The natural abundance of stable water isotopes method may overestimate deep-layer soil water use by trees

Wang, Shaofei; Gao, Xiaodong; Yang, Min; Huo, Gaopeng; Song, Xiaolin; Siddique, Kadambot H. M.; Wu, Pute; Zhao, Xining

doi:https://doi.org/10.5194/hess-2022-142

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https://doi.org/10.5194/hess-2022-142

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03 May 2022

Status: a revised version of this preprint is currently under review for the journal HESS.

The natural abundance of stable water isotopes method may overestimate deep-layer soil water use by trees

Shaofei Wang¹, Xiaodong Gao^2,3, Min Yang¹, Gaopeng Huo¹, Xiaolin Song⁴, Kadambot H. M. Siddique⁵, Pute Wu^2,3, and Xining Zhao^2,3 Shaofei Wang et al.

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¹Key Laboratory of Agricultural Soil and Water Engineering in Arid and Semiarid Areas, Ministry of Education, Northwest A&F University, 712100, Yangling, Shaanxi Province, China
²Institute of Soil and Water Conservation, Northwest A&F University, 712100, Yangling, Shaanxi Province, China
³Institute of Soil and Water Conservation, Chinese Academy of Sciences and Ministry of Water Resources, 712100, Yangling, Shaanxi Province, China
⁴State Key Laboratory of Crop Stress Biology for Arid Areas, College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China
⁵The UWA Institute of Agriculture, The University of Western Australia, Perth, WA 6001, Australia

Received: 09 Apr 2022 – Discussion started: 03 May 2022

Abstract. Stable water isotopes have been used extensively to study the water use strategy of plants in various ecosystems. In deep vadose zone (DVZ) regions, the rooting depth of trees can reach several meters to tens of meters. However, the existence of roots in deep soils does not necessarily mean the occurrence of root water uptake, which usually occurs at a particular time during the growing season. Therefore, quantifying the contribution of deep-layer soil water (DLSW) in DVZ regions using the natural abundance of stable water isotopes may not be accurate because this method assumes that trees always extract shallow- and deep-layer soil water. We propose a multi-step method for addressing this issue. First, isotopic labeling in deep layers identifies whether trees absorb DLSW and determines the soil layer depths from which trees derive their water source. Next, calculate water sources based on the natural abundance of stable isotopes to quantify the water use strategy of trees. We also compared the results with the natural abundance of stable water isotopes method. The 11- and 17-year-old apple trees were taken as examples for analyses on China’s Loess Plateau. Isotopic labeling showed that the water uptake depth of 11-year-old apple trees reached 300 cm in the blossom and young fruit (BYF) stage and only 100 cm in the fruit swelling (FSW) stage, whereas 17-year-old trees always consumed water from the 0–320 cm soil layer. Overall, apple trees absorbed the most water from deep soils (> 140 cm) during the BYF stage, and 17-year-old trees consumed more water in these layers than 11-year-old trees throughout the growing season. In addition, the natural abundance of stable water isotopes method overestimated the contribution of DLSW, especially in the 320–500 cm soil layer. Our findings highlight that determining the occurrence of root water uptake in deep soils helps quantify the water use strategy of trees in DVZ regions.

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Shaofei Wang et al.

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RC1:
'Comment on hess-2022-142', Anonymous Referee #1, 10 Jun 2022
With great pleasure I read your manuscript, where you investigate the water use strategy of apple trees by different ages. You injected labeled water (D2O) and studied at which depth the trees withdraw their waters. This analysis was carried out for three different growing stages. The paper is very well structured, easy to read, informative figures and in good English language. The applied method is correct and I have no comments about the conclusion. Hence my comments limit mostly to technical issues, except from the following two comments:

From a scientific point of view your work is really interesting, but the question remains what we can do with this information (social relevance). Furthermore, the study only looks at one growing cycle and ignores that water use strategies change depending on water availability (or climate). In case plants experience dry spells, their roots develop differently in comparison to plant that do not expericence dry spells. So there is also a long-term strategy, where plants (sometimes) can adapt to climate change. Could you discuss on this topic?

Data availablility: I don't think the current data-statement is sufficient. Data that is used in publications should preferable be available online and not "upon request". The latter is only possible in exeptional cases. If this is the case, this should be justified.

