The origin of carbonates in termite mounds of the Lubumbashi area, D.R. Congo

The origin of carbonate accumulations in termite mounds is a controversial issue. This study is an attempt to elucidate the processes of carbonate precipitation in Macrotermes mounds built on Ferralsols in Upper Katanga, D.R. Congo, whereby a differentiation between pedogenic and inherited carbonates is considered. Carbonate features were investigated for a 9 m deep termite-mound profile, and for an 18 m wide cross-section through a termite mound and the adjacent soil, using field and laboratory techniques. Field evidence for a pedogenic origin includes morphological type (soft powdery materials, nodules, and coatings on ped surfaces) and distribution patterns of the carbonates. Thin-section studies reveal that the carbonates occur predominantly as impregnative orthic nodules and less commonly as coatings, both clearly pedogenic; calcareous pellets are interpreted as locally reworked pedogenic carbonates. X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray spectrometry (SEM-EDS) and stable isotope (δ¹³C) analyses show that all isolated carbonate features consist of high-Mg calcite (4.9-12.3 mol% MgCO₃) with δ¹³C signatures ranging from − 13.2‰ to − 11.5‰. Weddellite (CaC₂O₄. 2H₂O) is identified in a thin-section and by XRD analysis, and appears to be locally transformed into calcite. The stable isotope composition of carbon suggests that calcite precipitated in equilibrium with soil CO₂ generated during decomposition of soil organic matter, and locally most likely during oxidation of oxalate. This study proves that carbonates which accumulated in Macrotermes mounds are pedogenic precipitates, whose deposition is partly related to microbial decay of organic matter, subsequently redistributed to some extent by abiotic dissolution-reprecipitation and termite activity.     

Effect of temperature and rhizosphere processes on pedogenic carbonate recrystallization: Relevance for paleoenvironmental applications

In soils of arid and semiarid climates, dissolution of primary (lithogenic) carbonate and recrystallization with CO₂ from soil air leads to precipitation of pedogenic carbonates and formation of calcic horizons. Thus, their carbon isotope composition represents the conditions prevailing during their formation. However, the widespread use of the isotopic signature (δ¹³C, δ¹⁸O, Δ¹⁴C) of pedogenic carbonates for reconstruction of local paleovegetation, paleoprecipitation and other environmental conditions lacks knowledge of the time frame of pedogenic carbonate formation, which depends on climatic factors. We hypothesized that temperature-dependent biotic processes like plant growth and root and rhizomicrobial respiration have stronger influence on soil CaCO₃ recrystallization than abiotic temperature-dependent solubility of CO₂ and CaCO₃.To assess the effect of temperature on initial CaCO₃ recrystallization rates, loess with primary CaCO₃ was exposed to ¹⁴CO₂ from root and rhizomicrobial respiration of plants labeled in ¹⁴CO₂ atmosphere at 10, 20 or 30 °C. ¹⁴C recovered in recrystallized CaCO₃ was quantified to calculate amounts of secondary CaCO₃ and corresponding recrystallization rates, which were in the range of 10⁻⁶-10⁻⁴ day⁻¹, meaning that 10⁻⁴-10⁻²% of total loess CaCO₃ were recrystallized per day. Increasing rates with increasing temperature showed the major role of biological activities like enhanced water uptake by roots and respiration. The abiotic effect of lower solubility of CO₂ in water by increasing temperature was completely overcompensated by biotic processes. Based on initial recrystallization rates, periods necessary for complete recrystallization were estimated for different temperatures, presuming that CaCO₃ recrystallization in soil takes place mainly during the growing season. Taking into account the shortening effect of increasing temperature on the length of growing season, the contrast between low and high temperature was diminished, yielding recrystallization periods of 5740 years, 4330 years and 1060 years at 10, 20 and 30 °C, respectively. In summary, increasing CaCO₃ recrystallization rates with increasing temperature demonstrated the important role of vegetation for pedogenic CaCO₃ formation and the predominantly biotic effects of growing season temperature.Considering the long periods of pedogenic carbonate formation lasting to some millennia, we conclude that methodological resolution of paleoenvironmental studies based on isotope composition of pedogenic carbonates is limited not by instrumental precision but by the time frame of pedogenic carbonate formation and hence cannot be better than thousands of years.     

