Clay-rich Soils Research Articles

Degradation and Microbial Uptake of C60 Fullerols in Contrasting Agricultural Soils.

The environmental fate of functionalized carbon nanomaterials (CNM) remains poorly understood. Using 13C-labeled nanomaterial we present the results of a study investigating the mineralization and microbial uptake of surface-functionalized C60 (fullerols) in agricultural soils with contrasting properties. Soil microcosms rapidly mineralized fullerol C, as determined by 13C-content in the respired CO2, with higher fullerol mineralization in an organic, clay-rich soil versus a silty, low C soil (∼56.3% vs ∼30.9% fullerol C mineralized over 65 days). By tracking the enriched 13C from fullerol into microbial phospholipid fatty acids (PLFA) we also report, for the first time, the incorporation of nanomaterial-derived C into soil microbial biomass, primarily by fungi and Gram-negative bacteria. While more fullerol C was incorporated into PLFA in the organic C-rich soil (0.77% vs 0.19% of PLFA C), this soil incorporated fullerol C into biomass less efficiently than the silty, low C soil (0.13% and 0.84% of assimilated fullerol C, respectively). These results demonstrate that, in contrast to pristine C60, surface functionalized C60 are unlikely to accumulate in surface soils and are readily mineralized by a range of soil microorganisms.

The relationship between intertidal soil composition and fiddler crab burrow depth

Fiddler crab burrow morphologies in tidal flat ecosystems depend largely on physical characteristics of the soil. Herein, we aimed to examine the relationship between soil texture profiles and the burrow depths and morphologies of four fiddler crab species: Tubuca arcuata, Gelasimus borealis, Xeruca formosensis, and Austruca lactea. We found that the burrow depths of T. arcuata, G. borealis, and A. lactea (range=8–50cm) were strongly affected by surface soil composition; only the burrow depth of X. formosensis (which reached 100cm in some cases) was influenced by the presence of deeper, clay-rich soils. Specifically, X. formosensis relied more heavily on fine soils, such as clay or silt, when they burrowed deeper into sand-covered mudflats. We hypothesize that deep burrowers, such as X. formosensis, may coat clay-rich soils on their burrow walls to impede flooding and to prevent burrow collapse, suggesting that surface soil composition does not necessarily determine the burrow depth and composition for all fiddler crab species.

Mineral mediated isotope fractionation of soil water.

The ability to recover the isotopic signature of water added to soil samples that have previously been oven-dried decreases with the increasing presence of silt and clay. The effects on the isotopic signature of water associated with physicochemical soil properties are not yet fully understood, for either hydration or dehydration of soil samples. The soil sample chemistry and the crystallinity of minerals were measured by X-ray fluorescence and X-ray diffraction. The organic carbon and the cation-exchange capacity were also determined. Water of known isotopic signature was used to spike an oven-dried substrate and subsequently extracted by cryogenic vacuum extraction at a temperature of 105°C. In addition, the soils were oven-dried at 205°C and water extractions were also performed at 205°C. The isotopic signatures of the water samples were determined by cavity-ring-down spectrometry. The isotope effects caused by the cryogenic vacuum extraction method applied to soils with elevated clay content were reduced. First, by increasing the extraction temperature to 205°C, we improved the precision of the cryogenic vacuum extraction method and the recovery of the known isotopic signature of the spike water. Secondly, the post-correction of data based on the physicochemical soil properties and a common extraction temperature of 105°C improved the measurement trueness. The isotopic signature of soil water is influenced by mineral-water interaction. During the hydration of clay, different minerals deplete free water in heavy isotopes. The extracted soil water (dehydration water) gathered from clay-rich soils is generally more depleted in the heavy isotopes than the spike water, making results obtained for different soil types difficult to compare. Isotope effects observed at the mineral-water interface highlight potential explanations for eco-hydrological separation of water pools. Copyright © 2016 John Wiley & Sons, Ltd.

