Clayey Oxisol Research Articles

Clay mineral composition drives soil structure behavior and the associated physical properties in Brazilian Oxisols

Soil aggregation controls several physical, chemical, and biological processes. Soil organic matter (SOM) and its stabilizing agents are regarded as the most important factors driving formation and stabilization of soil aggregates. However, aggregate stability in highly weathered soils may also be related to clay mineral composition and soil chemical properties. This study aims to evaluate the processes controlling soil aggregate stability and to understand the influence of soil chemical and clay mineral composition on the structural stability of highly weathered soils. Four Brazilian Oxisols were investigated: (P1) Xanthic Kandiustox, (P2) Rhodic Haplustox, (P3) Anionic Acrustox, and (P4) Typic Hapludox. Undisturbed and disturbed soil samples were collected from the Bw horizon under a native forest. Soil structural stability was evaluated using a variety of techniques and indices, including mean weight diameter (MWD) by hydraulic stress, mechanically-dispersible clay (MDC) by turbidimetry, tensile strength (TS) by crushing aggregates, and soil structural stability index (SSI) taking into account soil organic carbon (SOC). In general, P1 exhibited the highest MDC content (3.05 ± 0.54, Nephelometric Turbidity Unit, NTU/g L‐−1), while P4 had the highest MWD (10.26 ± 0.24 mm) and the highest TS (80.42 ± 18.54 kPa) within the 8–4 mm aggregate size class. The TS for the 4–2 mm and 2–1 mm aggregate size classes was found to be equal for P2 and P4, with values ranging from 158.17 ± 24.70 kPa to 148.04 ± 38.50 kPa in the 4–2 mm class, and from 459.51 ± 189 kPa to 328.35 ± 78.22 kPa in the 2–1 mm one. The SSI was found to be inadequate for evaluating the structural stability of the Oxisols. In general, SOC was found to be the main stabilizing agent of larger aggregates, while clay mineral composition determined the stability of smaller aggregates. Goethite associated with gibbsite was more effective in increasing the structural stability of P2 and P4. Furthermore, kaolinites with low crystallinity, which are found in clayey Oxisols, resulted in a high specific surface area, particularly in Rhodic Haplustox and Typic Hapludox soils, which promoted more interactions with other clay minerals (e.g., goethite and gibbsite) and SOC, thereby increasing the tensile strength in these Oxisols. In fact, the formation and stabilization of aggregates in highly weathered soils depends on several factors, but the influence of clay mineral composition stands out as the most pronounced.

Análise espacial da produtividade da cenoura irrigada em função das propriedades físicas do solo

ABSTRACT Understanding soil, plant attributes, and carrot crop dynamics is vital for optimizing site-specific management practices and enhancing crop yield and soil quality. This study aimed to ascertain the dependence and spatial variability between irrigated carrot yield and soil physical attributes in a clayey Oxisol. The research took place during the 2021 growing season in a commercial carrot production area in Campos Altos, MG, Brazil. One hundred sampling points were established within the irrigated area, distributed across a square expanse of 40,000 m², positioned at the centers of 20 × 20 m grids. The study revealed high yield in the irrigated carrot crop (99.74 t ha-1) with moderate variability. Soil attributes displayed a range of heterogeneity, ranging from low to extremely high, contingent on the variable assessed, as determined by the coefficient of variation. Among the variables exhibiting spatial dependence, particle density exhibited a minimum range of 40 m, which is recommended for further studies on irrigated carrot cultivation in a clayey Oxisol. Total soil water availability demonstrated a positive spatial correlation and proved to be the recommended variable for estimating the spatial variability of carrot crop yield.