Distribution of stable carbon isotopes in an agrochernozem during the transition from C3 vegetation to a corn monoculture

The distribution of carbon in an agrochernozem??s profile was studied by the natural ¹³C abundance method during the C₃-C₄ vegetation transition and the analysis of the soil phytolith complex under a continuous corn monoculture. A young pool of soil organic matter (SOM) formed during 43 years of monoculture growing was detected by the isotope analysis in the 0-to 60-cm layer, while the analysis of the phytolith complex identified this pool deeper: corn phytoliths were detected in the 0- to 80-cm layer. The maximum size of the young pool was found in the upper soil horizon; it reached 6.4% of the SOM in the 0- to 20-cm layer. The apparent time of the SOM turnover was 635 and 2225 years in the 0- to 20- and 40- to 60-cm layers, respectively. The high values of the mean residence time were related to the low input of plant residues to the soil at the growing of corn for silage and the high initial content of organic carbon in the chernozem. The changes in the isotope composition after the decalcification of the soil to remove carbonates and the variation of the ??¹³C in the corn biomass during the vegetation period significantly affected the calculated value of the mean residence time.     

Identification of carbonate pedofeatures of different ages in modern chernozems

Carbonate pedofeatures of three chernozemic soils developed from loesslike loams in the foreststeppe zone of Lipetsk oblast under fallow plot (Luvic Chernozem (Clayic, Pachic)) and under forest (Calcic Chernozem (Clayic, Pachic)) and in the steppe zone of Dnepropetrovsk oblast (Calcic Chernozem (Episiltic, Endoclayic, Pachic)) were studied in the field and laboratory with the use of a set of methods, including the radiocarbon method, mass spectrometry, and micro- and submicromorphology. The morphological diversity of carbonate pedofeatures in these soils was represented by carbonate veins, coatings, disperse carbonates (carbonate impregnations), soft masses (beloglazka), and concretions. In the forest-steppe soils, disperse carbonates and soft masses were absent. The radiocarbon age of carbonate pedofeatures in the forest-steppe soils varied within a relatively narrow range of 3–4.3 ka cal BP with a tendency for a younger age of carbonate concretions subjected to destruction (geodes). In the steppe chernozem, this range was larger, and the ¹⁴C ages of different forms of carbonate pedofeatures were different. Thus, soft masses (beloglazka) had the age of 5.5–6 ka cal BP; disperse carbonates, 17.5–18.5 ka cal BP; and hard carbonate concretions, 26–27 ka cal BP. Data on δ¹³C demonstrated that the isotopic composition of carbon in virtually all the “nonlabile” carbonate pedofeatures does not correspond to the isotopic composition of carbon of the modern soil organic matter. It was shown that the studied chernozemic soils are polygenetic formations containing carbonate pedofeatures of different ages: (a) recent (currently growing), (b) relict, and (c) inherited pedofeatures. The latter group represents complex pedofeatures that include ancient fragments integrated in younger pedofeatures, e.g., the Holocene soft carbonate nodules with inclusions of fragments of the ancient microcodium.     

Differential response of mineral-associated organic matter in tropical soils formed in volcanic ashes and marine Tertiary sediment to treatment with HCl,NaOCl, and Na4P2O7

Quantitative knowledge of the amount and stability of soil organic matter (SOM) is necessary to understand and predict the role of soils in the global carbon cycle. At present little is known about the influence of soil type on the storage and stability of SOM, especially in the tropics. We compared the amount of mineral-associated SOM resistant to different chemical treatments in soils of different parent material and mineralogical composition (volcanic ashes – dominated by short-range-order aluminosilicates and marine Tertiary sediments – dominated by smectite) in the humid tropics of Northwest Ecuador. Using ¹³C isotope analyses we traced the origin of soil organic carbon (SOC) in mineral-associated soil fractions resistant to treatment with HCl, NaOCl, and Na₄P₂O₇ under pasture (C₄) and secondary forest (C₃). Prior to chemical treatments, particulate organic matter was removed by density fractionation (cut-off: 1.6 g cm^?3). Our results show that: (1) independent of soil mineralogical composition, about 45% of mineral-associated SOC was resistant to acid hydrolysis, suggesting a comparable SOM composition for the investigated soils; (2) oxidation by NaOCl isolated a SOM fraction with enhanced stability of mineral-bound SOM in soils developed from volcanic ashes; while Na₄P₂O₇ extracted more SOC, indicating the importance of Al-humus complexes in these soils; and (3) recently incorporated SOM was not stabilized after land use change in soils developed from volcanic ashes but was partly stabilized in soils rich in smectites. Together these results show that the employed methods were not able to isolate a SOM fraction which is protected against microbial decay under field conditions and that the outcome of these methods is sensitive to soil type which makes interpretation challenging and generalisations to other soils types or climates impossible.     

