Alon Angert

Seasonal variations in the isotopic composition of near-surface water vapour in the eastern Mediterranean

Although the isotopic composition of precipitation is widely used in global climate change studies, use of water vapour isotopes is considerably more limited. Here we present the results from 9 yr of atmospheric vapour measurements in the Eastern Mediterranean, at a site in Israel. The measurements show a strong mean seasonal cycle of about 4‰ in 18O (peaking around July). This seasonality could not be adequately explained by changes in surface interactions or in air mass trajectories, as usually invoked for variations in local precipitation. We could explain this cycle only as a combination of three components: (1) rainout effects; (2) temperature and relative humidity control of the initial vapour and (3) seasonal variations in the vertical mixing across the top of the planetary boundary layer. This last component is emphasized in the current study, and it was shown to be a significant factor in the seasonal cycle features. The measurements were also compared with an isotope-enabled GCM (CAM2) run, which exhibited a markedly different seasonal cycle. Such comparisons with vapour isotopes data could help in constraining models better.

CO2 seasonality indicates origins of post-Pinatubo sink

was observed in the early 1990’s (http://www.cmdl.noaa.gov/ccgg/). In 1992, the year that followed the eruption ofMount Pinatubo (June, 1991), the growth rate was thelowest for the period 1983–2003 indicating an enhancedcarbon sink of 2PgC/yr [Bousquet et al., 2000]. Thisenhanced sink has been linked to the climatic effects ofthe eruption. Global temperatures dropped by 0.4 Cin1992. Superimposed over the global cooling, was additionalcooling over most of North-America, and warming andcooling over different regions of Eurasia [Hansen et al.,1996]. These climatic anomalies are believed to result fromthe injection of 20Mt of SO

Fractionation of oxygen isotopes by respiration and diffusion in soils and its implications for the isotopic composition of atmospheric O2

The 18O content of atmospheric O2 is an important tracer for past changes in the biosphere and has been used to estimate changes in the balance between terrestrial and marine productivity. Its quantitative use depends on knowledge of the isotopic fractionations associated with the various O2 production and consumption processes. Here we monitored oxygen concentration and δ18O of O2 in sandy and clayey soils to evaluate in situ 18O fractionation associated with soil respiration. In the clayey soil, O2 concentrations decreased as low as 1% at 150 cm depth, and δ18O values ranged from 0‰ to – 1.6‰ relative to atmospheric O2. In the sandy soil the O2 concentration was 20.38−20.53‰, and δ18O values were −0.06 ± 0.015‰ to 0.06 ± 0.015‰ relative to atmospheric O2. Using the observed [O2] and δ18O profiles and their change with time, together with a one-box analytical model and a five-box numerical model, a mean discrimination of 12 ± 1% was estimated for the two sites (including effects of concentration and temperature gradients). This low discrimination was consistent with that determined in closed-system soil incubation experiments (8.4–16.9‰). The current understanding of the composition of air O2 attributes the magnitude of the fractionation in soil respiration to biochemical mechanisms alone (about 18‰ and 25–30‰ in cyanide-sensitive and cyanide-resistant respiration, respectively). The low discrimination we report is significantly less than in dark respiration and is explained by diffusion limitation in soil aggregates and root tissues that results in low O2 concentration in the consumption site. Soil respiration is a major component of the global oxygen uptake, and the potential contribution of low discrimination, such as observed here, to the global Dole effect should be considered in global-scale studies.

Fractionation of oxygen isotopes by root respiration: Implications for the isotopic composition of atmospheric O2

Abstract The ratio of 18O/16O in atmospheric oxygen depends on the isotopic composition of the substrate water used in photosynthesis and on discrimination against 18O in respiratory consumption. The current understanding of the composition of air O2 attributes the magnitude of the respiratory fractionation to biochemical mechanisms alone. Thus the discrimination against 18O is assumed as 18‰ in normal dark respiration and 25‰ to 30‰ in cyanide resistant respiration. Here we report new results on the fractionation of O2 isotopes in root respiration. The isotopic fractionation was determined from the change in δ18O of air due to partial uptake by roots in closed containers. The discrimination in these experiments was in the range of 11.9‰ to 20.0‰ with an average of 14.5‰. This average is significantly less than the known discrimination in dark respiration. A simple diffusion-respiration model was used to explain the isotopic discrimination in roots. Available data show that O2 concentration inside roots is low due to slow diffusion. As a result, due to diffusion and biological uptake at the consumption site inside the root, the overall discrimination is small. Root respiration is an important component of the global oxygen uptake. Our new result that the discrimination against 18O is less than generally thought indicates that the mechanisms affecting δ18O of atmospheric oxygen should be re-evaluated.

