Francis Andreux

Organic matter dynamics and aggregation in soils under rain forest and pastures of increasing age in the eastern Amazon Basin

Abstract In the eastern Amazon basin, four neighbouring clayey Oxisols with similar particle-size distributions were selected, one under rain forest and three under pasture for 7, 12 and 17 years, respectively. These soils were sampled at depth intervals of 0.1 m down to 1 m. Although no clear effect of pasture establishment on aggregate stability was assessed, significant negative effects of cattle trampling on porosity and water retention and of vegetation change on clay water-dispersion were observed in the organic-rich horizons (0–0.40 m layers). Indirect evidence of a great change of the nature of organic materials was also suggested, particularly with (i) an increase in both negative surface charges and clay dispersibility, attributed to an increase in organic functional groups, and (ii) a decrease in clay fabric porosity, attributed to an increase in the hydrophilic-hydrophobic balance on organic surfaces (short-range adhesion forces during drying processes). Studies of soil organic matter (SOM) changes along this forest-to-pasture sequence were based on total C and 13C measurements, which allowed to calculate the distribution of C derived from forest (Cdff) and from pasture (Cdfp) throughout the profiles. The distribution of C and 13C in the whole soil, in water-stable (WSA), in not stable (NWSA) aggregates, and in particle fractions, was compared. Young organic residues derived from pasture were trapped in WSA, from where they were released by dispersion. After 17 years, the decrease in forest-derived SOM and the input of about 25% of pasture-derived SOM were suggested to be more effective on clay dispersability than on aggregate stability.

Inoculation of an atrazine-degrading strain, Chelatobacter heintzii Cit1, in four different soils: effects of different inoculum densities

The possibility to improve atrazine degradation in soils by bioaugmentation was studied. The atrazine-mineralizing strain, Chelatobacter heintzii Cit1, was inoculated in four sterile and four non-sterile soils, at varying inoculum densities. Two soils, which had shown enhanced atrazine mineralization, were used to determine which inoculum density was capable of restoring their original mineralizing capacity after sterilization. The two other soils, with intermediate and low capacity to mineralize atrazine, were used in order to demonstrate that atrazine mineralization in such soils could be improved by inoculation. Mineralization kinetics were fitted using the Gompertz model. In the case of soils adapted to atrazine mineralization, inoculation of C. heintzii did not accelerate the rate of atrazine mineralization, which was essentially performed by the indigenous microflora. However, with soils that did not mineralize atrazine, the introduction of 10(4) cfug(-1) resulted in a 3-fold increase of atrazine mineralization capacity.

Transformation of monuron photosensitized by soil extracted humic substances: energy or hydrogen transfer mechanism?

The humic and fulvic acids extracted from a Ranker type soil sensitize the transformation of monuron. When monuron is irradiated at 365 nm in the presence of the fulvic acid, its degradation is faster in deoxygenated medium than in air-saturated solution. Chloride ions are released, and the para-hydroxylated derivative is formed as upon direct photolysis. It is deduced that the consumption of monuron observed in the absence of oxygen is due to an energy transfer from reactive triplet states of the fulvic acid to monuron. Energy transfer reactions also take place when hydroquinone or acetophenone are used as sensitizers, showing that the energy level of the triplet state of monuron is lower than 311 kJ mol−1. The rate of monuron disappearance in the presence of the humic acid is lower in deoxygenated than in aerated medium. This oxygen effect indicates that there is no energy transfer. A hydrogen atom transfer from monuron to triplet states of the humic acid probably takes place.

Comparison of the photoinductive properties of commercial, synthetic and soil-extracted humic substances

Abstract The photochemical behavior of a fulvic acid (FA) and several humic acids (HAs) were studied. These synthetic, commercial or soils-extracted humic substances were compared according to their abilities to sensitize the formation of singlet oxygen and to photoinduce the transformation of aromatic compounds such as 2,4,6-trimethylphenol (TMP) and 1,1-dimethyl-3-phenylurea (fenuron). Singlet oxygen was detected by ESR experiments, and quantum yields of singlet oxygen formation were measured by using 1,2-difuranyl-2-hydroxyethanone (furion) as a probe molecule. The photoinductive capacities were evaluated from the kinetics of TMP and fenuron disappearance. The humic substances that showed the greatest ability to generate singlet oxygen were also most efficient in photoinducing the transformation of TMP and fenuron. The efficiencies were found to increase in the following order: synthetic HAs ⪡ Podzol HA Rendzina HA > Aldrich HA > Ranker HA > Ranker FA. Though humic substances formed under the same ecological conditions exhibited similar photoinductive properties, the structural characteristics (IR, elemental analyses, solid-state 13 C NMR) showed no apparent correlations with the photochemical behavior.

