Influence of mineral and organic fertilizer inputs on nitrogen cycling in Eucalypts plantations

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Influence of fertilizer inputs on soil solution chemistry in eucalypt plantations established on Brazilian sandy soils

International audienceThe present paper is part of a comprehensive approach currently developed in Brazil to study biogeochemical cycles at the ecosystem level in Eucalyptus grandis plantations. It aims at assessing changes occurring in water chemical composition throughout their transfer in the soil, during the first year after planting. A lysimetry was installed in a 5-year-old E. saligna stand. Lysimeters were strategically positioned within a compartment of E. saligna prior to the clear felling of the stand that would allow the assessment of a fertilization experiment planned on the same site for the next rotation. About 180 zero-tension lysimeters were installed in the upper soil layers and 136 ceramic cups were setup horizontally down to a 3 m depth and connected to an automatic vacuum pump. After the harvest of the E. saligna stand, a fertilization experiment of E. grandis improved seedlings was initiated using a complete randomized block design, with 6 blocks and 5 treatments. The objective was to compare the influence of different amounts of ammonium sulphate and sewage sludge fertilizations on biogeochemical cycling. At the end of the rotation, nutrient concentrations in soil solutions were low whatever the depth and the lysimeter type. After clear felling, soil solution ionic balances were dominated by NO3- and Al3+, whose concentrations increased substantially. No obvious change in concentrations was observed for all other elements. A proton unbalance, resulting from the interruption of NO3- uptake by plants after harvesting, might be responsible for the aluminium accumulation in soil solutions. After planting, fertilizer inputs were responsible for increasing concentrations of all elements applied until 1 m deep. Twelve months after planting E. grandis, the chemistry of soil solutions at 3 m deep had not developed. The monitoring of soil solution chemistry is going on in order to quantify the effects of these different fertilizations on deep drainage nutrient losses

Low nutrient losses by deep leaching after clearcutting and replanting Eucalyptus plantations in Brazil

International audienceSustainable management of highly productive eucalypt plantations requires the application of fertilizers to balance nutrient exports associated with biomass removal every 6-7 years. Although deep leaching after clear-cutting is an important component of input-output budgets of nutrients in forest soils, accurate quantifications are rare in tropical plantations. Our study aimed to assess the consequences of management practices in highly productive eucalypt plantations on nutrient losses by deep leaching in two hillslope positions with contrasting soil types. Soil solutions were continuously collected using lysimeters down to a depth of 3 m, in the last year before the clear-cutting, then in the first 3.5 years after replanting. Concentrations of the main cations and anions were determined monthly and fluxes of gravitational solutions at the depths where the lysimeters were installed were estimated using the Hydrus 1D model, calibrated in situ. Stand productivity was high in both landscape positions with a mean basal area at harvest of 35 m2 ha-1 at the bottom of the slope and 27 m2 ha-1 at the top of the slope. Soil solution chemistry in the topsoil was highly responsive to management practices with sharp increases in ionic charges after clearcutting and fertilizer applications. While leaching fluxes of mobile ions (especially N-NO3-, K+ and Mg2+) reached values greater than 30 kg ha-1 yr-1 at a depth of 1 m after clear-cutting, they remained lower than 3 kg ha-1 yr-1 at a depth of 3 m both in the upper and lower hillslope positions throughout the rotation. Our study suggests that splitting fertilizer applications may not be necessary in tropical Eucalyptus plantations established in very deep tropical soils

Mixing Eucalyptus and Acacia trees leads to fine root over-yielding and vertical segregation between species

The consequences of diversity on belowground processes are still poorly known in tropical forests. The distributions of very fine roots (diameter < 1 mm) and fine roots (diameter < 3 mm) were studied in a randomized block design close to the harvest age of fast-growing plantations. A replacement series was set up in Brazil with mono-specific Eucalyptus grandis (100E) and Acacia mangium (100A) stands and a mixture with the same stocking density and 50 % of each species (50A:50E). The total fine root (FR) biomass down to a depth of 2 m was about 27 % higher in 50A:50E than in 100A and 100E. Fine root over-yielding in 50A:50E resulted from a 72 % rise in E. grandis fine root biomass per tree relative to 100E, whereas A. mangium FR biomass per tree was 17 % lower than in 100A. Mixing A. mangium with E. grandis trees led to a drop in A. mangium FR biomass in the upper 50 cm of soil relative to 100A, partially balanced by a rise in deep soil layers. Our results highlight similarities in the effects of directional resources on leaf and FR distributions in the mixture, with A. mangium leaves below the E. grandis canopy and a low density of A. mangium fine roots in the resource-rich soil layers relative to monospecific stands. The vertical segregation of resource-absorbing organs did not lead to niche complementarity expected to increase the total biomass production