Contribution of soil respiration in tropical, temperate, and boreal forests to the 18O enrichment of atmospheric O2

[1] The 18 O content of atmospheric O2 is an important tracer for past changes in the biosphere. Its quantitative use depends on knowledge of the discrimination against 18 O associated with the various O2 consumption processes. Here we evaluated, for the first time, the in situ 18 O discrimination associated with soil respiration in natural ecosystems. The discrimination was estimated from the measured [O2] and d 18 Oo f O2 in the soilair. The discriminations that were found are 10.1 ± 1.5%, 17.8 ± 1.0%, and 22.5 ± 3.6%, for tropical, temperate, and boreal forests, respectively, 17.9 ± 2.5% for Mediterranean woodland, and 15.4 ± 1.6% for tropical shrub land. Current understanding of the isotopic composition of atmospheric O2 is based on the assumption that the magnitude of the fractionation in soil respiration is identical to that of dark respiration through the cytochrome pathway alone (� 18%). The discrimination we found in the tropical sites is significantly lower, and is explained by slow diffusion in soil aggregates and root tissues that limits the O2 concentration in the consumption sites. The high discrimination in the boreal sites may be the result of high engagement of the alternative oxidase pathway (AOX), which has high discrimination associated with it (� 27%). The intermediate discrimination (� 18%) in the temperate and Mediterranean sites can be explained by the opposing effects of AOX and diffusion limitation that cancel out. Since soil respiration is a major component of the global oxygen uptake, the contribution of large variations in the discrimination, observed here, to the global Dole Effect should be considered in global scale studies. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/ atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 1040 Geochemistry: Isotopic composition/chemistry; 1615 Global Change: Biogeochemical processes (4805); KEYWORDS: Dole Effect, oxygen isotopes, soil respiration Citation: Angert, A., et al., Contribution of soil respiration in tropical, temperate, and boreal forests to the 18 O enrichment of

Soil Phosphate Stable Oxygen Isotopes across Rainfall and Bedrock Gradients

The stable oxygen isotope compositions of soil phosphate (δ(18)O(p)) were suggested recently to be a tracer of phosphorus cycling in soils and plants. Here we present a survey of bioavailable (resin-extractable or resin-P) inorganic phosphate δ(18)O(p) across natural and experimental rainfall gradients, and across soil formed on sedimentary and igneous bedrock. In addition, we analyzed the soil HCl-extractable inorganic δ(18)O(p), which mainly represents calcium-bound inorganic phosphate. The resin-P values were in the range 14.5-21.2‰. A similar range, 15.6-21.3‰, was found for the HCl-extractable inorganic δ(18)O(p), with the exception of samples from a soil of igneous origin that show lower values, 8.2-10.9‰, which indicate that a large fraction of the inorganic phosphate in this soil is still in the form of a primary mineral. The available-P δ(18)O(p) values are considerably higher than the values we calculated for extracellular hydrolysis of organic phosphate, based on the known fractionation from lab experiments. However, these values are close to the values expected for enzymatic-mediated phosphate equilibration with soil-water. The possible processes that can explain this observation are (1) extracellular equilibration of the inorganic phosphate in the soil; (2) fractionations in the soil are different than the ones measured at the lab; (3) effect of fractionation during uptake; and (4) a flux of intercellular-equilibrated inorganic phosphate from the soil microbiota, which is considerably larger than the flux of hydrolyzed organic-P.

