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Despite the strong ecological importance of ectomycorrhizal (ECM) fungi, their vertical distribution remains poorly understood. To our knowledge, ECM structures associated with trees have never been reported in depths below 2 meters. In this study, fine roots and ECM root tips were sampled down to 4-m depth during the digging of two independent pits differing by their water availability. A meta-barcoding approach based on Illumina sequencing of internal transcribed spacers (ITS1 and ITS2) was carried out on DNA extracted from root samples (fine roots and ECM root tips separately). ECM fungi dominated the root-associated fungal community, with more than 90% of sequences assigned to the genus Pisolithus. The morphological and barcoding results demonstrated, for the first time, the presence of ECM symbiosis down to 4-m. The molecular diversity of Pisolithus spp. was strongly dependent on depth, with soil pH and soil water content as primary drivers of the Pisolithus spp. structure. Altogether, our results highlight the importance to consider the ECM symbiosis in deep soil layers to improve our understanding of fine roots functioning in tropical soils.

Background and aims Eucalyptus plantations cover 20 million hectares on highly weathered soils. Large amounts of nitrogen (N) exported during harvesting lead to concerns about their sustainability. Our goal was to assess the potential of introducing A. mangium trees in highly productive Eucalyptus plantations to enhance soil organic matter stocks and N availability. Methods A randomized block design was set up in a Brazilian Ferralsol soil to assess the effects of mono-specific Eucalyptus grandis (100E) and Acacia mangium (100A) stands and mixed plantations (50A:50E) on soil organic matter stocks and net N mineralization. Results A 6-year rotation of mono-specific A. mangium plantations led to carbon (C) and N stocks in the forest floor that were 44% lower and 86% higher than in pure E. grandis stands, respectively. Carbon and N stocks were not significantly different between the three treatments in the 0–15 cm soil layer. Field incubations conducted every 4 weeks for the two last years of the rotation estimated net soil N mineralization in 100A and 100E at 124 and 64 kg ha−1 yr−1, respectively. Nitrogen inputs to soil with litterfall were of the same order as net N mineralization. Conclusions Acacia mangium trees largely increased the turnover rate of N in the topsoil. Introducing A. mangium trees might improve mineral N availability in soils where commercial Eucalyptus plantations have been managed for a long time.

Harvest residue management is a key issue for the sustainability of Eucalyptus plantations established on poor soils. Soil microbial communities contribute to soil fertility by the decomposition of the organic matter (OM), but little is known about the effect of whole-tree harvesting (WTH) in comparison to stem only harvesting (SOH) on soil microbial functional diversity in Eucalyptus plantations. We studied the effects of harvest residue management (branches, leaves, bark) of Eucalyptus grandis trees on soil enzymatic activities and community-level physiological profiles in a Brazilian plantation. We measured soil microbial enzymatic activities involved in OM decomposition and we compared the community level physiological profiles (CLPP) of the soil microbes in WTH and SOH plots. WTH decreased enzyme activities and catabolic potential of the soil microbial community. Furthermore, these negative effects on soil functional diversity were mainly observed below the 0–5 cm layer (5–10 and 10–20 cm), suggesting that WTH can be harmful to the soil health in these plantations.

The objectives of the study were to assess changes in fine root anisotropy and specific root lengths throughout the development of Eucalyptus grandis (W. Hill ex Maiden) plantations and to establish a predictive model of root length density (RLD) from root intercept counts on trench walls. Fine root densities (<1 mm in diameter) were studied in 6-, 12-, 22-, 28-, 54-, 68- and 72-month-old E. grandis plantations established on deep Ferralsols in southern Brazil. Fine root intercepts were counted on 3 faces of 90-198 soil cubes (1 dm³ in volume) in each stand and fine root lengths (L) were measured inside 576 soil cubes, sampled between the depths of 10 cm and 290 cm. The number of fine root intercepts was counted on one vertical face perpendicular to the planting row (N t), one vertical face parallel to the planting row (N l) and one horizontal face (N h), for each soil cube sampled. An overall isotropy of fine roots was shown by paired Student's t-tests between the numbers of fine roots intersecting each face of soil cubes at most stand ages and soil depths. Specific root lengths decreased with stand age in the upper soil layers and tended to increase in deep soil layers at the end of the rotation. A linear regression established between N t and L for all the soil cubes sampled accounted for 36% of the variability of L. Such a regression computed for mean N t and L values at each sampling depth and stand age explained only 55% of the variability, as a result of large differences in the relationship between L and N t depending on stand productivity. The equation RLD = 1.89*LAI*N t, where LAI was the stand leaf area index (m² m⁻²) and N t was expressed as the number of root intercepts per cm², made it possible to predict accurately (R² = 0.84) and without bias the mean RLDs (cm cm⁻³) per depth in each stand, for the whole data set of 576 soil cubes sampled between 2 years of age and the end of the rotation.

