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The application of biochar (biomass-derived black carbon) to soil has been shown to improve crop yields, but the reasons for this are often not clearly demonstrated. Here, we studied the effect of a single application of 0, 8 and 20 t ha⁻¹ of biochar to a Colombian savanna Oxisol for 4 years (2003-2006), under a maize-soybean rotation. Soil sampling to 30 cm was carried out after maize harvest in all years but 2005, maize tissue samples were collected and crop biomass was measured at harvest. Maize grain yield did not significantly increase in the first year, but increases in the 20 t ha⁻¹ plots over the control were 28, 30 and 140% for 2004, 2005 and 2006, respectively. The availability of nutrients such as Ca and Mg was greater with biochar, and crop tissue analyses showed that Ca and Mg were limiting in this system. Soil pH increased, and exchangeable acidity showed a decreasing trend with biochar application. We attribute the greater crop yield and nutrient uptake primarily to the 77-320% greater available Ca and Mg in soil where biochar was applied.

Solanum paniculatum L., a species endemic to tropical America and used in folk medicine in the treatment of anemia, hepatic and digestive disorders, has been widely studied. However, to date, no previous studies on correlations between the contents of mineral nutrients in plant roots with their contents in soil have been reported. The aim of this study was to determine the main mineral nutrients found in S. paniculatum roots and the soil in their natural habitat. It was observed that S. paniculatum roots grow in soils rich in calcium and had a positive correlation in the concentration of Ca with Fe, Na and K and a negative correlation with Zn and Mg. The results contribute to the knowledge of mineral nutrients in S. paniculatum as well as to its cultivation.

The Cr(VI) removal efficiency of a Brazilian Oxisol collected from the A horizon (15.2% iron oxides, 4.6% organic matter, pH 4.3) was assessed and compared with synthesized iron minerals (SM), to mitigate contamination as a nature-based solution. Batch tests revealed that Cr(VI) removal efficiency was higher and faster for SM than for Oxisol: to remove ∼ 100% Cr(VI) from a solution containing 5 mg L − 1 Cr(VI), SM took 24 h, while for the same concentration Oxisol removed 80% Cr(VI) in 30 days. At a higher concentration (30 mg L − 1 ), Cr(VI) removal efficiency dropped to 90% in 24 h for SM and 22% in 30 days for Oxisol. The Cr(VI) removal of Oxisol and SM fitted, respectively, to the pseudo-first and pseudo-second order, reflecting the way Cr(VI) is removed - by adsorption for SM (since pH PZC > pH solution ) and by reduction to Cr(III) for Oxisol organic matter. To simulate a contamination event, Oxisol was incubated with 300 mg L − 1 of Cr(VI) for 5, 10 and 30 days, followed by sequential extraction. The results of this test revealed that all the chromium was reduced to Cr(III) in all the time periods, and was mostly bound to the less mobile fractions of the soil: organic matter (75–85%), iron oxide/hydroxide (∼ 15%), and the residual fraction (< 5%); and only 5–10% was bound to the exchangeable fraction (more mobile). Although Cr(VI) is better adsorbed by SM, in Oxisol chromium is found in the less toxic form (trivalent) and bound to the less mobile fractions, promoting a more sustainable solution.

PurposeImidacloprid is a widely used seed dressing insecticide in Brazil. However, the effects of this pesticide on non-target organisms such as soil fauna still present some knowledge gaps in tropical soils. This study aimed to assess the toxicity and risk of imidacloprid to earthworms Eisenia andrei and collembolans Folsomia candida in three contrasting Brazilian tropical soils.Materials and methodsAcute and chronic toxicity assays were performed in the laboratory with both species in a tropical artificial soil (TAS) and in two natural soils (Oxisol and Entisol), at room temperature of 25 °C. The ecological risk was calculated for each species and soil by using the toxicity exposure ratio (TER) and hazard quotient (HQ) approaches.Results and discussionAcute toxicity for collembolans and earthworms was higher in Entisol (LC50 = 4.68 and 0.55 mg kg−1, respectively) when compared with TAS (LC50 = 10.8 and 9.18 mg kg−1, respectively) and Oxisol (LC50collembolans = 25.1 mg kg−1). Chronic toxicity for collembolans was similar in TAS and Oxisol (EC50 TAS = 0.80 mg kg−1; EC50 OXISOL = 0.83 mg kg−1), whereas higher toxicity was observed in Entisol (EC50 = 0.09 mg kg−1). In chronic assays with earthworms, imidacloprid was also more toxic in Entisol (EC50 = 0.21 mg kg−1) when compared to TAS (EC50 = 1.89 mg kg−1). TER and HQ values indicated a significant risk of exposure of the species to imidacloprid in all soils tested, and the risk in Entisol was at least six times higher than in Oxisol or TAS.ConclusionsThe toxicity and risk of imidacloprid varied significantly between tropical soils, being the species exposure to this pesticide particularly hazardous in very sandy natural soils such as Entisol.

