Barium and Cadmium in Tropical Soils Cropped and Under Native Forest

Toxic elements pose a high environmental risk because of their long persistence in soil, water, and food chain. This study aimed to estimated potentially available and pseudototal contents of barium (Ba) and cadmium (Cd) in tropical soil under native forest vegetation, sugarcane and maize crops. Soil samples were collected at 0.00–0.20 m depth in different municipalities in São Paulo State, Brazil, and analyzed for fertility, texture, total iron, iron oxides, pseudototal and available Ba and Cd contents. Heavy metals were extracted using different extraction solutions (Mehlich-1, Mehlich-3, and DTPA). Data were subjected to descriptive and multivariate analyses. Correlations between soil clay content, mineralogy, and fertility were also investigated. Of the three extraction solutions tested, Mehlich-3 was the most effective to estimate the potential availability of Ba and Cd. Ba extracted by Mehlich-3 was negatively correlated with goethite, and pseudototal barium was positively correlated with pH CaCl2. Cd extracted by Mehlich-3 was positively correlated with pH CaCl2, and pseudototal cadmium was strongly correlated with iron oxide, clay, and organic matter contents.

Linkage of microbial living communities and residues to soil organic carbon accumulation along a forest restoration gradient in southern China

Background Forest restoration has been considered an effective method to increase soil organic carbon (SOC), whereas it remains unclear whether long-term forest restoration will continuously increase SOC. Such large uncertainties may be mainly due to the limited knowledge on how soil microorganisms will contribute to SOC accumulation over time. Methods We simultaneously documented SOC, total phospholipid fatty acids (PLFAs), and amino sugars (AS) content across a forest restoration gradient with average stand ages of 14, 49, 70, and > 90 years in southern China. Results The SOC and AS continuously increased with stand age. The ratio of fungal PLFAs to bacterial PLFAs showed no change with stand age, while the ratio of fungal AS to bacterial AS significantly increased. The total microbial residue-carbon (AS-C) accounted for 0.95–1.66 % in SOC across all forest restoration stages, with significantly higher in fungal residue-C (0.68–1.19 %) than bacterial residue-C (0.05–0.11 %). Furthermore, the contribution of total AS-C to SOC was positively correlated with clay content at 0–10 cm soil layer but negatively related to clay content at 10–20 cm soil layer. Conclusions These findings highlight the significant contribution of AS-C to SOC accumulation along forest restoration stages, with divergent contributions from fungal residues and bacterial residues. Soil clay content with stand age significantly affects the divergent contributions of AS-C to SOC at two different soil layers.

RESPONSE OF HETEROTROPHIC MICROBIAL COMMUNITIES TO AN ENVIRONMENTAL GRADIENT IN THE FLOODPLAIN OF MAPIRE RIVER, VENEZUELA

Soil physico-chemical characteristics of floodplains, particularly hydrology, influence microbiological activity. As such, each river floodplain system has a unique physico-chemical dynamic that in turn supports the microbial community. The Mapire River floodplain is a complex system in which seasonal flood pulses cause changes in the soil physico-chemical variables. We examined how these temporal and spatial differences are associated with the microbiological activity along a seasonally flooded gradient at the mouth of the Mapire River (Lower Orinoco, Venezuela). Soil samples were collected during three different seasons by a systematic sampling at 4 points of the gradient, defined by the intensity of flooding. The physico-chemical parameters of the soil were determined and related by the density and physiological profile of the microbial community through multivariate permutation analysis and gradient analysis. The results indicate that there is a spatial gradient determined by soil clay content and a temporal gradient influenced by moisture and total organic carbon. Significant differences were found among soil zones and seasons, with the interaction of both factors also significant. It was observed that microbial activity is decisive in phosphorus dynamics, even during flooding. It is concluded that amid the complex interactions between biotic and abiotic factors, microbial communities are able to respond to changes in the physico-chemical soil environment and maintain their activity throughout the hydroperiod.

Geostatistical Characterization of Soil Clay Patterns in a Typical Watershed of Cultivated Black Soil

Soil clay influences a range of processes, including vertical and lateral redistribution of water, runoff, and erosion. Knowledge of the spatial pattern of soil clay content (SCC) in cultivated soil region is important. The objective of this study was to determine the degree of spatial variability of SCC across depths in a cultivated black soil region in northeastern China. Data collected from each of 58 sites on a regular grid of 50 by 50 m were analyzed both statistically and geostatistically to describe the spatial distribution. The SCCs between different segments in soil profiles were significantly different. The 0- to 65-cm soil profile could be clustered into three segments: 0- to 30-cm, 30- to 35-cm, and 35- to 65-cm layers.The standard deviation (SD) and coefficients of variation (CV) for the SCC of the 30- to 35-cm segment were the largest. Experimental semivariograms of SCC were best fitted by spherical models. Nugget-to-sill ratios indicated a strong spatial dependence for SCC at other depths, except the 40- to 45-cm and 60- to 65-cm segments. The 35- to 40-cm layer had the largest spatial dependency compared with the other layers. Cross-validation of the kriging map indicated that prediction of SCC using semivariogram parameters was better than assuming the mean of the observed value for any unsampled location. The mapping displayed heterogeneity of SCC across the experimental site and revealed higher SCC close to the tail of an eroded gully and lower SCC neighbouring eroded gully margins. The fragmentation degree and clay-enriched patch amount increased from the near-surface down to 65-cm of depth, suggesting the higher evenness of SCC in the cultivated layer than that in the tillage pan.