The influence of soil processes on carbon isotope distribution and turnover in the British uplands

Understanding the natural variation of carbon within the soil, and between soil types, is crucial to improve predictive models of carbon cycling in high and mid-latitude ecosystems in response to global warming. We measured the carbon isotope distributions (¹²C, ¹³C and ¹⁴C) in soil organic matter (SOM) from Podzols, Brown Podzolic soils and Stagnohumic Gleysols from the British uplands, which were then compared with the total amounts and turnover of carbon in these soils. We did so by sampling at 2-cm intervals down six profiles of each soil type. The average amount of carbon stored in the top 28 cm of the Stagnohumic Gleysols is twice that of the other two soils. The ¹³C content and ¹⁴C age show a general increase with depth in all soils, and there is also a significant correlation between isotopic variation and the main pedogenic features. The latter suggests that soil-forming processes are significant in determining the carbon isotope signatures retained in SOM. Organic matter formed since 1960 is not found below 5 cm in any of the soils. Evidently organic detritus in the surface layers (LF and Oh) is rapidly mineralized. This accords with our modelled net annual C fluxes which show that more than 80% of the CO₂ emanating from these soils is derived from the top 5 cm of each profile. Although these soils contain much carbon, they do not appear to assimilate and retain SOM rapidly. The mean residence time of most of their carbon is in the 2–50 years range, so the soils are fairly ineffective sinks for excess CO₂ in the atmosphere. Under the predicted future ‘greenhouse’ climate, likely to favour more rapid microbial decomposition of organic materials, these soils are a potential source of CO₂ and are therefore likely to accelerate global warming.     

Total carbon and nitrogen in the soils of the world

The soil is important in sequestering atmospheric CO₂ and in emitting trace gases (e.g. CO₂, CH₄ and N₂O) that are radiatively active and enhance the ‘greenhouse’ effect. Land use changes and predicted global warming, through their effects on net primary productivity, the plant community and soil conditions, may have important effects on the size of the organic matter pool in the soil and directly affect the atmospheric concentration of these trace gases. A discrepancy of approximately 350 × 10¹⁵ g (or Pg) of C in two recent estimates of soil carbon reserves worldwide is evaluated using the geo-referenced database developed for the World Inventory of Soil Emission Potentials (WISE) project. This database holds 4353 soil profiles distributed globally which are considered to represent the soil units shown on a ¹/₂° latitude by ¹/₂° longitude version of the corrected and digitized 1:5 M FAO–UNESCO Soil Map of the World. Total soil carbon pools for the entire land area of the world, excluding carbon held in the litter layer and charcoal, amounts to 2157–2293 Pg of C in the upper 100 cm. Soil organic carbon is estimated to be 684–724 Pg of C in the upper 30 cm, 1462–1548 Pg of C in the upper 100 cm, and 2376–2456 Pg of C in the upper 200 cm. Although deforestation, changes in land use and predicted climate change can alter the amount of organic carbon held in the superficial soil layers rapidly, this is less so for the soil carbonate carbon. An estimated 695–748 Pg of carbonate-C is held in the upper 100 cm of the world's soils. Mean C: N ratios of soil organic matter range from 9.9 for arid Yermosols to 25.8 for Histosols. Global amounts of soil nitrogen are estimated to be 133–140 Pg of N for the upper 100 cm. Possible changes in soil organic carbon and nitrogen dynamics caused by increased concentrations of atmospheric CO₂ and the predicted associated rise in temperature are discussed.     

Verwertung organischer Substanzen durch mikrobielle Bodenbiomasse als eine Funktion chemisch-thermodynamischer Parameter

Carbon use efficiency of organic substances by soil microbial biomass as a function of chemical and thermodynamical parameters A simple calculation determining carbon use efficiency by soil microorganisms based on chemical and thermodynamical parameters of organic matter is proposed. The use efficiency characterizes carbon fraction of organic matter which is incorporated into the cells of the soil microbial biomass. The proposed approach is based on the transition of the organic matter enthalpy into the microbial biomass enthalpy considering the formation enthalpy of the end products of respiration like CO₂, NH₄⁺ and H₂O. The combustion energy content of organic matter was used for the calculations. This combustion energy content can be determined by simple analytical means. It can be derived from data given in the literature and for various agricultural products. The comparison of organic matter use efficiency data calculated as shown above with literature data produced by diverse methods showed a satisfactory correlation. The calculated enthalpy of formation and maintenance of a C-unit of soil microbial biomass under in situ conditions amounted to —153,3 kJ · gC^—1. This value is compared with the maintenance coefficient used in microbiology. The use of the suggested approach for the calculation of carbon use efficiency based on the substance composition of the organic matter allows a uniform, standardized procedure which is not dependent on specific experimental conditions.     

