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This study is aimed at evaluating C and N stocks in fractions of soil organic matter (SOM) in an area of bauxite mining under recovery with tree species. We have analyzed the long-term recovery of C and N stocks of organic matter fractions from five types of forest cover (Eucalyptus, Anadenanthera peregrina, mixed plantation of 16 native species, a mined area without vegetation cover as a control site, and a natural forest cover as a reference site). The total organic C (TOC) and N (TN) stocks and also organic matter fractions, particulate organic matter (POM), mineral-associated organic matter (MAOM), microbial biomass (MB), and labile C (LC), were determined, as well as the C/N ratio and the carbon management index (CMI). Although the stocks of TOC and LC, CMI, and MB did not differ between the types of forest cover in the 0-60 cm layer, they were lower than the values in the native forest. Forest cover increases the stocks of TOC, LC, MB, and CMI in the area of bauxite mining compared to the control site. In addition, we found that the TOC C and TN stocks and also SOM fractions (LC, C-MAOM, C-POM, N-MAOM, and N-POM) are positively correlated (r ≥ 0.71 for all cases) with volume of roots larger than 2 mm. Therefore, Eucalyptus, A. peregrina, and a mixed of 16 native trees contribute for restoring stocks of soil C and N following bauxite mining in the Brazilian Atlantic Forest.

The acceleration of K release in the eucalyptus rhizosphere is a key process for explaining plant K uptake in low-K tropical soils. We conducted a pot experiment to evaluate the short-term dynamics of soil K at the soil-root interface of eucalyptus plants. We hypothesized that eucalyptus roots are able to substantially modify the soil K supply by releasing K from more recalcitrant K pools. To test our hypothesis, we used five tropical Oxisols in which the remaining structural K forms were components of highly weathering-resistant minerals. The experiment was installed in a split-plot arrangement of treatments in a randomized block design with four replicates. The effect of the soils was studied in the main plots. In the subplots, we isolated or did not isolate eucalyptus roots from the soil. At the end of the experiment, we determined the total soil K, exchangeable K, and K extracted with Mehlich 1 (M1) and Mehlich 3 (M3) as well as the K content in plant shoots. Our main findings suggest that the acceleration of K release in the eucalyptus rhizosphere is a key process for explaining plant K uptake in low-K tropical soils. Understanding and quantifying the processes related to this phenomenon are necessary to provide a more mechanistic basis for K fertilization recommendations as well as to monitor the sustainability of natural and planted eucalypt forests

Relatively few studies have documented the impacts of afforestation, particularly production forestry, on belowground carbon dioxide (CO2) effluxes to the atmosphere. We evaluated the changes in the soil CO2 efflux—a proxy for soil respiration (Rs)—for three years following a native grassland conversion to eucalypt plantations in southern Brazil where minimum tillage during site preparation created two distinct soil zones, within planting row (W) and between-row (B). We used root-exclusion and carbon (C)- isotopic approaches to distinguish Rs components (heterotrophic-Rh and autotrophic-Ra respirations), and a CO2 profile tube (1-m deep) to determine the concentration ([CO2]) and isotopic C signature of soil CO2 (δ13[CO2]). The soil CO2 efflux in the afforested site averaged 0.37 g CO2 m−2 h−1, which was 56% lower than the soil CO2 efflux in the grassland. The δ13CO2 in the afforested site ranged from − 14.1‰ to − 29.4‰, indicating a greater contribution of eucalypt-derived respiration (both Rh and Ra) over time. Higher soil CO2 efflux and lower [CO2] were observed in W than B, indicating that soil preparation creates two distinct soil functional zones with respect to C cycling. The [CO2] and δ13[CO2] decreased in both zonal positions with eucalypt stand development. Although the equilibrium in C fluxes and pools across multiple rotations is needed to fully account for the feedback of eucalypt planted forests to climate change, we provide quantitative information on soil CO2 dynamics after afforestation and show how soil preparation can leverage the feedback of planted forests to climate change.

The extent by which the contribution of eucalypt harvest residues (HR) to soil organic carbon (SOC) is impacted by soil disturbance during and/or after harvesting is unclear. We addressed this question by following the decomposition of HR in microplots (15-cm diameter, 15-cm height) and determining their contribution to SOC in 10 sites across southeastern Brazil. The experiment was set up according to a complete randomized block design arranged into a 3 × 2 factorial scheme including: HR removal (HR0), only bark removal (HR − B) and HR including bark (HR + B) applied to: undisturbed soil (US) with HR left on the soil surface or disturbed soil with HR mixed into the first 5 cm of the topsoil. We had eight blocks as replicates. Following a 12-month field decomposition, soil samples were fractionated to isolate SOC within the particle-size fractions (PSF) greater and lower than 53 µm. Subsequently, we used 13C to quantify the effects of the treatments in C3-derived SOC content (C3-SOC) within each PSF. For both PSF, their C3-SOC content increased in response to HR − B or HR + B relative to HR0, depending on soil disturbance and HR half-life time (t0.5). For HR − B, net increments in C3-SOC within the PSF > 53 µm increased significantly with HR t0.5, regardless of soil disturbance. Otherwise, the C3-SOC content within the PSF

