Soil Water Stable Isotopes Research Articles

Alterations in soil moisture dynamics due to open-pit coal mining semi-arid regions: Perceptions based on soil water stable isotopes and underground water conductivity analysis

Alterations in soil moisture dynamics due to open-pit coal mining semi-arid regions: Perceptions based on soil water stable isotopes and underground water conductivity analysis

Characterization of spatiotemporal patterns of soil water stable isotopes at an agricultural field

Spatiotemporal variations in soil water content (SWC) and soil water stable isotopic compositions (SWSIC; 2H/1H (δD) and 18O/16O (δ18O)) provide critical information on elucidating land surface processes across scales. Meanwhile, little is known about the spatiotemporal characteristics of SWSIC and its driving factors. Therefore, it's necessary to improve tracer techniques of SWSIC by interpreting their spatiotemporal variability patterns as well as the correlations with other factors such as texture, soil depth and vegetation. To this end, the spatiotemporal variations in SWC and SWSIC along with their controlling factors were jointly investigated based on seven field campaigns over roughly a two-year period at an agricultural field in North China Plain. Two transects, vegetated and bared, were considered. The results of vegetated transect showed that both SWC and SWSIC exhibited considerable spatiotemporal variabilities at the field scale of ~100 m, with SWSIC displaying more complex patterns. Overall, the spatial variations in SWSIC were larger in wet seasons than in dry seasons, which decreased with increasing soil depth, largely due to less impacts of precipitation inputs and soil evaporation on SWSIC dynamics at deeper depths. The temporal stability analysis (TSA) showed that there existed temporal persistence of the spatial structure of SWSIC, particularly at deeper soil depths. Moreover, the SWSIC data in our study showed that the effect of vegetation on the SWSIC dynamics was noticeable with shading effects, root distribution and water uptake, which caused much lesser degrees of soil evaporation at the vegetated transect. What's more, the representative sites for monitoring spatial average δD values were identified, demonstrating the viability of using the TSA method to estimate the spatial average SWSIC values at field scales. These findings can improve the interpretation of SWSIC data for practical applications.

Deuterium depletion in xylem water and soil isotopic effects complicate the assessment of riparian tree water sources in the seasonal tropics

AbstractRiparian trees located in seasonally dry environments may be reliant on groundwater supplies, but the prevalence and magnitude of groundwater uptake is often unclear. Using soil water matric potential and water stable isotopes, we examined the relative contributions of soil water and groundwater to the dry season water uptake of five riparian tree species along an intermittent river of tropical northern Australia. Because xylem water was depleted in deuterium relative to source water (average offset −14.0‰), we numerically removed this offset and assessed the effect of the correction on mixing model results. We also estimated the isotopic composition of unbound soil water (i.e., the portion of soil water not tightly bound to soil particles) from bulk soil water data by using an empirical formulation from the literature and tested whether considering unbound soil water as a source would affect our results. Despite the hot and dry surface environment, we found that soil moisture was available for trees at relatively shallow (~0.7–1.5 m) depths. When unbound soil water and corrected xylem water data were considered, most tree species used a combination of this soil moisture source and groundwater from the capillary fringe. However, not correcting for isotopic effects resulted in large underestimations of the groundwater contributions to tree water uptake. Our findings suggest that ignoring soil isotopic effects and deuterium depletion in xylem water may reduce the validity of source water partitioning assessments. Further research is needed on the likely causes for deuterium depletion in xylem water.