Distribution and spatial autocorrelation of physical-water attributes of an Oxisol

Spatial autocorrelation, which in this work was calculated using Moran's bivariate analysis, can be defined as the coincidence of similar values in nearby locations, or the absence of randomness of a variable due to its spatial distribution. Therefore, the objective of this study is to analyze the distribution and spatial autocorrelation of physical attributes of an Oxisol. The experiment was carried out in the irrigation and drainage area of the Universidade Federal de Viçosa, in Viçosa, Minas Gerais, Brazil. The soil in which the experimental meshes were installed was classified as a sandy clayey Oxisol. The attributes were determined: soil moisture on a dry basis, % (DB), soil moisture on a wet basis, % (WB), volumetric soil moisture, % (VS), particle density, g cm-1 (PD), sampled at different depths and within a grid of 90 georeferenced points. For spatial autocorrelation, the global Moran and local Moran indexes (LISA) were used as statistical tools. Bivariate analysis revealed that soil volumetric moisture is closely related to wet and dry basis moisture. It was also found that the surface particle density is related to the deeper layers of the soil, thus reinforcing that the solid fraction of a soil sample, without considering porosity, tends to remain constant. This happens because the predominant mineral constituents in soils are quartz, feldspars, and colloidal aluminum silicates, whose particle densities are around 2.65 g cm-3.

Is just considering texture enough to define compaction on the basis of soil density?

The bulk density values indicative of compaction in clayey soils correspond to a wide range of values (1.30-1.47 Mg m-3), due to the diverse types of soil management. Our hypothesis is that if we consider bulk density values within similar management groups (those that fall to the ground and those that do not fall), the values will be more accurate within each management group. Our objective in this work was to analyze using the concepts of (Yates & Cochran, 1938; Whitehead, 2002) what is a suitable statistical way of grouping these results to reach a consensus regarding a tested hypothesis, the greatest number of density values surveyed between 1977 and 2021, considered for the study of compaction in clayey Oxisols under different managements, to verify whether our hypothesis will be confirmed. We selected 1,521 studies with bulk densities in the literature, corresponding to 44 years (1977 and 2021), which were analyzed by a statistical technique that integrates the results of two or more independent studies on the same subject and combines them into a summary measure (portion forestry). From the data collected in the literature, it was possible to separate two groups of bulk density and management in clayey soils indicative of compaction: the managements that perform little or no manipulation of the soil, such as pasture and no-tillage, densities between 1.41-1.45 Mg.m-3 (average 1.43 Mg m-3), and for the others, such as conventional plowing and minimum tillage, soil densities between 1.31-1.38 Mg.m-3 (average 1.35 Mg m-3). Finally, we conclude that compaction must be analyzed considering the texture and type of soil management.

Soil legacy phosphorus after 10 years of annual dairy liquid manure application in a no‐tillage system

AbstractThe benefits of manure as a source of nutrients for plants and to improve soil quality are well‐known. Monitoring of manure application is needed if environmental issues are to be prevented. In particular, the availability and accumulation of phosphorus (P) has to be subject to rigorous monitoring. This study aims to both evaluate the efficacy of the resin method in extracting inorganic labile P in soils under the long‐term application of dairy liquid manure (DLM), and verify the influence of DLM on the recovery of applied P and soil legacy P. To realize our objectives, two long‐term field experiments were established under a no‐tillage system with annual DLM application on sandy clay loam (sandy Oxisol) and clayey (clayey Oxisol) soils. Treatment consisted of DLM applications (0, 60, 120, 180 m3 ha−1 year−1), independent of mineral fertilizer. Soil samples were taken from the 0–5, 5–10 and 10–15 cm layers after 10 years from the beginning of the trial. A single extraction with resin underestimated inorganic labile P; however, successive extractions were able to take up 43% and 26% of the total P in sandy and clayey Oxisols, respectively, whereas in a single extraction the take‐up was 17% and 8% from the same soils, respectively. The resin method was more effective in extracting P from the sandy Oxisol. Thus, when interpreting soil P contents for fertilizer planning, the soil texture should be taken into account. DLM application decreased P recovered from applied P, ranging from 54% to 83% (sandy), and 43% to 67% (clayey), and substantially increased soil legacy P.

Sugarcane Straw Contribution to Mitigating the Impacts of Cultivation on the Physical Quality of Oxisol

ABSTRACT Sugarcane areas show a reduction in soil physical quality. However, few studies demonstrate the effect of sugarcane straw maintenance on soil physical quality along the crop cycle. This study aimed to evaluate and compare the soil physical quality in areas cultivated with sugarcane where straw was kept on soil, after the first and fourth harvests, with that of an area under native forest. Two areas cultivated with sugarcane, representative of the beginning and end of the production cycle, were evaluated in a clayey Oxisol. One area was evaluated after the first mechanical harvest (C1), while the other was evaluated after the fourth harvest (C4). These areas were compared to an area of native forest (NF). The data were subjected to multivariate principal component analysis. Agricultural areas cultivated with sugarcane showed reduction in soil physical quality compared to NF, presenting higher soil penetration resistance (SPR), soil bulk density (Sd) and microporosity (Mic). Sd was 13.7% higher in the C1 area, while in the C4 area such increase was only 5.1% compared to NF. The negative impact of sugarcane cultivation on soil physical quality is reduced after four harvests in areas with mechanical harvesting and where straw was kept on the soil, especially in the 0.00–0.20 m layer.

