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Although many studies have shown that soil solution chemistry can be a reliable indicator of biogeochemical cycling in forest ecosystems, the effects of litter manipulations on the fluxes of dissolved elements in gravitational soil solutions have rarely been investigated. We estimated the fluxes of NH4-N, NO3-N, K, Ca, Mg, Na, Cl, dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) over the first two years after re-planting Eucalyptus trees in the coastal area of Congo. Two treatments were replicated in two blocks after clear-cutting 7-year-old stands: in treatment R, all the litter above the mineral soil was removed before planting, and in a double slash (DS) treatment, the amount of harvest residues was doubled. The soil solutions were sampled down to a depth of 4 m and the water fluxes were estimated using the Hydrus 1D model parameterized from soil moisture measurements in 4 plots. Isotopic and spectroscopic analytical techniques were used to assess the changes in dissolved organic matter (DOM) properties throughout the transfer in the soil. The first year after planting, the fluxes of NH4-N, K, Ca, Mg, Na, Cl and DOC in the topsoil of the DS treatment were 2–5 times higher than in R, which showed that litter was a major source of dissolved nutrients. Nutrient fluxes in gravitational solutions decreased sharply in the second year after planting, irrespective of the soil depth, as a result of intense nutrient uptake by Eucalyptus trees. Losses of dissolved nutrients were noticeably low in these Eucalyptus plantations despite a low cation exchange capacity, a coarse soil texture and large amounts of harvest residues left on-site at the clear cut in the DS treatment. All together, these results clarified the strong effect of litter manipulation observed on eucalypt growth in Congolese sandy soils. DOM fluxes, as well as changes in δ13C, C:N and aromaticity of DOM throughout the soil profile showed that the organic compounds produced in the litter layer were mainly consumed by microorganisms or retained in the topsoil. Below a depth of 15 cm, most of the DOC and the DON originated from the first 2 cm of the soil and the exchanges between soil solutions and soil organic matter were low.

African savannas developed an efficient strategy of nutrient uptake and control of nitrification making it possible to avoid large losses of nutrient by drainage. The conversion of native savanna into commercial forest plantations is an important global change driver that potentially impacts element cycles. Afforestation of native Congolese savannas by Eucalyptus started 30 years ago. Large amounts of nutrients are removed through biomass harvesting every 6-7 years. Losses of nutrients by deep drainage might be a serious threat for the sustainability of these plantations, established on sandy soils with high hydraulic conductivities. We compared the soil N-mineralisation, the nutrient fluxes and deep drainage beneath savanna and Eucalyptus plantation in Congo. Then, we discussed the strategy of nutrient recycling. The water fluxes at a depth of 400 cm were approximately 20% higher in the savanna than in Eucalyptus plantation. Although the nitrification rate and solution chemistry exhibited strong modifications during the first year following both savanna afforestation and the harvesting of the Eucalyptus stand, the losses of nutrients by deep drainage remained unexpectedly low. The largest fluxes of drainage at a depth of 6 m were found for NH4+-N, which reached a maximum of 0.4 g m(-2) yr(-1) in the second year following savanna afforestation and I g m(-2) yr(-1) in the first year after clear cutting. The deep drainage of NO3--N, Mg2+, Ca2+ and K+ did not exceed 0.2 g m(-2) yr(-1) in the savanna and at any stage of plantation development. These results are discussed regarding (i) the roots distribution in the soil of each ecosystem and (ii) the nutrient accumulation in biomass. The limited changes between the nutrient fluxes in both ecosystems were the result of fast root growth in the deep soil layers after planting, combined with an intense uptake of the tree roots to satisfy the large nutrient and water requirements for the development of tree crowns. Intense uptake and cycling of nutrients mitigated the risk due to clear cut and N fertilisation in this forest plantation. Nevertheless, forest managers must carefully fit fertilisation regimes to the nutrient requirements of new clone selected by breeding programmes and reduce as much as possible the delay between harvesting and re-planting to avoid high losses by deep drainage. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.

The objectives of this study were: (i) to test whether near infrared reflectance spectroscopy (NIRS) can be used to assess site fertility in sandy and ferralitic soils of the Congo; (ii) to evaluate the comparative advantages of this methodology with respect to classical regressions with chemical and physical analysis of soils; and (iii) to assess the ability of NIRS to predict concentrations of individual nutrients. Sixty-two plots were sampled in 20 selected forest stands with varying wood production. Site indices (Hlim) were estimated from the measured dominant height in the plots. Soil samples (N = 353) were analysed by NIRS, while physical and chemical analyses were carried out on 210 subsamples. Partial Least Squares regression (PLS) was used to model the site index from NIRS, while Variable Importance in Projection (VIP) allowed relating the main explicative frequencies to nutrient availability. A validation was performed based on an independent field trial (not used in the calibration set) in which soils were collected and analysed by NIRS. The measured dominant height growth was compared with the dominant height growth simulated from the site indices predicted using NIR spectra. Multiple regressions between site indices (Hlim) and soil properties failed to explain the variability in Hlim (R-2 = 0.36). This variability was better predicted by NIR spectra, with an R-2 = 0.64 and a predicted Root Mean Square Error of 7% for the mean value of Hlim. Based on analysis of VIP frequencies, the most important factors determining the site indices were organic matter, phosphorus, CEC, clay content, and pH. The comparison between measured and simulated dominant height in the validation set gave similar R-2 (0.61) than in the calibration set. These results demonstrate that mapping of site indices could be performed at an affordable cost in this region. The methodology developed here, which links PLS and VIP to assess the main factors determining soil fertility, is not specific to this study and could be used elsewhere. (C) 2011 Elsevier B.V. All rights reserved.