Soil Water Stable Isotopes Reveal Surface Soil Evaporation Loss Dynamics in a Subtropical Forest Plantation

Line-conditioned excess (lc-excess), the deviation of the relationship between δD and δ18O in soil water from that of precipitation, is often used to indicate soil evaporation loss, but the conditions of using lc-excess under the influences of precipitation infiltration or percolation had not been identified. The interaction effects of climate, soil and vegetation on soil evaporation in forests are not well known. We collected soil water at 0–5, 15–20 and 40–45 cm depths and event-based precipitation from 2011 to 2015 in a subtropical forest plantation and calculated the lc-excess. Precipitation on the sampling day and percolation of upper soil water with low lc-excess affected the capacity of the lc-excess to indicate the soil evaporation fractionation signals. Lc-excess of soil water at 0–5 cm depth indicated a reliable soil evaporation loss estimate over 30 days prior to the sampling day. Soil evaporation loss was dominated by the cumulative soil temperature (Tss) during drought periods and was dominated by the relative soil water content (RSWC) during non-drought periods. High Tss decreased soil evaporation loss by increasing transpiration and relative humidity. Our results emphasize the importance of sampling the upper-most soil layer when there is no rain and vegetation during drought periods in forests when studying soil evaporation loss dynamics.

Recharge and Infiltration Mechanisms of Soil Water in the Floodplain Revealed by Water-Stable Isotopes in the Upper Yellow River

The stable isotopes (δD and δ18O) in soil water allow tracing of the flow and transportation of water in the soil. However, there are few studies on the use of soil water stable isotopes to explore the soil water in the floodplain, especially in determining the soil water source and infiltration mechanism. The Bayesian mixing model (MixSIAR) was integrated with the line conditioned excess (lc-excess) of stable isotopes (δD and δ18O) in precipitation, soil water (0–150 cm), river water, and groundwater to determinate the source and recharge mechanisms of two different soil profile types in the floodplain of the upper Yellow River in Lanzhou, China. The results showed that soil water below 80 cm was affected by river water recharge, affecting soil water content and stable isotopic composition at S1 sampling points (profile parallel to river water); this effect was not observed at S2 (profile is higher than the river water) sampling points. The isotopic compositions of the soil water sources at the two sampling points (S1: δD = −77.41‰, δ18O = −11.01‰; S2: δD = −74.02‰, δ18O = −10.56‰) were depleted more than those in the long-term amount-weighted precipitation isotopes (δD = −56.30‰, δ18O = −8.17‰). The isotope signatures of soil water sources are similar to the isotope characteristics of some high-intensity precipitation events (≥30 mm/day), indicating that soil water originates from a fraction of the total precipitation. The piston flow (60%) and the preferential flow (40%) coexist, but soil moisture and rainfall intensity will affect the sequence of the two infiltration methods. This study provides insights for understanding the hydrological process of the upper Yellow River and evaluating groundwater quality and protecting the floodplain environment.

Spatiotemporal variations of soil water stable isotopes in a small karst sinkhole basin

Spatiotemporal variations of soil water stable isotopes in a small karst sinkhole basin

Using Soil Water Stable Isotopes to Investigate Soil Water Movement in a Water Conservation Forest in Hani Terrace

Water conservation forests significantly contribute to the stability of mountain agricultural ecosystems in Hani Terrace. In this study, we analyzed the relationship between the stable isotopic composition of soil water and precipitation to determine the mechanisms of soil water movement in the small watershed of Quanfuzhuang. We observed significant seasonal variations in soil water sources: antecedent precipitation was the dominant supply during the dry season, and current precipitation dominated during the rainy season. The recharge ratio of precipitation to soil water in the grassland was significantly higher than that in the arbor land and shrubland. The influence of water infiltration, old and new soil water mixing, and soil evaporation on the soil water stable isotopes gradually decreased from the surface (0–20 cm) to the deep (60–80 cm) soil. We observed significant seasonal variability in average soil water δ18O in the upper 0–60 cm and lower variability at 60–100 cm. The average soil water δ18O was generally higher in the dry season than in the rainy season. The mixing of old and new water is a continuous and cumulative process that is impacted by soil structure, soil texture, and precipitation events. We therefore identified a significant time delay in soil water supply with increasing soil depth. Moreover, the piston flow of soil water co-occurred with preferential flow, and the latter was the dominant supply during the rainy season.

The Soil Water Isotope Storage System (SWISS): An integrated soil water vapor sampling and multiport storage system for stable isotope geochemistry.