Soil fertility status of some of the intensive crop growing areas under major agroecological zones of Bangladesh

Laboratory studies on soil fertility evaluation was carried out across major agroecological zones (AEZs) of Bangladesh to know the nutrient status of soils and to relate those with soil properties like pH, organic matter, CEC, and clay content. Thirty five composite soil samples were collected from intensive crop growing sites, which covered 17 AEZs of Bangladesh. After proper processing, the samples were analyzed for texture, pH, organic carbon, CEC, exchangeable cations (K, Ca, Mg and Na), total N, available P and S following standard methods. The textural class of the soils collected from AEZ 12 and 13 appeared to be mostly clay. Clay loam soil was found in AEZ 4, 8, 9, 11, 25 and 28. Loamy soil was seen in AEZ 1 while AEZ 22, 23 and 29 were mostly sandy textured. The results revealed that 65.7% of the tested soil was acidic while 25.7% was alkaline in nature. All the tested soils showed lower pHKCl compared to pHH2O thus possessed negative charge. About 68.6% of the collected soils contained low (1.10-1.70%) level of organic matter, 25.7% soils retained it at medium level (1.71-2.40) and 5.7% soils at very low level (<1.0%). All the tested soils appeared to be deficient (< 0.12%) in nitrogen content. 68.6% soil samples had the low level of available P while only 8.6% retained it an optimum amount. About 80% of the tested soils contained low level of available S (7.9- 14.7 mg kg -1) although coastal regions soils hold higher amount of available S. High CEC (20-38 cmol kg-1) was found in clay rich soils of AEZ 10, 11, 12, and 13. Study revealed that 40% of the collected soils were very low, 31.4% were low, 8.6% each of medium and optimum, and 11.4% contained high level of exchangeable K. The calcareous soils (AEZ 10, 11, 12 and 13) contained very high level of Ca. Non calcareous soils also showed fairly good level of Ca content except AEZ 1, 3, 23 and 29. Sandy textured soils of greater Dinajpur, Rangpur, Moulvibazar showed lower level of exchangeable Mg. About 86% of the tested soils had the lower (< 2%) potassium saturation percentage (KSP), which needs K application for sustainable crop production. Estimate showed that 44% variability for CEC may be attributed by clay content and the relationship was significant (p = 0.05). Again, 50.4 and 65.6% variability in exchangeable K and Mg, respectively may be governed by clay content of the soils, while such relationship for Ca was non-significant. CEC may contribute 62.2, 92.3 and 83.9% variability for exchangeable K, Ca and Mg content in soils, respectively. The fertility status of most of the studied soils (except AEZ 10, 12, 13 and to some extent 11) appeared to be low to very low, which demand judicious management in order to achieve food security and to conserve the soil fertility.Bangladesh J. Agril. Res. 41(4): 735-757, December 2016

Gully erosion and freeze-thaw processes in clay-rich soils, northeast Tennessee, USA

This study examines gully erosion in northeast Tennessee hillslopes in the Southern Appalachian Valley and Ridge physiographic province, where a thick sequence of red clay Ultisols (Acrisol, according to the World Reference Base for Soil) overlies dolomite and limestone bedrock. The role of freeze-thaw processes in gully erosion was examined weekly from 6/3/2012 to 9/17/2014 using a network of n=78 erosion pins in three geomorphic areas: channels, interfluves, and sidewalls. Freeze-thaw days were identified using meteorological data collected on site. When freeze-thaw days occurred, erosion and deposition increased and gully conditions were more dynamic. When daily temperature did not plunge below freezing, more stable gully conditions persisted. Ordinary Least Square regression models of erosion pin length using freeze-thaw events explained significant portions of variability in channels (R²=0.113, p<0.01), interfluves (R²=0.141, p<0.01), and sidewalls (R²=0.263, p<0.01). Repeat analysis on only the winter-spring months minimally improved the sidewall model (R²=0.272, p<0.01). Erosion in interfluves exhibited a lagged effect, and was best correlated to freeze-thaw events during the prior period while erosion in channels and sidewalls was related to freeze-thaw events in the current week. Of the three geomorphic areas studied, sidewall erosion was best modeled by freeze-thaw events which contribute to widening of gullies through mobilization of sediment and mass wasting. This research demonstrates that freeze-thaw processes are a significant contributor to erosion in gully channels, interfluves, and especially sidewalls, and therefore temperature variability should be considered in erosion studies in similar climates.

