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The mineralogy of bulk and rhizosphere soils was compared to assess the effect of roots on mineral weathering in a Typic Dystrochrept supporting Norway spruce (Picea abies [L.] Karst) and oak (Quercus sessiliflora Smith). In an experimental forest site (Breuil-Chenue, France), systematic soil sampling was performed in forty pits. The soil adhering to the roots was considered as rhizosphere soil. The remaining material was regarded as bulk soil. The mineralogy of the clay-sized particles of both fractions was determined by x-ray diffraction (XRD). Amorphous solid phases were estimated by extracting Fe and Al with hot Na-citrate and dithionite-citrate-bicarbonate. Total chemical analyzes were performed on the clay-sized particles via Ba[Cl.sub.2]-saturation and an iso-zirconium weathering balance was calculated. The XRD demonstrated an increase of illite-like minerals in the rhizosphere of both species and the selective extractions revealed a decrease in the amorphous phases. The total chemical analyzes showed that the rhizosphere clay-sized fraction contained significantly more Si and K (for oak only) and less Fe and Al than the bulk soil. By way of the iso-zirconium weathering balance, these losses of Fe and AI in the rhizosphere were estimated at several tens of kilograms by hectare for the surface horizon (0-3 cm). This study demonstrates that, despite the short duration of the contact between the active part of a root and the solid mineral phase, the intensity of the processes occurring in the rhizosphere significantly increases mineral weathering. Abbreviations: B, bulk soil; BS, base saturation; subscript c, hot Na-citate treatment; CEC, cation exchange capacity; subscript d, dithionite-citrate-bicarbonate treatment; EA, exchangeable acidity; EG, ethylene glycol treatment; HIS, hydroxy-interlayered smectite; HIV, hydroxy-interlayered vermiculite; ICP-AES, inductively coupled plasma spectrometry-atomic emission spectrometry; ICP-MS, inductively coupled plasma spectrometry- mass spectrometry; R, rhizosphere; subscript [tot], total; XRD, x-ray diffraction.

Chicory (Cichorium intybus L.), a deep-rooted forb, has potential for inclusion in pasture mixtures because of its reported drought tolerance and high productivity during summer months. This study examined how adding chicory to pasture mixtures affected forage yield, root growth, and soil moisture extraction under drought. The experiment was planted in August 2002. Movable rainout shelters were used to control water application in the field. Adding chicory to orchardgrass (Dactylis glomerata L.)-white clover (Trifolium repens L.) or perennial ryegrass (Lolium perenne L.)--white clover mixtures increased drought tolerance in 2003 when chicory constituted 24 to 39% of harvested forage biomass. Chicory mortality was high, decreasing from 39% of harvested forage yield under drought stress in early summer 2003 to 9% in late summer 2004. Improved yield under drought stress was not observed in 2004 when chicory constituted only 9 to 16% of the mixture. The three-species mixtures in 2003 had greater root counts than the two-species mixtures at soil depths below about 70 cm under well-watered conditions, but a greater proliferation of roots at depths below 70 cm was observed for the two-species mixture under drought stress. Both mixtures appeared capable of nighttime transfer of soil water from deep to shallow soil layers, thereby improving water availability to shallow roots. However, improved drought tolerance of the three-species mixture was probably related to improved water use efficiency rather than to greater access to and extraction of deep soil moisture.

Rhizosphere conditions are of great importance fur nutrient uptake and thus plant growth. Nutrient availability in the rhizosphere may differ substantially from that of the bulk soil and specific sampling of the rhizosphere soil is crucial to the understanding of plant growth, particularly where roots are restricted in growth due to high bulk density of the soil. The purpose of this study was to develop suitable methods for investigating the chemical composition of soil around pores with and without roots in soils with high bulk density. Two different methods were undertaken: one approach was to perform sequential extractions with [H.sub.2]O, 0.01, 0.1, and 1 M N[H.sub.4]N[O.sub.3] on soil samples taken at different distances from the pore wall to determine differences in element availability. Another approach was to analyze resin-impregnated, undisturbed soil samples from the same site for elemental composition of pure wall material using a scanning electron microscope (SEM) equipped with an x-ray micro analyzer. In general, the sequential extraction detected few statistically significant differences in nutrient availability between pores containing routs compared with pores without roots. The SEM analysis showed lower amounts of easily weatherable minerals around pores containing roots. Both methods are suitable for investigating differences between rhizosphere anti bulk soil characteristics.

