F. Bartoli

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.

EFFECT OF INOCULATION WITH BACILLUS POLYMYXA ON SOIL AGGREGATION IN THE WHEAT RHIZOSPHERE: PRELIMINARY EXAMINATION

Gouzou, L., Burtin, G., Philippy, R., Bartoli, F. and Heulin, T., 1993. Effect of inoculation with Bacillus polymyxa on soil aggregation in the wheat rhizosphere: preliminary examination. In: L. Brussaard and M.J. Kooistra (Editors), Int. Workshop on Methods of Research on Soil Structure I Soil Biota Interrelationships. Geoderma, 56: 479–491. The present paper reports the results of a combined physical and microbial approach to studying the aggregation state of a silt loam soil in the wheat rhizosphere. Non-rhizosphere soil aggregate size distribution was attributed to self-fragmentation of clods. In contrast, aggregate size distribution was uni-modal and centered at 0.2-2 mm for soil adhering to wheat roots. On the other hand, inoculation of wheat with a rhizosphere strain of Bacillus polymyxa increased the mass of soil adhering to the roots by 57%. Comparison of aggregate size distributions suggested a more porous structure for the inoculated rhizosphere soil than the uninoculated. An avidin-biotin indirect ELISA procedure, which allows the detection of B. polymyxa in its natural environment, indicated that the population remained at a constant level whatever the treatment or the size of aggregates. Aggregate stability in water, which was low, showed no statistical difference between treatments or between aggregate size fractions. The water-stable aggregates studied here represent the elementary rhizosphere aggregates characterized by the association of clays, silts, fine sands and B. polymyxa cells. No effect on the population level of B. polymyxa or on the stabilization of aggregates was observed after inoculation of soil and seeds. However, a more porous structure developed within the rhizosphere soil. We are continuing to investigate this aspect of the soil structure process which could be related to a spatial extension of the rhizosphere soil. Inoculation by B. polymyxa could play an important role in water retention and nutrient transfer in the rhizosphere through increasing porosity.

Influence of organic matter on aggregation in Oxisols rich in gibbsite or in goethite. II. Clay dispersion, aggregate strength and water-stability

Abstract The structural implications of the influence of organic matter on Oxisol surface charges were further explored in this study. In Oxisol aggregates rich in gibbsite, amounts of dispersed clay were only 20–40% with moderate mechanical input energy and 30–70% with ultrasonic disruption. The stability was mainly attributed to the importance of non-electrostatic intra-aggregate bonds. Organic aggregates were also much more stable than mineral aggregates attributable to lower porosities and to greater hydrophobicity. Tensile strength of air-dried aggregates was relatively low in comparison with temperate soil counterparts and increased with necessary organic matter content. Consequently, organic aggregates had much more complex populations than mineral aggregates. The influences of mineralogy, structure (pore volume and heterogeneity) and intra-aggregate bonds on both aggregate tensile strength and water-stability are discussed.

Fungal ectomycorrhizal community and drought affect root hydraulic properties and soil adherence to roots ofPinus pinaster seedlings

Pinus pinaster seedlings were grown in a sandy dune soil either inoculated withHebeloma cylindrosporum or let to natural colonisation. Six months later, half of the seedlings of both treatments were subjected to a 3-week moderate drought. Root colonisation analysis showed that root tips were colonised to almost 100% independent of the inoculation. DNA determination of the ectomycorrhizal morphotypes showed that inoculated seedlings were extensively mycorrhized byH. cylindrosporum (more than 75%) whereas non-inoculated seedlings were mycorrhized by the exotic speciesThelephora terrestris (50%) andLaccaria bicolor (30%) and to a lesser extent byH. cylindrosporum (20%). Drought did not affect these frequencies. Total plant biomass was not affected by the mycorrhizal status or by drought but the root/shoot biomass ratio as well as the root/leaf surface area ratio were much lower in seedlings extensively colonised byH. cylindrosporum. Root hydraulic conductivity was higher in plants mainly mycorrhized byH. cylindrosporum, showing that this fungus improved the water uptake capacity of the root system as compared toT. terrestris and/orL. bicolor. This positive effect was also found under drought but to a lesser extent.H. cylindrosporum also increased the amount of root-adhering soil as compared to the other fungal symbionts, illustrating the performance of this association in aggregating sandy soil particles and developing the rhizosheath. The origin of the reduced root hydraulic resistance byH. cylindrosporum mycorrhization is discussed for the whole path including soil, soil-root interface and root cortex.