Reconstruction of Holocene paleohydrological and sedimentological characteristics of Umm Ar-Rimam basin (northern Kuwait)

The paleohydrological and sedimentological characteristics of a playa lake in northern Kuwait (Arabian Gulf) are reconstructed using sedimentological, geochemical, and isotopic techniques. The sequence consists of up to 8 cycles of S-poor, alluvial sediments capped by a thin organic soil interbedded with gravity-fall calcrete sediments. The succession is locally derived from mainly Quaternary sediments and is regressive with upsection filling of the subsiding basin by cycles of sheetwash flow in response to climatic change. There is no natural, open-water lake water as indicated by low total organic carbon (TOC) data, but the presence of incised calcrete yardangs suggests that more extensive open-water conditions were operative in the past. Stable isotope (δ18O‰ and δ13C‰) values of the authigenic carbonates indicate the following three distinct processes: evaporation, meteoric fluid infiltration, and rapid per-descensum flow (rapid downward movement of water and playa sediment through pipes) through a porous, clastic sequence. Because evaporites are scarce, other factors besides evaporation action control chemical and isotopic compositions of the per-decensum lake fluids. Consequently, the isotopic composition cannot be interpreted exclusively as an indicator of salinity or evaporation ratio. The degassing of CO2 during groundwater discharge may explain the enriched carbon isotope values for the authigenic carbonates precipitated in the sediments. Hydrologically closed lake water bodies tend to show low negative carbonate oxygen and carbon isotopic signatures. Isotopically negative δ13C values imply a strong input of soil-zone carbon to the groundwater of the top 60 cm of the sediment. Lakes that are hydrologically closed and evaporate or equilibrate with atmospheric CO2 will tend to have low negative δ18O and δ13C values in the carbonates as reported by Talbot (Chem Geol: Isotope Geosci Sect 80(4):261–279, 1990). Biologically active lakes will tend toward lower δ13C of dissolved carbon due to the photosynthetic effects of 12C withdrawal as reported by Dunagan and Driese (J Sed Res 69:772–783, 1999). Increased biological activity during sedimentation may account for low carbon isotope values where plants were abundant, but in shrinkage-dominated systems (those of clay-rich soil subjecting to wet-dry conditions), carbon isotopes will be largely inherited from the calcretic limestones in the land extending landward of the coast and not influenced by coastal processes (known as hinterland), such as Umm Ar-Rimam depression. This basin does not fit the classic shallow playa-type basins of the Arabian Peninsula but rather the recharge playas of the southwestern USA.

Soil CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; efflux in an old-growth southern conifer forest (&amp;lt;i&amp;gt;Agathis australis&amp;lt;/i&amp;gt;) – magnitude, components and controls

Abstract. Total soil CO2 efflux and its component fluxes, autotrophic and heterotrophic respiration, were measured in a native forest in northern Aotearoa–New Zealand. The forest is dominated by Agathis australis (kauri) and is on an acidic, clay rich soil. Soil CO2 efflux, volumetric soil water content and soil temperature were measured bi-weekly to monthly at 72 sampling points over 18 months. Trenching and regression analysis was used to partition total soil CO2 efflux into heterotrophic and autotrophic respiration. The effect of tree structure was investigated by calculating an index of local contribution (Ic, based on tree size and distance to the measurement location) followed by correlation analysis between Ic and total soil CO2 efflux, root biomass, litterfall and soil characteristics. The measured mean total soil CO2 efflux was 3.47 µmol m−2 s−1. Autotrophic respiration accounted for 25 % (trenching) or 28 % (regression analysis) of total soil CO2 efflux. Using uni- and bivariate models showed that soil temperature was a poor predictor of the temporal variation in total soil CO2 efflux (&lt; 20 %). In contrast, a stronger temperature sensitivity was found for heterotrophic respiration (around 47 %). We found significant positive relationships between kauri tree size (Ic) and total soil CO2 efflux, root biomass and mineral soil CN ratio within 5–6 m of the sampling points. Using multiple regression analysis revealed that 97 % of the spatial variability in total soil CO2 efflux in this kauri-dominated stand was explained by root biomass and soil temperature. Our findings suggest that biotic factors such as tree structure should be investigated in soil carbon related studies.

