Given the groundwater isotope data available at all sites apart from Dorset, there is little evidence that deeper water sources can help explain the xylem samples not potentially related to soil water sources . Furthermore, at Dorset the thin soil cover overlies what seems to be relatively unfractured bedrock. It is possible that some trees have roots that are tapping water held in fractures, but given the geology it is unlikely that there is sufficient storage to sustain a significant fraction of evapotranspiration.It is clear that some of the observed changes in xylem water throughout the growing season are related to phenological changes . This temporal correspondence partly reflects the “switching on” of plants in the spring as photosynthesis and transpiration increase as well as the availability and isotopic composition of soil water. Previous work by Sprenger et al. showed that variations in soil water isotopic composition at the study sites were mainly driven by precipitation and snowmelt over the preceding weeks, although there was also an effect of evaporation on kinetic fractionation of isotope ratios during summer. These dependencies highlight the importance of precipitation frequency and intensity, infiltration,30 plant pot soil wetness and the mixing interactions that govern soil water residence time distributions . The way in which processes and interactions relate to plant demand highlights the importance of the temporal integration of root uptake and water transport into the main plant stems.
The non-stationary travel times from uptake to transpiration may average many months , with tailing in the travel time distribution potentially a result of plant-stored water contributing to transpiration under dry conditions and possible mixing of xylem water with other plant water . The temporal trajectory of the xylem waters varied relative to soil water through the growing season, but this differed between angiosperms and gymnosperms. Also, inter-site contrasts between the angiosperm and gymnosperm differences were apparent: For Bruntland Burn, soil and xylem water signals were most similar in spring, deviated more strongly in summer and then returned to greater overlap in autumn for angiosperms. However, this was not the case for gymnosperms which showed dissimilarity throughout the year. For angiosperms at Dorset, there was a degree of overlap to start with, but depletion increased through summer and then closed again in autumn. In contrast, gymnosperm xylem waters became more 2 H- and 18O-enriched. At Dry Creek, there was a large difference through the autumn and winter for both angiosperms and gymnosperms until spring, but compositions became increasingly similar in summer. At Krycklan, angiosperms were most similar in the spring and early summer, but became increasingly different as the summer progressed. At Wolf Creek, there was an offset at the beginning of spring but samples then increasingly converged. This post-winter offset, also evident at Dry Creek, may relate to desiccation and/or diffusion within the plant during the biologically inactive period .
Inclusion of longer antecedent periods for soil isotope data generally improved overlaps within the boundary polygons for most sites, especially for angiosperms. The “sampling window” over which soil water may have been a source for plant uptake and contributed to xylem water in the trunk at breast height is unknown, and is likely to be non-stationary given seasonal variations in soil moisture and plant physiology. However, the greater overlaps for the longer antecedent period would support the hypothesis that xylem water at any point in time represents an integrated sample of soil water accumulating over preceding months, rather than soil water on the sampling day which will be most influenced by the most recent rainfall. In this sense, the results are similar to those of Allen et al. who demonstrated that trees throughout Switzerland predominantly use soil water derived from winter precipitation for summer transpiration. In our study, however, findings across sites and plant species were not consistent. Regardless, results from both studies suggest that caution should be used when constructing conceptual models of how plants access soil water based on synoptic, space-based sampling. Our phenologically-timed sampling strategy, particularly at such high latitude sites, is novel. However, more frequent sampling would likely be advantageous providing more nuanced insights into the phenological controls and short-term dynamics of xylem isotopes, particularly in relation to short term soil moisture dynamics and periods of higher atmospheric moisture demand . Nevertheless, higher-frequency sampling will still likely show that the xylem samples indicate stronger fractionation which has been widely shown for many vegetation types around the world . This focuses attention on potentially fractionating processes linked to small-scale interactions at the root-soil pore interface, especially close to the soil surface where most fine roots are present and where labile nutrients are also highest in acidic, organic soils.
