Models relating soil pH measurements in H2O, KCl and CaCl2 for volcanic ash soils of Cameroon (2023)

Heavy metals from anthropogenic sources accumulate slowly but steadily, leading to high metal concentration levels in soil. However, the effect of each heavy metal on soil bacterial communities is usually assessed in laboratories by a single application of individually spiked metals. We evaluated the differences between single individual application and repeated individual applications of Cr, Cu, Ni, Pb, and Zn on bacterial communities, through pollution-induced community tolerance (PICT), using bacterial growth as the endpoint (³H-leucine incorporation method). We found that PICT development was higher when soil was spiked in individual single application than individual repeated applications for Cu, Ni and Zn. In contrast, bacterial communities did not show different tolerance between singly or repeatedly when soil was spiked with Cr. In the case of Pb any increase of bacterial community tolerance to this metal was found despite high doses applied (up to 2000mgkg⁻¹). These results are relevant for the interpretation of the effects of heavy metals on soil microbes in order to avoid laboratory overestimations of the real effects of heavy metals on soil microbes.

Artisanal gold (Au) mining is the activity with the highest consumption of mercury (Hg) and the main source of environmental contamination by this element, which is a recurring problem in the Amazon. In this study, contamination and risks caused by Hg to the environment and human health were evaluated in different forms of artisanal Au mining in the Brazilian Amazon. For this purpose, 25 samples of soils and tailings were collected in three types of artisanal mine and one native forest. The mineralogical analysis revealed that there is no occurrence of minerals constituted by Hg. However, the concentrations of Hg in underground mining tailings were very high and exceeded the prevention values established by Brazilian environmental legislation, indicating elevated risk to the ecosystem and human health. The enrichment factor indicated that underground mining tailings are enriched by Hg, submitted to cyanidation or not, suggesting anthropogenic source for the high concentrations of Hg. The geoaccumulation index and the contamination factor showed that the colluvial mining tailings are moderately contaminated, and the tailings from underground mining are highly to extremely contaminated, leading to very high risks to the environment and the health of children from the region. These results represent a great contribution to the Amazon, since they provide subsidies for the definition of policies to mitigate environmental contamination and associated risks.

Agricultural activities pose a significant risk of groundwater pollution. Indeed, fertilizers and treated wastewater used for irrigation are, in part, responsible for the deterioration of groundwater and surface water quality. In some cases, soil may provide a protective barrier against this pollution, but this depends on the nature of the soil and the contaminant. This work presents the effect of the soil clay content on the retention of four different pollutants in order to evaluate the risk they represent for the groundwater. These contaminants are generated by two main agricultural activities: 1/soil fertilization with phosphate and nitrate fertilizers and 2/irrigation with treated wastewater in which heavy metals such as nickel and lead are persistent. Firstly, the characterization of the sand and clay used in this work was performed and showed a cation exchange capacity of 1.24 and 25 meq/100 g, a specific surface area of 0.12 and 67.98 m²/g and a percentage of organic matter of 0.15 and 2% for sand and clay, respectively. The retention isotherms on all pollutants and the Langmuir, Freundlich, Freundlich-Langmuir, Hill and Koble-Corrigan models were applied. All experimental isotherms have been successfully adjusted using the Koble-Corrigan expression. The amounts of nitrates, phosphates, nickel and lead retained by the sandy soil, for an initial pollutant concentration equal to 1 meq/L, were evaluated at 0.29, 3.89, 5.97 and 8.56 μeq/g respectively. In contact with a soil containing 30% clay, the adsorbed amounts were estimated at 3.55, 15.00, 6.97 and 8.79 μeq/g for nitrates, phosphates, nickel and lead, respectively. These results mean that the pollutants that pose the greatest risk of groundwater contamination when carried by water through sandy soil are classified as follows lead < nickel < phosphate < nitrate while for a clayey soil, the classification becomes as follows: phosphates < lead < nickel < nitrate.

Geoderma Regional, Volume 14, 2018, Article e00187

To obtain better knowledge on how soil organic matter (SOM) decomposition is regulated by different soil properties, 13 soil samples amended with maize residues were incubated for 163 days. Carbon mineralization kinetics was modelled by fitting five different kinetic models, from which the first-order two pools model was selected for further analysis. The results showed that residue addition increased the rate of C mineralization, particularly in sand samples, where four times as much C was released as CO₂ in amended samples as in control samples. Residue addition also affected the kinetic parameters of the two C pools modelled. The mean residence time (MRT) averaged 31.9 days for the fast C pool and 65.1 days for the slow C pool phase in the amended soils, whereas in the control soils MRT₁ and MRT₂ averaged 52.3 and 94.2 days, respectively. The kinetics of C decomposition was found to be influenced to the greatest extent by soil organic carbon (SOC) content and soil texture. The texture primarily affected the size of the fast carbon pool (C₁): in both the amended and control samples C₁ was increased by the clay content (r = 0.574 and r = 0.554, respectively) and decreased by the sand content (r = −0.771 and r = −0.583, respectively). Our results confirm the fact that texture has a significant role in soil organic matter mineralization, however other parameters also play a crucial role in the decomposition due to the complexity of the processes.

