Nitrogen patterns in subsurface waters of the Yzeron stream: effect of combined sewer overflows and subsurface-surface water mixing.

Urbanization subjects streams to increased nitrogen loads. Therefore studying nitrogen forms at the interface between urban stream and groundwater is important for water resource management. In this study we report results on water δ(18)O and nitrogen forms in subsurface waters of a stream (Yzeron, France). The sites studied were located upstream and downstream of combined sewer overflows (CSO) in a rural area and a periurban area, respectively. Water δ(18)O allowed us to follow the mixing of subsurface water with surface water. Dissolved organic nitrogen and organic carbon of fine sediment increased by 20-30% between rural and periurban subsurface waters in the cold season, under high flow. The highest nitrate levels were observed in rural subsurface waters in the cold season. The lowest nitrate levels were found in periurban subsurface waters in the warm season, under low flow. They corresponded to slow exchange of subsurface waters with channel water. Thus reduced exchange between surface and subsurface waters and organic-matter-rich input seemed to favor nitrate reduction in the downstream, periurban, subsurface waters impacted by CSO.

A single-substrate model to interpret intra-annual stable isotope signals in tree-ring cellulose.

The carbon and oxygen stable isotope composition of wood cellulose (delta(13)C(cellulose) and delta(18)O(cellulose), respectively) reveal well-defined seasonal variations that contain valuable records of past climate, leaf gas exchange and carbon allocation dynamics within the trees. Here, we present a single-substrate model for wood growth to interpret seasonal isotopic signals collected in an even-aged maritime pine plantation growing in South-west France, where climate, soil and flux variables were also monitored. Observed seasonal patterns in delta(13)C(cellulose) and delta(18)O(cellulose) were different between years and individuals, and mostly captured by the model, suggesting that the single-substrate hypothesis is a good approximation for tree ring studies on Pinus pinaster, at least for the environmental conditions covered by this study. A sensitivity analysis revealed that the model was mostly affected by five isotopic discrimination factors and two leaf gas-exchange parameters. Modelled early wood signals were also very sensitive to the date when cell wall thickening begins (t(wt)). Our model could therefore be used to reconstruct t(wt) time series and improve our understanding of how climate influences this key parameter of xylogenesis.

Seasonal variations in soil, grass and shrub water status in a West African humid savanna.

Despite their overwhelming influence on (1) variations in the physiological activity of vegetation (2) plant phenology, and (3) potential competitive interactions between coexisting species, comprehensive information on water relations and soil water uptake patterns is still lacking for grasses and shrubs in African humid savannas. Over 2 years, seasonal variations in plant water status were measured with the pressure bomb technique for two deciduous shrub species (Cussonia barteri and Crossopteryx febrifuga) and a perennial grass species (Hyparrhenia diplandra) in a humid savanna. Concurrently, soil moisture was surveyed with the neutron probe technique in grassy areas and under shrub clumps. Absence of transpiration at dawn was assessed by surveys of the leaf water isotopic signal, and plant water relations were characterized during rainy and dry periods with the pressure-volume curve technique. The seasonal changes in predawn and minimum shoot water potentials were pronounced for the grass species and the shrub C. febrifuga, but weak for the shrub C. barteri. Relationships between plant and soil water status showed that both the grass species and the shrub C. febrifuga did not maintain high water potentials when soil moisture in the upper soil layers decreased. In contrast, the shrub C. barteri exhibited a predawn water potential around -0.5 MPa when the 0 to 60-cm soil layer was at the permanent wilting point. During the early dry season, leaf dehydration was observed for C. febrifuga, but not for C. barteri. We conclude that the grass H. diplandra and the shrub C. febrifuga had very poor and limited access to deep soil layers (below 60 cm), respectively while the shrub C. barteri was able to access this water resource. The relatively low osmotic potential and tissue elasticity measured for C. febrifuga as compared to C. barteri were consistent with the use of water from upper soil layers by this species, since these characteristics could help water withdrawal and promote turgor maintenance at lower shoot water potentials. Different water uptake patterns could explain the contrasted leaf shedding patterns reported for the two shrub species. These results emphasize that the water economy of deciduous shrub species of African humid savannas can differ significantly and that both grasses and some shrub species acquire water from the upper soil layers even during dry spells. These findings will aid understanding of interspecific competition and modelling ecosystem function in this kind of humid savanna.

Spatial partitioning of the soil water resource between grass and shrub components in a West African humid savanna.