Medium-Term No-Tillage, Additional Compaction, and Chiseling as Affecting Clayey Subtropical Soil Physical Properties and Yield of Corn, Soybean and Wheat Crops

Understanding the soil-plant relationship is important to determine critical limits of soil properties that influence crop growth and yield. The objectives were to quantify the influence of soil compaction levels on physical properties and crop growth and yield in a clayey Oxisol. An experiment was performed having in the main plots, levels of soil compaction (NT: no-tillage during six years, NTC: NT + four passes of a 10 Mg machine in the agricultural year 2003/2004, and Chisel: NT + chiseling and harrowing in the agricultural year 2003/2004), and in the sub-plots, the summer crops soybean and corn, and wheat in the winter season. We measured soil physical and mechanical properties, along with yield of corn, soybean and wheat, and root growth of that last two crops. After four passes of a 10 Mg machine, the soil resistance to penetration increased to a 0.12 m depth, while Chisel disrupted the 0–0.20 m soil layer, with the effects persisting for at least nine months. Soil compaction in no-tillage concentrated in the 0.05–0.15 m layer. Corn yield was similar among the treatments, soybean yield was highest in NT, and the highest yield of wheat was in the sequence with soybean under NT.

Mineralogy and pore size distribution of clayey Oxisols with granular structure and the effect of management systems

There is a general consensus in the tropical literature on the crucial role played by clay mineralogy in the formation of the very stable granular structure of Oxisols, which has been mainly attributed to gibbsite. Gibbsite hinders the face-to-face adjustment of kaolinite plates responsible for the very common block structure. While kaolinite predominates in the clay fraction of most Oxisols, gibbsite occurs in variable contents. Studies have reported that the granular structure is secondarily associated to goethite and hematite, soil organic matter and biological activity. However, the variation of clay mineralogy on the alteration of the pore size distribution of clayey Oxisols with granular structure remains unclear. So, the main aim of this work was to study several granular Oxisols from the Brazilian Cerrado biome, under native vegetation, with clayey texture and variable clay mineralogy, focusing on the influence of the latter on pore size distribution and related physical and hydric attributes, and to assess how these attributes are affected by management practices. Results showed that greater contents of gibbsite led to an increase in the micropores/total pores ratio and a higher frequency of smaller pores distribution. Low mesoporosity and water availability values were common in the studied Oxisols. A case study was conducted to evaluate the influence of management practices in physical and hydric attributes when such soils are incorporated to agricultural production systems. Overall, clayey Oxisols with similar granular structure can have distinguishing physical and hydric environments and soil management systems may constitute important strategies to overcome the intrinsic low water availability of these soils.

Biochar After Thirteen Years of Agricultural Crops on the Physical Attributes and Organic Carbon of a Yellow Oxisol in Central Amazon

Biochar has been identified as a conditioner for the physical, chemical and biological properties of the soil. In this perspective, it is reckoned that when added in the long term, this material may condition improvements in the physical properties of agricultural soils. As a result, the objective of this work was to quantify changes in the physical attributes of the soil after thirteen years of the addition of biochar in a Yellow Dystrophic Oxisol in Central Amazon, in the agricultural ecosystem in Brazil. In a thirteen-year experiment (2006-2019) with rotation of agricultural crops, the physical properties of a clayey Yellow Oxisol with the addition of increasing doses of biochar, were studied. The experiment was conducted in randomized blocks, with four replications and four treatments, making up sixteen experimental units with a size of 10 m × 10 m (100 m2). The analysis of the results did not indicate a positive effect on most of the physical attributes studied, however, there was significance only for two sets of data. Compared to soil without biochar, there was a decrease in the density of soil particles, and an increase in soil resistance to penetration into the surface layer, in soil with biochar. No difference was found in the subsurface layer for all evaluated attributes.

