Thierry Becquer

Chromium availability in ultramafic soils from New Caledonia

The sources and potential availability of chromium (Cr) on soils formed on ultramafic rocks were investigated with mineralogical studies and selective chemical extractions. Soil solutions were collected in the field (i) along a soil toposequence under natural vegetation with ceramic cups; (ii) under grass in a mandarin trees plantation with tension-free tube lysimeters. On selected soil solutions, the Cr(VI) was determined colorimetrically with the s-diphenylcarbazide method and total Cr by ICP-AES and speciation of Cr(VI) was performed with the MINEQL+ V 4.5 software. The main mineralogical sources of Cr were Cr-substituted goethite and chromite. Up to 90 mg kg(-1) of Cr was extracted by KH(2)PO(4), whereas KCl extractable Cr was very low, indicating that exchangeable Cr was mainly in the highly toxic Cr(VI) form in these soils. Under natural vegetation, the Cr concentrations in the soil solutions remained relatively low (<20 microg l(-1)) due to the high retention of the Cr(VI) anions by Fe-oxides. The Cr concentrations were larger in well aerated colluvial soils, where high levels of Mn-oxides are able to oxidize Cr(III) to Cr(VI), than in piedmont soil where the Mn-oxide content is lower, or in alluvial soils from the lowlands, where waterlogging occurs. Cr concentrations reached 700 microg l(-1) in the field that was fertilized with high amount of phosphorus, due to the exchange of Cr(VI) with phosphate. In such conditions, toxicity phenomena for crops can be expected.

Mn-oxide: a major source of easily mobilisable Co and Ni under reducing conditions in New Caledonia Ferralsols

Chemical and bacterial reduction and dissolution of Fe and Mn-oxide and the concomitant solubilisation of Co and Ni were studied in a surface horizon of a New-Caledonia Ferralsol. Chemical extractions showed that Mn and Co were in a large part associated in Mn-oxides. The main part of Ni was associated with goethite, but a very small fraction was also associated with Mn-oxides. Anaerobic reducing bacterial activity was responsible for Fe solubilisation at a smaller extent than for Mn solubilisation and consequently for associated metal release. Submicroscopic investigations revealed the presence of a Mn-oxide containing Co, Ni and Al, close to a lithiophorite–asbolane mixed-layers Mn-oxide, which can be considered as a main source of easily available metals in this soil. To cite this article: C. Quantin et al., C. R. Geoscience 334 (2002) 273–278.

Mineralogical, chemical and charge properties of Geric Ferralsols from New Caledonia

The mineralogical, chemical and surface charge properties of Geric Ferralsols of New Caledonia were examined. These soils, which corresponded to two soil mantles formed either on ultramafic rocks or volcanic ejecta and ashes, were respectively dominated by iron and aluminium oxides. The electric charge characteristics were studied by measuring retention of Ca (2+) and Cl- different pH values ranging from 3 to 7. The cation exchange capacity (CEC,) increased with soil organic carbon and pH and varied from 0 to 35 cmol(c) kg(-1) soil. The anion exchange capacity (AEC) reached 4.25 cmol(c) kg(-1) soil in Bo horizons at pH 4. The magnitude of the CECT and AEC variations was modelled according to CECT = 10(a1pH) * 10(B1) and AEC = - 10(a2pH) * 10(B2). Parameters a(1) and a(2) were low for samples with high organic carbon (from 0.10 to 0.19 and from - 0.44 to - 0.66, respectively) and could increase to 1.25 and - 0.39, respectively, when organic carbon content decreased. The parameters a(1) and a(2) could be also useful to regroup soil types according to the nature of the properties that can be manipulated for their management

Nitrification and nitrate uptake: leaching balance in a declined forest ecosystem in eastern France.

Nitrate uptake and leaching were measured during one year in a declined fir forest on the Vosges highlands (eastern France), in order to investigate whether excess nitrification could be responsible for a deleterious acidification of the ecosystem. Nitrate uptake by the vegetation was active mainly from spring to early fall, and then reached about 66 kg N ha-1. No significant leaching loss occurred during the growth period of the vegetation. Significant nitrate leaching occurred in winter (about 17 kg N ha-1). During fall and winter the nitrification rate was of the same magnitude as values reported for other ecosystems, and, thus, was not considered to be abnormaly strong. No abnormal temporal discoupling of nitrate production and nitrate uptake occurred in the ecosystem, and forest decline must therefore have some other cause.

A simple dynamic method to estimate anion retention in an unsaturated soil

Abstract The anions Br− and NO3− are commonly adsorbed non-specifically by the highly weathered soils found in tropical regions. We used a simple dynamic technique to determine their retention, using the so-called “Perroux tube”, and we compared these results with those obtained from leaching experiments. The method was tested on a ferrallitic soil from New Caledonia which is rich in aluminium and iron oxides. From the resident concentrations observed, we obtained retardation values of 1.1 and 1.15 for bromide and nitrate respectively, using respective input concentrations of 0.025 and 0.1 M. A numerical model using these results predicted the flux concentrations obtained from the leaching experiments quite well, thereby proving the utility of this simple technique.

Importance of the Nitrogen and Sulfur Cycles in the Proton Budget in a Declining Fir Forest and its Relation to Aluminium Toxicity

In a fir stand showing symptoms of forest decline from Eastern France, the net H+ budget was strongly inflluenced by the net NO3- production/uptake/leaching balance. Aluminium toxicity was mainly related to seasonal variations of the NO3- fluxes and corresponding proton budget.