Organic phosphorus (P o ) plays an important role in tropical forest nutrition on strongly-weathered soils. However, of its role in Eucalyptus nutrition little is known. For this purpose, soil P forms were determined by NaOH-Na 2 EDTA extraction and 31 P nuclear magnetic resonance spectroscopy ( 31 P NMR) at 10 sites of Eucalyptus plantations in Brazil. The objectives were (1) to determine the amounts and forms of soil P o ; (2) to build a structural model of the soil P cycle to provide quantitative estimates of the transformation processes of soil P. Soil P was found to consist of large amounts of Monoesters-P (on average 43.7 mg kg −1 ) followed by Diesters-P in the form of DNA (on average 4.2 mg kg −1 ), as well as containing high reserves of P i species (on average 101.6 mg kg −1 of the ortho -P; 2.7 mg kg −1 of the pyrophosphate). Thestructural model indicated that the P cycle is regulated by clay, soil organic carbon, total P and pH. High total P o concentrations (on average 48 mg kg −1 ) reveal a high potential of P supply to Eucalyptus trees, in which Diester-P (DNA) would be the main source P for the available P pool. The close interconnection between ortho -P, pyrophosphate, DNA and Monoester-P compounds provides evidence of a strong influence of soil biological activity on the distribution and accumulation of these P compounds and therefore on the availability of P. Thus, these results are relevant to establish soil fertility management strategies for low-fertilizer input Eucalyptus systems in soils with low fertility.

Micronutrients participate in various plant metabolic pathways, including the synthesis of carbohydrates, lignin, and phenols, which are necessary for plant growth and defense against pests and pathogens. In this study, we evaluated the effects of nutrient solutions deplete in B, Cu, or Zn on the nutrition, growth, and production of biochemical compounds in Eucalyptus urophylla cuttings. The experiment was carried out in a greenhouse for 90 days, with clonal cuttings of E. urophylla at four months of age. The treatments evaluated were complete fertilization (CF) and individual omissions of B, Cu, and Zn in the nutrient solution. The omission of Cu and Zn increased the concentration of carbohydrates in the leaves by 10% and reduced the rate of CO2 assimilation (A) by more than 30%. Cu omission decreased the lignification process compared to complete fertilization. Flavonoid production in plants with complete fertilization was 45–75% higher than that in plants maintained under B, Cu, or Zn deficiencies. Collectively, these results suggest that micronutrient deficiencies may reduce plant productivity as well as plant resistance to pests and pathogens.

Potassium has important physiological functions in eucalypt plantations, increasing their productivity when applied to soil via mineral fertilizers. There is interest in identifying alternative sources to KCl owing to its high cost and limited reserves. The aim of the study was to test the effect of replacing KCl with NaCl and phonolite rock powder. Two comparisons were made: (1) application of 283 kg ha−1 of KCl compared with that of 2125 kg ha−1 of phonolite rock powder (equivalent to 170 kg ha−1 of K2O in both treatments); (2) application of 139 kg ha−1 of NaCl compared with that of 183 kg ha−1 of KCl (equivalent to 2.33 kmol Na and K, respectively). Radial growth, soil water content, leaf water potential (Ψ), accumulated transpiration, stem volume and biomass increment, as well as water use efficiency (WUE) were evaluated. In the first comparison, both fertilizations presented equal values for all characteristics evaluated. In the second, the accumulated transpiration in trees fertilized with KCl was 17% higher than that in plants fertilized with NaCl. In contrast, the WUE was 20% higher in the trees fertilized with NaCl than in those fertilized with KCl, reflecting the lower water consumption for the same increment in stem volume and biomass. We conclude that phonolite rock powder and NaCl are possible substitutes for conventional K fertilization performed with KCl.