Although several grasses have been evaluated for cadmium (Cd) phytoextraction, there are few studies assessing how Cd is accumulated and distributed in the tissues of Panicum maximum grown in mildly spiked soils. The evaluation of tillering, nutritional status and biomass yield of this grass, mainly along successive shoot regrowths, is not well studied so far. Thus, P. maximum Jacq. cv. Massai was grown for two periods in an Oxisol presenting bioavailable Cd concentrations varying from 0.04 (control) to 10.91 mg kg−1 soil. Biomass yield of leaves and stems’ growth has decreased under the highest Cd exposure, but it did not occur in the regrowth period, indicating that Cd-induced toxicity is stronger in the early stages of development of P. maximum. The tillering was not compromised even the basal node presenting Cd concentrations higher than 100 mg kg−1 DW. We identified a restriction on Cd transport upwards from basal node, which was the main localization of Cd accumulation. Apparently, P, K, Mg, S and Cu are involved in processes that restrict Cd translocation and confer high tolerance to Cd in P. maximum. The Cd-induced nutritional disorders did not negatively correlate with factors used to calculate phytoextraction efficiency. However, the nutritional adjustments of P. maximum to cope with Cd stress restricted the upward Cd transport, which decreased the phytoextraction efficiency from the available Cd concentration of 5.93 mg kg−1 soil.

Purpose This study aimed to assess the impact of two biochars applied at the rate of 15 t haâ1 on physico-chemical parameters of an oxisol in Cameroon. Method The biochars were made from slow pyrolysis (~300 °C, 4 h) of eucalyptus tree bark and corncobs and then incorporated into the top 15 cm of the soil with or without straw. The soil tillage mode was either fat plots or furrows and ridges. Soil porosity, bulk density, saturated hydraulic conductivity, available water content, pH, nitrogen, potassium, phosphorus, cation exchange capacity and electrical conductivity were analyzed before biochar application, then 6 and 12 months after. Results None of the measured soil physical parameters were affected by the presence or type of biochar. The total porosity was lower during the second production period compared to the first, while available water content and van Genuchten parameters increased during the second production period. No significant difference was observed between soil nitrogen, phosphorus, potassium, cation exchange capacity and electrical conductivity of control and treated plots. Conclusion We recommend that straw be pyrolysed and the resulting biochar incorporated into soil instead of burying straw (as is actually done in furrow and ridges tillage mode).

 This work evaluated the potential of lead (Pb) phytoextraction by forage peanut, Arachis pintoi, cultivated on an Oxisol and a Gleysol. The samples of these soils received Pb as lead acetate heptahydrate (Pb(C2H3O2)2.7H2O) at rates of 0, 150 and 300 mg kg-1. The plants were cultivated in pots filled with Pb contaminated soil, and 105 days after planting were collected to determine shoot and root dry matter. Lead contents in soil and plant samples were determined using an atomic absorption spectrophotometer. Lead did not affect shoot dry matter yield in both soil classes, but linearly reduced root dry matter. The highest Pb concentration in the plant was detected in the A. pintoi shoot. This species is not a Pb accumulator but may be considered Pb tolerant and is indicated for revegetation programs in Pb-polluted soils.