The effects of different root systems on vegetation communities are assessed. Shallow-rooted trees enhanced grass productivity, while deep-rooted trees lowered grass productivity and lengthened fire return intervals; shallow-rooted grasses created smaller soil carbon pools, while deep-rooted grasses enhanced nitrogen uptake and the growth of soil organic matter pools.

Research indicates that free-air carbon dioxide enrichment (FACE) enhances organic matter inputs to soil carbon pools, but may also encourage belowground carbon losses due to increased soil respiration. Data were obtained from a loblolly pine forest, which also demonstrated increased fine root increment and litterfall mass after FACE.

1 Nutrient-rich patches can occur in soils at a wide range of spatial and temporal scales. Differences in the root proliferation response between species may be due to differing abilities of root systems to locate and recognize patches of differing size and nutrient concentration. 2 We investigated the root proliferation responses of seven co-occurring herbaceous perennial British woodland species (Ajuga reptans, Glechoma hederacea, Oxalis acetosella, Silene dioica, Stachys sylvatica, Veronica montana and Viola riviniana) and the effect of mycorrhizal colonization on any response. 3 Plants were planted in nutrient-poor sand in the centre of rectangular pots, with a nutrient-rich patch to one side and a control (sand) patch on the other. Size and nutrient concentration of the patches were varied between treatments. 4 Species differed in the size of their root systems and in their ability to respond to localized nutrient-rich patches. Oxalis acetosella and Viola riviniana, which produced the smallest root systems, showed similar root growth in nutrient-rich and control patches. All other species responded to the presence of a nutrient-rich patch by various combinations of root proliferation, changes in root branching pattern and by an increase in specific root length. 5 In some species the response was affected by patch attributes: Silene dioica and Veronica montana were sensitive to the nutrient concentration of the patch, and Glechoma hederacea did not respond to the smallest sized patch. 6 Mycorrhizal colonization had little effect on root proliferation. Only one species (Oxalis acetosella) could be shown to benefit from colonization by increased phosphate uptake. 7 The soil is a heterogeneous environment in terms of nutrient availability; differences between species in the ability to exploit this heterogeneity may affect their distribution, and could be a mechanism that reduces interspecific root competition. Key-words: arbuscular mycorrhizal fungi, plasticity, root architecture, roots, soil heterogeneity Journal of Ecology (1999) 87, 849-859

Summary 1 We report on an experimental investigation into the effects of contrast in patch quality and patch size upon the performance of the clonal herb Glechoma hederacea in nutritionally heterogeneous environments. 2 There were six treatments providing different degrees of patch contrast. These ranged from maximum contrast, in which patches consisted entirely of compost or sand, to homogeneity, in which all patches contained 50% of each. These were combined factorially with two patch size treatments. Overall, all treatments provided the same quantity of compost and sand, and the same area and volume of higher and lower quality patches. 3 Clones in the large-patch environments produced significantly more root biomass in both rich- and poor- quality patches than the clones in the small-patch environments. At both scales there was a decline in root biomass in the rich patches, accompanied by an increase in root biomass in the poor patches, as patch quality converged. 4 The proportion of the root biomass of clones that was located in nutrient-rich patches was greatest at high contrast and declined gradually to become equal in all patches, as contrast between patches diminished to homogeneity. This effect was independent of patch scale. There was a close match between root placement and nutrient availability in different quality patches in all treatments except that with the highest contrast. 5 At both patch scales, root:shoot ratios of clone parts in rich patches rose (and those in poor patches declined) as patch contrast increased. This species is therefore capable of morphological specialization at a local level when clones grow in heterogeneous environments. These effects were also greater at the larger patch scale. 6 The effect of patch contrast on total clone yield was significantly modified by patch scale. At lower contrast, yield was similar at both scales, whereas at higher contrast, yield was significantly greater in the large-patch treatment. 7 Although G. hederacea can match its morphology to environmental heterogeneity in some habitats, it is less able to do so when patches are small-scale and highly contrasting. These limitations cause considerable differences in the yield that can be achieved from a given quantity of resource provided in different configurations. Key-words: clonal plants, foraging, morphological plasticity, patch selection, root: shoot ratio Journal of Ecology (1999) 87, 860-872