Analysis of long- and short-term temporal variations of the diffuse CO2 emission from Timanfaya volcano,Lanzarote, Canary Islands

Reported herein are the results of eight soil CO₂ efflux surveys performed from 2006 to 2011 at Timanfaya Volcanic Field (TVF), Lanzarote Island with the aim of evaluating the long- and short-term temporal variations of the diffuse CO₂ emission. Soil CO₂ efflux values ranged from non-detectable up to 34.2 g m⁻² d⁻¹, with the highest values measured in September 2008. Conditional sequential Gaussian simulations (sGs) were applied to construct soil CO₂ efflux distribution maps and to estimate the total CO₂ output from the studied area at the TVF. Soil CO₂ efflux maps showed a high spatial and temporal variability. Total CO₂ emission rates ranged between 41 and 518 t d⁻¹, February 2011 (winter) being the season when maximum diffuse CO₂ emission rates were observed. To investigate the influence of external variables on the soil CO₂ efflux, a geochemical station (LZT01) was installed at TVF to measure continuously the soil CO₂ efflux between July 2010 and March 2012 Since external factors such as barometric pressure, rainfall, soil water content, soil and air temperatures, and wind speed influence strongly the observed soil CO₂ effluxes, multiple regression analysis was applied to the time series recorded by the automatic geochemical station LZT01 to remove the contribution of these external factors. The influence of meteorological variables on soil CO₂ efflux oscillations accounts for 13% of total variance, with barometric pressure, rainfall and/or soil water content having the most influence in the control of the soil CO₂ efflux. These observations along with the results from the eight soil gas surveys performed at TVF indicate that the short and long-term trends in the diffuse CO₂ degassing are mainly controlled by environmental factors.     

Soil/landform relationships surrounding the Harappa archaeological site,Pakistan

A soil survey around the archaeological site of Harappa, Pakistan revealed alluvial deposits of five distinct ages based on relative position in the landscape and degree of soil profile development. the youngest deposit (age 1) is in the lowest landscape position and has received flood waters as recently as 1988. Soils there are in an incipient stage of development: only organic carbon and soluble salts have accumulated at the surface of the profile. the age 2 deposit has not undergone significant pedogenic change, but is in a slightly higher landscape position than the youngest deposit. Elevated concentrations of P, and the presence of sand-sized pottery and brick fragments, indicate that this deposit was derived at least partially from archaeological material. the presence of small, soft calcite nodules (Stage II) and some soluble salt translocation are the primary pedogenic changes observed in the age 3 deposit. the age 4 deposit shows evidence of both carbonate and gypsum accumulation. Presence of large gypsum nodules in deep By horizons suggests that a high groundwater table has altered these soils. the oldest deposit, age 5, forms a late Pleistocene stream terrace of the Ravi River. the soil formed in this deposit exhibits considerable carbonate accumulation, with large, dense nodules (Stage II + ) and an argillic horizon. A ¹⁴C date from pedogenic calcite gives an age of 7080 ± 90 years B.P., indicating a minimum age of early Holocene. the soil survey suggests that the ancient city of Harappa was built on an age 5 stream terrace remnant, surrounded by Holocene floodplains and a meandering channel of the Ravi River.     

Hohokam Canal Irrigation and the Formation of Irragric Anthrosols in the Middle Gila River Valley,Arizona, USA

Irragric anthrosols form as a result of prolonged deposition of fine sediments from irrigation water. Ancient irragric soils centuries to millennia old occur in several world regions, especially in arid environments of Asia and the Americas. This article presents evidence for an ancient irragric anthrosol in the North American Southwest, along the Snaketown Canal System in the Middle Gila River Valley, Arizona. This pedostratigraphic unit was formed as a result of a millennium of irrigation by Hohokam farmers from A.D. 450 to 1450. The irragric soil consists of a mantle of silty‐to‐loamy textures with minimal soil formation overlying a natural argillic horizon on a Pleistocene stream terrace. A soil mapped independently by the United States Department of Agriculture‐Natural Resources Conservation Service with these horizons corresponds closely with the canal system. Soil within the canal system tends to be lower in salt, sodium, and pH compared with external soils. This suggests that the irragric process improved soil for crop production through long‐term leaching and additions of fresh sediments with the irrigation water. This anthropogenic process of canal sedimentation has had a long‐lasting impact on the sedimentary record and soils in this arid environment.     