Technical issues:

L60: BYF is note explained in the main text (only in the abstract). I think it's good practice to define abbreviations the first time you mention them in the main text.

L888: unit of annual rainfall in mm/y

Table1: I would recommend to change the way the units are provided. I would skip the '/' and use brackets.

L155-and further: all variables/parameters should be in italic.

L156: "mean annual Pt": this is long-term Pt? If so, provide period. Furthermore unit should be mm/y

L156-157: but the monthly rainfall can differ a lot (see figure2b). So is 2019 a normal year?

Fig 2: unit of precipitation is mm/day (LEFT) and mm/month (RIGHT)

Fig 5: I would rotate this figure 90 degrees, so you can more easily compare the figures with fig 3, 4, and 6

L212: "with relative higher reliance": I am not fully understand this sentence. Could you explain?
Citation: https://doi.org/10.5194/hess-2022-142-RC1
- AC1: 'Reply on RC1', Xiaodong Gao, 29 Jul 2022
  
  Anonymous Referee #1
  With great pleasure I read your manuscript, where you investigate the water use strategy of apple trees by different ages. You injected labeled water (D₂O) and studied at which depth the trees withdraw their waters. This analysis was carried out for three different growing stages. The paper is very well structured, easy to read, informative figures and in good English language. The applied method is correct and I have no comments about the conclusion. Hence my comments limit mostly to technical issues, except from the following two comments:
  Response: Thank you very much for the constructive and encouraging comments and giving us an opportunity to revise this paper. Corrections have been made based on the recommendations, and the detailed response to each comment is presented as follow.
  
  Comment 1
  From a scientific point of view your work is really interesting, but the question remains what we can do with this information (social relevance). Furthermore, the study only looks at one growing cycle and ignores that water use strategies change depending on water availability (or climate). In case plants experience dry spells, their roots develop differently in comparison to plant that do not experience dry spells. So there is also a long-term strategy, where plants (sometimes) can adapt to climate change. Could you discuss on this topic?
  Response: Thanks for your comments. Here are our responses.
  (1) The Loess Plateau is the largest apple tree cultivation zone globally and apple yield accounts for 27% of global production, involving more than 10 million farmers (Gao et al., 2021). However, drought and water shortage in this region are serious and most orchards have no irrigation water source, causing apple trees to heavily rely on deep-layer soil water (DLSW). Therefore, it is of great significance to determine the utilization time of DLSW for orchard water management and apple yield improvement. We will add the social relevance of the findings in the Discussion section (L 320-324).
  References
  Gao, X., Zhao, X., Wu, P., Yang, M., Ye, M., Tian, L., Zou, Y., Wu, Y., Zhang, F., and Siddique, K. H. M.: The economic–environmental trade-off of growing apple trees in the drylands of China: A conceptual framework for sustainable intensification, J. Clean Prod., 296, 126497, https://doi.org/10.1016/j.jclepro.2021.126497, 2021.
  (2) We agree that water use strategy can be changed by climate. The apple trees are expected to use more soil water in deeper soils in dry spells. We will add more details in the Discussion section to explain this point (L 329-344).
  “Our results show that apple trees switch their water sources between different soil layers to adapt to the changing water environments on the Loess Plateau, which is particularly important in the context of future climate change. Special attention should be directed to water consumption in deep soils—we found that apple trees absorbed the most water from deep soils during the BYF stage, with 17-year-old apple trees consuming more water in these layers than 11-year-old trees throughout the growing season. This result is in accordance with previous observation in this region that soil water availability gradually decreased with increasing stand age, and then apple trees absorbed more water from deeper soil layers (Li et al., 2019). Similarly, Barbeta et al. (2015) found that trees increased their use proportion of deep soil water and groundwater following a long-term (12 years) experimental drought. However, this may not be sustainable, especially in deep vadose zone (DVZ) regions. The result of the tritium peak method suggested that it took more than 50 years for soil water migration to 6 m depth in apple orchards in DVZ regions (Li et al., 2018). Thus, once deep-layer soil water is depleted, it cannot be replenished within a short timeframe, reducing the tree’s ability to resist water stress. Also, Wu et al. (2021) observed that soil water generated by precipitation was the primary water source for apple trees when deep soil water was depleted, dominating their transpiration. In this case, trees were likely to encounter irreversible embolism, increasing the risk of drought-induced mortality, threatening the sustainability development of vegetation and changing regional hydrological cycle (Brodribb et al., 2020; Zhang et al., 2020). Therefore, we suggest that long-term and high-frequency monitoring of isotopes in soil and xylem water is necessary, especially at large geographical scale, to further understand the long-term changes of plant water use strategy and evaluate their adaptability under climate change.”
  References
  Barbeta, A., Mejia-Chang, M., Ogaya, R., Voltas, J., Dawson, T. E., and Penuelas, J.: The combined effects of a long-term experimental drought and an extreme drought on the use of plant-water sources in a Mediterranean forest, Global Change Biol., 21, 1213-1225, 10.1111/gcb.12785, 2015.
  Brodribb, T.J., Powers, J., Cochard, H., and Choat, B.: Hanging by a thread? Forests and drought, Science, 368(6488), 261-266, DOI:10.1126/science.aat7631, 2020.
  Li, H., Si, B., and Li, M.: Rooting depth controls potential groundwater recharge on hillslopes, J. Hydrol., 564, 164-174, 10.1016/j.jhydrol.2018.07.002, 2018.
  Li, H., Si, B., Wu, P., and McDonnell, J. J.: Water mining from the deep critical zone by apple trees growing on loess, Hydrol. Process., 33, 320-327, 10.1002/hyp.13346, 2019.
  Wu, W., Li, H., Feng, H., Si, B., Chen, G., Meng, T., Li, Y., and Siddique, K. H. M.: Precipitation dominates the transpiration of both the economic forest (Malus pumila) and ecological forest (Robinia pseudoacacia) on the Loess Plateau after about 15 years of water depletion in deep soil, Agr. Forest Meteorol., 297, 108244, https://doi.org/10.1016/j.agrformet.2020.108244, 2021.
  Zhang, Z., Huang, M., Yang, Y., and Zhao, X.: Evaluating drought-induced mortality risk for Robinia pseudoacacia plantations along the precipitation gradient on the Chinese Loess Plateau, Agr. Forest Meteorol., 284, 107897, https://doi.org/10.1016/j.agrformet.2019.107897, 2020.
  