Spatial distribution of soil phosphorus fractions in a clayey Oxisol submitted to long-term phosphate fertilization strategies

Due to the strong interaction of phosphorus (P) with soil constituents, P fertilizer placement can significantly affect how crops take up this nutrient. Nonetheless, few studies address the spatial distribution of P at the row-interrow scale according to management strategies. In a 16-yr no-tillage (NT) field experiment involving two different P fertilizer application methods (broadcast or band application) and two P sources (triple superphosphate – TSP or reactive Gafsa phosphate rock-RPR), plus a control treatment, the spatial distribution of P fractions was assessed in two occasions: after the 8th and the 16th crops. This was done vertically to a depth of 30 cm and horizontally from the crop row to the center of the interrows. Broadcast treatments showed total and Mehlich-1 P accumulation at the soil surface while for band application this accumulation was in the crop row region. A small P movement down the soil profile was observed from the 8th to the 16th crop with broadcast application, whereas with band fertilizer this effect was more noticeable, showing increased soil volume under P fertilizer influence even without soil tillage; it is likely that this was partly due to biological P turnover and application at depth. After 16 crops, the soil volume under the influence of P fertilizer was greater under band application while the volume above Mehlich-1 P critical levels for the 0–20 cm layer in regional studies was higher under broadcast application, independent of the P source. Soil organic carbon (SOC) contents were not affected by P placement or source. However, a significant accumulation of SOC was seen from the soil surface downwards after the last eight crops. The spatial distribution of P and SOC was better correlated under broadcast treatments, with high values for both variables at the soil surface; this may explain similar yields to those obtained with band application, where P fertilizer is placed near the main roots in an attempt to reduce P adsorption to the soil solid phase.

Utilização de resíduo de sangue como fonte alternativa de fósforo no cultivo de girassol (Helianthus annuus L.)

ABSTRACT The aim of this work was to increase the aerial part, biometry, and yield of achenes from sunflower plants on the effect of increasing doses of blood residue, as an alternative source of phosphorus (P), in a clayey-textured red Oxisol. The experimental design used was randomized blocks, with six treatments and four replications. The treatments consisted of five doses of blood residue (0, 6, 12, 24 and 48m3 ha-1) and a control treatment that received phosphate chemical fertilization, 80kg ha-1 of P2O5. The increasing doses of blood residue and chemical fertilization do not show disproportionality for P concentration in soil, leaf, plant height (PH), stem diameter (SD), chapter diameter (CD) and achene production. The analysis of the principal component (PC) revealed that the first PC presents the greatest accumulated variation (74%). Based on the first PC, the dose of 12m-3 ha-1 has greater influence for PH, SC, and CD. Due to the greater influence on the biometric characteristics of sunflower plants, the blood residue (12m-3 ha-1 = 98.4kg P ha-1), can be used as an alternative source of phosphorus for the cultivation of sunflower in clayey oxisols.

Changes in soil temperature and moisture due to sugarcane straw removal in central-southern Brazil

Crop residues left in the field cover and protect the soil surface, and regulate key processes and functions, such as gas and water exchanges. However, the Brazilian sugarcane (Saccharum officinarum L.) sector has begun to use straw as feedstock to produce bioenergy. We conducted a field study to evaluate the effects of sugarcane straw removal in soil temperature and moisture changes at three sites (with different soil textures: Site 1 - clayey Oxisol, Site 2 - medium texture Oxisol, and Site 3 - sandy Ultisol) in the state of São Paulo, Brazil. The experimental design was randomized blocks with four rates of straw removal: i) no removal (NR); ii) moderate removal (MR); iii) substantial removal (SR), and iv) total removal (TR). Soil temperature was measured by sensors in the 0- to 5- and 5- to 10-cm soil layers. Undisturbed soil samples were collected from the 0- to 20- and 20- to 40-cm layers to determine soil moisture. Intensive straw removal (HR and TR) increased the soil temperature between 2 and 3 °C and the thermal amplitude between 5 and 9 °C in the 0- to 5-cm layer, compared to MR and NR. Soil moisture decreased between 0.03 and 0.07 g g–1 in the 0- to 20-cm layer with intensive straw removal. The sandy soil was more susceptible to straw removal. Therefore, straw maintenance on the soil surface plays an essential role in temperature regulation and preservation of higher soil moisture, especially in regions with severe water deficits and long periods of water stress.