Use of Phosphate Oxygen Isotopes for Identifying Atmospheric‑P Sources: A Case Study at Lake Kinneret

The input of phosphorus (P) through atmospheric deposition can be a major source of P to fresh water bodies and may strongly affect their biogeochemistry. In Lake Kinneret (LK), northern Israel, dust deposition provides a significant fraction of the bioavailable P input. Here, we demonstrate that the oxygen isotopic composition of resin-extractable inorganic phosphate (δ(18)OP) in dust particles can be used to identify the phosphate source. Samples of soils with both natural vegetation and agricultural cover were collected upwind of LK and found to have distinct δ(18)OP value ranges (17.4-18.2‰ and 19.3-22.1‰, respectively). The δ(18)OP values for dust, collected continuously over LK during June 2011 to March 2012, were in the same range as agricultural soils. The dust concentration in the air decreased from the dry to the wet season and was correlated with a decrease in P concentration in air, yet no correlation was found between these parameters and dust δ(18)OP. Dust deposited during short-term desert dust events was characterized by a combination of high δ(18)OP values ranging from 22.2‰ to 22.7‰ and high concentrations of dust in the air. The data we present demonstrates a new application of δ(18)OP measurements for direct estimation of dust-P sources to lakes, as well as the potential for tracing dust-P on larger scales.

A method for analyzing the δ18O of resin‐extractable soil inorganic phosphate

Improved tools for tracing phosphate transformations in soils are much needed, and can lead to a better understanding of the terrestrial phosphorus cycle. The oxygen stable isotopes in soil phosphate are still not exploited in this regard. Here we present a method for measuring the oxygen stable isotopes in a fraction of the soil phosphate which is rapidly available to plants, the resin-extractable P. This method is based on extracting available phosphate from the soil with anion-exchange membranes, soil organic matter removal by a resin, purification by precipitation as cerium phosphate, and finally precipitation as silver phosphate. The purified silver phosphate samples are then measured by a high-temperature elemental analyzer (HT-EA) coupled in continuous flow mode to an isotope ratio mass spectrometer. Testing the method with Mediterranean and semi-arid soils showed no artifacts, as well as good reproducibility in the same order as that of the HT-EA analytical uncertainty (0.3‰).

What's the flux? Unraveling how CO2 fluxes from trees reflect underlying physiological processes

