Radiation Measurements 30 (1999) 207). However, the shift in D_E's that is seen in the Lagoa East samples is most likely due to the postburial downward movement of fully and partially bleached grains from the surface, with possible contributions from the downward movement of grains that received low doses while buried in overlying horizons. The simplest explanation for our results is that grains carrying zero or small D_E's have been moved downward during bioturbation events. These results suggest that downward bioturbation in ancient soil profiles can be identified based on the OSL characteristics of quartz grains. We are also able to provide a minimum age of 24.4±3.2 ka for the Upper Palaeolithic lithic assemblage at the site.     

Size distribution of condensed aromatic rings in various soil humic acids

Condensed aromatic rings are important skeletal components with regard to the recalcitrant nature of humic acids (HAs) in the environment. However, they have not been extensively studied. The relative content and composition (size distribution) of condensed aromatic rings in HAs were obtained from various soils subjected to transmission electron microscopy (TEM) and X-ray diffraction (XRD) profile analysis. In the XRD profiles of all the HAs, the 11 band that was derived from the carbon layer planes was clearly observed. Analysis of the 11 band indicated that the size of the carbon layer planes in HAs ranged from 0.48-1.68 nm, corresponding to 4- to 37-ring condensed aromatic structures. The contents of the total and larger carbon layer planes were larger in HAs with darker color and larger aromatic carbon content. At the same time, the carbon layer planes in HAs were smaller than those in a carbon black reference (from 0.24 to >3.66 nm). In the TEM analysis, fringes observed in HAs were less distinct and less ordered than those in carbon black, which was in agreement with the XRD result.     

Contribution of maize root derived C to soil organic carbon throughout an agricultural soil profile assessed by compound specific C analysis

Cutin and suberin structural units might be stabilized in subsoils and contribute to the aliphatic structures observed in stabilized soil organic matter (SOM). We studied their dynamics in subsoils by measuring the concentrations and ¹³C contents of cutin and suberin markers in soil profiles under wheat (C₃) and after 9 years of maize cropping (C₄ plant). Alkandioic acids were considered as markers for roots, mid-chain hydroxy acids were only present in shoots and ω-hydroxy acids were identified in both roots and shoots. The diacid concentrations greatly increased below the ploughed layer after 9 years of maize cropping, possibly due to a higher root density of maize compared to wheat or to a faster turnover of fine roots and increased exudation of maize compared to wheat. From 0-75 cm, 9 years of maize cropping did not affect the distribution of shoot biomarkers but increased their concentrations. By contrast, below 75 cm, the shoot marker concentrations drastically decreased from the wheat control to the 9 year maize soil. The difference of δ¹³C observed for shoot markers was always lower than that observed for ω-hydroxy acids, and below 15 cm, it was close to that observed for SOC. The difference in δ¹³C of diacids was much more variable along the profile. Since the concentrations of the different markers were not at equilibrium, it was not possible to estimate their turnover. This study suggests several caveats for the use of molecular markers of roots and shoots to study the dynamics of SOM in deep soils: the higher heterogeneity compared to the ploughed layer, the presence of long history record of past vegetation that may hinder the short time scale changes tracked with the ¹³C isotope technique, and the difficulty in evaluating root inputs in the soil systems.     

Effects of soil organic matter composition on unfrozen water content and heterotrophic CO2 production of frozen soils

Several recent studies have highlighted the importance of soil organic matter (SOM) mineralization at high latitudes during winter for ecosystem carbon (C) balances, and the ability of the soil to retain unfrozen water at sub-zero temperatures has been shown to be a major determinant of C mineralization rates. Further, SOM is believed to strongly influence the liquid water contents in frozen surface layers of boreal forest soils and tundra, but the mechanisms and specific factors involved are currently unknown. Here we evaluate the effects of the chemical composition of SOM on the amount of unfrozen water, the pore size equivalents in which unfrozen water can exist, and the microbial heterotrophic activity at sub-zero temperatures in boreal forest soils. To do this, we have characterized the chemical composition of SOM in forest soil samples (surface O-horizons) using solid state CP-MAS (cross polarization magic angle spinning) NMR spectroscopy. The acquired information was then used to elucidate the extent to which different fractions of SOM can explain the observed variations in unfrozen water content, pore size equivalents, and biogenic CO₂ production rates in the examined soil samples under frozen conditions (−4 °C). The data evaluation was done by the use of principal component analysis (PCA) and projections to latent structures by means of partial least square (PLS). We conclude that aromatic, O-aromatic, methoxy/N-alkyl and alkyl C are the major SOM components affecting frozen boreal forest soil’s ability to retain unfrozen water and sustain heterotrophic activity (95% confidence level). Our results reveal that solid carbohydrates have a significant negative impact (95% confidence level) on CO₂ production in frozen boreal spruce forest soils, in contrast to the positive effects of carbohydrate polymers during unfrozen conditions. We conclude that the hierarchy of environmental factors controlling SOM mineralization changes as soils freeze. The effect of SOM composition on pore size distribution and unfrozen water content has a superior influence on SOM mineralization and hence on heterotrophic CO₂ production of frozen soils.     