The contribution of biogas residues to soil organic matter formation and CO2 emissions in an arable soil

The biogas production process generates as side-products biogas residues containing microbial biomass which could contribute to soil organic matter formation or induce CO₂ emissions when applied to arable soil as fertilizer. Using an isotope labelling approach, we labelled the microbial biomass in biogas residues, mainly G⁺ bacteria and methanogenic archaea via KH¹³CO₃, and traced the fate of microbial biomass carbon in soil with an incubation experiment lasting 378 days. Within the first seven days, 40% of the carbon was rapidly mineralized and after that point mineralization continued, reaching 65% by the end of the experiment. Carbon mineralization data with 93% recovery could be fitted to a two-pool degradation model which estimated proportions and degradation rate constants of readily and slowly degrading pools. About 49% of the carbon was in the slowly degrading pool with a half-life of 1.9 years, suggesting mid-term contribution to living and non-living soil organic matter formation. Biogas residues caused a priming effect at the beginning, thus their intensive application should be avoided.     

Coatings in cryoaridic soils and other records of landscape and climate changes in the Ak-Khol Lake basin (Tyva)

An assemblage of coatings in cryoaridic soils (Skeletic Cambisols Protocalcic) of southwestern Tyva is considered as a key block of soil memory, which is an intrasoil archive of landscape and climate changes in regional geosystems in the Holocene. The results of hierarchical macro-, meso-, and micromorphological studies of a large assemblage of coatings and the data on the X-ray microanalysis of coatings and composition of stable carbon and oxygen isotopes, as well as on radiocarbon dating of coatings are presented. The synthesis of the results made it possible to reconstruct the main evolutionary phases of cryoaridic soils in the Holocene and landscape and climate changes that induced their alteration. The following climatogenic evolutionary phases of pedogenesis were distinguished: (1) formation of microsparite–micritic dense silica-containing coatings due to short-term fluctuations of the shallow alkaline bicarbonate groundwater level in the semiarid–arid climate; (2) formation of sparitic dense coatings under the slow accumulation of carbonates from low-mineralized bicarbonate water at the higher lake level as compared to the present one in the less arid conditions; (3) the eluvial-illuvial formation of micritic loose coatings under stable automorphic semiarid conditions; (4) formation of Fe-humus coatings in cool humid climate (Al–Fe-humus phase of pedogenesis); (5) the recommencement of the eluvial-illuvial formation of micritic loose coatings under aridization of the last thousand years of the Holocene.     

Incorporation of C-labeled rice-straw-derived carbon into microbial communities in submerged rice field soil and percolating water

Rice straw is a major organic material applied to rice fields. The microorganisms growing on rice-straw-derived carbon have not been well studied. Here, we applied ¹³C-labeled rice straw to submerged rice soil microcosms and analyzed phospholipid fatty acids (PLFAs) in the soil and percolating water to trace the assimilation of rice-straw-derived carbon into microorganisms. PLFAs in the soil and water were markedly enriched with ¹³C during the first 3 days of incubation, which indicated immediate incorporation of rice-straw-derived carbon into microbial biomass. The enrichment of PLFAs in the percolating water with ¹³C suggested that microorganisms other than the population colonizing rice straw also assimilated rice-straw-derived carbon or that some bacterial groups were selectively released from the straw. The microbial populations could be categorized into two communities based on the carbon isotope data of the PLFAs: those derived from rice straw and those derived from soil organic matter (SOM). The composition of the PLFAs from the two communities differed, which indicated the assimilation of rice-straw-derived carbon by a subset of microbial populations. The composition of rice-straw-derived PLFAs in the percolating water was also distinct from that in the soil.     