Fluorescence of soil humic acids and their fractions obtained by tandem size exclusion chromatography–polyacrylamide gel electrophoresis

Humic acids (HAs) extracted from soils of different origin (chernozem, ferralsol and ranker) and their fractions (A, B and C+D) obtained by tandem size exclusion chromatography–polyacrylamide gel electrophoresis were investigated by steady-state fluorescence spectroscopy in the emission mode. Independently of HA source, high molecular size fractions A and B are shown to be weakly fluorescent. The main fluorophores, especially those emitting at long wavelength (around 500–510 nm), are contained in the polar and low molecular size fractions C+D. As indicated by the observed pH effect, aromatic structures bearing carboxylate and OH substituents may be involved in these longer wavelength emissions.

Soil organic carbon dynamics in cleared temperate forest spodosols converted to maize cropping

In southwest France, sandy spodosols have developed from Quaternary sandy eolian deposits. On these soils, numerous forest lands have been converted to continuous intensive maize cropping. A chronosequence study is realized by comparing organic C pools and 13C natural abundance of one forested and 6 agricultural sites, whose ages of cultivation range from 4 to 32 yr. δ13C ratio is found to increase with time of cultivation. After 3 decades of intensive maize cropping, about half of the initial organic C content in the forest topsoil layer has disappeared. The fraction of C derived from maize crop increases during the first decades of cultivation, but its level is significantly lower than those observed in other soils, which indicates a high mineralization rate of organic C. In this context, soil characteristics associated to intensive agricultural practices lead to a rapid and large loss of C, whereas inputs from maize seem to have only a very small long-term contribution.

Localization of atrazine non-extractable (bound) residues in soil size fractions

Abstract Three different soils were incubated under field conditions with 14 C-ring labelled atrazine. After six months, the soils were exhaustively extracted with methanol and sonicated in water. The dispersed material was then fractionated by sieving, sedimentation and centrifugation, and each fraction was separated into humin, fulvic and humic acids. In all soils, the well humified organic matter and the atrazine residues were mainly located in the 20-2 and 2-0.2 μm fractions. There was a very large concentration of bound residues in the coarsest fractions, especially in the 200-50 μm fraction. These could be related to the active degradation of coarse plant residues, or to bioconcentration by soil actinomycetes and fungi.

Interactions of diuron with dissolved organic matter from organic amendments.

Diuron is frequently detected in some drinking water reservoirs under the Burgundy vineyards, where organic amendments are applied. The environmental effect of these amendments on pesticide transport is ambiguous: on the one hand it could enhance their retention by increasing soil organic carbon content; on the other hand, dissolved organic matter (DOM) could facilitate their transport. Elutions were performed using columns packed with glass beads in order to investigate DOM-diuron interactions, and the possible co-transport of diuron and DOM. Four organic amendments (A, B, C and D) were tested; C and D were sampled at fresh (F) and mature (M) stages. An increase in diuron leaching was observed only for A and D(F) amendments (up to 16% compared to the DOM-free blank samples), suggesting a DOM effect on diuron transport. These results could be explained by the higher DOM leaching for A and D(F) compared to B, C(F), C(M) and D(M) increasing diuron-DOM interactions. These interactions seem to be related to the aromatic and aliphatic content of the DOM, determining formation of hydrogen and non-covalent bonds. The degree of organic matter maturity does not seem to have any effect with amendment C, while a reduction in diuron leaching is observed between D(F) and D(M). After equilibrium dialysis measurement of diuron-DOM complexes, it appeared that less than 3% of the diuron applied corresponded to complexes with a molecular weight >1000 Da. Complexes <1000 Da could also take part in this facilitated transport.

Characterization of organic matter in topsoils under rain forest and pasture in the eastern Brazilian Amazon basin

Abstract In topsoils under forest and 7-, 12- and 17-year-old pastures, organic matter was characterized by analysing C and N distribution in particle-size fractions, the C decomposition rates of soil and particle-size fractions and by employing density-fractionation of macro-organic matter (>150 μm). The C and N associated with clay fractions increased with increasing age of pasture. The weight (%) of macro-organic matter and its heavy fractions (>1.37 g cm–3) also increased with increasing age of pasture. However, in a long-term incubation (100 days), these changes seemed to involve an increase in the C decomposition rate in the topsoil of the oldest pasture. Using the C decomposition rates of particle-size fractions, it appeared that silt and clay contributed differently to C decomposition in the whole soil. C associated with silt contributed to the C decomposition rate during the first 40 days of incubation, while C associated with clay contributed to C decomposition in the long-term incubation (after 40 days), especially when the clay fraction appeared to reach saturation point with respect to its ability to bind organic compounds and thus protect the soil from C loss.

Photoinductive efficiency of soil extracted humic and fulvic acids

Humic and fulvic acids extracted from soils of different genesis were investigated for their ability to photoinduce the transformation of fenuron (2 x 10(-4) mol(-1)) at 365 nm. The ratio of the initial rate of fenuron consumption over the rate of light absorption by humic substances was found to be higher for fulvic acids (range 2.0 x 10(-3) to 9.0 x 10(-5)) than for humic acids (range 1.7 x 10(-4) to - 3.6 x 10(-5)). Within the FAs population, this ratio decreased as the specific absorption coefficient at 365 nm increased. It seems therefore that most of 365-nm absorbing components have no photoinductive activity and even reduce that of photoinductive chromophores.