Geoderma, Volume 337, 2019, pp. 350-358

Efficient phosphorus (P) fertilization strategies are essential for intensive crop production with minimal negative environmental impacts. A key factor in sustainable P use is assessment of the plant available soil P pool using soil P tests. This study determined isotopically exchangeable P after six days of reaction with ³³PO₄ (P-E (6 d)) to determine how accurately two commonly used P tests, Olsen and AL (acid ammonium acetate lactate) can quantify the amount of labile P. Soil samples were taken from both highly P-amended and unamended plots at six sites within the Swedish long-term soil fertility experiments. According to P K-edge XANES spectroscopy, the P speciation was dominated by Al-bound P and organic P, with additional contributions from Fe-bound P and Ca phosphates in most soils. The results showed that the AL test overestimated P-E (6 d) by a factor of 1.70 on average. In contrast, the Olsen test underestimated P-E (6 d), with the mean ratios of P-Olsen to P-E (6 d) being 0.52 for high-P and 0.19 for low-P soils. The ³³P/³¹P ratio in the Olsen extract of a ³³PO₄ spiked soil was closer to that of a 0.005 mol L⁻¹ CaCl₂ soil extract than the corresponding ratio in the AL extract, suggesting that AL extraction solubilized more non-labile P. In conclusion, the AL and Olsen methods are not suitable for direct quantification of the isotopically exchangeable soil P pool after 6 days of equilibration. However, based on the results, Olsen may be superior to AL for classification of soil P status, due to its even performance for calcareous and non-calcareous soils and lower extraction of non-labile P.

Geoderma Regional, Volume 14, 2018, Article e00186

In Rio Grande do Sul (RS) State, Brazil, the soils are not prone to salinization due to the large amounts of rainfall, which are sufficient to leach the soluble salts. However, flooded rice crops grown in the coastal plains of RS may be affected by soil salinity, once the soils in this region are formed by marine, fluvial-lacustrine sediments. The use of water for irrigation of crops from sources connected with the Atlantic Ocean, especially Patos Lagoon, can lead to deposition of excessive amounts of sodium in the soil, especially in southern portion, by the proximity to channel linking the lagoon to the sea. This study aimed to map the occurrence of salt-affected soils in the coastal plains of RS State, Brazil. The study was performed in three regions: Internal Coastal Plain (ICP), External Coastal Plain (ECP) to Patos Lagoon and North Shore. 766 georreferenced soil samples were collected and levels of exchangeable sodium, exchangeable sodium percentage (ESP) and electrical conductivity (EC) of saturated extract (ECse) were analyzed. The resulting analytical data of the sampling points were submitted to a descriptive statistic and used to generate maps aiming to depict the continuous spatial variation of each measured variable. Most of soil samples showed indicators of low soil salinity, especially in most of the ICP and North Shore. The soils most affected are concentrated in regions overlapping the paleochannels sub-surface previously identified in the sub-surface of the ECP and in areas closer to links between sources of freshwater and sea. In the ICP, the salinity problem was restricted to only some areas due to the use of saline water from Patos Lagoon.

Geoderma Regional, Volume 14, 2018, Article e00184

This study examined the impact of land use changes on phosphorus (P) transformations in soils of different pedogenesis with little or no addition of P in the humid tropical region. The sequential extraction method was used to determine P fractions, and structural equations modeling was employed to investigate the P cycle in soils under plantations of rubber tree (Hevea brasiliensis), rubber tree + cocoa (Theobroma cacao), rubber tree + acai palm (Euterpe oleracea), rubber tree + cupuassu (Theobroma grandiflorum) and cocoa + erythrina (Erythrina glauca) as well as pastures (Brachiaria decumbens) and natural forests. The distribution of P fractions in soil appears to be affected by land use in all soil orders. Agroforestry systems of rubber tree + acai palm and rubber tree + cocoa showed high capacity to increase the concentrations of all the P fractions in soil compared to natural forest in soils of advanced stage of weathering. Pastures and rubber plantations provided higher concentrations of inorganic P fractions. The P fractionation method revealed the potential of labile fractions (resin-Pi + NaHCO₃-Pi + Po) to supply appropriately the demand of all vegetation types evaluated in all soil orders. The structural model enabled to identify functional pools of P in soil, and to identify transformation processes of P in soil, in which organic P pool was the main P source for the available P pool and which part of the occluded P pool (recalcitrant) can be available for the plants.