Soil Respiration, Microbial Biomass C and N Availability in a Sandy Soil Amended with Clay and Residue Mixtures

Crop yields in sandy soils can be increased by addition of clay-rich soil, but little is known about the effect of clay addition on nutrient availability after addition of plant residues with different C/N ratios. A loamy sandy soil (7% clay) was amended with a clay-rich subsoil (73% clay) at low to high rates to achieve soil mixtures of 12%, 22%, and 30% clay, as compared to a control (sandy soil alone) with no clay addition. The sandy-clay soil mixtures were amended with finely ground plant residues at 10 g kg−1: mature wheat (Triticum aestivum L.) straw with a C/N ratio of 68, mature faba bean (Vicia faba L.) straw with a C/N ratio of 39, or their mixtures with different proportions (0%–100%, weight percentage) of each straw. Soil respiration was measured over days 0–45 and microbial biomass C (MBC), available N, and pH on days 0, 15, 30, and 45. Cumulative respiration was not clearly related to the C/N ratio of the residues or their mixtures, but C use efficiency (cumulative respiration per unit of MBC on day 15) was greater with faba bean than with wheat and the differences among the residue mixtures were smaller at the highest clay addition rate. The MBC concentration was lowest in sole wheat and higher in residue mixtures with 50% of wheat and faba bean in the mixture or more faba bean. Soil N availability and soil pH were lower for the soil mixtures of 22% and 30% clay compared to the sandy soil alone. It could be concluded that soil cumulative respiration and MBC concentration were mainly influenced by residue addition, whereas available N and pH were influenced by clay addition to the sandy soil studied.

Phosphate treatment by onsite wastewater systems in nutrient-sensitive watersheds of North Carolina's Piedmont.

The goal of this study was to gain a better understanding of the PO4-P treatment efficiency of onsite wastewater systems (OWS) installed in nutrient-sensitive watersheds of the North Carolina Piedmont. Four OWS including two conventional and two single-pass sand filter (SF) systems were evaluated at sites with clay-rich soils. Piezometers were installed near all of the OWS, and down-gradient from the conventional OWS for groundwater collection and characterization. Septic tanks, groundwater, SF effluent, and surface waters were sampled each season during 2015 (five times) and analyzed for PO4-P and Cl concentrations and for various environmental parameters. The conventional and SF OWS reduced PO4-P concentrations by an average of 99% and 90%, respectively, before discharge to surface waters. Mass-load reductions of PO4-P were also greater for the conventional OWS (mean 95%), relative to SF (83%) systems. The effluents discharged by SF OWS were influencing surface water quality. Additional treatment of the effluent from single-pass SF with reactive media is suggested, along with monitoring of the final effluent for PO4-P concentrations. This research provides important information that is absent from the published literature concerning PO4-P contributions to water resources from OWS in clay soils.

Soil types influence predictions of soil carbon stock recovery in tropical secondary forests

Tropical forests are major sinks of terrestrial carbon (C) both above- and below-ground. As a consequence their destruction and degradation is considered the second largest anthropogenic source of carbon dioxide to the atmosphere. Also contributing to the changing dynamics of the global carbon cycle is the widespread and significant expansion of secondary forest. Secondary forests that colonise abandoned agricultural lands can potentially recover above-ground C stocks to historical levels in a few decades. However, the dynamics of below-ground C stored as soil C stocks are unaccounted for in several tropical regions. Similarly, although parent materials are known to differ in chemical and physical properties, little is known about the relationships of soil C stocks with environmental predictors and whether they interact with soil types during natural forest regeneration. We investigated whether soil organic carbon (SOC) stocks change with secondary forest age in two contrasting soil types (derived from either basalt or granite). Soil and vegetation parameters were analysed to determine the best predictors of SOC stock changes in secondary forests. SOC stocks from 24 secondary forests (up to 69years since pasture abandonment) were compared with those from active pastures and mature forests. We found that clay-rich soils (originating from basalt parent material) store higher amounts of SOC, although these stocks remain unchanged as secondary forests matured. In contrast, SOC stocks in granite soils tend to be lower in young secondary forests and increase rapidly to levels comparable to mature forests. Moreover, our analysis indicated that soil pH and woody plant diversity are strong candidates as predictors of SOC stock variations, yet it appears this is within the context of soil type. Our results support the contention that models predicting SOC stocks during forest succession should not rely only on secondary forest age. Instead, predictions of SOC stocks can be improved with the inclusion of basic information on vegetation cover and soil type (especially soil texture).

