Soil evaporation and organic matter turnover in the Sub-Taiga and Forest-Steppe of southwest Siberia.

Southwest Siberia encompasses the forest-steppe and sub-taiga climatic zones and has historically been utilized for agriculture. Coinciding with predicted changes in climate for the region is the pressure of agricultural development; however, a characterization of the soil water and carbon dynamics is lacking. We assessed current soil water properties and soil organic carbon turnover in forests and grasslands for two sites that span the forest steppe and sub-taiga bioclimatic zones. Soil evaporation was 0.62 ± 0.17 mm d-1 (mean ± standard error) in grasslands and 0.45 ± 0.08 mm d-1 in the forests of the forest-steppe site. Evaporation at the sub-taiga site was 1.80 ± 1.70 mm d-1 in grasslands and 0.96 ± 0.05 mm d-1 in forest plots. Evaporation was significantly greater at the sub-taiga site than the forest-steppe site. The density of fine roots explained the soil water isotopic patterns between vegetation types and sites. We found soil organic matter turnover to be three times faster in the sub-taiga site than in the forest-steppe site. Our results show that while climate factors, in particular snow levels, between the two sites are drivers for water and carbon cycles, site level hydrology, soil characteristics, and vegetation directly interact to influence the water and carbon dynamics.

Influence of root exudation of white lupine (Lupinus albus L.) on uranium phytoavailability in a naturally uranium-rich soil.

Mechanisms of uranium (U) transfer from soil to plants remain poorly understood. The kinetics of supply of U to the soil solution from solid phases could be a key point to understand its phytoavailability and implications for environmental risk assessment. Root activity, particularly the continuous release of organic acids in the rhizosphere, could have an effect on this supply. We tested the impact of citrate exudation by roots of Lupinus albus, either P-sufficient (P+) or P-deficient (P-), on the phytoavailability of U from a naturally contaminated soil (total content of 413 mg U kg-1) using a rhizotest design. Combined effects of P (P-/P+ used to modulate plant physiology) and citrate (model exudate) on the solubilization of U contained in the soils were tested in closed reactors (batch). The batch experiment showed the existence of a low U available pool (0.4% total U) and high accessibility (kd' around 20 L kg-1) which was not significantly affected by P treatment or citrate concentrations. Analysis of U, Fe, Ca, P and citrate concentrations in the batches suggested a complex combination of mechanisms and factors including desorption, resorption, precipitation, co-sorption. On rhizotest, L. albus plants extracted 0.5-0.75% of the total U and between 25 and 40% of the estimated available U present in the rhizotest in 5 days. Uranium accumulation at the whole plant level (20 mg U kg-1d.w., shoot to root ratio around 10-3) seemed to be dependent neither on the plant P nutrition status nor citrate exudation level, possibly in relation with the equivalent accessibility of U whatever the growth conditions. Yet differential translocation to shoots seemed to be positively correlated to citrate exudation.