The response of soil CO<Subscript>2</Subscript> efflux to desertification on alpine meadow in the Qinghai–Tibet Plateau

Assessing the global C budget requires a better understanding of the effect of environmental factors on soil CO₂ efflux from both experiments and theoretical research, especially in different desertified lands in the Qinghai–Tibet Plateau. Based on the enclosed chamber method, soil CO₂ efflux in four different desertified lands and one control [alpine meadow (AM)] were measured in June, August and September, 2008, respectively. Soil CO₂ efflux rates at the top, the middle, the bottom of a slope and the flat in front of the slope were obtained at Maduo County. The results showed that average daily soil CO₂ efflux rates were 3.72, 2.65, 2.68, 0.59 and 0.37 g m⁻² day⁻¹ in the AM, lightly (LDL), moderately (MDL), severely (SDL) and very severely desertified lands (VSDL) during the growing season, respectively. Soil CO₂ efflux decreased with the change of desertification. The response of soil CO₂ efflux to environmental factors was adequately described by the linear model; models accounted for 76, 65, 72, 59 and 71% of the variability on soil CO₂ efflux in the AM, LDL, MDL, SDL and VSDL, respectively. Any environmental factor, however, was insufficient to explain the soil CO₂ efflux; the common influence could perfectly reflect soil CO₂ efflux response to the desertification change.     

Holocene soils and soil-geomorphic relations in an arid region of southern New Mexico

A study area in an arid region of southern New Mexico is in basin-and-range topography and includes both a river valley and a closed basin. Holocene soils occur in valley fills and low terraces between Pleistocene fans, in and near drainageways on the fan-piedmont, on ridges, and in dunes. Holocene soils suggest the character of initial development in soils that are much older and more complex, and record the beginnings of various soil horizons. Noncalcareous brown or reddish brown B horizons have formed in low-carbonate parent materials of stable sites. Incipient development of the argillic horizon and the Haplargids occurs at stable sites in very gravelly materials that are about 1–2000 yr old. The cambic horizon and Camborthids occur in adjacent low-gravel materials of the same age. The argillic horizon occurs continuously in soils of earliest Holocene, particularly in very gravelly materials. Where soils have been truncated, as in areas affected by landscape dissection, argillic and cambic horizons are usually absent and the soils are Torripsamments, Torriorthents, or Torrifluvents depending on content of sand, gravel, and organic carbon. In high-carbonate parent materials, noncalcareous, reddish brown B horizons have not formed at any time in the Holocene. Most of these soils are Torriorthents or Torrifluvents although an incipient calcic horizon has formed in some of the oldest Holocene soils; the latter are Calciorthids. Horizons of carbonate accumulation are the best and most common pedogenic indicators of soil age. Stage I carbonate horizons are a major feature of pedogenesis in the Holocene. Because of additions of carbonate from the atmosphere, carbonate horizons are morphologically similar whether they have formed in high or low-carbonate alluvium. The carbonate accumulations are illuvial.Some Holocene deposits apparently resulted from changes in climate. Others, such as the youthful deposits of coppice dunes, apparently were caused by man's introduction of cattle and subsequent overgrazing and seed dispersal.     

Annual soil CO2 efflux in a wet meadow during active layer freeze–thaw changes on the Qinghai-Tibet Plateau

Soil CO₂ efflux from an ecosystem responds to the active layer thawing depth (H) significantly. A Li-8100 system was used to monitor the CO₂ exchange from a wet meadow ecosystem during a freeze–thaw cycle of the active layer in a permafrost region on the Qinghai-Tibet Plateau. An exponential regression equation ( $ F_{\text{soil\, flux}} = 1.84e^{0.023H} + 5.06\,R^{2} = 0.96 $ ) has been established on the basis of observed soil CO₂ efflux versus the thawed soil thickness. Using this equation, the total soil CO₂ efflux during an annual freeze–thaw cycle has been calculated to be approximately 8.18 × 10¹⁰ mg C. The results suggest that freeze–thaw cycles in the active layer play an important role in soil CO₂ emissions and that thawed soil thickness is the major factor controlling CO₂ fluxes from the wet meadow ecosystem in permafrost regions on the Qinghai-Tibet Plateau. It can be concluded that with active layer thickening due to permafrost degradation, massive amounts of soil carbon would be emitted as greenhouse gases, and the permafrost region would become a carbon source with a positive feedback effect on climate warming. Hence, more attention should be paid to the influences of the active layer changes on soil carbon emission from these permafrost regions.     