Geoderma Regional, Volume 22, 2020, Article e00315

It is critical to have a soil test that allows for the measurement of potential toxicity. A laboratory experiment was undertaken to investigate the effects of changing the concentration of salt in the extract and extraction time of the standard CaCl₂ and KCl soil Al tests on the Al concentrations extracted from 13 soils from four New Zealand soil orders. Al extracted by KCl (Al_KCl) was 13 times higher than extracted by CaCl₂ (Al_CaCl2) across all soils. The effect of changing extract salt concentration and extraction time on Al extracted differed among the four soil orders tested for the two extraction methods. Increasing the concentration of CaCl₂ in the extract increased (P<.001; P<.05) the amount of Al_CaCl2 (by 0.02–0.13 cmol_c/kg) for the four soil orders, while increasing extraction time resulted in a difference only in the Allophanic soils, where Al_CaCl2 decreased (P<.01). The interaction of the concentration of salt in the extract and extraction time for the CaCl₂ extraction did not affect (P>.05) the Al_CaCl2 extracted from all soils. An increase in the concentration of KCl in the extract up to 1M increased Al_KCl (P<.01) (by 0.2–0.8 cmol_c/kg) on the Allophanic, Brown and Pumice soils, with no increase (P>.05) with a further increase in concentration. Extraction time affected Al_KCl (P<.001) for Pallic soils, while the interaction of concentration of KCl in the extract and extraction time resulted in differences (P<.001) only for the Pallic soils. These findings suggest that the Al concentrations measured by the two extraction methods are affected by specific soil properties in the topsoil related to soil order. This means that when measuring the Al bioavailability in soils, extreme care must be taken when interpreting soil Al test results.

Soil Biology and Biochemistry, Volume 151, 2020, Article 108051

Soil texture is well known to directly affect bioavailability of organic matter to heterotrophs, but it also steers their activity by moderating soil moisture fluctuation. Disentangling these direct and indirect textural controls is, however, not trivial and attempts to do so are very scarce. Most attention has just gone to the stimulation of soil carbon (C) mineralization by soil moisture fluctuation per se. To quantify the indirect moisture-mediation control of soil texture on C mineralization, we monitored maize straw degradation in various soil texture/moisture regime combinations. Moisture levels were firstly kept fixed at 32% WFPS (experiment Fixed₃₂) in a sand, sandy loam and silt loam soil or allowed to fluctuate between 20% and 50% water-filled pore space (WFPS, Dry-wet_20-50). Total maize-C (C_maize) mineralized was highly similar between these three textures and thus the direct textural control was minor. On the contrary with the fluctuating moisture level, around threefold more added C_maize was mineralized (P<0.01) in the sand (86%) than in the silt loam (25%) soil. We owe this boost in C_maize mineralization to the rewetting of larger pores in the sandy soil which should contain most of the ground maize residue. This determining control of texture on distribution of moisture as well as the maize substrate clearly exceeded the direct impact of texture on organic matter stability. In a third and fourth experiment, timing and dose to remoisten a silt loam soil back to 50% (experiment Equal_20-50) or 35% WFPS (experiment Equal_20-35) when dried out to 20% WFPS were additionally mimicked in the sand and sandy loam soils. These scenarios correspond more closely to a field situation in which nearby differently textured soils all receive the same precipitation input. After several rewetting cycles, the silt loam soil eventually had a 13.9% higher WFPS than the sandy soils. By then moisture stress clearly limited C_maize mineralization in the sandy soil (0.13 (Equal_20-50) and 0.05 (Equal_20-35) mg kg⁻¹ h⁻¹) as it proceeded at only half of the rate as in the silt loam soil (0.21 (Equal_20-50) and 0.12 (Equal_20-35) mg kg⁻¹ h⁻¹). The amount of C_maize mineralized after 120 days in the silt loam soil was simulated to surpass that in the sandy soil. We conclude that the effect of soil texture on decomposition of a fresh substrate is largely indirect, i.e. through mediation of soil water content and its distribution in the soil matrix. Moreover, our data suggests that in the event of prolonged drought C_maize mineralization will be less limited in finer textured soil, contradicting the widespread idea that organic matter would degrade more rapidly in coarser textured soils.