Influence of loading rate and modes on infiltration of treated wastewater in soil-based constructed wetland

ABSTRACTOver the last 10 years soil-based constructed wetlands for discharge of treated wastewater (TWW) are commonly presented as a valuable option to provide tertiary treatment. The uncomplete knowledge in soil modifications and a lack of clear design practices laid the foundation of this work. The aim of this study was to determine optimal hydraulic loads and to observe the main critical parameters affecting treating performances and hydraulic loads acceptance. For this purpose, a soil rich in clay and backfill was chosen to perform column infiltration tests with TWW. Two loading rates and two loading modes were compared to study the influence of an intermittent feeding. Inlet and outlet waters were periodically analysed and columns were instrumented with balances, tensiometers, O2 and temperature probes. Soil physico-chemical characteristics were also taken into account to better understand the modification of the soil. One of the main expectations of tertiary treatment is to improve phosphate removal. A particular attention was thus given to phosphorus retention. The interest of an intermittent feeding in presence of a soil with high clay content was showed. This study highlighted that an intermittent feeding could make possible the use of a clay-rich soil for water infiltration.

Advantages of Using Microwave Satellite Soil Moisture over Gridded Precipitation Products and Land Surface Model Output in Assessing Regional Vegetation Water Availability and Growth Dynamics for a Lateral Inflow Receiving Landscape

To improve the understanding of water–vegetation relationships, direct comparative studies assessing the utility of satellite remotely sensed soil moisture, gridded precipitation products, and land surface model output are needed. A case study was investigated for a water-limited, lateral inflow receiving area in northeastern Australia during December 2008 to May 2009. In January 2009, monthly precipitation showed strong positive anomalies, which led to strong positive soil moisture anomalies. The precipitation anomalies disappeared within a month. In contrast, the soil moisture anomalies persisted for months. Positive anomalies of Normalized Difference Vegetation Index (NDVI) appeared in February, in response to water supply, and then persisted for several months. In addition to these temporal characteristics, the spatial patterns of NDVI anomalies were more similar to soil moisture patterns than to those of precipitation and land surface model output. The long memory of soil moisture mainly relates to the presence of clay-rich soils. Modeled soil moisture from four of five global land surface models failed to capture the memory length of soil moisture and all five models failed to present the influence of lateral inflow. This case study indicates that satellite-based soil moisture is a better predictor of vegetation water availability than precipitation in environments having a memory of several months and thus is able to persistently affect vegetation dynamics. These results illustrate the usefulness of satellite remotely sensed soil moisture in ecohydrology studies. This case study has the potential to be used as a benchmark for global land surface model evaluations. The advantages of using satellite remotely sensed soil moisture over gridded precipitation products are mainly expected in lateral-inflow and/or clay-rich regions worldwide.

Salisbury biochar did not affect the mobility or speciation of lead in kaolin in a short-term laboratory study

Salisbury biochar (produced from British broadleaf hardwood) with two different particle sizes (≤2mm and ≤0.15mm) was applied on a kaolin with three different lead (Pb2+) contamination levels (50mg/kg, 300mg/kg and 1000mg/kg) at the dosage of 1% in w/w. The short-term impact of biochar on the mobility and speciation of Pb2+ in the kaolin was investigated using attenuation periods of 1, 7 and 28 days. The leachability and extractability of Pb2+ in carbonic acid leaching and EDTA extraction tests as well as the speciation of Pb2+ in soils were not significantly affected by biochar treatment during all periods. The insignificant effects of biochar on Pb2+ immobilisation were most likely attributed to the high adsorption capacity of Pb2+ on the kaolin and biochar failed to competitively adsorb Pb2+ against kaolin. The kaolin immobilised Pb2+ primarily through cation exchange, which represents the readily bioavailable fractions of Pb2+ in soils and may still pose environmental risks. This paper suggests the inefficiency of biochar treament on heavy-metal contaminated clay-rich soils. Therefore a laboratory treatablity study with respect to the soil type may be crucial when large-scale biochar applications in heavy-metal associated soil remediation are evaluated.