Atmospheric acid deposition remains a widespread problem that may influence the protection of carbon (C) in soil by altering organo-mineral interactions. However, the impacts of additional acidity on organo-mineral interactions and soil C sequestration in naturally acidic tropical soils with a high content of reactive iron (Fe) phases have not been well studied. Here we conducted a long-term (9.5-year) field experiment with a gradient of acidity treatments (3, 9.6, 32, 96 mol H+ ha−1 year−1 as nitric + sulfuric acid, added to ambient deposition) to examine how acidification alters organo-mineral interactions and soil organic carbon (SOC) pools in a tropical forest of China where soils are already highly acidic (pH ≈ 4). As expected, soil acidification significantly enhanced the leaching of base cations (e.g., Ca2+), and it also altered the solubility and composition of Fe and Al phases. The acidity treatments decreased total Fe and converted more crystalline Fe (oxyhydr) oxides to short-range-ordered phases, evidenced by an increase in Fe extracted by citrate-ascorbate solution and 0.5 M hydrochloric acid, and a decrease in the difference in Fe extracted by citrate-dithionite and citrate-ascorbate. Together, these changes led to a large increase in Fe-bound C versus a relatively small decrease in Ca-bound C. Overall, the acidity treatments increased the mineral-associated C stock to 32.5–36.4 Mg C ha−1 versus 28.8 Mg C ha−1 in the control, accounting for 71–83% of the observed increase in total SOC stock. These findings highlight the importance of pH-sensitive geochemical changes and the key roles of Fe in regulating the response of SOC to further inputs of acid deposition even in highly weathered and naturally acidic soils. The magnitude of SOC changes observed here further reveals the necessity of including pH-sensitive geochemistry in Earth system models to better predict ecosystem C budgets under future acid deposition scenarios.

Application of organic fertilizers and charcoal increase nutrient stocks in the rooting zone of crops, reduce nutrient leaching and thus improve crop production on acid and highly weathered tropical soils. In a field trial near Manaus (Brazil) 15 different amendment combinations based on equal amounts of carbon (C) applied through chicken manure (CM), compost, charcoal, and forest litter were tested during four cropping cycles with rice (Oryza sativa L.) and sorghum (Sorghum bicolor L.) in five replicates. CM amendments resulted in the highest (P < 0.05) cumulative crop yield (12.4 Mg ha-¹) over four seasons. Most importantly, surface soil pH, phosphorus (P), calcium (Ca), and magnesium (Mg) were significantly enhanced by CM. A single compost application produced fourfold more grain yield (P < 0.05) than plots mineral fertilized in split applications. Charcoal significantly improved plant growth and doubled grain production if fertilized with NPK in comparison to the NPK-fertilizer without charcoal (P < 0.05). The higher yields caused a significantly greater nutrient export in charcoal-amended fields, but available nutrients did not decrease to the same extent as on just mineral fertilized plots. Exchangeable soil aluminum (Al) was further reduced if mineral fertilizer was applied with charcoal (from 4.7 to 0 mg kg-¹). The resilience of soil organic matter (SOM) in charcoal amended plots (8 and 4% soil C loss, mineral fertilized or not fertilized, respectively) indicates the refractory nature of charcoal in comparison to SOM losses over 20 months in CM (27%), compost amended (27%), and control plots (25% loss).

The thermochemical transformation of sewage sludge (SS) to biochar (SSB) allows exploring the advantages of SS and reduces possible environmental risks associated with its use. Recent studies have shown that SSB is nutrient-rich and may replace mineral fertilizers. However, there are still some questions to be answered about the residual effect of SSB on soil nutrient availability. In addition, most of the previous studies were conducted in pots or soil incubations. Therefore, the residual effect of SSB on soil properties in field conditions remains unclear. This study shows the results of nutrient availability and uptake as well as maize yield the third cropping of a three-year consecutive corn cropping system. The following treatments were compared: (1) control: without mineral fertilizer and biochar; (2) NPK: with mineral fertilizer; (3) SSB300: with biochar produced at 300 °C; (4) SSB300+NPK; (5) SSB500: with biochar produced at 500 °C; and (6) SSB500+NPK. The results show that SSB has one-year residual effects on soil nutrient availability and nutrient uptake by maize, especially phosphorus. Available soil P contents in plots that received SSB were around five times higher than the control and the NPK treatments. Pyrolysis temperature influenced the SSB residual effect on corn yield. One year after suspending the SSB application, SSB300 increased corn yield at the same level as the application of NPK. SSB300 stood out and promoted higher grain yield in the residual period (8524 kg ha−1) than SSB500 (6886 kg ha−1). Regardless of pyrolysis temperature, biochar boosted the mineral fertilizer effect resulting in higher grain yield than the exclusive application of NPK. Additional long-term studies should be focused on SSB as a slow-release phosphate fertilizer.