International audience; The main characteristics of soils under commercial Eucalyptus plantations on the Atlantic coast of the Congo were studied down to a depth of 5 m. The objectives were to invest gate the mineralogical assemblage and distinguish between current versus old pedogenetic processes, and to find evidence of relations between minerals and cation retention in highly weathered sandy soil. These soils, developed from Plio-Pleistocene sandy deposits, were characterized by mineralogical uniformity over depth, with quartz contents ranging from 850 to 930 g kg(-1), a clay fraction dominated by kaolinite and a cation exchange capacity (CEC) < 0.5 cmol(c) kg(-1) whatever the soil layer. Whilst dioctahedral vermiculite amounted to only 3% of the clay fraction, it played a key role in soil cation retention contributing to 49% of the mineral CEC. Vermiculitisation of muscovite was a current process showed by the changes in muscovite/vermiculite ratios with soil depth. The simple mineralogical assemblage of this soil made it possible to estimate CEC values for kaolinite ranging from 6.2 to 6.9 cmol(c) kg(-1) for the 0-2 mu m particle size. Their morphologies and saturation index of soil solutions showed that kaolinite had different possible origins: i) current precipitation from soil solution, ii) pseudomorphic transformation of pre-existing 2:1 phyllosilicates and iii) old particles present in the original deposit. Measured fine earth CEC below estimations summing together the contributions of all the mineralogical fractions showed that particle aggregation greatly influenced the cation retention capacity, even in these poorly structured soils. Pedogenetic differentiations were weak in these soils because the elapsed time since deposition was too short to differentiate 'mineralogical horizons' from weathered materials. Our study showed that beyond the properties that minerals confer to soil, partially controlling the bioavailability of nutrients, their mineralogical and chemical characteristics infer current and former pedogenetic processes in this highly weathered tropical soil. (C) 2011 Elsevier B.V. All rights reserved.

International audience; The sustainability of fast-growing tropical Eucalyptus plantations is of concern in a context of rising fertilizer costs, since large amounts of nutrients are removed with biomass every 6–7 years from highly weathered soils. A better understanding of the dynamics of tree requirements is required to match fertilization regimes to the availability of each nutrient in the soil. The nutrition of Eucalyptus plantations has been intensively investigated and many studies have focused on specific fluxes in the biogeochemical cycles of nutrients. However, studies dealing with complete cycles are scarce for the Tropics. The objective of this paper was to compare these cycles for Eucalyptus plantations in Congo and Brazil, with contrasting climates, soil properties, and management practices. The main features were similar in the two situations. Most nutrient fluxes were driven by crown establishment the two first years after planting and total biomass production thereafter. These forests were characterized by huge nutrient requirements: 155, 10, 52, 55 and 23 kg ha_1 of N, P, K, Ca and Mg the first year after planting at the Brazilian study site, respectively. High growth rates the first months after planting were essential to take advantage of the large amounts of nutrients released into the soil solutions by organic matter mineralization after harvesting. This study highlighted the predominant role of biological and biochemical cycles over the geochemical cycle of nutrients in tropical Eucalyptus plantations and indicated the prime importance of carefully managing organic matter in these soils. Limited nutrient losses through deep drainage after clear-cutting in the sandy soils of the two study sites showed the remarkable efficiency of Eucalyptus trees in keeping limited nutrient pools within the ecosystem, even after major disturbances. Nutrient input–output budgets suggested that Eucalyptus plantations take advantage of soil fertility inherited from previous land uses and that long-term sustainability will require an increase in the inputs of certain nutrients.

International audience; Eucalyptus plantations have been introduced since 1978 on savannah soils of the coastal plains of Congo, but there is still little information on the effect of silvicultural practices on soil organic carbon dynamics after afforestation on these savannahs. The objectives of this study were to assess the effects of two experimental site preparation treatments on soil CO2 efflux, tree growth and soil carbon balance during the first year following plantation establishment. One treatment involved mechanical soil disturbance with disk harrowing (D), whereas in the second treatment (H), savannah grasses were killed by herbicide application before planting, without mechanical soil disturbance. Soil respiration and soil water content were monitored for 1 year following treatment application, at 2-week intervals. We hypothesized that mechanical soil disturbance would increase soil CO2 efflux, but the results did not support this hypothesis. The cumulated soil CO2 efflux over 1 year was not significantly different in the two treatments and averaged 658 g C m-2. In contrast, tree growth was significantly increased by disk harrowing, maybe as a result of decreased soil penetration resistance. Carbon inputs to the soil from savannah residues (428 g C m-2) were outweighed by the annual carbon outputs through heterotrophic respiration (505 and 456 g C m-2 in the H and D treatments, respectively) leading to a slightly negative soil carbon budget in both treatments 1 year after afforestation.