However, methodological issues may at least partly explain some of the difference. These are discussed in the following section. Dry Creek stands out as an anomalous site in many results, most of which can be explained by its warm, dry conditions and high seasonality. Wolf Creek, however, the coldest site, shares similar results. The two sites obscure an otherwise clear relationship between plotting position along the GMWL and the mean annual temperature , they show the most overlap between xylem and soil water isotopes in bulk and at various depths , and they have the highest negative lc-excess values for both xylem and soil water . They also have the lowest May-August relative humidity at 38% and 63%, as well as precipitation at 19mm and 44mm, for Dry Creek and Wolf Creek, respectively . The relatively dry conditions shared by both sites expose soil waters to sustained evaporative environments, which may cause hydro-patterning of roots . Roots grow where water is available, which tends to be in less conductive pores where water has longer residence times and likely more isotopic fractionation due to evaporation. This evaporatively-enriched soil water also has limited potential for mixing with isotopically-different incoming precipitation that would alter its isotopic composition, partly because the growing-season precipitation at these sites is low. Accordingly, plant roots in dry environments have fewer soil water source options, so xylem water and bulk soil water will trend towards similar isotopic compositions.As noted above, one possibility is that exchange between the soil liquid and vapour phase is complex and may affect root water uptake. This may be either through roots being able to access a fractionated vapour phase and/or condensation onto soil surfaces from the soil atmosphere increasing the likelihood that plants take up water depleted in heavier isotopes, especially deuterium. Both recent field and modelling studies have highlighted the plausibility of such mechanisms, but mechanistic studies to test such a hypothesis are limited and urgently needed. Similarly, the complex interactions in the symbiotic relationship between mycorrhiza and plant roots cause uptake of more 2 H- and 18O-depleted water compared to bulk soil water. In particular, widespread arbuscular mycorrhizal fungi which penetrate the cortical cells in the roots of vascular plants may be an effective mechanism that can facilitate fractionation of root water uptake . This occurs as part of the complex symbiosis of nutrient exchange that also affects plant-water relationships and is focused in the upper soil horizons. Such mycorrhizal interactions are particularly important in nutrient poor minerogenic northern soils, and may have strong effects at sites like Bruntland Burn, Dorset and Krycklan. Again,grow raspberries in a pot more specific process-based studies are required to test this hypothesis in contrasting soil-plant systems. Finally, diffusion and evaporation through bark may be important biophysical processes, especially during winter when there is no transpiration . This is potentially a factor in northern regions where winter conditions preclude transpiration but can expose vegetation to arid conditions with high wind speeds and low humidity at sites like Dry Creek and Wolf Creek . Isotope transport through bark may explain why the gymnosperms at Dry Creek showed much greater overlap with the isotopic composition of soil water sampled over a range of antecedent intervalsin spring compared with Bruntland Burn, Dorset, and Krycklan where there was very little overlap.
However, this inter-site difference was less pronounced for angiosperms . b) Extraction of vegetation and soil water: We do not fully know what kind of vegetation water is mobilized by the cryogenic extraction, although it is usually assumed to characterise xylem water. However, it is likely that some of the water that gets extracted is part of live cells subject to potentially fractionating biophysical processes that are independent of the hydrological cycle. Zhao et al. saw large differences between xylem sap, extracted with a syringe, and twig water extracted via cryogenic extraction with the former being more enriched in 2 H compared to the latter. In such cases, differences in the ratio of cell water to xylem water, which would depend on soil wetness, could have an effect on the differences between the isotopic composition of plant water and cryogenically extracted water . Barbeta et al. support this interpretation and call for more specific characterisation of what is assumed to be extracted xylem water. Very recent experimental work by Chen et al. showed that cryogenic extraction can enhance deuterium exchange with organically bound water and contribute to the deuterium depletion. Moreover, they showed the effect can be greatest under more moisture-limited conditions which may explain the tendency for more negative swexcess values as sites become drier. Physiological and biochemical differences between angiosperms and gymnosperms may also contribute to differences in extraction effects . As with vegetation water extraction, differences from contrasting soil extraction techniques may explain some of the mis-match between observed xylem water and soil sources. For example, the similarities between soil and xylem water at Dry Creek involved cryogenic extraction of soils, whereas all other sites used equilibration. However, at Bruntland Burn cryogenic and equilibration methods gave similar results for peaty soils, and reasonable agreement with xylem water . Extraction focusing on small-scale moisture isotope dynamics at the root – soil interface may be needed, including scalable methods to explore the phase change/mycorrhizal mechanisms suggested above. Our findings, based on bulk soil field measurements, underline the major difficulties associated with relating potential water sources to plant water stable isotope compositions. Even under controlled laboratory conditions, Orlowski et al. could not confidently link relate the soil water to root crown isotopic compositions, but reported similar 2 H depletion as we found in Dandelions growing on sandy soils. c) Differences between angiosperms vs gymnosperms: A clear finding of our study is that the extracted xylem waters of angiosperms and gymnosperms have a very different isotopic composition at most sites, with gymnosperms generally showing a greater degree of fractionation. In this regard, several hypotheses could be tested. Firstly, root networks and root-mycorrhizal networks of different species may be able to access different pore sizes. For example, gymnosperms may have greater potential to mobilize water that has undergone some fractionation during the interactions among water, gas, and solid phases of the soil. Secondly, storage and mixing of water within plant tissues may be greater in softwood gymnosperms, as suggested in recent modelling work . The generally slower metabolism and transpiration rates for gymnosperms might exacerbate this mechanism. Interestingly, Amin et al. showed little difference between angiosperms and gymnosperm xylem waters for cold and temperate environments in their meta-analysis, whereas angiosperms in arid regions were offset in δ2 H compared to gymnosperms. Understanding the mechanisms of adaptive evolution in pathogenic bacteria is central to long-term disease control. One major focus of research into adaptive bacterial evolution has been lateral gene transfer , usually defined as the transfer of genes across species boundaries . Until recently, discussions of LGT focused on the transfer of novel genes, as exemplified by the discovery of the plasmid-mediated transfer across species of the genes coding for penicillin resistance ; however, with the increasing availability of genomic sequence data, it has become apparent that the transfer of homologous gene copies is also widespread . These two kinds of exchange, the transfer of novel genes or novel alleles, are fundamentally different. The acquisition of novel genes can result in the acquisition of a completely new trait that has already been refined in other taxa by natural selection . It can determine critical traits such as virulence, antibiotic resistance, and ecological niche , even though most of the material transferred appears to be evolutionarily transient .