A study of the soil geochemistry of the Platreef in the Bushveld Complex, South Africa

A study of the soil profiles in the central Transvaal showed that in most areas a thick top layer of transported soils is present. Since this transported horizon has moved for considerable distances and because mixing took place in the process, the use of soil geochemistry in the search for hidden ore bodies tends to yield erratic results. In the present investigation an attempt was made to use volatile elements such as Hg, As, S and C as CO₂ as geochemical indicator elements and to establish whether epigenetic haloes of these elements are imprinted on the transported soils. The geochemical behaviour of these elements are compared with elements such as Ni, Co, Cu and Fe, which would tend to migrate with the transported soil.The geochemical study was done on the Platreef in the Bushveld Complex, north of Potgietersrus. The Platreef consists of a stratified horizon which contains appreciable amounts of chalcopyrite, pyrrhotite, pentlandite and platinum-group elements. Four traverses across the ore body were investigated and the results indicate that in cases where the soil cover is relatively thin, approximately 1 to 2 metres, all the elements studied can be used to locate the ore body. Those traverses where the soil cover is thicker and where transported soils are definitely present, only mercury yields significant anomalies, while siderophile elements give erratic and poor results. The mercury anomalies are usually not displaced and even tend to delineate mineralized sub-zones of the Platreef.Arsenic, sulphur and carbon measured as total CO₂, also give erratic results in the cases where a thick soil cover is present. The fact that arsenic tends to be fixed relatively easily in soils, either in laterite particles or in surface limestone, apparently is the prime reason for its erratic behaviour. The results indicate that both S and C would yield unsatisfactory results if these elements are measured as total S and C in the soils. The use of gas species may be more successful.A surprising but consistent result, was the tendency for mercury to be concentrated in the soils above faults, which intersect the ore body at depth. The behaviour of mercury in soils taken along the strike of a fault indicated that a logarithmic relationship exists between the mercury concentration in the soil and the depth of the ore body.     

Soil,archaeological, biotic,and climatic relationships for the late holocene of the Wyoming basin: The case of the Garrett Allen (Elk Mountain) site (48CR301)

Soils at the Garrett Allen (Elk Mountain) site (48CR301) along Quealy Spring in Carbon County, Wyoming, indicate nearly continuous, although episodic, meadow soil formation during much of Neoglacial or post-Altithermal time at a mountain-basin edge (elev. 2237 m) of the Wyoming Basin. Seven relict and buried meadow soils now under sagebrush (Haplaquents or Fluvaquents, Haplaquolls, and Argiaquolls) of the T₁ terrace date less than 200 B.P. as well as around 200, 600, 900, 1700, and two soils at greater than 3100 B.P. Composite-superposed soils of two or more of the individual soils are common. Some soils have E horizons and the older ones have intensely gleyed cambic or argillic subsoils. Development of meadow soils and deposition of their respective parent materials seem to coincide with glacial episodes in the region when the climate may have been somewhat humid. Occupations of the meadow by Archaic and younger cultural groups, whose artifacts occur mainly in A(O?) horizons of meadow soils, appear to coincide with these humid–glacial climates when the meadow grew and flourished. Changes in subsistence and settlement of these peoples may have been partly influenced by humid climates during glacial episodes when rangeland and habitat were able to support large human and animal populations. However, four major erosional unconformities separate some soils. These unconformities may coincide with times of lessened glacial activity when the climate may have become drier and the meadow decreased in size. During these times, human groups may have abandoned the meadow, left the region, or only sparsely occupied the plains and basins. The youngest unconformity developed when Quealy Spring cut from T₁ to the T₀ level in the last century. This appears to coincide with sagebrush invasion across portions of the meadow, although a small relict meadow still exists. Sagebrush invasion and downcutting of the drainage may relate to climatic drying in the last century. Fluvents are now forming in alluvium of the T₀ level along the Quealy Spring drainage.     