Soil organic matter fractions in an Oxisol under tillage systems and winter cover crops for 26 years in the Brazilian subtropics

The improvement of carbon (C) accumulation in soils has been one of the main purposes of the conservation systems in agricultural production. This study aimed to assess the long-term effect of conventional tillage (CT) and no-tillage (NT) combined with winter cover crops, black oat and oilseed radish, and fallow on C accumulation and stabilization in a very clayey Oxisol in Southern Brazil. Soil samples were collected in the 0-0.05, 0.05-0.10 and 0.10-0.20 m layers of a 26-year-old experiment. Distribution of size-class aggregates, C stock in aggregates, total C stock, and C stocks in the physical fractions, free particulate organic matter (free-POM), occluded particulate organic matter (occluded-POM) and mineral-associated organic matter (min-OM) were assessed. NT had a higher percentage of macroaggregates and C stock in this size-class, and also higher C stock in bulk soil, free-POM and occluded-POM fractions than CT in 0-0.05 m (Tukey’s test p < 0.05), due to higher input of biomass and minimum soil mobilization in NT. Oat and radish had higher C stock in macroaggregates than fallow in 0.05-0.10 m (Tukey’s test p < 0.05). Radish had the highest C stock in the free-POM (0-0.05 m). Fallow decreased the stabilization of macroaggregates and C accumulation in free-POM, due to the lower C input from aboveground biomass over the years. In conclusion, NT after 26 years improved C accumulation and stabilization, mainly in the superficial layer and in POM fractions, and winter cover crops favored the formation and stability of macroaggregates.

Exchangeable potassium reserve in a Brazilian savanna Oxisol after nine years under different cotton production systems

Adequate potassium (K) fertilization is essential for agricultural production in soils of the Brazilian Savanna (Cerrado biome) due to the high demand by crops (especially cotton), likelihood of leaching losses, and the dependence on fertilizer importations. Therefore, sustainability requires improvements in the soil efficiency management. This study evaluated the influence of soil management and crop succession or rotation combinations with cotton on K dynamics and exchangeable reserves in the soil profile, and the partial balance of K after nine years of cultivation in a clayey Oxisol in the Cerrado. The soil was sampled in layers up to 100 cm depth in four cotton production systems treatments: 1) conventional soil tillage (CST) with cotton monoculture; 2) CST with annual cotton-soybean-cotton succession; 3) CST with cotton/soybean/maize rotation; and 4) no-tillage system (NTS) with cotton/soybean/maize rotation and Urochloa ruziziensis (ruzigrass) as a cover crop in the off-season after grain crops. The experiment was conducted using a randomized block design with four replications. CST with cotton monoculture favored the leaching of K surplus from fertilization. Crop rotation including ruzigrass in the NTS allowed better control of K dynamics, ensuring its circulation in the soil-plant compartments and reducing leaching. The increased organic matter (OM) in this system enlarges the storage capacity of K in the topsoil. The adoption of no-tillage crop systems integrating ruzigrass is viable to improve the efficiency of K fertilizers in cotton cultivation in highly weathered tropical soils.

Soil enzymatic activity under coffee cultivation with different water regimes associated to liming and intercropped brachiaria

ABSTRACT: This research evaluated the effects of coffee cultivation with two different water regimes associated or not with liming and the presence/absence of brachiaria as intercrop on the activities of the soil enzymes β-glucosidase, arylsulfatase and acid phosphatase. The study was carried out at the experimental farm of Embrapa Cerrados, using the cultivar IAC 144 (Coffea arabica L.), under a clayey dystrophic Cerrado Oxisol. Two water regimes (WR) were considered, WR1 with irrigation shifts throughout the year and WR3 with controlled water stress, for about 70 days, in the dry season. In each water regime, effects of lime application (with/without) and the presence/absence of brachiaria cultivated between the lines of coffee plants were evaluated. The activities of the enzymes β-glucosidase, arylsulfatase and acid phosphatase were evaluated during the rainy and dry seasons. Liming and intercropped brachiaria positively affected the activities of the three enzymes assessed in this study at varying degrees, depending on season and/or the WR. Our findings evidenced that intercropped brachiaria in coffee rows was the factor that most positively impacted soil enzymes activities.