Water and nutritional restrictions are limiting factors for the growth of Eucalyptus trees in tropical climates. In the dry season, boron (B) uptake is severely affected. The objectives of this study were to evaluate the phloem mobility of B and whether its deficiency can increase plant sensitivity to osmotic stress. It was also tested to what extent foliar application of B could mitigate the negative effects of drought under low B supply. Seedlings of a drought tolerant Eucalyptus urophylla (Blake, S. T.) clone were grown in nutrient solution, subjected to low availability of B for 25 days, and then submitted to a progressive osmotic stress. After imposition of osmotic stress, B was applied to young or mature leaves. B applications, mainly to mature leaf, stimulated root growth and delayed dehydration under osmotic stress and led to an increased B translocation and carbon isotopic composition. The expression of B transporters and pectin metabolism genes were also increased in water-stressed plants supplied with B by foliar application. B deficiency led to increased plant dehydration and decreased root growth under osmotic stress. The application of B to mature leaf of water-stressed plants proved effective in mitigating the negative effects of water deficit in root growth.

Microbial solubilization of rock phosphate (RP) is mainly achieved by the production of organic acids and medium acidification through H+ release. During RP solubilization, mineral nutrient availability is likely to be critical for determining how much carbon is channeled either for metabolite synthesis or for microbial growth, influencing organic acid release by microorganisms. Thus, the objective of this work was to study the relationships between the concentration of mineral nutrients in the growth medium and the efficiency of RP solubilization by Aspergillus niger FS1. For this, the fungus was grown in Czapek medium containing 0, 1, 2, 10, 50, and 100 % of its original concentration of mineral nutrients. Decreasing mineral availability in the growth medium led to decreases in fungal biomass and solubilized P, and increases in titratable acidity and solubilization efficiency as expressed by mg solubilized P per g fungal biomass (YP/B), indicating a shift in fungal metabolism from biomass production to organic acid release. The transfer of pre-grown biomass to media with or without added minerals confirmed that lower mineral availability increases YP/B and led to the solubilization of 76 % of P present in Patos RP. These observations open new perspectives on improving RP solubilization systems by manipulating mineral nutrient availability in the medium, with consequent gains in cost reduction.

The effects of different doses of rock phosphate (RP), sucrose, and (NH(4))(2)SO(4) on the solubilization of RP from Araxá and Catalão (Brazil) by Aspergillus niger, Penicillium canescens, Eupenicillium ludwigii, and Penicillium islandicum were evaluated in a solid-state fermentation (SSF) system with sugarcane bagasse. The factors evaluated were combined following a 2(3) + 1 factorial design to determine their optimum concentrations. The fitted response surfaces showed that higher doses of RP promoted higher phosphorus (P) solubilization. The addition of sucrose did not have effects on P solubilization in most treatments due to the presence of soluble sugars in the bagasse. Except for A. niger, all the fungi required high (NH(4))(2)SO(4) doses to achieve the highest level of P solubilization. Inversely, addition of (NH(4))(2)SO(4) was inhibitory to P solubilization by A. niger. Among the fungi tested, A. niger stood out, showing the highest solubilization capacity and for not requiring sucrose or (NH(4))(2)SO(4) supplementation. An additional experiment with A. niger showed that the content of soluble P can be increased by adding higher RP doses in the medium. However, P yield decreases with increasing RP doses. In this experiment, the maximal P yield (approximately 60 %) was achieved with the lower RP dose (3 g L(-1)). Our results show that SSF can be used to obtain a low cost biofertilizer rich in P combining RP, sugarcane bagasse, and A. niger. Moreover, sugarcane bagasse is a suitable substrate for SSF aiming at RP solubilization, since this residue can supply the C and N necessary for the metabolism of A. niger within a range that favors RP solubilization.

Variations in genotype rankings among screenings for Al tolerance in hydroponics may be related to differences in the composition of the solutions. In the present study, we investigated the involvement of Mg ions in modifying Al rhizotoxicity in soybeans. Root elongation was strongly inhibited by Al in a simple, 800 μM CaSO4 solution, but elongation increased noticeably when the solutions also contained Mg. Amelioration of Al rhizotoxicity was not associated with an increase in ionic strength of treatment solutions because Al3+ activities were kept constant. Concentration series experiments indicated that the Mg effect occurred in the μM range, while Ca amelioration of Al toxicity occurred at mM concentrations. The positive effect of Mg on root elongation was greatest for Al-sensitive genotypes and minimized genotypic differences for Al-tolerance. The Mg protection against Al rhizotoxicity apparently does not occur with all species, because it was not observed in Atlas and Scout 66 wheat varieties. The ability of Mg to ameliorate Al toxicity in soybean at μM levels suggests the involvement of distinct physiological factors.