Carbon losses in soils previously exposed to elevated atmospheric CO2 in a chaparral ecosystem: Potential implications for a sustained biospheric C sink

Between 1996 and 2001 an experimental set up in a chaparral community near San Diego, CA, examined various plant and ecosystem responses to CO₂ concentrations ranging from 250 to 750 μl l^− 1. These experiments indicated a significant increase in soil C sequestration as CO₂ rose above the ambient levels. In 2003, two years after the cessation of the CO₂ treatments, we returned to this site to examine soil C dynamics with a particular emphasis on stability of specific pools of C. We found that in as little as two years, C content in the surface soils (0–15 cm) of previously CO₂ enriched plots had dropped to levels below those of the ambient and pretreatment soils. In contrast, C retained in response to CO₂ enrichment was more durable in the deeper soil layers (> 25 cm deep) where both organic and inorganic C were on average 26% and 55% greater, respectively, than C content of ambient plots. Using stable isotope tracers, we found that treatment C represented 25% of total soil C and contributed to 55% of soil CO₂ efflux, suggesting that most of treatment C is readily accessible to decomposers. We also found that, C present before CO₂ fumigation was decomposed at a faster rate in the plots that were exposed to elevated CO₂ than in those exposed to ambient CO₂ levels. To our knowledge, this is the first report that allows for a detail accounting of soil C after ceasing CO₂ treatments. Our study provides a unique insight to how stable the accrued soil C is as CO₂ increases in the atmosphere.     

Analysis of the spatial and temporal changes in soil CO<Subscript>2</Subscript> flux in alpine meadow of Qilian Mountain

Field experiments on the CO₂ flux of alpine meadow soil in the Qilian Mountain were conducted along the elevation gradient during the growing season of 2004 and 2005. The soil CO₂ flux was measured using the Li-6400-09 soil respiration chamber attached to the Li-6400 portable photosynthesis system. The effects of water and heat and roots on the soil CO₂ flux were statistically analyzed. The results show that soil CO₂ flux along the elevation gradient gradually decreases. The soil CO₂ flux was low at night, with lowest value occurring between 0200 and 0600 hours, started to rise rapidly during 0700–0830 hours, and then descend during 1600–1830 hours. The peak CO₂ efflux appears during 1100–1600 hours. The diurnal average of soil CO₂ efflux was between 0.56 ± 0.32 and 2.53 ± 0.76 μmol m⁻² s⁻¹. Seasonally, soil CO₂ fluxes are relatively high in summer and autumn and low in spring and winter. The soil CO₂ efflux, from the highest to the lowest in the ranking order, occurred in July and August (4.736 μmol m⁻² s⁻¹), June and September, and May and October, respectively. The soil CO₂ efflux during the growing season is positively correlated with soil temperature, root biomass and soil water content.     

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.     

An application of soil humic substances to geochemical exploration

The wet temperate climate of Tasmania causes strong leaching of soils and conventional “B” horizon soil geochemistry yields unreliable results. For this reason, most surveys of recent times have been based on “C” horizon sampling which is expensive and often difficult to accomplish in the strongly dissected, heavily vegetated terrain of western Tasmania.A system has been developed based on sampling of the “A” soil horizon, extraction of the humic substances with 0.5 M NH₄OH and determination of the associated metals by atomic absorption spectrometry. The humic substances content of the extract is obtained from quantitative wet oxidation of an aliquot of the sample with 0.4N K₂ Cr₂ O₇.In trials to date this system has yielded results compatible with those obtained from the “C” horizon. It offers a cost effective method of exploration by soil geochemistry and can be carried out with minimal disturbance to the environment.     

A pedogenic goethite record of soil CO2 variations as a response to soil moisture content

The terrestrial carbon cycle and the role of atmospheric CO₂ concentrations in controlling global temperatures can be inferred from the study of ancient soils (paleosols). Soil-formed goethite and calcite have been the primary minerals used as a geochemical proxy for reconstructing atmospheric pCO₂ from ancient terrestrial records. In the case of goethite, optimum sampling strategies for reconstructing pCO₂ focus on the portion of the soil profile that displays steep gradients in both soil CO₂ concentration and δ¹³C values of soil CO₂ such that a keeling plot can be developed for a given soil and atmospheric pCO₂ can be calculated from it. We report data from a Carboniferous paleosol that depart from the expected linear trends. The results indicate that pedogenic goethite is sensitive to variations in the isotopic composition of soil CO₂, over a range of timescales, and can record these variations in the carbon isotope composition and mole fraction of Fe(CO₃)OH in solid solution with goethite. We explore possible environmental conditions that can drive these changes as a function of either moisture controlled variations in soil respired CO₂ or in the residence time of carbon in soils. The implications of this result are overestimation of paleoatmospheric pCO₂ from pedogenic goethite.     