Is soil natural organic matter a sink or source for mobile radioiodine (I) at the Savannah River Site?

¹²⁹I is one of the three major radiation risk contributors to the public as a consequence of past nuclear processing activities at Department of Energy (DOE) facilities. Elevated levels of ¹²⁹I are present in the surface soils of F-Area of Savannah River Site, which used to be an isotope separation facility for the production of nuclear weapons components. The ¹²⁹I in soils is thought to be bound predominantly to soil organic matter (SOM). Measurements of stable ¹²⁷I and radioactive ¹²⁹I in humic acids (HAs) and fulvic acids (FAs) obtained by five successive alkaline, two glycerol and one citric acid-alkaline extraction, demonstrated that these extractable humic substances (HS) together account for 54-56% and 46% of the total ¹²⁷I and ¹²⁹I in the soil, respectively. The remainder was likely bound to residual SOM. The iodine content (μg-I/g-C) generally decreased with each subsequent extract, while ¹²⁹I/¹²⁷I increased concurrently. The coincident variations in chemical compositions, aromaticity (estimated by UV spectroscopy), functional groups (e.g., aliphatic), degree of humification, relative migration in the hydrophobic interaction column, and molecular weight indicated that: (1) iodine in different HAs was bound to a small-size aromatic subunit (∼10 kDa); (2) the large-size subunit (∼90 kDa), which likely linked the small-size unit through some weak chemical forces (hydrogen bonds, hydrophobic or electrostatic interactions), determined the relative mobility of iodine bound to organic matter; (3) from the strong correlation between iodine content and aromaticity in the HAs, we suggested that iodine incorporation into the SOM via covalent aromatic C-I bond is the key mechanism controlling iodine behavior in this system. However, this relationship is not universal for all fractions of organic matter as evidenced from the different slopes of this relationship at the two sampling sites, as well as from the different relationships for HAs and FAs, respectively. These differences in iodination are due to different SOM molecular sizes, compositions, and availability of preferred iodination sites. ¹²⁹I in the soil downstream from the contaminated site and near a wetland abruptly dropped below our detection limit (0.5 pCi-¹²⁹I/g-soil), which suggests that the high SOM in the plume soil around the ¹²⁹I-contaminated F-Area might be a natural barrier to scavenge radioiodine released from the nuclear waste repository by forming organo-iodine compounds. Soil resuspension experiments showed that mobile ¹²⁹I was mostly associated with a low average molecular weight amphiphilic organic carrier (13.5-15 kDa). SOM clearly behaves as a sink for iodine at the Savannah River Site F-Area. However, this work demonstrates that a small fraction of the SOM can also behave as a source, namely that a small fraction that may be readily dispersible under some environmental conditions and presumably release iodine in the organic-colloidal form. This radioiodinated organo-colloid likely can get into the groundwater through infiltration or surface runoff where it might migrate further into the wetlands. Results from this study provide the geochemical basis for future ¹²⁹I migration controls, remediation, and/or land-groundwater management strategies.     

Application of carbon isotope analysis to ancient maize agriculture in the Petexbatún region of Guatemala

The ancient Maya subsisted in an environment limited by shallow soils and unpredictable weather patterns until their collapse ∼A.D. 800–900. Ancient subsistence can be a difficult subject, with little physical evidence of agricultural artifacts and structures. This study characterized soil profiles and utilized changes in stable carbon isotope ratios of soil organic matter (SOM) to locate and interpret areas of ancient C₄ plant growth and maize (Zea mays) cultivation among the Maya. The investigation indicated some of the challenges the Maya faced, including shallow and sloped soils in some areas. The C₄ plant signature was found in seasonal wetland soils on the opposite side of the Laguneta Aguateca from the ruins of Aguateca, but not in the perennial wetlands on the immediate side. No C₄ plant signature was detected in the shoulder and backslope soils. Based on these findings, the ancient Maya of Aguateca probably adapted to their environment by farming rich toeslope soils. It is possible that maize was also grown in the seasonal wetlands adjacent to the site. If the steep backslope soils around Aguateca were used in ancient agriculture, the evidence has probably eroded away. © 2007 Wiley Periodicals, Inc.     