Hydrocarbon biostimulation and bioaugmentation in organic carbon and clay-rich soils

Hydrocarbon-contaminated organic carbon-rich clayey soils are challenging for bioremediation stakeholders since the pollutant is heterogeneously distributed and poorly bioavailable due to its strong adsorption on clay and organic particles. In addition, biodegradation rates are restricted by limited diffusion of oxygen and nutrients to hydrocarbon-degrading aerobes. This study assessed the benefits of bioaugmentation with the strain Rhodococcus erythropolis T902.1 versus those from biostimulation and anaerobic natural attenuation in terms of hydrocarbon (HC) degradation efficiency and changes in the bacterial community structure in a diesel-polluted clay-rich soil. Three soil samples with a similar total organic content but with a different HC concentration (0.2, 1.0 and 6.5 g/kg) were compared in a microcosm experiment. Despite a limitation in oxygen transfer, R. erythropolis T902.1 enhanced a greater HC degradation compared to the biostimulation treatment. However, this advantage decreased with time as the proportion of Rhodococci declined from 25% initially to 1% of the global community after 80 days of treatment. Similarly, the alkB gene proportion in bioaugmented soils decreased to levels close to those of biostimulated soils. Consequently, further engineering was suggested to improve the resilience of the inoculum to ensure its long-term presence and activity in such polluted environments.

Possible effects of irrigation with wastewater on the clay mineralogy of some Australian clayey soils: laboratory study

Potassium is common in a wide variety of wastewaters and in some wastewaters is present at several hundred to several thousand mg L–1. Potassium is taken up by expandable clays leading to its fixation and illitisation of smectitic and vermiculitic layers. Hence the addition of wastewaters to soils may lead to mineralogical changes in the soils that affect their physico-chemical properties. Winery wastewater was equilibrated with clay-rich soils from Southern Australia. X-ray diffraction patterns and chemical composition of clays extracted from untreated and treated soils were determined. In three of the four soils, shifts in peak positions occurred towards more illitic components along with increases in K and sometimes also Mg and Na contents of soil clays. Peak decomposition showed trends towards the formation of interstratifications of illite with smectite at the expense of smectite and an alteration of poorly crystallised illite into its more well-ordered forms. The results show that illitisation may occur as a result of the addition of K-rich wastewaters to clayey soils.

Influence of clay concentration, residue C/N and particle size on microbial activity and nutrient availability in clay-amended sandy soil

The low fertility of sandy soils can be ameliorated by addition of clay-rich soil, but the effect of clay may differ between high and low C/N residues and could be modulated by residue particle size. An incubation experiment was conducted with addition of a clay-rich subsoil (73% clay) to a sandy soil (10% clay). The final clay concentrations were 10, 15, 20 and 30% (w/w). The residues of young kikuyu shoots (C/N 14) and mature wheat shoots (C/N >120) in two particle sizes (0.2-2 and 3-4 mm) were added at 10 g kg-1 soil. Soil respiration, pH, available N, microbial biomass C (MBC), N and P were measured. Cumulative respiration was up to 4-fold higher with kikuyu than with wheat and 30% lower at the highest clay concentration. The MBC concentration was up to 2-fold higher with kikuyu than with wheat. The available N concentration was up to 2-fold higher with kikuyu than with wheat and up to 50% lower at highest clay concentration. Thus, clay addition to sandy soils may reduce nutrient availability by reducing accessibility of plant residues to microbes and binding of nutrients, but this clay effect is not influenced by residue C/N or particle size.

Interactions Between Bacteria, Protozoa and Nematodes in Soil

Bacteria, protozoa and nematodes interact closely in soil ecosystems. Protozoa and nematodes eat bacteria (and occasionally each other), while bacteria defend themselves using chemical substances, resistant cell walls, irregular shapes and motility. Protozoa and nematodes are very different types of organisms, and hence apply very different feeding mechanisms; thus many protozoa can pick and choose individual bacterial cells, whereas nematodes ingest bacterial patches more uncritically. Protozoa and nematode are both aquatic organisms whose activity depends on available soil water, but differences in size, motility, resting stages and reproductive strategies mean that the soil physico-chemical environment influences the activity of protozoa and nematodes differently. For example, the relative importance of protozoa compared to nematodes may shift towards protozoa in very clay-rich soils. The interactions between the three organism groups have major ecological consequences such as modification of the bacterial communities and increased nitrogen mineralisation, both of which affect plant growth. Increased nitrogen mineralisation will usually be beneficial for plant growth, whereas the grazing induced changes in the bacterial communities can be both beneficial and detrimental to plants. Selective protozoan grazing can favour plant inhibiting bacteria. This may be a problem in clay rich soils where protozoa have better life conditions than nematodes.

Threshold Responses to Soil Moisture Deficit by Trees and Soil in Tropical Rain Forests: Insights from Field Experiments.