Carbon and nitrogen mineralization in acidic, limed and calcareous agricultural soils: Apparent and actual effects

Soil pH and calcium carbonate contents are often hypothesized to be important factors controlling organic matter turnover in agricultural soils. The aim of this study was to differentiate the effects of soil pH from those related to carbonate equilibrium on C and N dynamics. The relative contributions of organic and inorganic carbon in the CO₂ produced during laboratory incubations were assessed. Five agricultural soils were compared: calcareous (74% CaCO₃), loess (0.2% CaCO₃) and an acidic soil which had received different rates of lime 20 years ago (0, 18 or 50 t ha⁻¹). Soil aggregates were incubated with or without rape residues under aerobic conditions for 91 days at 15 °C. The C and N mineralized, soil pH, O₂ consumption and respiratory quotient (RQ=ΔCO₂/ΔO₂) were monitored, as well as the δ¹³C composition of the evolved CO₂ to determine its origin (mineral or organic). Results showed that in non-amended soils, the cumulative CO₂ produced was significantly greater in the limed soil with a pH>7 than in the same soil with less or no lime added, whereas there was no difference in N mineralization or in O₂ consumption kinetics. We found an exponential relationship between RQ values and soil pH, suggesting an excess production of CO₂ in alkaline soils. This CO₂ excess was not related to changes in substrate utilization by the microbial biomass but rather to carbonates equilibrium. The δ¹³C signatures confirmed that the CO₂ produced in soils with pH>7 originated from both organic and mineral sources. The contribution of soil carbonates to CO₂ production led to an overestimation of organic C mineralization (up to 35%), the extent of which depended on the nature of soil carbonates but not on the amount. The actual C mineralization (derived from organic C) was similar in limed and unlimed soil. The amount of C mineralized in the residue-amended soils was ten times greater than in the basal soil, thus masking the soil carbonate contribution. Residue decomposition resulted in a significant increase in soil pH in all soils. This increase is attributed to the alkalinity and/or decarboxylation of organic anions in the plant residue and/or to the immobilization of nitrate by the microbial biomass and the corresponding release of hydroxyl ions. A theoretical composition (C, O, H, N) of residue and soil organic matter is proposed to explain the RQ measured. It emphasizes the need to take microbial biomass metabolism, O₂ consumption due to nitrification and carbon assimilation yield into account when interpreting RQ data.     

Nitrogen isotope discrimination in denitrification of nitrate in soils

Nitrogen isotope discrimination during denitrification in soils of nitrate containing natural concentrations of ¹⁴N and ¹⁵N was studied by determining the amount and the ¹⁵N content of nitrate-N and (nitrate + nitrite)-N in nitrate-treated soils incubated under anaerobic conditions (He atmosphere) for various times after treatment with glucose to promote denitrification. Analyses performed showed that the nitrate-N lost on incubation of these soils could largely be accounted for as products of denitrification (nitrite, NO. N₂O and N₂).The studies reported show that marked discrimination between ¹⁴N and ¹⁵N occurs during denitrification of nitrate in soils and that significant N isotope effects occur both in reduction of nitrate to nitrite and in reduction of nitrite to gaseous forms of N. They also indicate that the overall N isotope effect during denitrification of nitrate in soil will depend upon the tendency of the soil to accumulate nitrite under conditions that induce denitrification.It is concluded that discrimination between ¹⁴N and ¹⁵N during denitrification in soils of nitrate containing natural concentrations of these isotopes is of sufficient magnitude to invalidate the use of N isotope-ratio analyses for assessment of the contributions of soil and fertilizer N to nitrate in surface or ground waters or to nitrous oxide in the atmosphere.     

Role of aggregate surface and core fraction in the sequestration of carbon from dung in a temperate grassland soil

The sequestration of dung carbon in soil depends on the location and rate at which it is immobilized in soil aggregates. Here C₄ dung (δ¹³C = ?16.1‰) or C₃ dung (δ¹³C = ?26.8‰) were applied to a temperate permanent pasture C₃ soil (δ¹³C = ?27.9‰). Triplicate samples were taken from C₃ and C₄ dung remaining at the surface, and in the 0–1 and 1–5 cm soil layers in the unamended control and under the C₃ and C₄ dung patches after 7, 14, 29, 42 and 70 days after the application of the dung. Macroaggregates (≥ 4 mm) at the lower depth (1–5 cm) were mechanically fractionated into surface and core fractions by a combination of shock freezing followed by wet sieving. Neither overall nor differential carbon isotope fractionation occurred in the dung remaining at the surface. The incorporation of C₄ dung significantly increased the δ¹³C content of the 0–1 cm layer of the C₃ soil. Dung C sequestration did not exceed 10% for the 0–1 cm layer and was only 20% for the whole soil (0–30 cm) during the 7‐day experiment. Only 32–66% of the C from dung in the 1–5 cm layer was sequestered in the aggregates; the major proportion was initially preferentially attached to their surfaces, but incorporated into aggregates within the following 14 days. The majority of dung, however, soon resided between the aggregates, pointing to the important role of the inter‐aggregate fraction in short‐term C dynamics of dung in this pasture soil.     

