The Global Land-Atmosphere Climate Experiment-Coupled Model Intercomparison Project phase 5 (GLACE-CMIP5) is a multimodel experiment investigating the impact of soil moisture-climate feedbacks in CMIP5 projections. We present here first GLACE-CMIP5 results based on five Earth System Models, focusing on impacts of projected changes in regional soil moisture dryness (mostly increases) on late 21st century climate. Projected soil moisture changes substantially impact climate in several regions in both boreal and austral summer. Strong and consistent effects are found on temperature, especially for extremes (about 1-1.5 K for mean temperature and 2-2.5 K for extreme daytime temperature). In the Northern Hemisphere, effects on mean and heavy precipitation are also found in most models, but the results are less consistent than for temperature. A direct scaling between soil moisture-induced changes in evaporative cooling and resulting changes in temperature mean and extremes is found in the simulations. In the Mediterranean region, the projected soil moisture changes affect about 25% of the projected changes in extreme temperature. Key Points GLACE-CMIP5 quantifies soil moisture feedbacks in climate projections Impacts on late 21st century temperature and precipitation mean and extremes Effects of about 25% for temperature extremes in Mediterranean region © 2013 The Authors. Geophysical Research Letters published by Wiley on behalf of the American Geophysical Union.

Impact of soil moisture-climate feedbacks on CMIP5 projections: First results from the GLACE-CMIP5 experiment

. Evaporation ( E ) and transpiration ( T ) respond differently
to ongoing changes in climate, atmospheric composition, and land use. It is
difficult to partition ecosystem-scale evapotranspiration (ET) measurements
into E and T , which makes it difficult to validate satellite data and land
surface models. Here, we review current progress in partitioning E and T and
provide a prospectus for how to improve theory and observations going
forward. Recent advancements in analytical techniques create new
opportunities for partitioning E and T at the ecosystem scale, but their
assumptions have yet to be fully tested. For example, many approaches to
partition E and T rely on the notion that plant canopy conductance and
ecosystem water use efficiency exhibit optimal responses to atmospheric
vapor pressure deficit ( D ). We use observations from 240 eddy covariance flux
towers to demonstrate that optimal ecosystem response to D is a reasonable
assumption, in agreement with recent studies, but more analysis is necessary
to determine the conditions for which this assumption holds. Another
critical assumption for many partitioning approaches is that ET can be
approximated as T during ideal transpiring conditions, which has been
challenged by observational studies. We demonstrate that T can exceed 95 %
of ET from certain ecosystems, but other ecosystems do not appear to reach
this value, which suggests that this assumption is ecosystem-dependent with
implications for partitioning. It is important to further improve approaches
for partitioning E and T , yet few multi-method comparisons have been
undertaken to date. Advances in our understanding of carbon–water coupling
at the stomatal, leaf, and canopy level open new perspectives on how to
quantify T via its strong coupling with photosynthesis. Photosynthesis can be
constrained at the ecosystem and global scales with emerging data sources
including solar-induced fluorescence, carbonyl sulfide flux measurements,
thermography, and more. Such comparisons would improve our mechanistic
understanding of ecosystem water fluxes and provide the observations
necessary to validate remote sensing algorithms and land surface models to
understand the changing global water cycle.

/pdf/reviews-and-syntheses-turning-the-challenges-of-partitioning-1fbk4vumon.pdf

Reviews and syntheses: Turning the challenges of partitioning ecosystem evaporation and transpiration into opportunities

For the past decade, observations of carbonyl sulfide (OCS or COS) have been investigated as a proxy for carbon uptake by plants. OCS is destroyed by enzymes that interact with CO2 during photosynthesis, namely carbonic anhydrase (CA) and RuBisCO, where CA is the more important one. The majority of sources of OCS to the atmosphere are geographically separated from this large plant sink, whereas the sources and sinks of CO2 are co-located in ecosystems. The drawdown of OCS can therefore be related to the uptake of CO2 without the added complication of co-located emissions comparable in magnitude. Here we review the state of our understanding of the global OCS cycle and its applications to ecosystem carbon cycle science. OCS uptake is correlated well to plant carbon uptake, especially at the regional scale. OCS can be used in conjunction with other independent measures of ecosystem function, like solar-induced fluorescence and carbon and water isotope studies. More work needs to be done to generate global coverage for OCS observations and to link this powerful atmospheric tracer to systems where fundamental questions concerning the carbon and water cycle remain.

/pdf/reviews-and-syntheses-carbonyl-sulfide-as-a-multi-scale-rw8hcitzfa.pdf

Reviews and syntheses: Carbonyl sulfide as a multi-scale tracer for carbon and water cycles

The hydrogen isotope ratio of water cryogenically extracted from plant stem samples (δ2Hstem_CVD) is routinely used to aid isotope applications that span hydrological, ecological, and paleoclimatological research. However, an increasing number of studies have shown that a key assumption of these applications—that δ2Hstem_CVD is equal to the δ2H of plant source water (δ2Hsource)—is not necessarily met in plants from various habitats. To examine this assumption, we purposedly designed an experimental system to allow independent measurements of δ2Hstem_CVD, δ2Hsource, and δ2H of water transported in xylem conduits (δ2Hxylem) under controlled conditions. Our measurements performed on nine woody plant species from diverse habitats revealed a consistent and significant depletion in δ2Hstem_CVD compared with both δ2Hsource and δ2Hxylem. Meanwhile, no significant discrepancy was observed between δ2Hsource and δ2Hxylem in any of the plants investigated. These results cast significant doubt on the long-standing view that deuterium fractionation occurs during root water uptake and, alternatively, suggest that measurement bias inherent in the cryogenic extraction method is the root cause of δ2Hstem_CVD depletion. We used a rehydration experiment to show that the stem water cryogenic extraction error could originate from a dynamic exchange between organically bound deuterium and liquid water during water extraction. In light of our finding, we suggest caution when partitioning plant water sources and reconstructing past climates using hydrogen isotopes, and carefully propose that the paradigm-shifting phenomenon of ecohydrological separation (“two water worlds”) is underpinned by an extraction artifact.