Many tropical rain forest regions are at risk of increased future drought. The net effects of drought on forest ecosystem functioning will be substantial if important ecological thresholds are passed. However, understanding and predicting these effects is challenging using observational studies alone. Field-based rainfall exclusion (canopy throughfall exclusion; TFE) experiments can offer mechanistic insight into the response to extended or severe drought and can be used to help improve model-based simulations, which are currently inadequate. Only eight TFE experiments have been reported for tropical rain forests. We examine them, synthesizing key results and focusing on two processes that have shown threshold behavior in response to drought: (1) tree mortality and (2) the efflux of carbon dioxdie from soil, soil respiration. We show that: (a) where tested using large-scale field experiments, tropical rain forest tree mortality is resistant to long-term soil moisture deficit up to a threshold of 50% of the water that is extractable by vegetation from the soil, but high mortality occurs beyond this value, with evidence from one site of increased autotrophic respiration, and (b) soil respiration reaches its peak value in response to soil moisture at significantly higher soil moisture content for clay-rich soils than for clay-poor soils. This first synthesis of tropical TFE experiments offers the hypothesis that low soil moisture–related thresholds for key stress responses in soil and vegetation may prove to be widely applicable across tropical rain forests despite the diversity of these forests.

Thixotropic and anti-thixotropic behaviors of fine-grained soils in various flocculated systems

Time-dependent yield stress fluids display hysteresis with respect to reversible and irreversible structural changes. We report an experimental study of the rheological behavior of clay-rich soils (i.e., Mediterranean Sea and bentonite clays) and clay-poor material (i.e., iron mine tailings) that includes steady state, dynamic response, and hysteresis measurement. The Mediterranean Sea and bentonite clays are low- and high-activity clays, respectively. For the materials examined, different thixotropic behaviors are observed as a function of the time of shearing. These materials typically exhibit strong thixotropic behavior for a given shear rate in the limited time of shearing. In general, larger grain sizes produce larger hysteresis loop areas. However, the bentonite clay experiences an increase in viscosity with shear time, regardless of the salinity effect, for sufficiently long shearing times under continuous shear conditions at the shear rate of 1200s−1 (the maximum value in the present study). As a result, the flow behavior of clayey soils is strongly influenced by the imposed shearing time.

Effect of structural modification on heat transfer through man-made soils in urban green areas

This study examined the characteristics of heat and water transfer in structurally modified urban soils. To satisfy our goals, we measured the temperature and moisture content of anthropogenic soils to a depth of 50 cm. Field observations was carried out for three sets (each of two pedons) of soils in the Tokyo Metropolitan area. Each pedon had the same turf coverage but different profile modifications in the green areas. Soil temperature, soil moisture, and precipitation data were collected during the summer (July–Sept) and winter (Oct–Feb) every 10 min. From the results, we calculated the thermal diffusivity and thermal conductivity in each pedon. Soil temperature showed a clear daily variation down to 30 cm depth. Temperature transmission to deeper layers was faster in pedon having stronger soil compaction and more artificial fragments than in pedons with weaker soil compaction and fewer concrete fragments. This finding suggests that strongly compacted soil has a relatively high thermal conductivity, and easily transfers heat to deeper soil. In pedons composed of soft, organic-rich, and clay-rich soil, water retention impedes the increase in soil temperature during daytime, whereas nighttime cooling is prevented by the lower heat transmission due to the larger porosity. Throughout the observations, the water content ranged from 0.1 to 0.45 m3 m−3. The thermal diffusivity was obtained as 1.2–5.0 × 10−3 cm2 s−1 in pedons without artifacts, but was higher (2.5–7.3 × 10−3 cm2 s−1) in all pedons containing larger volumes of concrete artifacts. Although the directions of heat flow by time within the profiles having lithological disturbance was not much different with that of natural soils, the observation data revealed that heat flow per time differed by structural properties of the profiles. Furthermore, thermal properties such as thermal diffusivity and thermal conductivity of the soils characterized with lithological disturbance were significantly higher than those of natural soils and it was notable that they were not influenced by either volume fraction of water or air in the soil. The fact suggested that the anthropogenic soils containing a large amount of modifiers and concrete artifacts have small capacity of water retention due to specific macro pores. Artificial materials and compaction regulate the drainage and water retention of the soil, although the water transfer behavior in the studied pedons could be rather complicated.