Forum Commentary What’s the flux? Unraveling how CO 2 fluxes from trees reflect underlying physiological processes Tree stems and branches emit carbon dioxide (CO 2 ) at rates that per unit area can rival emissions from leaves or the soil surface and summed over a forest stand can comprise 14–30% of the total CO 2 efflux (Chambers et al., 2004; Ryan et al., 2009). Stem CO 2 fluxes have predictable patterns of variation with growth rate, stand age, and elevation (Chambers et al., 2004; Ryan et al., 2009; Robertson et al., 2010). Over the past decade observations of diel covariation of CO 2 efflux with sapflux rates measured in tree stems have led to the conclusion that internal transport of CO 2 within the stem strongly influences the measured CO 2 efflux at the surface (Teskey et al., 2008). In this issue of New Phytologist, Bloemen et al. (pp. 555–565) report on a tracer experiment that demonstrates not only upward transport of 13 CO 2 added to the transpiration stream, and emission of this label along the stem, but also fixation of a significant fraction of the added CO 2 in canopy branches, petioles and, to a minor extent, leaves. The study of Bloemen et al. adds to the growing literature that demonstrates the utility of isotope labeling studies to understand allocation and carbon (C) cycling in trees (Powers & Marshall, 2011; Epron et al., 2012). ‘Dynamic approaches for measuring continuous diurnal CO 2 fluxes and transport in the transpiration stream need to be more widely applied.’ Processes influencing stem CO 2 efflux A number of factors can influence the efflux of CO 2 measured by a flux chamber covering a segment of tree stem (Fig. 1). The cambium is the site of formation of new tissue, that is, of growth, while maintenance respiration produces CO 2 in all living tissues. The C being respired may derive from recent photosynthetic products transported in the phloem (e.g. Powers & Marshall, 2011) and from storage reserves. The pathways for respiration may vary with time or tree species: recently 18 O/ 16 O measurements in oxygen (O 2 ) provided the first evidence for the alternative oxidase pathway contributing to respiration in some tree stems (Angert et al., 2012a). CO 2 may also be locally fixed by photosynthetic tissues found under the bark before it is lost to the atmosphere. O 2013 The Authors New Phytologist O 2013 New Phytologist Trust Low rates of diffusion, especially across the cambium, can cause high CO 2 concentrations in stems, and internal O 2 concentrations can drop to very low levels (Spicer & Holbrook, 2005; Teskey et al., 2008). CO 2 is highly soluble, and will dissolve in (or exsolve from) stem water, depending on local saturation conditions, which in turn are controlled by factors such as temperature and pH. Uptake of CO 2 directly from the soil atmosphere, once thought potentially important, has largely been shown to be minor (see summary in Bloemen et al.). Hence the source of CO 2 emitted to the atmosphere from the bark surface can reflect a combination of local growth and maintenance respiration, other local processes producing CO 2 (including potentially decomposition in heartwood) or CO 2 from respiration in other tissues (e.g. roots) that has been transported into the volume beneath a chamber in solution. However, there can also be net export in the xylem water stream, as indicated by the fate of the tracer added by Bloemen et al. The measured chamber flux at any given time is thus the complex result of transport in, transport out and respiration minus photosynthesis in local tissues. Use of a dark chamber will exclude local photosynthesis. Observations of a relationship between sapflux and CO 2 efflux provide a clue as to whether CO 2 is net imported or exported from the volume of stem under a chamber attached to the stem surface (see Fig. 1, modified from Teskey et al., 2008). Other evidence for net CO 2 transport away from the region of efflux measurement comes from lower-than-expected efflux rates compared with what is expected given the construction costs of wood (Ryan et al., 2009), and potentially from higher efflux rates in canopy branches (Teskey et al., 2008). Changes in local temperature and/or pH can change respiration rates and also cause changes in CO 2 solubility (Kunert & Mercado Ca´rdenas, 2012). Stem anatomy, including bark thickness and tree hydraulics, likely influences the importance of the mechanisms and can help explain observations such as changes in CO 2 efflux with stand age or tree size, or differences between similar trees growing in different environments (Ryan et al., 2009). Bloemen et al. report results from labeling Populus deltoides, the eastern cottonwood tree, which has very high transpiration rates and generally is found in riparian zones. As noted by Ubierna et al. (2009) most studies that have reported relationships between sapflux and CO 2 efflux have been made in tree species with high sapflux rates and small conducting area. By contrast, the large conifer trees investigated by Ubierna et al. (2009), with lower overall sapflux, did not demon- strate such relationships, and even crown removal did not change the rates of CO 2 efflux from stems they studied. What do these results mean for interpretation of other ecosystem CO 2 efflux measurements? A major conclusion of Bloemen et al. is that the transport of the tracer from the tree base to the canopy indicates that root respiration New Phytologist (2013) 197: 353–355 353 www.newphytologist.com

Carbon dioxide emitted from live stems of tropical trees is several years old

Storage carbon (C) pools are often assumed to contribute to respiration and growth when assimilation is insufficient to meet the current C demand. However, little is known of the age of stored C and the degree to which it supports respiration in general. We used bomb radiocarbon ((14)C) measurements to determine the mean age of carbon in CO2 emitted from and within stems of three tropical tree species in Peru. Carbon pools fixed >1 year previously contributed to stem CO2 efflux in all trees investigated, in both dry and wet seasons. The average age, i.e., the time elapsed since original fixation of CO2 from the atmosphere by the plant to its loss from the stem, ranged from 0 to 6 years. The average age of CO2 sampled 5-cm deep within the stems ranged from 2 to 6 years for two of the three species, while CO2 in the stem of the third tree species was fixed from 14 to >20 years previously. Given the consistency of (14)C values observed for individuals within each species, it is unlikely that decomposition is the source of the older CO2. Our results are in accordance with other studies that have demonstrated the contribution of storage reserves to the construction of stem wood and root respiration in temperate and boreal forests. We postulate the high (14)C values observed in stem CO2 efflux and stem-internal CO2 result from respiration of storage C pools within the tree. The observed age differences between emitted and stem-internal CO2 indicate an age gradient for sources of CO2 within the tree: CO2 produced in the outer region of the stem is younger, originating from more recent assimilates, whereas the CO2 found deeper within the stem is older, fueled by several-year-old C pools. The CO2 emitted at the stem-atmosphere interface represents a mixture of young and old CO2. These observations were independent of season, even during a time of severe regional drought. Therefore, we postulate that the use of storage C for respiration occurs on a regular basis challenging the assumption that storage pools serve as substrates for respiration only during times of limited assimilation.