Declines in Plant Productivity Drive Carbon Loss from Brackish Coastal Wetland Mesocosms Exposed to Saltwater Intrusion

Coastal wetlands, among the most productive ecosystems, are important global reservoirs of carbon (C). Accelerated sea level rise (SLR) and saltwater intrusion in coastal wetlands increase salinity and inundation depth, causing uncertain effects on plant and soil processes that drive C storage. We exposed peat-soil monoliths with sawgrass (Cladium jamaicense) plants from a brackish marsh to continuous treatments of salinity (elevated (~?20 ppt) vs. ambient (~?10 ppt)) and inundation levels (submerged (water above soil surface) vs. exposed (water level 4 cm below soil surface)) for 18 months. We quantified changes in soil biogeochemistry, plant productivity, and whole-ecosystem C flux (gross ecosystem productivity, GEP; ecosystem respiration, ER). Elevated salinity had no effect on soil CO₂ and CH₄ efflux, but it reduced ER and GEP by 42 and 72%, respectively. Control monoliths exposed to ambient salinity had greater net ecosystem productivity (NEP), storing up to nine times more C than plants and soils exposed to elevated salinity. Submersion suppressed soil CO₂ efflux but had no effect on NEP. Decreased plant productivity and soil organic C inputs with saltwater intrusion are likely mechanisms of net declines in soil C storage, which may affect the ability of coastal peat marshes to adapt to rising seas.     

Factors affecting the dissolution kinetics of volcanic ash soils: dependencies on pH,CO2, and oxalate

Laboratory experiments were conducted with volcanic ash soils from Mammoth Mountain, California to examine the dependence of soil dissolution rates on pH and CO₂ (in batch experiments) and on oxalate (in flow-through experiments). In all experiments, an initial period of rapid dissolution was observed followed by steady-state dissolution. A decrease in the specific surface area of the soil samples, ranging from 50% to 80%, was observed; this decrease occurred during the period of rapid, initial dissolution. Steady-state dissolution rates, normalized to specific surface areas determined at the conclusion of the batch experiments, ranged from 0.03 μmol Si m⁻² h⁻¹ at pH 2.78 in the batch experiments to 0.009 μmol Si m⁻² h⁻¹ at pH 4 in the flow-through experiments. Over the pH range of 2.78–4.0, the dissolution rates exhibited a fractional order dependence on pH of 0.47 for rates determined from H⁺ consumption data and 0.27 for rates determined from Si release data. Experiments at ambient and 1 atm CO₂ demonstrated that dissolution rates were independent of CO₂ within experimental error at both pH 2.78 and 4.0. Dissolution at pH 4.0 was enhanced by addition of 1 mM oxalate. These observations provide insight into how the rates of soil weathering may be changing in areas on the flanks of Mammoth Mountain where concentrations of soil CO₂ have been elevated over the last decade. This release of magmatic CO₂ has depressed the soil pH and killed all vegetation (thus possibly changing the organic acid composition). These indirect effects of CO₂ may be enhancing the weathering of these volcanic ash soils but a strong direct effect of CO₂ can be excluded.     

Spatiotemporal changes in CO<Subscript>2</Subscript> emissions during the second ZERT injection,August–September 2008

This study reports the first field test of a multi-channel, auto-dilution, steady-state, soil–CO₂ flux monitoring system being developed to help understand the pathways by which fugitive CO₂ from a geologic sequestration site migrates to the surface. The test was conducted from late August through mid-October 2008 at the Zero Emissions Research and Technology project site located in Bozeman, MT. Twenty steady-state and five non-steady-state flux chambers were installed in a 10 × 15 m area, one boundary of which was directly above a shallow (2-m depth) horizontal injection well located 0.5 m below the water table. A total flux of 52 kg CO₂ day⁻¹ was injected into the well for 13 days and the efflux from the soil was monitored by the chambers before, during, and for 33 days after the injection. The results showed a rapid increase in soil efflux once injection started, with maximal values reached within 3–7 days in most chambers. Efflux returned to background levels within a similar time period after injection ceased. A radial efflux pattern was observed to at least 2 m from the injection well, and evidence for movement of the CO₂ plume during the injection, presumably due to groundwater flow, was seen. The steady-state chambers yielded very stable data, but threefold to fivefold higher fluxes than the non-steady-state chambers. The higher fluxes were attributed to vacuum induced in the steady-state chambers by narrow vent tubes. High winds resulted in significant decreases in measured soil CO₂ efflux, presumably by enhancing efflux from soil outside the chambers.     