Soil water stable isotopes are a powerful tool for tracking interactions among the hydrosphere, geosphere, atmosphere, and biosphere. The challenges associated with creating high-temporal-resolution soil water stable isotope datasets from a diversity of sites have limited the utility of stable isotope geochemistry in addressing a range of complex problems. A device that can enable further development of high-temporal-resolution soil water isotope datasets that are created with minimal soil profile disruption from remote sites would greatly expand the utility of soil water stable isotope analyses. We designed a method for sampling and storing soil water vapor for stable isotope analysis that leverages recent advances in soil water sampling strategies. Here, we test the reliability of the storage system by introducing water vapor of known oxygen and hydrogen isotopic composition into the storage system, storing the water vapor for a predetermined amount of time, and then measuring the stable isotope composition of the vapor after the storage period. We demonstrate that water vapor stored in our flasks reliably maintains its isotope composition within overall system uncertainty (±0.5‰ for δ18 O values and ±2.4‰ for δ2 H values) for up to 30 days. This method has the potential to enable the collection of high-temporal-resolution soil water isotope datasets from remote sites that are not accessed daily in a time- and cost-effective manner. All the components used in the system can be easily controlled using open-source microcontrollers, which will be used in the future to automate sampling routines for remote field deployment. The system is designed to be an open-source tool for use by other researchers.

Identification of suites of traits that explains drought resistance and phenological patterns of plants in a semi-arid grassland community

Grassland ecosystems are comprised of plants that occupy a wide array of phenological niches and vary considerably in their ability to resist the stress of seasonal soil-water deficits. Yet, the link between plant drought resistance and phenology remains unclear in perennial grassland ecosystems. To evaluate the role of soil water availability and plant drought tolerance in driving phenology, we measured leaf hydraulic conductance (Ksat), resistance to hydraulic failure (P50), leaf gas exchange, plant and soil water stable isotope ratios (δ18O), and several phenology metrics on ten perennial herbaceous species in mixed-grass prairie. The interaction between P50 and δ18O of xylem water explained 67% of differences in phenology, with lower P50 values associated with later season activity, but only among shallow-rooted species. In addition, stomatal control and high water-use efficiency also contributed to the late flowering and late senescence strategies of plants that had low P50 values and relied upon shallow soil water. Alternatively, plants with deeper roots did not possess drought-tolerant leaves, but had high hydraulic efficiency, contributing to their ability to efficiently move water longer distances while maintaining leaf water potential at relatively high values. The suites of traits that characterize these contrasting strategies provide a mechanistic link between phenology and plant-water relations; thus, these traits could help predict grassland community responses to changes in water availability, both temporally and vertically within the soil profile.

Oxygen and hydrogen stable isotope compositions of soil water in deep loess profile under different land use types of northern Shaanxi, China

Investigation of stable isotope composition under different land use types is helpful for understanding soil water movement and hydrological effects of land use change. We collected soil samples in profiles ＞ 15 m deep under four land use types (i.e. farmland, grassland, Salix cheilophila and Populus sp.) in the loess deposits of northern Shaanxi. We measured hydrogen and oxygen stable isotope composition of soil water to explore the mechanism of soil water movement and the impacts of land use types. The isotope compositions of soil water under four land use types were significantly different. The δD values of soil water under farmland, grassland, S. cheilophila and Populus sp. were -81.1‰--60.1‰, -91.2‰--61.0‰, -87.4‰--63.6‰ and -73.5‰--62.2‰, while the δ18O values were -11.2‰--7.6‰, -12.6‰--8.2‰, -11.5‰--8.1‰ and -9.9‰--7.7‰, respectively. The soil water stable isotopes fluctuated across the profiles. The soil water isotope compositions in the layers of 0-3 m changed sharply, with the δD values being -80.2‰--61.8‰, -75.9‰--65.5‰, -76.0‰--63.6‰ and -73.5‰--62.2‰, respectively. In the layers of 3-12 m, the isotope profiles of farmland and grassland were parabolic, whereas those of S. cheilophila and Populus sp. were relatively stable. Soil water isotope compositions in the layers deeper than 12 m were generally stable with the δD values of -80.8‰--71.5‰, -83.0‰--67.5‰, -87.4‰--76.0‰ and -67.5‰--64.3‰, respectively. Across the four land use types, soil water stable isotope compositions were not significantly different either in the shallow layers or in the deep soil layers, but their differences in the layers of 3-12 m were significant. Soil moisture was mainly recharged from precipitation with piston flow as the main form of soil water movement. Soil water under four land use types might be recharged by wet events of different intensities. Soil water under farmland and grassland could be recharged by wet events of small intensity, but that under S. cheilophila and Populus sp. may be mainly recharged by the rainstorm in summer and autumn.