  Comment 2
  Data availability: I don't think the current data-statement is sufficient. Data that is used in publications should preferable be available online and not "upon request". The latter is only possible in exceptional cases. If this is the case, this should be justified.
  Response: We agree. We will make changes in the revised manuscript (L 361).
  “The data that support the findings of this study are provided in Supplement.”
  
  Technical issues:
  L60: BYF is note explained in the main text (only in the abstract). I think it's good practice to define abbreviations the first time you mention them in the main text.
  Response: We agree. We will make changes in the revised manuscript (L 60).
  “Wang et al. (2020b) stated that the heavy absorption of deep soil water only occurred during the blossom and young fruit (BYF) stage in apple orchards”.
  
  L88: unit of annual rainfall in mm/y.
  Response: We agree. We will make changes in the revised manuscript (L 88).
  “Mean annual precipitation in the study region is 507.9 mm/y”.
  
  Table1: I would recommend to change the way the units are provided. I would skip the '/' and use brackets.
  Response: We agree. We will make changes in the revised manuscript (L 96).
  The Table 1 is given in the supplemental document attached.
  
  L155-and further: all variables/parameters should be in italic.
  Response: We agree. We will make changes in the revised manuscript (L 155-156).
  “Figure 2 shows the total precipitation (P_t) and growing season (April to September) precipitation (P_g). P_t and P_g in 2019 were 522.1 mm and 442.3 mm, respectively, similar to the multiyear (1999–2018) mean (507.9 mm/y for P_t and 407.5 mm/y for P_g).”
  
  L156: "mean annual P_t": this is long-term P_t? If so, provide period. Furthermore unit should be mm/y.
  Response: Yes, “mean annual P_t” is long-term P_t. The period will be added in the revised manuscript (L 155-157).
  “P_t and P_g in 2019 were 522.1 mm and 442.3 mm, respectively, similar to the multiyear (1999–2018) mean (507.9 mm/y for P_t and 407.5 mm/y for P_g).”
  