Coppicing is a widely adopted management system for forest plantations; however, little information is available pertaining to responses to fertilizer application. Our objective was therefore to assess the effect of individual and conjunct omissions of N, P, K, Ca, Mg, B, and Cu on the growth of a highly productive Eucalyptus urophylla × E. grandis hybrid clone, managed for coppicing at the second rotation, in two soils of contrasting fertility in southeastern Brazil. Two commercial sites of approximately 7 years of age, one in an Arenosol and the other in a Ferralsol, were harvested and the experiments installed. At the Arenosol site, the yield from the fertilizer treatment in the coppice system (R2) was the same as determined for the first rotation (high forest; R1), at approximately 40 m3 ha−1 year−1 at 6 years old. In contrast, the yield from the fertilizer treatment in R2 at the Ferralsol site was 11% lower compared with R1. Despite some alterations in leaf nutrient concentrations, other than for K at the Arenosol site (where the K omission treatment reduced wood volume at 6 years by 21%), no yield reduction was found for any no-fertilizer treatments. Due to the widely distributed root system already established in coppiced plantations, ensuring a large volume of soil exploration, coppiced eucalypts only responded to K application. This is due to low soil K availability and the high K demand in eucalypt plantations.

Fine-root functioning is a major driver of plant growth and strongly influences the global carbon cycle. While fine-root over-yielding has been shown in the upper soil layers of mixed-species forests relative to monospecific stands, the consequences of tree diversity on fine-root growth in very deep soil layers is still unknown. Our study aimed to assess the consequences of mixing Acacia mangium and Eucalyptus grandis trees on soil exploration by roots down to the water table at 17 m depth in a tropical planted forest. Fine roots (diameter < 2 mm) were sampled in a randomized block design with three treatments: monospecific stands of Acacia mangium (100A), Eucalyptus grandis (100E), and mixed stands with 50% of each species (50A50E). Root ingrowth bags were installed at 4 depths (from 0.1 m to 6 m) in the three treatments within three different blocks, to study the fine-root production over 2 periods of 3 months. Down to 17 m depth, total fine-root biomass was 1127 g m(-2) in 50A50E, 780 g m(-2) in 100A and 714 g m(-2) in 100E. Specific root length and specific root area were 110-150% higher in 50A50E than in 100A for Acacia mangium trees and 34% higher in 50A50E than in 100E for Eucalyptus grandis trees. Ingrowth bags showed that the capacity of fine roots to explore soil patches did not decrease down to a depth of 6 m for the two species. Belowground interactions between Acacia mangium and Eucalyptus grandis trees greatly increased the exploration of very deep soil layers by fine roots, which is likely to enhance the uptake of soil resources. Mixing tree species might therefore increase the resilience of tropical planted forests through a better exploration of deep soils.

Eucalyptus plantations in the Brazilian savanna (Cerrado) are challenged by acidic soil and water-deficit conditions. Supply of Ca and S via application of dolomitic limestone and gypsum may stimulate root development (in terms of depth) and also enhance access to water stored in the subsurface layers. Aerial and root growth, Ca2+, Mg2+, K+, and S–SO42− migration, and soil profile of Eucalyptus urophylla were evaluated following dolomitic limestone and gypsum application. Five treatments were used: (i) no application of Ca2+, Mg2+, and S–SO42− sources (CONTROL); (ii) 1200 kg ha−1 of gypsum (GYP); (iii) 2000 kg ha−1 of dolomitic limestone (DOL); (iv) DOL plus 1200 kg ha−1 of gypsum (DOL + GYP); and (v) DOL plus 2400 kg ha−1 of gypsum (DOL + 2GYP). Compared with CONTROL, Gypsum and dolomitic limestone application in DOL + 2GYP increased the concentrations of Ca2+ and Mg2+ in the soil surface and of S–SO42− in the subsurface by five, four, and three times, respectively. Fertilization with gypsum alone or in combination with dolomitic limestone did not promote an increase in fine root density (FRD) in the deeper layers. Concentrations of Ca, S, and K in the leaves increased by 16–43%. Furthermore, applying limestone 49 months after planting increased E. urophylla whole wood volume and the mean annual increment (MAI) by 8%, as compared to those in the CONTROL. Thus, gypsum and dolomitic limestone positively impacted soil fertility and may be used to acclimatize Eucalyptus to water-deficit conditions.