The Role of Matric Potential, Solid Interfacial Chemistry, and Wettability on Isotopic Equilibrium Fractionation

Core Ideas Matric potential controls the equilibrium fractionation factor between soil water and water vapor. Surface chemistry determined by X‐ray spectroscopy affects the equilibrium fractionation factor. A conceptual isotope retention characteristic approach is presented. Soil water stable isotopes are widely used for geo‐ and ecohydrological applications. However, the signature of the soil water isotopic composition in the environment depends on various factors. While recent work has shown matric potential effects on equilibrium fractionation, little work has examined other soil parameters concerning soil water energy status like the surface wettability, usually quantified in terms of contact angle. We simultaneously explored the role of matric potential, contact angle, and soil surface chemistry effects on the equilibrium fractionation factor during soil water evaporation. We present a simple laboratory experiment with four different soils of various textures. Subsamples of each texture class were treated with dichlorodimethylsilane to modify surface wetting properties. Additionally, we tested two natural soil samples to explore wettability effects. Samples were dried at temperatures between 40 and 550°C to produce chemically modified surface properties. All samples were spiked with water of known isotopic composition at different water contents. The isotopic signature was determined using the vapor‐bag equilibration method. The matric potential of each sample was measured with a soil water potential meter, the contact angle was determined with the sessile drop method, and the surface chemistry by X‐ray photoelectron spectroscopy. In addition to temperature and soil matric potential, the elemental composition has apparently some control on the equilibrium fractionation factor. Based on findings, we introduce a new soil water isotope retention characteristic approach to summarize how these factors (matric potential, contact angle, and soil surface chemistry) each control the equilibrium fractionation factor for 18O/16O and 2H/H. Corresponding retention curve approach parameters are promising to be applied in the future to predict soil water fractionation effects under natural and non‐stationary conditions.

Examination of deep root water uptake using anomalies of soil water stable isotopes, depth-controlled isotopic labeling and mixing models