  L156-157: but the monthly rainfall can differ a lot (see figure2b). So is 2019 a normal year?
  Response: The study area is located in China’s Loess Plateau, its most significant climatological characteristics are distinctly seasonal precipitation, approximately 55-78% of which falls in June through September (Fu et al., 2017; Jia et al., 2017). In 2019, 74.9% of the precipitation in study area fell in June through September, according with the seasonal distribution characteristics of precipitation in the Plateau. In addition, the total precipitation (P_t) and growing season (April to September) precipitation (P_g) in 2019 were 522.1 mm and 442.3 mm, respectively, similar to the multiyear (1999–2018) mean (507.9 mm/y for P_t and 407.5 mm/y for P_g). Thus, 2019 was considered a normal precipitation year. We will make changes in the revised manuscript (L 157-159).
  References
  Fu, B., Wang, S., Liu, Y., Liu, J., Liang, W., and Miao, C.: Hydrogeomorphic Ecosystem Responses to Natural and Anthropogenic Changes in the Loess Plateau of China, Annu. Rev. Earth Pl. Sc., 45(1), 223-243, DOI:10.1146/annurev-earth-063016-020552, 2017.
  Jia, X., Shao, M., Zhu, Y., and Luo, Y.: Soil moisture decline due to afforestation across the Loess Plateau, China, J. Hydro., 546, 113-122, DOI:10.1016/j.jhydrol.2017.01.011, 2017.
  
  Fig 2: unit of precipitation is mm/day (LEFT) and mm/month (RIGHT).
  Response: We agree. The Figure 2 will be revised in the revised manuscript (L 163).
  The Figure 2 is given in the supplemental document attached.
  
  Fig 5: I would rotate this figure 90 degrees, so you can more easily compare the figures with fig 3, 4, and 6.
  Response: We agree. The Figure 5 will be revised in the revised manuscript (L 191).
  The Figure 5 is given in the supplemental document attached.
  
  L212: "with relative higher reliance": I am not fully understand this sentence. Could you explain?
  Response: It means that the contribution proportion of water from 140–320 cm soil layer both exceeded 48% in BYF stage for 11- and 17-year-old apple trees, which was higher than other stages. To clarify it, we will reorganize the sentence (L 216).
  “The BYF stage produced more negative isotopic values in xylem water for 11- and 17-year-old apple trees (Fig. 7), which mainly utilized water from 140–320 cm soil layer (more than 48%)”.
  
  Citation: https://doi.org/10.5194/hess-2022-142-AC1
RC2:
'Comment on hess-2022-142', Anonymous Referee #2, 09 Aug 2022
General comment

The authors of this manuscript designed a tracer experiment based on stable isotopes of hydrogen and oxygen to identify the depth of soil water taken up by apple trees at three different stages (blossom and young fruit, fruit swelling and fruit maturation stage). The topic of this manuscript is timely and potentially interesting for the readers of Hydrology and Earth System Sciences. Overall, the manuscript is well written and structured, but I found that various and important methodological details (e.g., a detailed description of the extraction method used for soil and vegetation material, and the isotopic composition of the injected water) were not described in the manuscript. Furthermore, the authors should also have considered in the introduction and the discussion recent literature on isotopic fractionation and offset which presents the current technical limitations for the application of stable isotopes in ecohydrological studies. Finally, a section about the limitation of the methodological approach should be added before the conclusions.

Specific comments

Lines 44-45: The authors should mention in the introduction that isotopic fractionation has been observed in various studies (e.g., Poca et al., 2019; Vargas et al., 2017; Barbeta et al., 2019), along the soil-root-stem-twig-leaf pathway. The factors affecting such fractionation/offset are still unclear, but they seem to be mainly related to the water extraction technique, and particularly to cryogenic vacuum distillation (e.g., Zhao et al., 2016; Barbeta et al., 2022).

Lines 50-52: The authors should mention limitations due to the extraction technique applied to soils (e.g., cryogenic vacuum distillation, Orlowski et al. (2016a,b, 2018)).

Lines 90-92: What is the depth to the water table in the study area? Do the roots of the apple trees have access to shallow groundwater?