Root:soil adhesion in the maize rhizosphere: the rheological approach

This study was designed to investigate the strength of attachment of plant seedling roots to the soil in which they were grown. The study also assessed the effects of differing soil textures and differing soil matric potentials upon the strength of the root:soil attachment. A device for growing roots upon a soil surface was designed, and was used to produce roots which were attached to the soil. In order to quantify root:soil adhesion, roots of maize seedlings, grown on the soil surface, were subsequently peeled off using a universal test machine, in conjunction with simultaneous time-lapse video observation. To clarify the partitioning of energy in the root:soil peeling test, separate mechanical tests on roots, and on two adherent remoulded topsoil balls were also carried out. The seedling root was characterised by a low bending stiffness. The energy stored in bending was negligible, compared to the root:soil adhesion energy. The mechanical properties of two adherent remoulded topsoil balls were a decrease of the soil:soil adhesion energy as the soil:soil plastic energy increased. These two parameters were therefore interdependent. Using a video-camera system, it was possible to separate the different processes occurring during the root:soil peeling test, in particular, the seed:soil adhesion and the root:soil soil adhesion. An interpretation of the complex and variable force:displacement curves was thus possible, enabling calculation of the root:soil interfacial rupture energy. At a given suction (10 kPa), the results of the peeling test showed a clear soil texture effect on the value of the root:soil interfacial rupture energy. In contrast, for the same silty topsoil, the effect of the soil water suction on the value of the interfacial rupture energy was very moderate. The root:soil interfacial rupture energy was controlled mainly by a product of microscopic soil specific surface area and the macroscopic contact surface area between the root and the soil. Biological and physical interactions contributing to root:soil adhesion such as root:soil interlocking mechanics were also analysed and discussed.

Effects of inoculation of EPS-producing Pantoea agglomerans on wheat rhizosphere aggregation

The effect of bacteria secreting an extracellular polysaccharide (EPS) on the physical properties of rhizosphere soil has been investigated as a function of soil water content by using an approach in which wheat seedlings were inoculated with a strain Pantoea agglomerans (NAS206) selected from the rhizosphere of wheat (Triticum durum L.) growing in a Moroccan vertisol. Colonization by strain NAS206 occurred both on the rhizoplane and the root-adhering soil as opposed to the bulk soil. The intense colonization of the wheat rhizosphere by these EPS-producing bacteria was associated with significant aggregation and stabilization of root-adhering soil, as shown by the combined increases of (i) aggregate mean weight diameter (MWD), (ii) aggregate macro-porosity (pore throat diameter between 10 and 80 μm), (iii) adhering soil:root mass ratio (RAS/RT), (iv) water-stable >200 μm aggregates and (v) 0.1–2 μm elementary clayey micro-aggregates. Biological exudation and capillary pressure interactions leading to root-adhering soil aggregation are also analysed and discussed.

Influence of organic matter on aggregation in Oxisols rich in gibbsite or in goethite. I. Structures: the fractal approach

Abstract The structural implications of the influence of organic matter on surface charges in Oxisols were studied. Oxisols rich either in goethite or in gibbsite have irregular and self-similar aggregates of elementary ordered card-house-type assemblages of rigid kaolinite-goethite or kaolinite-gibbsite associations. The size of the elementary structural pattern decreases as a function of organic matter content and positive charge neutralization by organic matter which infills pores The fractal approach also appears to be a useful tool for understanding the underlying mechanisms in the creation of Oxisol aggregates. The compactness of these aggregate structures increases towards the surface of the soil due to rearrangements during drying events of repeated wet-dry cycles. A correspondence between fractal interface and both aggregation kinetics during air-drying of wet aggregates and surface tension at the interface is suggested.