Most savanna water balance models assume water partitioning between grasses and shrubs in a two-layer hypothesis, but this hypothesis has not been tested for humid savanna environments. Spatial partitioning of soil water between grasses and shrubs was investigated in a West African humid savanna by comparing the isotopic composition (oxygen-18 and deuterium) of soil water and plant stem water during rainy and dry conditions. Both grass and shrub species acquire most of their water from the top soil layer during both rainy and dry periods. A shift of water uptake pattern towards deeper horizons was observed only at the end of the dry season after shrub defoliation. The mean depth of water uptake, as determined by the isotopic signature of stem water, was consistent with grass and shrub root profiles and with changes in soil water content profiles as surveyed by a neutron probe. This provides evidence for potentially strong competition between shrubs and grasses for soil water in these humid savannas. Limited nutrient availability may explain these competitive interactions. These results enhance our understanding of shrub-grass interactions, and will contribute to models of ecosystem functioning in humid savannas.

Spatial and temporal variability of (7)Be and (210)Pb wet deposition during four successive monsoon storms in a catchment of northern Laos.

Fallout radionuclides (7)Be and (210)Pb have been identified as potentially relevant temporal tracers for studying soil particles dynamics (surface vs. subsurface sources contribution; remobilization of in-channel sediment) during erosive events in river catchments. An increasing number of studies compared (7)Be: (210)Pb activity ratio in rainwater and sediment to estimate percentages of freshly eroded particles. However, the lack of data regarding the spatial and temporal variability of radionuclide wet deposition during individual storms has been identified as one of the main gaps in these estimates. In order to determine these key parameters, rainwater samples were collected at three stations during four storms that occurred at the beginning of the monsoon (June 2013) in the Houay Xon mountainous catchment in northern Laos. Rainwater (7)Be and (210)Pb activities measured using very low background hyperpure Germanium detectors ranged from 0.05 to 1.72 Bq L(-1) and 0.02 to 0.26 Bq L(-1), respectively. Water δ(18)O were determined on the same samples. Total rainfall amount of the four sampled storms ranged from 4.8 to 26.4 mm (51 mm in total) at the time-fractionated collection point. Corresponding cumulative (7)Be and (210)Pb wet depositions during the sampling period were 17.6 and 2.9 Bq m(-2), respectively. The (7)Be: (210)Pb activity ratio varied (1) in space from 6 to 9 for daily deposition and (2) in time from 3 to 12 for samples successively collected. Intra-event evolution of rainwater (7)Be and (210)Pb activities as well as δ(18)O highlighted the progressive depletion of local infra-cloud atmosphere radionuclide stock with time (washout), which remains consistent with a Raleigh-type distillation process for water vapour. Intra-storm ratio increasing with time showed the increasing contribution of rainout scavenging. Implications of such variability for soil particle labelling and erosion studies are briefly discussed and recommendations are formulated for the collection of rainwater signature in studies based on the (7)Be: (210)Pb ratio method, especially in tropical areas under high erosive pressure.

Non-steady-state, non-uniform transpiration rate and leaf anatomy effects on the progressive stable isotope enrichment of leaf water along monocot leaves.

This study focuses on the spatial patterns of transpiration-driven water isotope enrichment (Delta(lw)) along monocot leaves. It has been suggested that these spatial patterns are the result of competing effects of advection and (back-)diffusion of water isotopes along leaf veins and in the mesophyll, but also reflect leaf geometry (e.g. leaf length, interveinal distance) and non-uniform gas-exchange parameters. We therefore developed a two-dimensional model of isotopic leaf water enrichment that incorporates new features, compared with previous models, such as radial diffusion in the xylem, longitudinal diffusion in the mesophyll, non-uniform gas-exchange parameters and non-steady-state effects. The model reproduces well all published measurements of Delta(lw) along monocot leaf blades, except at the leaf tip and given the uncertainties on measurements and model parameters. We show that the longitudinal diffusion in the mesophyll cannot explain the observed reduction in the isotope gradient at the leaf tip. Our results also suggest that the observed differences in Delta(lw) between C(3) and C(4) plants reflect more differences in mesophyll tortuosity rather than in leaf length or interveinal distance. Mesophyll tortuosity is by far the most sensitive parameter and different values are required for different experiments on the same plant species. Finally, using new measurements of non-steady-state, spatially varying leaf water enrichment we show that spatial patterns are in steady state around midday only, just as observed for bulk leaf water enrichment, but can be easily upscaled to the whole leaf level, regardless of their degree of heterogeneity along the leaf.