Nathalie Kruyts

Soil organic horizons as a major source for radiocesium biorecycling in forest ecosystems.

Here we review some of the main processes and key parameters affecting the mobility of radiocesium in soils of semi-natural areas. We further illustrate them in a collection of soil surface horizons which largely differ in their organic matter contents. In soils, specific retention of radiocesium occurs in a very small number of sorbing sites, which are the frayed edge sites (FES) born out of weathered micaceous minerals. The FES abundance directly governs the mobility of trace Cs in the rhizosphere and thus its transfer from soil to plant. Here, we show that the accumulation of organic matter in topsoils can exert a dilution of FES-bearing minerals in the thick humus of some forest soils. Consequently, such accumulation significantly contributes to increasing 137Cs soil-to-plant transfer. Potassium depletion and extensive exploration of the organic horizons by plant roots can further enhance the contamination hazard. As humus thickness depends on both ecological conditions and forest management. our observations support the following ideas: (1) forest ecosystems can be classified according to their sensitivity to radiocesium bio-recycling, (2) specific forest management could be searched to decrease such bio-recycling.

Mobility of radiocesium in three distinct forest floors.

The degree of mixing of organic matter with minerals in organic and hemi-organic horizons of forest soils largely differs between humus types. As clay minerals might control the mobility of radiocesium in these forest floor horizons, plant contamination could greatly vary with the kind of humus. We measured the mobility of radiocesium in the upper O, OAh and Ah horizons of three acid forest soils with three distinct humus types: eumoder, dysmoder and fibrimor. We used two different approaches: a physico-chemical test quantifying the radiocesium interception potential (RIP) and a biological assay simulating an experimental rhizosphere. The results show that the (137)Cs horizon-to-plant transfer is directly governed by RIP, and thus by frayed edge sites born by weathered micaceous minerals. The inverse relationship between RIP and organic matter content indicates that in the three sites investigated the mixing of organic residues with Cs-fixing minerals is a key process in 137Cs mobility. These Cs-fixing clay minerals indeed decrease in the sequence eumoder > dysmoder > fibrimor because they are more diluted in forest floor with less bioturbation. Our results suggest that humus type might be an important parameter in classifying forest soils with respect to their ability to transfer radiocesium to the above standing vegetation.

Labile zinc concentration and free copper ion activity in the rhizosphere of forest soils

Water-soluble and acid-extractable Cu and Zn, water-soluble organic carbon (WSOC), pH, differential pulse anodic stripping voltammetry-labile Zn (ZnL), Zn2+ activity (Windemere humic aqueous model [WHAM]; http://chess.ensmp.fr/ chemsites.html), and Cu2+ activity with an ion-selective electrode were compared between the rhizosphere and the bulk components of nine acidic forest sites from southeastern Canada. At all sites, the WSOC contents were higher in the rhizosphere than in the bulk component. Acidity was also higher in the rhizosphere, although pH differences were significant at only five sites. The concentrations of Zn in water extracts and ZnL contents (at six sites) were higher in the rhizosphere, whereas acid-extractable Zn was only marginally increased in the rhizosphere. Calculations with WHAM indicated that free Zn2+ ion activities were higher in the rhizosphere than in the bulk soil but that the fraction of total dissolved Zn in water extracts that is present as free Zn2+ did not differ significantly between the two components. The concentration of Cu in the water extract was higher in the rhizosphere for all sites, but acid-extractable Cu levels did not differ. The fraction of water-soluble Cu present as Cu2+ was higher in the bulk soil, although Cu2+ activities did not significantly vary with proximity to roots. These results showed that the processes acting in the rhizosphere of forest soils strongly affected the concentrations of dissolved Zn and Cu and that this microenvironment should be considered when estimating the bioavailability and the ecological risks of metals in soils.

Uptake, Assimilation and Translocation of Mineral Elements in Monoxenic Cultivation Systems

While searching for optimal means to study the transport processes of nutrients and non-essential elements, diverse in vivo systems were developed using bi-compartmental containers where extraradical mycelium (ERM) of mycorrhizal fungi was separated from the plant roots. These systems generated some major results (reviewed in Smith and Read 1997), but suffered several limitations such as (1) the presence of undesirable micro-organisms which could influence element bioavailability or the transport processes; (2) the difficulty to visualize the ERM dynamic development and the bidirectional translocation processes in hyphae; (3) the difficulty to collect ERM and to distinguish thin hyphae of arbuscular mycorrhizal (AM) fungi from other fungi. As a consequence, some innovative approaches were tried out, such as the system of Pearson and Tinker (1975), used by Cooper and Tinker (1978) to study the transport of P, Zn and S by AM fungi. This system was based on a bi-compartment Petri plate in which a mycorrhizal plant (Trifolium repens L.) was grown on sterilized soil, while the ERM was allowed to cross the partition wall to develop in an agar medium without roots. This ingenious system kept the plant and the AM fungus (Glomus mosseae) under sterile conditions but, as the seedling developed, a hole was made in the lid and the plant grew out from it, making the system difficult to maintain free from other micro-organisms. This system was improved by St Arnaud et al. (1996), by growing the AM fungus in bi-compartmental Petri plates under monoxenic culture conditions on

Understanding Root Uptake of Nutrients, Toxic and Polluting Elements in Hydroponic Culture

The understanding of plant uptake (nutrients, toxic and polluting elements) is crucial for the future food needs of humanity given the explosive growth of the world population and the anthropogenic pressure on the environment which significantly modify the homeostasis of the balanced global cycles. Rice and banana are of fundamental interest for development policy since they are two major foods for the world population. The understanding of the mechanisms and the optimal conditions of nutrient uptake by these plants is thus important to ensure biomass production. Furthermore, the transfer of toxic and polluting elements in the soil-plant system can influence the nutrient uptake and plant growth, and thus has strong agronomic consequences, in addition to the large environmental consequences.