Seasonal variation in soil CO2 emission and leaf gas exchange of well‐managed commercial Citrus sinensis (L.) orchards

Purpose To investigate whether soil clay content, cultivar and seasonal variation have any effect on soil CO2 emission rates and leaf CO2 assimilation rates in a drip-irrigated commercial Citrus sinensis orchard. Methods The study was carried out in the field as a randomised complete block design in a 2 × 2 factorial consisting of two soil types and two citrus cultivars on a drip-irrigated commercial Citrus sinensis orchards with 2-week interval measurements of soil CO2 emission and leaf gas exchanges for a year. Results Soil clay content did not influence plant CO2 assimilation rates and soil CO2 emission rates in irrigated citrus. However, seasonal variation significantly influenced both processes. Soil CO2 emission rates were highest in summer and were more than double the rates observed in winter while leaf CO2 assimilation rates were highest in autumn and four times higher than the winter season rates. Mean seasonal soil CO2 emission rates were strongly influenced by mean minimum seasonal temperatures while leaf CO2 assimilation rates only showed a relatively weak relationship with mean maximum seasonal temperatures. Conclusions Soil clay content did not influence soil CO2 emission and assimilation rates in drip irrigated citrus suggesting a non-significant effect of clay content for soils subjected to similar management practices. Citrus CO2 assimilation rate peaks in the autumn while soil CO2 emission rates peak in summer. A snapshot analysis of CO2 sequestration rates suggests that irrigated citrus orchards are net sinks of CO2 in summer, autumn and winter season.

Seasonal variation in soil CO2 emission and leaf gas exchange of well-managed commercial Citrus sinensis (L.) orchards

Purpose To investigate whether soil clay content, cultivar and seasonal variation have any effect on soil CO2 emission rates and leaf CO2 assimilation rates in a drip-irrigated commercial Citrus sinensis orchard. Methods The study was carried out in the field as a randomised complete block design in 2x2 factorial consisting of two soil types and two citrus cultivars on a drip-irrigated commercial Citrus sinensis orchards with 2-week interval measurements of soil CO2 emission and leaf gas exchanges over period of a year. Results Soil clay content did not influence plant CO2 assimilation rates and soil CO2 emission rates in irrigated citrus. However, seasonal variation significantly influenced both processes. Soil CO2 emission rates were highest in summer and were more than double the rates observed in winter while leaf CO2 assimilation rates were highest in autumn and four times higher than the winter season rates. Mean seasonal soil CO2 emission rates were strongly influenced by mean minimum seasonal temperatures while leaf CO2 assimilation rates only showed a relatively weak relationship with mean maximum seasonal temperatures. Conclusions Soil clay content did not influence soil CO2 emission and assimilation rates in drip irrigated irrigated citrus. Citrus CO2 assimilation rate peaks in the autumn while soil CO2 emission rates peak in summer.Snapshot analysis of CO2 sequestration rates suggests that irrigated citrus orchards are net sinks of CO2. Empirically measured CO2 flux rates in a commercial drip-irrigated citrus orchard are presented.

Improved Shear Strength Performance of Compacted Rubberized Clays Treated with Sodium Alginate Biopolymer

This study examines the potential use of sodium alginate (SA) biopolymer as an environmentally sustainable agent for the stabilization of rubberized soil blends prepared using a high plasticity clay soil and tire-derived ground rubber (GR). The experimental program consisted of uniaxial compression and scanning electron microscopy (SEM) tests; the former was performed on three soil–GR blends (with GR-to-soil mass ratios of 0%, 5% and 10%) compacted (and cured for 1, 4, 7 and 14 d) employing distilled water and three SA solutions—prepared at SA-to-water (mass-to-volume) dosage ratios of 5, 10 and 15 g/L—as the compaction liquid. For any given GR content, the greater the SA dosage and/or the longer the curing duration, the higher the uniaxial compressive strength (UCS), with only minor added benefits beyond seven days of curing. This behaviour was attributed to the formation and propagation of so-called “cationic bridges” (developed as a result of a “Ca2+/Mg2+ ⟷ Na+ cation exchange/substitution” process among the clay and SA components) between adjacent clay surfaces over time, inducing flocculation of the clay particles. This clay amending mechanism was further verified by means of representative SEM images. Finally, the addition of (and content increase in) GR—which translates to partially replacing the soil clay content with GR particles and hence reducing the number of available attraction sites for the SA molecules to form additional cationic bridges—was found to moderately offset the efficiency of SA treatment.

Amplicons and isolates: Rhizobium diversity in fields under conventional and organic management

BackgroundThe influence of farming on plant, animal and microbial biodiversity has been carefully studied and much debated. Here, we compare an isolate-based study of 196 Rhizobium strains to amplicon-based MAUI-seq analysis of rhizobia from 17,000 white clover root nodules. We use these data to investigate the influence of soil properties, geographic distance, and field management on Rhizobium nodule populations.ResultsOverall, there was good agreement between the two approaches and the precise allele frequency estimates from the large-scale MAUI-seq amplicon data allowed detailed comparisons of rhizobium populations between individual plots and fields. A few specific chromosomal core-gene alleles were significantly correlated with soil clay content, and core-gene allele profiles became increasingly distinct with geographic distance. Field management was associated with striking differences in Rhizobium diversity, where organic fields showed significantly higher diversity levels than conventionally managed trials.ConclusionsOur results indicate that MAUI-seq is suitable and robust for assessing nodule Rhizobium diversity. We further observe possible profound effects of field management on microbial diversity, which could impact plant health and productivity and warrant further investigation.