Spatial variability of topsoil characteristics within one silty soil type. Effects on clay migration

Abstract In order to understand and to model soil runoff erosion as well as to optimize sampling schemes, improved understanding of spatial variability of clay and some other soil erosion parameters is needed. For this purpose, two complementary approaches to the study of spatial variability of silty topsoil characteristics, the pedological approach and the fractal approach applied to geostatistics, were carried out in the context of soil erosion within the intensive cereal agriculture zone of northwestern Europe. Fractal geometry provides one synthetic key to the description of classical geostatistical tools such as variograms. Spatial structures of soil properties of each of the three topsoil pedological units were mostly characterized along the slope by the ranges of the fractal one-dimensional space domains and their scale invariants: the fractal dimensions. Results suggest that, within each topsoil pedological unit, these scale invariants are relevant qualifiers of the intrinsic topsoil variability, which can be modelled as a fractal Brownian process and should be incorporated in simple recursive or complex network soil erosion models. Different surface fractal dimensions, in a one-dimensional space, have been found within these three topsoil units for each soil parameter studied (multifractals). All the data have been aggregated within the whole one-dimensional slope transect in order to obtain both possible general scale laws on clays and other soil characteristics and possible evidence concerning underlying soil erosion mechanisms by particle runoff.

Silty topsoil structure and its dynamics: the fractal approach

Abstract This paper describes the application of fractal geometry to the structure and dynamics of tilled silty topsoil. The soil structure of each topsoil sample has been experimentally quantified directly by image analysis and indirectly by both water retention and mercury porosimetry. Any fractal scaling laws were mostly determined within a relevant common pore radius yardstick scale. Their scale invariants were fractal dimensions of either the matrix ( D m ) or the solid:pore interfaces ( D s ). We showed that the D m or D s values computed from water retention data were higher than their D m or D s value counted from mercury porosimetry data, which were themselves much higher than their microscopic fractal dimension value counterparts (only D m ). This was attributed to (i) partial pore-volume filling by either water or mercury and (ii) hysteresis between water drainage and mercury intrusion. A positive relationship between the fractal dimensions of the solid:pore interfaces and their solid mass fractal dimension counterparts have also been found, with the value of D m being higher than the corresponding value of D s , characterizing complex fractal structures with interconnected pores. We also showed that clay content has a positive effect on both D m and D s values from either water retention or mercury porosimetry data as well on the positive relationship occurring between the D s values and their D m value counterparts. The physical process underlying this behaviour is proposed to be partial volume-pore filling by clays and, concomitantly, an increase in the rugosity of the microscopic solid: pore interfaces. In contrast, the temporal variability of either D m or D s values was moderate and can be attributed to both increase of pore connectivity and water hysteresis which occurred during the drying and wetting cycles of the studied cultivation period.

Influence of fir root zone on soil structure in a 23 m forest transect: the fractal approach

Abstract This paper reports the results of a joint colloidal chemistry and fractal approach to the study of aggregate structures in the A 1 (0–10 cm) and A 1 (B) (20–30 cm) horizons of a sandy acid brown soil collected along a 23 m transect under three fir trees (eastern France, declined fir ecosystem). The amounts of both organic carbon and Al oxalate clearly discriminate between the two horizons studied but no significant root zone differences were observed. However, the A 1 horizons collected in the fir root zone contained much more polysaccharides than the corresponding non-root zone horizons, and root zone soil polysaccharides had both a plant and a microbial origin. The impact of a poorly-ordered Al hydrous oxides-organic matter balance (Al ox -C significant negative correlation) on both physico-chemical and structural characteristics of soil millimetric aggregates was demonstrated. The soil zero point of charge (ZPC) was mainly correlated both with positively charged poorly-ordered Al hydrous oxides and with negatively charged organic matter: ZPC = 7.3Al ox (%)−0.14C(%)+3.69 ( r =0.85; P More porous and water-stable aggregates as fractal domains over a larger range of spatial scales were observed in the A 1 , organic horizon rather than in the deeper Al-rich A 1 (B) horizon. Finally, the surface fractal dimension D s (as determined by mercury porosimetry) was found to be a better discriminator than the porosity between the root zone and non-root zone of the soils.