Continuous flow isotope ratio mass spectrometry of carbon dioxide trapped as strontium carbonate

Abstract

The isotopic signal provided by differential discrimination against atmospheric carbon dioxide (¹³CO₂) by C₃ and C₄ plant photosynthetic pathways is being widely used to study the processes of carbon (C) fixation, soil organic matter formation, and mineralization in nature. These studies have been facilitated by the availability of automated C and nitrogen (N) combustion analyzers (ANCA) combined with continuous flow isotope ratio mass spectrometers (CFIRMS). Analysis of ¹³CO₂ in these instruments requires consistent sample mass for best precision, a requirement that is easily satisfied for soil and tissue samples by adjusting sample weight. Consistent CO₂ sample size is much more difficult to achieve using gas handling systems for samples of headspace gases when CO₂ concentrations vary widely. Long storage of gaseous samples also is difficult. Extended respiration studies are most easily conducted by trapping CO₂ in alkali and conversion to an insoluble carbonate. Thermal decomposition of the carbonate in an on‐line ANCA allows consistent and optimal CO₂ sample mass to be obtained. The use of precipitated carbonates also facilitates storage of samples and enables full automation of sample analysis using an ANCA interfaced to a CFIRMS. Calcium (Ca), strontium (Sr), and barium (Ba) carbonates were tested. Strontium carbonate (SrCO₃) with the addition of vanadium pentoxide (V₂O₅) as a combustion catalyst was found most suitable.     

Inorganic and organic carbon dynamics in forested soils developed on contrasting geology in Slovenia—a stable isotope approach

Purpose

Soil carbon dynamics were studied at four different forest stands developed on bedrocks with contrasting geology in Slovenia: one plot on magmatic granodiorite bedrock (IG), two plots on carbonate bedrock in the karstic-dinaric area (CC and CD), and one situated on Pleistocene coalluvial terraces (FGS).

Materials and methods

Throughfall (TF) and soil water were collected monthly at each location from June to November during 2005–2007. In soil water, the following parameters were determined: T, pH, total alkalinity, concentrations of Ca²⁺ and Mg²⁺, dissolved organic carbon (DOC), and Cl^? as well as δ¹³C_DIC. On the other hand, in TF, only the Cl^? content was measured. Soil and plant samples were also collected at forest stands, and stable isotope measurements were performed in soil and plant organic carbon and total nitrogen and in carbonate rocks. The obtained data were used to calculate the dissolved inorganic carbon (DIC) and DOC fluxes. Statistic analyses were carried out to compare sites of different lithologies, at different spatial and temporal scales.

Results and discussion

Decomposition of soil organic matter (SOM) controlled by the climate can explain the ¹³C and ¹⁵?N enrichment in SOM at CC, CD, and FGS, while the soil microbial biomass makes an important contribution to the SOM at IG. The loss of DOC at a soil depth of 5 cm was estimated at 1 mol m^?2 year^?1 and shows no significant differences among the study sites. The DOC fluxes were mainly controlled by physical factors, most notably sorption dynamics, and microbial–DOC relationships. The pH and pCO₂ of the soil solution controlled the DIC fluxes according to carbonate equilibrium reactions. An increased exchange between DIC and atmospheric air was observed for samples from non-carbonate subsoils (IG and FGS). In addition, higher δ¹³C_DIC values up to ?19.4?‰ in the shallow soil water were recorded during the summer as a consequence of isotopic fractionation induced by molecular diffusion of soil CO₂. The δ¹³C_DIC values also suggest that half of the DIC derives from soil CO₂ indicating that 2 to 5 mol m^?2 year^?1 of carbon is lost in the form of dissolved inorganic carbon at CC and CD after carbonate dissolution.

Conclusions

Major difference in soil carbon dynamics between the four forest ecosystems is a result of the combined influence of bedrock geology, soil texture, and the sources of SOM. Water flux was a critical parameter in quantifying carbon depletion rates in dissolved organic and inorganic carbon forms.