Topographic controls on the depth distribution of soil CO2 in a small temperate watershed

Accurate measurements of soil CO₂ concentrations (pCO₂) are important for understanding carbonic acid reaction pathways for continental weathering and the global carbon (C) cycle. While there have been many studies of soil pCO₂, most sample or model only one, or at most a few, landscape positions and therefore do not account for complex topography. Here, we test the hypothesis that soil pCO₂ distribution can predictably vary with topographic position. We measured soil pCO₂ at the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), Pennsylvania, where controls on soil pCO₂ (e.g., depth, texture, porosity, and moisture) vary from ridge tops down to the valley floor, between planar slopes and slopes with convergent flow (i.e., swales), and between north and south-facing aspects. We quantified pCO₂ generally at 0.1–0.2 m depth intervals down to bedrock from 2008 to 2010 and in 2013. Of the variables tested, topographic position along catenas was the best predictor of soil pCO₂ because it controls soil depth, texture, porosity, and moisture, which govern soil CO₂ diffusive fluxes. The highest pCO₂ values were observed in the valley floor and swales where soils are deep (≥0.7 m) and wet, resulting in low CO₂ diffusion through soil profiles. In contrast, the ridge top and planar slope soils have lower pCO₂ because they are shallower (≤0.6 m) and drier, resulting in high CO₂ diffusion through soil profiles. Aspect was a minor predictor of soil pCO₂: the north (i.e., south-facing) swale generally had lower soil moisture content and pCO₂ than its south (i.e., north-facing) counterpart. Seasonally, we observed that while the timing of peak soil pCO₂ was similar across the watershed, the amplitude of the pCO₂ peak was higher in the deep soils due to more variable moisture content. The high pCO₂ observed in the deeper, wetter topographic positions could lower soil porewater pH by up to 1 pH unit compared to porewaters equilibrated with atmospheric CO₂ alone. CO₂ is generally the dominant acid driving weathering in soils: based on our observations, models of chemical weathering and CO₂ dynamics would be improved by including landscape controls on soil pCO₂.     

Effects of age on the chemical structure of paleosol humic acids and fulvic acids

Humic acids and fulvic acids were extracted from six paleosols in Southern Italy. Humic acids (HAs) constituted between 96.5 and 99.2% of the total extracts; the remaining materials consisted of fulvic acids (FAs). Radiocarbon ages of the HAs ranged from about 6,000 to close to 29,000 years B.P., δ¹³C values averaged $\text{?25.6 ± 0.3‰}$ The HAs were characterized by chemical (elemental and functional group analyses) and spectroscopic (IR, ESR, ¹³CNMR, E₄/E₆ ratios) methods. FAs were characterized by chemical methods, E₄/E₆ ratios and IR spectra.The chemical and spectroscopic analyses showed practically no differences in the chemical structure and composition of the six HAs and FAs, so that age appeared to have little effect on these parameters. The paleosols were found to be closed systems with low polysaccharide and protein contents, thus providing unfavorable substrates for microbial activity. The preservation of the humic materials in the paleosols may have been due to low biological activity and/or to retention by amorphous minerals. The HAs did not appear to be affected by temperatures higher than 170–200°C over the 23,000 year period which we observed.     

A comparative study of the molecular composition of a grassland soil with adjacent unforested and afforested moorland ecosystems

On-line thermally assisted hydrolysis and methylation (THM) in the presence of both unlabelled and ¹³C-labelled tetramethylammonium hydroxide (TMAH) was used to assess the relative contributions of phenolics (lignin, demethylated lignin and non-lignin phenolics) in a peaty gley soil profile beneath an unimproved grassland (LL), from a study site located at Harwood (Northumberland, northeast England, UK). This site also includes an unforested moorland (ML) and a second rotation Sitka spruce stand (SS). The common lignin proxies have been corrected for contributions of non-lignin phenols and demethylated lignin in the LL ecosystem and then compared with those from the ML and SS ecosystems. The phenolic compositions from the contributory vegetation inputs (i.e. grasses, heather and Sitka spruce) to all three soils (LL, ML and SS) were also analysed. By using ¹³C-labelled TMAH it was possible to show that the chemical composition of soil organic matter (SOM) reflected the different vegetation inputs in each of the L/F layers but these characteristics were lost from the deeper organic and mineral layers. Similar changes in the yield of lignin monomers (Λ) with increasing soil depth were displayed in the LL soil profile as reported previously in the ML soil in that no maxima were observed in these amount-depth profiles. The tannin input to the LL soil is low and as a consequence, unlike the ML and SS soils, there is no progressive decrease in the amounts of these non-lignin phenolics with increasing depth. Finally the methylated carbohydrate derivatives (MC) become more abundant relative to the phenolics with increasing soil depth in all three ecosystems (LL, ML and SS).