Knowledge on the water uptake depths of vegetation is crucial for understanding water transport processes of the soil-vegetation atmosphere continuum and relevant for many applications (e.g. the estimation of groundwater recharge, irrigation planning and the parameterization of (eco-) hydrological models). The identification and quantification of water uptake from deep soil layers and groundwater remain challenging. This study uses a combined framework based on natural abundances of stable water isotopes and isotopic labeling experiments with deuterium oxide (2H2O) to study root water uptake and identify uptake from deep soil in a semi-arid environment.Between 2013 and 2016, more than 1000 soil (isotope depth profiles); plant (xylem and transpiration) and water (precipitation and groundwater) samples for the analysis of isotope ratios were collected. Two experiments using isotopic labeling were carried out in order to assess root water uptake depths. Herein, we i) present series of deep soil water isotope depth profiles, interpret water transport dynamics and ecohydrological feedbacks; ii) examine the suitability of natural isotope depth profiles for identifying deep root water uptake; iii) apply the Bayesian mixing model MixSIAR to quantify deep root water uptake and iv) constrain water uptake depths using isotopic labeling experiments and derive an active root water uptake distribution.Our results show that the form of isotope depth profiles of soil in water-limited environments follows characteristic shapes for the end of the rainy and dry seasons, respectively. Isotope ratios in the upper 4 m of the soil are heavily dependent on the character of the respective rainy season. Under certain conditions – e.g. droughts or weak rainy seasons – the isotope depth profile displays an enrichment in heavy isotopes up to 4 m depth. Such pronounced anomalies provide an opportunity for studies on source water partitioning. In the present experiment, the studied individuals of Acacia erioloba were found to obtain 37% [24–52%] of their water from deep soil (>4 m) and groundwater at the end of the dry season of 2015. All other investigated trees (individuals of B. plurijuga, S. luebertii, T. sericea and C. collinum) mainly utilize water originating from 1 m to 2.5 m depth. Under “average” rainy season conditions, the similarity of isotope ratios of potential plant water sources hinders a conclusive identification of water uptake depths.Deep natural isotope depth profiles in dry environments can – under certain conditions – be used to identify and quantify access of vegetation to deep water resources. Isotopic labeling enables to determine active root distributions for the lateral root zone. Combined frameworks contribute to a better understanding of deep water uptake. A differentiated consideration of water uptake from the lateral root zone and deep, potentially groundwater-tapping roots is required in order to fully investigate ecohydrological feedbacks and for a proper parameterization of models.

Comparison of pore water samplers and cryogenic distillation under laboratory and field conditions for soil water stable isotope analysis

ABSTRACTWe used pore water samplers (PWS) to sample for isotope analysis (1) only water, (2) soil under laboratory conditions, and (3) soil in the field comparing the results with cryogenic extraction (CE). In (1) and (2), no significant differences between source and water extracted with PWS were detected with a mean absolute difference (MAD) always lower than 2 ‰ for δ2H and 1 ‰ for δ18O. In (2), CE water was more enriched than PWS-extracted water, with a MAD respect to source water of roughly 8 ‰ for δ2H and 4 ‰ for δ18O. In (3), PWS water was enriched relative to CE water by 3 ‰ for δ2H and 0.9 ‰ for δ18O. The latter result may be due to the distinct water portions sampled by the two methods. Large pores, easily sampled by PWS, likely retain recent, and enriched, summer precipitation while small pores, only sampled by CE, possibly retain isotopically depleted water from previous winter precipitation or irrigation inputs. Accuracy and precision were greater for PWS relative to CE. PWS is therefore suggested as viable tool to extract soil water for stable isotope analysis, particularly for soils used in this study (sandy and silty loams).

Soil water stable isotopes reveal evaporation dynamics at the soil–plant–atmosphere interface of the critical zone

Abstract. Understanding the influence of vegetation on water storage and flux in the upper soil is crucial in assessing the consequences of climate and land use change. We sampled the upper 20 cm of podzolic soils at 5 cm intervals in four sites differing in their vegetation (Scots Pine (Pinus sylvestris) and heather (Calluna sp. and Erica Sp)) and aspect. The sites were located within the Bruntland Burn long-term experimental catchment in the Scottish Highlands, a low energy, wet environment. Sampling took place on 11 occasions between September 2015 and September 2016 to capture seasonal variability in isotope dynamics. The pore waters of soil samples were analyzed for their isotopic composition (δ2H and δ18O) with the direct-equilibration method. Our results show that the soil waters in the top soil are, despite the low potential evaporation rates in such northern latitudes, kinetically fractionated compared to the precipitation input throughout the year. This fractionation signal decreases within the upper 15 cm resulting in the top 5 cm being isotopically differentiated to the soil at 15–20 cm soil depth. There are significant differences in the fractionation signal between soils beneath heather and soils beneath Scots pine, with the latter being more pronounced. But again, this difference diminishes within the upper 15 cm of soil. The enrichment in heavy isotopes in the topsoil follows a seasonal hysteresis pattern, indicating a lag time between the fractionation signal in the soil and the increase/decrease of soil evaporation in spring/autumn. Based on the kinetic enrichment of the soil water isotopes, we estimated the soil evaporation losses to be about 5 and 10 % of the infiltrating water for soils beneath heather and Scots pine, respectively. The high sampling frequency in time (monthly) and depth (5 cm intervals) revealed high temporal and spatial variability of the isotopic composition of soil waters, which can be critical, when using stable isotopes as tracers to assess plant water uptake patterns within the critical zone or applying them to calibrate tracer-aided hydrological models either at the plot to the catchment scale.