     

Early degradation of plant alkanes in soils: A litterbag experiment using C-labelled leaves

We monitored the carbon isotope composition of bulk leaves and specific long-chain alkanes during a four-year litterbag experiment using ¹³C-labelled leaves and unlabelled reference leaves of the European beech tree (Fagus sylvatica L.). Whereas the isotope composition of alkanes from ¹³C-enriched leaves exhibited a marked decrease in ¹³C-content, the isotope composition of unlabelled reference leaves remained nearly constant. We interpreted this difference as evidence for a microbial contribution to the long-chain alkane pool of the decomposing leaves and related it to the progressive invasion of leaves by soil organisms which was revealed upon microscopic examination. These results suggest that long-chain alkanes may not provide an unaltered record of organic carbon isotope composition in soils and sediments.     

Carbon allocation in grassland communities under drought stress followed by 14C pulse labeling

Although extreme climatic events such as drought have important consequences for belowground carbon (C) cycling, their impact on the plant-soil system of mixed plant communities is poorly understood. Our objective was to study the effect of drought on C allocation and rhizosphere-mediated CO₂ fluxes under three plant species: Lolium perenne, Festuca arundinacea and Medicago sativa grown in monocultures or mixture. The conceptual approach included ¹⁴CO₂ pulse labeling of plants grown under drought and optimum water conditions in order to be able to follow above- and belowground C allocation. After ¹⁴C pulse labeling, we traced ¹⁴C allocation to shoots and roots, soil and rhizospheric CO₂, dissolved organic carbon (DOC) and microbial biomass.Drought and plant community composition significantly affected assimilate allocation in the plant-soil system. Drought conditions changed the source sink relationship of monocultures, which transferred a relatively larger portion of assimilates to their roots compared to water sufficient plants. In contrast, plant mixture showed an increase in ¹⁴C allocation to shoots when exposed to drought.Under drought stress, root respiration was reduced for all monocultures except under the legume species. Microbial respiration remained similar in all cases showing that microbial activity was less affected by drought than root activity. This may be explained by strongly increased assimilate allocation to easily available exudates or rhizodeposits under drought. In conclusion, plant community composition may modify the impact of climatic changes on carbon allocation and belowground carbon fluxes. The presence of legume species attenuates drought effects on rhizosphere processes.     

褐土碳酸钙结核的胶体特性研究

碳酸钙是黄土母质发育土壤的重要胶结物质,对土壤团粒结构的形成具有重要作用。本文采集了碳酸盐褐土中的碳酸钙结核,采用物理分散法和化学分散法分别提取得到褐土碳酸钙结核纳米颗粒和褐土碳酸钙结核胶体,并以工业纳米碳酸钙作为对照对其胶体特性进行研究。采用X射线衍射仪、zeta电位仪和动态光散射仪对褐土碳酸钙结核胶体和工业纳米碳酸钙的矿物组成、zeta电位和胶体稳定性进行了表征。结果表明：褐土碳酸钙结核胶体、褐土碳酸钙结核纳米颗粒和工业纳米碳酸钙的初始颗粒直径分别为224.24、88.01和98.50nm,而褐土碳酸钙结核胶体和褐土碳酸钙结核纳米颗粒的多分散度高于工业纳米碳酸钙。褐土碳酸钙结核胶体中方解石含量为70.3%,其次含有石英、长石和伊利石等矿物;褐土碳酸钙结核纳米颗粒主要含有方解石和伊利石,含量分别为48%和45%。3种碳酸钙胶体表面均带负电荷,其zeta电位绝对值均随着溶液pH的增大而增大。褐土碳酸钙结核胶体在NaCl和CaCl₂溶液中的临界聚沉浓度分别为538.01mmol/L和2.08mmol/L,褐土碳酸钙结核纳米颗粒在NaCl和CaCl₂     

土壤中碳酸盐的碳氧同位素分析

本文采用磷酸法分析土壤中碳酸盐的碳、氧同位素组成,对分析方法和制样装置作了叙述,比较了经不同预处理土壤的碳、氧同位素值,指出土壤有机碳的存在不影响碳酸盐同位素组成的精密测定,有氧灼烧预处理土壤样品对分析结果有明显影响,使δ~13C和δ~18O值降低。测定多种标准样品表明,此方法准确、可靠,重复制样测定的标准偏差在0.1‰以内。