Effects of land use, tillage management, and crop diversification on soil physical quality in Cerrado agricultural systems

AbstractSustainable agricultural intensificationhas been considered one of the solutions for increasing food production and mitigating climate changes. Nevertheless, soil physical degradation induced by intense mechanization may threaten those benefits in large‐scale agricultural regions such as the Brazilian Cerrado. Thus, our study aimed to assess the effects of land use, tillage management, and crop diversification on soil physical quality of two long‐term experiments in Cerrado through on‐farm visual evaluation of soil structure (VESS) and soil physical parameters. Soil sampling was performed in 0‐to‐10‐ and 10‐to‐20‐cm layers of four agricultural systems: soybean [Glycine max(L.) Merr.]–fallow under conventional tillage (SF); soybean–fallow under no‐till (SN); soybean–maize (Zea maysL.) succession under no‐till (SM); and crop rotation under no‐till including soybean, maize, grasses, and legumes (CR). An additional area of native Cerrado vegetation (NV) was included as reference. Land transition from Cerrado to agricultural systems declined the soil physical quality, detected by VESS assessment and physical parameters in both layers. Tillage management did not alter soil physical parameters, which was likely associated with the medium time adoption in the experiments (9 yr) and high resilience capacity of clayey Oxisols. Unexpectedly, SM and CR negatively affected the porosity‐ and water‐associated soil parameters in the 10‐to‐20‐cm layer compared with SN. It suggests that the soil physical damage induced by higher frequency of machinery operations required to manage a more diverse crop sequence may prevail over the benefits provided by crop diversification. Finally, this study highlights that intensified agricultural systems need to be coupled with strategies to reduce and/or control intensive machinery traffic.

Variability in soil physical–chemical properties along the root-explored profile in deep Oxisols of commercial eucalypt plantations

While deep soils commonly characterize tropical forests, direct measurements of soil physical–chemical properties, fine root distributions, and relationships among them are scarce. In this paper, we report results from a project in Brazil that were obtained from experimental areas selected for their representative tropical climatic and edaphic features. Our objective was to evaluate vertical and spatial soil physical–chemical properties along the root-explored profile of Eucalyptus plantations to better understand the relationship between each component. Soil was collected from four sites (Três Lagoas-Site 13, Mogi Guaçu-20, Telemaco Borba-22, and Bocaiuva-30) and characterized for physical–chemical properties (pH, texture, organic matter, cation exchange capacity, and nutrient content) and fine root density (FRD). Soil was sampled up to 2 m beyond the deepest observed roots, or until a cemented horizon or R layer was detected. Spatial variability was investigated through a geostatistical approach using ordinary kriging. Data were subjected to ANOVA as well as bi- and multivariate statistics using a correlation matrix and robust principal factor analysis (PFA), respectively. Results showed that investigated Oxisols reached the maximum depth of 19 (Site 13), 8 (20), 11 (22), and 8 (30) m. In most investigated sites (20, 22, and 30) and for most soil properties, a typical soil horizonation was observed along the first 2.5/3.0 m. Deeper horizons (>3.0 m) of clayey Oxisols were characterized by the presence of typical porthole shapes dramatically interrupting soil horizonation, indicating spatial pedovariability and demonstrating underestimates of vertical and spatial pedovariability can occur under standard soil investigations to 2.0 m in depth. Correlation matrix and PFA showed that Eucalyptus roots strongly affected most investigated soil properties. Our study considered vertical and spatial dimensions of soil physical–chemical properties as well as the deep-rooting properties of Eucalyptus, which is critical for improving soil management in infertile Eucalyptus plantations.

Ecotoxicity of imidacloprid to soil invertebrates in two tropical soils with contrasting texture.