10Be distribution in soils from Merced River terraces,California

The distribution and residence time of cosmogenic ¹⁰Be in clay-rich soil horizons is fundamental to understanding and modelling the migration of ¹⁰Be on terrestrial sediments and in groundwater solutions. We have analyzed seven profiles of clay-rich soils developed from terrace sediments of the Merced River, California. The terraces and soils of increasing age are used to compare the ¹⁰Be inventory with a simple model of accumulation, decay and erosion. The data show that the distribution of ¹⁰Be varies with soil horizon clay content, that the residence time of ¹⁰Be in these horizons exceeds 10⁵ years, and that to a rough approximation the inventory of ¹⁰Be in a thoroughly sampled soil profile fits the equation: N = (q − Em)(1 − e^−λι)/λ where q is delivery rate, E is erosion rate, m is the concentration of ¹⁰Be in the eroding surface layer, λ is the decay constant, and t is the age of the depositional unit from which the soil has developed. The general applicability of this model is uncertain and warrants further testing in well-calibrated terrace sequences.     

Soil CO2 concentration in biological soil crusts and its driving factors in a revegetated area of the Tengger Desert,Northern China

Biological soil crusts (BSCs) are an important cover in arid desert landscapes, and have a profound effect on the CO₂ exchange in the desert system. Although a large number of studies have focused on the CO₂ flux at the soil–air interface, relatively few studies have examined the soil CO₂ concentration in individual layers of the soil profile. In this study, the spatiotemporal dynamics of CO₂ concentration throughout the soil profile under two typical BSCs (algae crusts and moss crusts) and its driving factors were examined in a revegetated sandy area of the Tengger Desert from Mar 2010 to Oct 2012. Our results showed that the mean values of the vertical soil CO₂ concentrations under algal crusts and moss crusts were 600–1,200 μmol/mol at the 0–40 cm soil profiles and increased linearly with soil depth. Daily CO₂ concentrations showed a single-peak curve and often had a 1–2 h time delay after the maximum soil temperature. During the rainy season, the mean soil CO₂ concentration profile was 1,200–2,000 μmol/mol, which was 2–5 times higher as compared to the dry season (400–800 μmol/mol). Annually, soil moisture content was the key limiting factor of the soil CO₂ concentration, but at the daily time scale, soil temperature was the main limiting factor. Combined with infiltration depth of crusted soils, we predicted that precipitation of 10–15 mm was the most effective driving factor in arid desert regions.     

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.     

The soils on the calcareous sand dunes southeast of South Australia

 The properties of soils on previously dated sand dunes from Robe to Naracoorte in South Australia were examined. In these areas younger sand dunes are composed of fresh sand, but older sand dunes are composed of calcarenited sand. The soils on the sand dunes developed successionally by the age of sand dunes. The soil properties of these sand dunes differ depending on the ages of the sand dunes. The properties of sand particles in soils are as follows: (1) On the sand dunes of 4300 years B.P., A/C profile developed (Rendzina). On the sand dunes older than 125 000 years B.P. and on the plateau of Tertiary limestone, soil profiles of A₁/AB/B/C on the sand dunes of 83 000 years B.P. and A₁/A₃/B₁/B₂/C (Terra rossa) are well developed. (2) Within the sand of A/C horizons of the sand dunes with the age of 4300 year B.P., the calcite grain content is about 64%, and the quartz content is about 35%. Within the B horizons of soils on the dunes from 83 000 years B.P. to 347 000 years B.P., the calcite grain content is only 1–2%; however, the quartz grain content is about 92%. In the B₂ horizons of soils on the dune of 690 000 years B.P. and on the Tertiary plateau, there are some calcite grains but the quartz grain content is about 96%. (3) The average size of quartz grains in the soils on the sand dunes from 4300 B.P. to 347 000 years B.P. is generally smaller, but the average size of quartz on the sand dunes of 690 000 year B.P. becomes larger and the grains are well rounded. On the Tertiary limestone plateau, the average quartz size becomes again smaller, and the grains are more rounded. (4) Fe_t in B₂ horizon of the soil profiles increases clearly corresponding to the age. Iron activity expressed by Fe_o/Fe_d also shows a close relation to the chronological sequence. The B horizon of the soil profiles shows a drastic decrease of Fe_o/Fe_d according to the age. Iron crystalinity, (Fe_d-Fe_o)/Fe_t, has a tendency for a positive relation with increasing age. Received: 1 June 1995 · Accepted: 4 December 1995