Illuminating hydrological processes at the soil‐vegetation‐atmosphere interface with water stable isotopes

AbstractWater stable isotopes (18O and 2H) are widely used as ideal tracers to track water through the soil and to separate evaporation from transpiration. Due to the technical developments in the last two decades, soil water stable isotope data have become easier to collect. Thus, the application of isotope methods in soils is growing rapidly. Studies that make use of soil water stable isotopes often have a multidisciplinary character since an interplay of processes that take place in the vadose zone has to be considered. In this review, we provide an overview of the hydrological processes that alter the soil water stable isotopic composition and present studies utilizing pore water stable isotopes. The processes that are discussed include the water input as precipitation or throughfall, the output as evaporation, transpiration, or recharge, and specific flow and transport processes. Based on the review and supported by additional data and modeling results, we pose a different view on the recently proposed two water world hypothesis. As an alternative to two distinct pools of soil water, where one pool is enriched in heavy isotopes and used by the vegetation and the other pool does not undergo isotopic fractionation and becomes recharge, the water gets successively mixed with newly introduced rainwater during the percolation process. This way, water initially isotopically enriched in the topsoil loses the fractionation signal with increasing infiltration depth, leading to unfractionated isotopic signals in the groundwater.

Review on soil water isotope-based groundwater recharge estimations

Groundwater recharge estimations are necessary for sustainable management of groundwater resources. Among various hydraulic, hydrogeological and hydrological approaches, tracer methods are proposed to improve groundwater recharge estimations. Soil water isotope-based estimates of groundwater recharge rates were developed in the 1960s and have been applied in more than 40 studies reviewed within this manuscript. This comprehensive review research distinguishes three fundamental research approaches: (1) labelling method with tritium or deuterium, which can be used to trace unsaturated zone water movement, (2) tracing soil water movement using a seasonal signal of stable isotopes in precipitation, mainly applied in humid zones, and (3) interpretation of an evaporation signal that impacts soil water stable isotope composition, mainly applicable in arid and semiarid regions. More recently, advances in soil water extraction and analytical methods allow a higher sampling resolution and in situ measurements of soil water stable isotopes. Analytical advances may strengthen and improve the applicability of stable isotope methods for quantitative recharge. We present, compare and discuss studies of unsaturated zone isotope profiles with an emphasis on necessary prerequisites, such as a substantial thick porous sedimentary layer, or negative influences through high clay and silt contents or dry soil conditions. Copyright © 2015 John Wiley & Sons, Ltd.

Tracking water pathways in steep hillslopes by δ18O depth profiles of soil water

Summary Assessing temporal variations in soil water flow is important, especially at the hillslope scale, to identify mechanisms of runoff and flood generation and pathways for nutrients and pollutants in soils. While surface processes are well considered and parameterized, the assessment of subsurface processes at the hillslope scale is still challenging since measurement of hydrological pathways is connected to high efforts in time, money and personnel work. The latter might not even be possible in alpine environments with harsh winter processes. Soil water stable isotope profiles may offer a time-integrating fingerprint of subsurface water pathways. In this study, we investigated the suitability of soil water stable isotope (δ18O) depth profiles to identify water flow paths along two transects of steep subalpine hillslopes in the Swiss Alps. We applied a one-dimensional advection–dispersion model using δ18O values of precipitation (ranging from −24.7 to −2.9‰) as input data to simulate the δ18O profiles of soil water. The variability of δ18O values with depth within each soil profile and a comparison of the simulated and measured δ18O profiles were used to infer information about subsurface hydrological pathways. The temporal pattern of δ18O in precipitation was found in several profiles, ranging from −14.5 to −4.0‰. This suggests that vertical percolation plays an important role even at slope angles of up to 46°. Lateral subsurface flow and/or mixing of soil water at lower slope angles might occur in deeper soil layers and at sites near a small stream. The difference between several observed and simulated δ18O profiles revealed spatially highly variable infiltration patterns during the snowmelt periods: The δ18O value of snow (−17.7 ± 1.9‰) was absent in several measured δ18O profiles but present in the respective simulated δ18O profiles. This indicated overland flow and/or preferential flow through the soil profile during the melt period. The applied methods proved to be a fast and promising tool to obtain time-integrated information on soil water flow paths at the hillslope scale in steep subalpine slopes.

