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.

Impact of biochar application dose on soil microbial communities associated with rubber trees in North East Thailand.

Biochar is a potential tool to mitigate climate change by enhancing C sequestration in soils, but its use as a soil amendment to improve soil fertility and crop yields is still a contentious subject. In North East (NE) Thailand, biochar has been promoted to restore soil fertility in rubber tree plantations. Despite this, there is scarce information on the impact of biochar application on the soil biota, particularly on microbial communities associated with rubber trees. The effects of increasing doses of biochar on microbial communities were investigated in a rubber tree plantation in NE Thailand, 28 months after application. Biochar application resulted in increases of soil pH and nutrient contents and also had an impact on both bacterial and fungal communities. Changes in microbial composition and structure were observed although fungal communities were more markedly affected than bacterial communities. The nature and magnitude of the observed changes were strongly related to soil properties (pH, soil moisture and P content), while biochar dose (5, 10 or 20 tons/ha) effect was not significant. Our results highlight the need for additional research for a better understanding of the impact of biochar application on soil microbial communities and further cascading effects on ecosystem functions.

Effect of land use and hydrological processes on Escherichia coli concentrations in streams of tropical, humid headwater catchments.

Lack of access to clean water and adequate sanitation continues to be a major brake on development. Here we present the results of a 12-month investigation into the dynamics of Escherichia coli, a commonly used indicator of faecal contamination in water supplies, in three small, rural catchments in Laos, Thailand and Vietnam. We show that land use and hydrology are major controlling factors of E. coli concentrations in streamwater and that the relative importance of these two factors varies between the dry and wet seasons. In all three catchments, the highest concentrations were observed during the wet season when storm events and overland flow were highest. However, smaller peaks of E. coli concentration were also observed during the dry season. These latter correspond to periods of intense farming activities and small, episodic rain events. Furthermore, vegetation type, through land use and soil surface crusting, combined with mammalian presence play an important role in determining E. coli loads in the streams. Finally, sampling during stormflow revealed the importance of having appropriate sampling protocols if information on maximum contamination levels is required as grab sampling at a fixed time step may miss important peaks in E. coli numbers.

Diversity of root-associated arbuscular mycorrhizal fungal communities in a rubber tree plantation chronosequence in Northeast Thailand.

Rubber tree (Hevea brasiliensis) is of major economic importance in Southeast Asia and for small land holders in Thailand in particular. Due to the high value of latex, plantations are expanding into unsuitable areas, such as the northeast province of Thailand where soil fertility is very low and therefore appropriate management practices are of primary importance. Arbuscular mycorrhizal fungi (AMF) contribute to plant growth through a range of mechanisms and could play a key role in a more sustainable management of the rubber plantations. We described the diversity of AMF associated with rubber tree roots in Northeast Thailand in relation to tree age and soil parameters along a chronosequence of rubber tree plantations. Cassava fields were included for comparison. Rubber tree and cassava roots harbored high diversity of AMF (111 Virtual Taxa, VT), including 20 novel VT. AMF VT richness per sample was consistently high (per site mean 16 to 21 VT per sample) along the chronosequence and was not related to soil properties. The composition of AMF communities differed between cassava and rubber tree plantations and was influenced by soil texture and nutrient content (sand, K, P, Ca). AMF community composition gradually shifted with the age of the trees. Our results suggest that the high diversity of AMF in this region is potentially significant for maintaining high functionality of AMF communities.

Influence of temperature on the electrical conductivity of leachate from municipal solid waste.

A bioreactor landfill is designed to manage municipal solid waste, through accelerated waste biodegradation, and stabilisation of the process by means of the controlled addition of liquid, i.e. leachate recirculation. The measurement of electrical resistivity by Electrical Resistivity Tomography (ERT) allows to monitor water content present in the bioreactors. Variations in electrical resistivity are linked to variations in moisture content and temperature. In order to overcome this ambiguity, two laboratory experiments were carried out to establish a relationship between temperature and electrical conductivity: the first set of measurements was made for leachate alone, whereas the second set was made with two different granular media saturated with leachate. Both experiments confirm a well known increase in conductivity of about 2% degrees C(-1). However, higher suspended matter concentrations lead to a lower dependence of electrical conductivity on temperature. Furthermore, for various porous media saturated with an identical leachate, the higher the specific surface of the granular matrix, the lower the effective bulk electrical conductivity. These observations show that a correct understanding of the electrical properties of liquids requires the nature and (in particular) the size of the electrical charge carriers to be taken into account.

Magnetic resonance sounding applied to aquifer characterization.

Magnetic resonance sounding (MRS) is distinguished from other geophysical tools used for ground water investigation by the fact that it measures a magnetic resonance signal generated directly from subsurface water molecules. An alternating current pulse energizes a wire loop on the ground surface and the MRS signal is generated; subsurface water is indicated, with a high degree of reliability, by nonzero amplitude readings. Measurements with varied pulse magnitudes then reveal the depth and thickness of water saturated layers. The hydraulic conductivity of aquifers can also be estimated using boreholes for calibration. MRS can be used for both predicting the yield of water supply wells and for interpolation between boreholes, thereby reducing the number of holes required for hydrogeological modeling. An example of the practical application of MRS combined with two-dimensional electrical imaging, in the Kerbernez and Kerien catchments area of France, demonstrates the efficiency of the technique.