Imidacloprid is one of the most commercialized insecticides in agriculture in the world, with a broad spectrum of action. However, little is known about the effects of commercial formulations containing this active ingredient (a.i.) on non-target organisms in tropical soils. Our objective was to assess the toxicity based on the predicted environmental concentration (PEC) of imidacloprid, in the avoidance behaviour of earthworms and collembolans as well as in the reproduction of collembolans, in two representative soils of the Brazilian Cerrado with contrasting texture (clayey Oxisol and sandy Entisol). Ecotoxicity tests were carried out according to ISO protocols to assess the avoidance behaviour of earthworms (Eisenia andrei) and avoidance and reproduction of collembolans (Folsomia candida). In the earthworm's avoidance test, more than 80% of the individuals were found in the control, in all tested concentrations, indicating a possible habitat function loss in both soils. The avoidance behaviour of collembolans was observed in both soils, being more expressive (up to 75% of escape) in Oxisol. In Entisol, only the two highest concentrations were avoided (up to 63%). There was a negative effect on the reproduction of collembolans in both soils, with a higher EC50 value (0.255 mg kg-1) in Oxisol than in Entisol (0.177 mg kg-1), demonstrating higher toxicity in the sandy soil. These differences were attributed to the contrasting texture of the studied soils, probably due to lower retention of the a.i. in the sandy soil, causing an increased bioavailability. This study demonstrated that imidacloprid can be highly toxic to soil invertebrates, even in soil concentrations lower than those expected from recommended dose, causing an impact on the edaphic organisms and, consequently, compromising its functions in the soil ecosystem.

Contribution of tillage systems and crop succession to soil structuring

The use of conservational systems of soil cultivation has gained importance due to the need for soil preservation and carbon sequestration from the atmosphere. With the objective of evaluating soil structuring, this study determined the physical attributes and organic carbon contents of a clayey Oxisol in an area of 10 years under four different land uses: i.e., afforestation (AF), no-tillage system with grass-grass succession (corn/corn), no-tillage systems with grass-legume succession (corn/Crotalaria juncea L.) and conventional soil tillage system (corn/fallow). The evaluated areas are located in the Cerrado biome of southeastern Brazil. Each area was sampled at 20 points and undisturbed and disturbed samples were collected in the layers of 0.00−0.10 m, 0.10−0.20 m and 0.20−0.30 m. The attributes evaluated were related to soil aggregation and porosity and, due to the structure of dependence of the variables, the data were subjected to multivariate factor analysis. We observed that, after a long period, the no-tillage system promoted soil aggregation and porosity similar to those of the planted forest and superior to those of the conventional tillage system. The type of crop succession in the no-tillage system caused differences in soil structure, and grass-grass succession (corn/corn) promoted better soil structure compared to grass-legume succession (corn/Crotalaria juncea L.). The greater soil porosity generated by the conventional tillage system was limited to the most superficial layer (0.00−0.10 m) and this effect can be considered ephemeral, due to the presence of a large amount of smaller aggregates that, after the operation, can occupy the generated pores.

Adsorption and desorption of arsenic and its immobilization in soils

ABSTRACT: Arsenic (As) is a naturally occurring chemical element considered toxic and carcinogenic by health and environmental protection agencies. Studies of As adsorption/desorption behavior in soils are important to predictions of As’ potential mobility in natural systems. The aim of this study was to assess the adsorption of As(V) in soils from Minas Gerais, Brazil, and determine its immobilization rate in order to identify soils with characteristics more favorable to its deployment as an As geochemical barrier. The adsorption experiment was performed using different As concentrations and the data pertaining to the maximum adsorption capacity of As(V) (MACAs) were determined by Langmuir and Freundlich isoterms. The Oxisols, due to their more oxidic mineralogy, especially more gibbsitic, and clayey texture, showed the highest MACAs, followed by Ultisols, Inceptisols, and Entisols. In terms of the desorption of As the Inceptisols were the soils that showed the most As desorption. Both As desorption and mobility was lower in the more oxidic and clayey soils. In all soils, the total amount of As was desorbed in due course, but the As release ratio tended to decrease with the passage of time. In general, soils with higher MACAs did not necessarily show less As desorption. For use as a geochemical barrier, as important as a high adsorption capacity of As by the soil is a low As desorption rate. The increase in As mobility may increase the risks of contaminating the supplies of water. To be a good As geochemical barrier the soil has to be a clayey Oxisol, with relatively high amounts of Fe and Al oxides, especially gibbsite.