https://www.pnas.org/content/pnas/117/52/33345.full.pdf

Stem water cryogenic extraction biases estimation in deuterium isotope composition of plant source water

A growing number of field studies report isotopic offsets between stem water and its potential sources that prevent the unambiguous identification of plant water origin using water isotopes. We explored the causes of this isotopic offset by conducting a controlled experiment on the temperate tree species Fagus sylvatica. We measured δ2 H and δ18 O of soil and stem water from potted saplings growing on three soil substrates and subjected to two watering regimes. Regardless of substrate, soil and stem water δ2 H were similar only near permanent wilting point. Under moister conditions, stem water δ2 H was 11 ± 3‰ more negative than soil water δ2 H, coherent with field studies. Under drier conditions, stem water δ2 H became progressively more enriched than soil water δ2 H. Although stem water δ18 O broadly reflected that of soil water, soil-stem δ2 H and δ18 O differences were correlated (r = 0.76) and increased with transpiration rates indicated by proxies. Soil-stem isotopic offsets are more likely to be caused by water isotope heterogeneities within the soil pore and stem tissues, which would be masked under drier conditions as a result of evaporative enrichment, than by fractionation under root water uptake. Our results challenge our current understanding of isotopic signals in the soil-plant continuum.

/pdf/an-explanation-for-the-isotopic-offset-between-soil-and-stem-3nv759bmyg.pdf

An explanation for the isotopic offset between soil and stem water in a temperate tree species.

. We investigated plant water sources of an emblematic refugial
population of Fagus sylvatica (L.) in the Ciron river gorges in south-western France using
stable water isotopes. It is generally assumed that no isotopic
fractionation occurs during root water uptake, so that the isotopic
composition of xylem water effectively reflects that of source water.
However, this assumption has been called into question by recent studies
that found that, at least at some dates during the growing season, plant
water did not reflect any mixture of the potential water sources. In this
context, highly resolved datasets covering a range of environmental
conditions could shed light on possible plant–soil fractionation processes
responsible for this phenomenon. In this study, the hydrogen ( δ2H ) and oxygen ( δ18O ) isotope compositions of all
potential tree water sources and xylem water were measured fortnightly over
an entire growing season. Using a Bayesian isotope mixing model (MixSIAR),
we then quantified the relative contribution of water sources for F. sylvatica and
Quercus robur (L.) trees. Based on δ18O data alone, both species used a mix
of top and deep soil water over the season, with Q. robur using deeper soil water
than F. sylvatica. The contribution of stream water appeared to be marginal despite the
proximity of the trees to the stream, as already reported for other riparian
forests. Xylem water δ18O could always be interpreted as a
mixture of deep and shallow soil waters, but the δ2H of xylem
water was often more depleted than the considered water sources. We argue
that an isotopic fractionation in the unsaturated zone and/or within the
plant tissues could underlie this unexpected relatively depleted δ2H of xylem water, as already observed in halophytic and xerophytic
species. By means of a sensitivity analysis, we found that the estimation of
plant water sources using mixing models was strongly affected by this
 δ2H depletion. A better understanding of what causes this
isotopic separation between xylem and source water is urgently needed.

https://hess.copernicus.org/articles/23/2129/2019/hess-23-2129-2019.pdf

Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest

Disentangling the rates of carbonyl sulphide (COS) production and consumption and their dependency with soil properties across biomes and land use types

. We investigated plant-water sources of an emblematic refugial population of Fagus sylvatica (L.) in the Ciron river gorges in South-Western France using stable isotopes. The stable isotopes of water are a powerful tracer of water fluxes in the soil-plant-atmosphere continuum. It is generally assumed that no isotopic fractionation occurs during root water uptake, and that xylem water isotopes effectively reflect source water isotopes. However, recent studies showed that under certain conditions the isotopes in plant water do not reflect any of the potential sources considered. Highly resolved datasets covering a range of environmental conditions could shed light on possible plant-soil fractionations processes. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured bi-weekly over an entire growing season. Using Bayesian isotope mixing models (MixSIAR), we then quantified the contribution of the considered sources to xylem water of F. sylvatica and Quercus robur (L.) trees. Based on δ18O data alone, both species used a mix of top and deep soil water over the season, with Q. robur using soil water relatively deeper than F. sylvatica. The contribution of stream water appeared to be marginal despite the proximity of the trees to the stream, as already reported for other riparian forests. Xylem water δ18O could always be interpreted as a mixture of deep and shallow soil waters, but the δ2H of xylem water was often more depleted than any other possible water source. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected δ2H depletion of xylem water, as already observed in halophytic and xerophytic species. By means of a sensitivity analysis, we found that the estimation of plant-water sources using isotope mixing models was largely affected by this isotopic δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.

/pdf/hydrogen-isotope-fractionation-affects-the-identification-10r7g1y0tp.pdf

Steve Wohl

Papers

Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest

Disentangling the rates of carbonyl sulphide (COS) production and consumption and their dependency with soil properties across biomes and land use types

Hydrogen isotope fractionation affects the identification andquantification of tree water sources in a riparian forest