Seasonal patterns of fine root production and turnover in a mature rubber tree (Hevea brasiliensis Müll. Arg.) stand- differentiation with soil depth and implications for soil carbon stocks

Fine root dynamics is a main driver of soil carbon stocks, particularly in tropical forests, yet major uncertainties still surround estimates of fine root production and turnover. This lack of knowledge is largely due to the fact that studying root dynamics in situ, particularly deep in the soil, remains highly challenging. We explored the interactions between fine root dynamics, soil depth, and rainfall in mature rubber trees (Hevea brasiliensis Müll. Arg.) exposed to sub-optimal edaphic and climatic conditions. A root observation access well was installed in northern Thailand to monitor root dynamics along a 4.5 m deep soil profile. Image-based measurements of root elongation and lifespan of individual roots were carried out at monthly intervals over 3 years. Soil depth was found to have a significant effect on root turnover. Surprisingly, root turnover increased with soil depth and root half-life was 16, 6–8, and only 4 months at 0.5, 1.0, 2.5, and 3.0 m deep, respectively (with the exception of roots at 4.5 m which had a half-life similar to that found between depths of 1.0 and 2.5 m). Within the first two meters of the soil profile, the highest rates of root emergence occurred about 3 months after the onset of the rainy season, while deeper in the soil, root emergence was not linked to the rainfall pattern. Root emergence was limited during leaf flushing (between March and May), particularly within the first two meters of the profile. Between soil depths of 0.5 and 2.0 m, root mortality appeared independent of variations in root emergence, but below 2.0 m, peaks in root emergence and death were synchronized. Shallow parts of the root system were more responsive to rainfall than their deeper counterparts. Increased root emergence in deep soil toward the onset of the dry season could correspond to a drought acclimation mechanism, with the relative importance of deep water capture increasing once rainfall ceased. The considerable soil depth regularly explored by fine roots, even though significantly less than in surface layers in terms of root length density and biomass, will impact strongly the evaluation of soil carbon stocks

Evaluation of consensus method for the culture of Burkholderia pseudomallei in soil samples from Laos.

Background: We have previously shown that PCR following enrichment culture is the most sensitive method to detect Burkholderia pseudomallei in environmental samples. Here we report an evaluation of the published consensus method for the culture of B. pseudomallei from Lao soil in comparison with our conventional culture method and with PCR with or without prior broth enrichment. Methods: One hundred soil samples were collected from a field known to contain B. pseudomallei and processed by: (i) the conventional method, (ii-iii) the consensus method using media prepared in either Laos or Thailand, and (iv) the consensus method performed in Thailand, as well as by (v) PCR following direct extraction of DNA from soil and (vi) PCR following broth pre-enrichment. Results: The numbers of samples in which B. pseudomallei was detected were 42, 10, 7, 6, 6 and 84, respectively. However, two samples were positive by the consensus method but negative by conventional culture, and one sample was negative by PCR following enrichment although B. pseudomallei was isolated by the conventional culture method. Conclusions/Discussion: The results show that no single method will detect all environmental samples that contain B. pseudomallei. People conducting environmental surveys for this organism should be aware of the possibility of false-negative results using the consensus culture method. An approach that entails screening using PCR after enrichment, followed by the evaluation of a range of different culture methods on PCR-positive samples to determine which works best in each setting, is recommended

Distribution of Burkholderia pseudomallei within a 300-cm deep soil profile: implications for environmental sampling.

The environmental distribution of Burkholderia pseudomallei, the causative agent of melioidosis, remains poorly understood. B. pseudomallei is known to have the ability to occupy a variety of environmental niches, particularly in soil. This paper provides novel information about a putative association of soil biogeochemical heterogeneity and the vertical distribution of B. pseudomallei. We investigated (1) the distribution of B. pseudomallei along a 300-cm deep soil profile together with the variation of a range of soil physico-chemical properties; (2) whether correlations between the distribution of B. pseudomallei and soil physico-chemical properties exist and (3) when they exist, what such correlations indicate with regards to the environmental conditions conducive to the occurrence of B. pseudomallei in soils. Unexpectedly, the highest concentrations of B. pseudomallei were observed between 100 and 200 cm below the soil surface. Our results indicate that unravelling the environmental conditions favorable to B. pseudomallei entails considering many aspects of the actual complexity of soil. Important recommendations regarding environmental sampling for B. pseudomallei can be drawn from this work, in particular that collecting samples down to the water table is of foremost importance, as groundwater persistence appears to be a controlling factor of the occurrence of B. pseudomallei in soil

Multi-spectral imaging of rhizobox systems: New perspectives for the observation and discrimination of rhizosphere components

In this issue of Plant and Soil Nakaji et al. (Plant Soil, this volume, 2008) report a novel approach for automatically identifying roots and other rhizosphere components in rhizosphere images acquired using a multi-spectral (visible—VIS- and nearinfrared—NIR-) imaging system. The images are acquired through a root-window observation device and the study highlights the perspectives offered by this imaging system. An outstanding outcome of this research is that the new approach can be applied to effectively separate soil litter from the purely mineral phase and distinguish root tissues that differ in physiological status, i.e. live (different age classes), senescent and dead. If achievable routinely, such a detailed classification of rhizosphere components could greatly improve our appraisal of root turnover and associated organic matter input to the soil, information of paramount importance for an improved understanding of many essential processes such as global geochemical cycles. Minirhizotrons (MR) systems have been increasingly used in global change studies because they are a convenient way to frequently and nondestructively quantify root length production and mortality (Norby and Jackson, New Phytol, 147:3–12, 2000; Hendrick and Pregitzer, Ecology, 73:1094–1104, 1992). However, the MR technique still has many limitations, including the lack of a standard, accurate and rapid procedure to extract and classify rhizosphere components from the MR images obtained. The recent work by Nakaji et al. (Plant Soil, this volume, 2008) provides convincing evidence that the inclusion of a VIS-NIR multispectral capability into conventional MR systems could substantially improve this method, and extend its adoption by the wider plant scientist community as a standard research tool

Avant-propos

Giami Alain, Pierret Janine. Avant-propos. In: Sciences sociales et santé. Volume 9, n°4, 1991. Sexualité et société, sous la direction de Alain Giami et Janine Pierret. pp. 5-6