Estimation of soil water evaporative loss after tillage operation using the stable isotope technique

Abstract Application of stable isotopes in soil studies has improved quantitative evaluation of evaporation and other hydrological processes in soil. This study was carried out to determine the effect of tillage on evaporative loss of water from the soil. Zero tillage and conventional tillage were compared. Suction tubes were installed for soil water collection at the depths 0.15, 0.50, and 1.0 m by pumping soil water with a peristaltic pump. Soil water evaporation was estimated using stable isotopes of water. The mean isotopic composition of the soil water at 0.15 m soil depth were -1.15‰ (δ18O) and -0.75‰ (δD) and were highly enriched compared with the isotopic compositions of the site precipitation. Soil water stable isotopes (δ18O and δD) were more enriched near the surface under zero tillage while they were less negative down the profile under zero tillage. This suggests an occurrence of more evaporation and infiltration under conventional then zero tillage, respectively, because evaporative fractionation contributes to escape of lighter isotopes from liquid into the vapour phase leading to enrichment in heavy isotopes in the liquid phase. The annual evaporation estimated using the vapour diffusion equation ranges from 46-70 and 54-84 mm year-1 under zero and conventional tillage, respectively, indicating more evaporation under conventional tillage compared with zero tillage. Therefore, to reduce soil water loss, adoption of conservation tillage practices such as zero tillage is encouraged.

Spectral analysis software improves confidence in plant and soil water stable isotope analyses performed by isotope ratio infrared spectroscopy (IRIS)

Previous studies have demonstrated the potential for large errors to occur when analyzing waters containing organic contaminants using isotope ratio infrared spectroscopy (IRIS). In an attempt to address this problem, IRIS manufacturers now provide post-processing spectral analysis software capable of identifying samples with the types of spectral interference that compromises their stable isotope analysis. Here we report two independent tests of this post-processing spectral analysis software on two IRIS systems, OA-ICOS (Los Gatos Research Inc.) and WS-CRDS (Picarro Inc.). Following a similar methodology to a previous study, we cryogenically extracted plant leaf water and soil water and measured the δ(2)H and δ(18)O values of identical samples by isotope ratio mass spectrometry (IRMS) and IRIS. As an additional test, we analyzed plant stem waters and tap waters by IRMS and IRIS in an independent laboratory. For all tests we assumed that the IRMS value represented the "true" value against which we could compare the stable isotope results from the IRIS methods. Samples showing significant deviations from the IRMS value (>2σ) were considered to be contaminated and representative of spectral interference in the IRIS measurement. Over the two studies, 83% of plant species were considered contaminated on OA-ICOS and 58% on WS-CRDS. Post-analysis, spectra were analyzed using the manufacturer's spectral analysis software, in order to see if the software correctly identified contaminated samples. In our tests the software performed well, identifying all the samples with major errors. However, some false negatives indicate that user evaluation and testing of the software are necessary. Repeat sampling of plants showed considerable variation in the discrepancies between IRIS and IRMS. As such, we recommend that spectral analysis of IRIS data must be incorporated into standard post-processing routines. Furthermore, we suggest that the results from spectral analysis be included when reporting stable isotope data from IRIS.