Lines 92-93: What is the average, minimum and maximum distance between the trees? Does the tree distance affect the root density and possibly the water uptake?

Table 1: It is unclear whether height, trunk and crown size data represent an average or single values. The authors should add the minimum and maximum values for all characteristics, as well as the number of trees used in this study.

Lines 108-109: The authors should report the isotopic composition of the water mixture used for injection/irrigation, as well as the total amount of irrigation water applied per each tree.

Lines 109-111: I do not understand the aim of this sentence. Based on it, I understand that the authors may have injected a water amount that could be too small to detect a soil water content variation and perhaps, also an isotopic variation.

Lines 112-113: Details about the sample size should be reported in the text here or in an additional table. Furthermore, more details are needed to understand how the background value was computed (is it an average of how many samples?), and where these unlabelled trees are located compared to the trees used for the experiment.

Lines 132-133: I am not familiar with this cryogenic vacuum distillation system, and I am not sure it is manufactured by Los Gatos Research. I recommend adding here a detailed description of the extraction method, as well as information about the extraction efficiency.

Lines 133-135: Details about the uncertainty in the isotopic analyses for each instrument should be added here. Did the authors use specific practises to mitigate the memory effect in the isotopic measurements?

Line 143: Considering recent literature, the no isotopic fractionation assumption is a strong assumption that should be tested. Did the authors check whether their isotopic data present an offset compared to the isotopic composition of local water sources (e.g., precipitation, soil water before the tracer experiment and shallow groundwater) and the water used for the tracer experiment?

Lines 145-148: I do not understand the purpose of this index, S. The description of this index should be improved, e.g. by adding what positive and negative values indicate, and the ranges used for the soil water content.

Figure 5: The isotopic composition of the injected water should be plotted, and I suggest showing the background value also without the 2 SD. Furthermore, the sample size should be reported in the caption.

Figure 7: In the plots for “FSW for 11yr” and “BYF for 17yr” there may be an isotopic offset for some xylem water samples; I suggest checking whether there is a significant deviation from the soil water isotopic line, by also considering the uncertainty due to the isotopic analyses. In these plots, the authors should add the isotopic composition of the background values (in soil and xylem waters) and of the injected water. Equation of the LMWL and details about sample size and when the samples were collected should be added in the caption.

Figure 9: Please remove the regression line when there are only three samples used for the analysis.

Section 3.6: I expected to read here the results concerning the application of the index S, but they are not reported. Therefore, I suggest adding here such results, or removing the description of S at Lines 145-148.

Section 4.3: Please add in this section (or in a new one) the limitations of the experimental approach. I think the limitations of this work are mainly related to the assumptions of no isotopic fractionation and negligible spatial variability in the isotopic composition of unlabelled and labelled trees, and to the extraction method (cryogenic vacuum distillation system).

Technical corrections

Line 43: I think the authors should write “Analytical techniques based on stable isotopes…”, and furthermore, they should consider that many sampling techniques are destructive because they require the collection of soil and vegetation material (e.g., leaves, twigs, wood cores etc.).

Line 51: Please replace “confusion of” with “unclear”.

Title of section 2.2: I suggest changing it with “Sample collection”.

Title of section 2.2.2: I suggest changing it with “Collection of soil and vegetation samples for isotopic analysis”.

Figure 4: Please add in the caption when the soil water content was determined (before, during or after the tracer injection).

Figure 8: Please remove from the caption “Seasonal patterns of” because the results refer only to three specific tracer injections.

References

Barbeta A., Burlett R., Martín-Gómez P., Fréjaville B., Devert N., Wingate L., Domec J.-C., Ogée J., 2022. Evidence for distinct isotopic compositions of sap and tissue water in tree stems: consequences for plant water source identification. New Phytol., 233, 1121-1132. DOI:10.1111/nph.17857

Barbeta A., Jones S.P., Clavé L.,Wingate L., Gimeno T.E., Fréjaville B., Wohl S., Ogée J.:, 2019. Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest. Hydrol. Earth Syst. Sci., 23, 2129-2146. DOI:10.5194/hess-23-2129-2019

Orlowski N., Breuer L., McDonnell J.J., 2016a. Ecohydrology Bearings – Invited Commentary Critical issues with cryogenic extraction of soil water for stable isotope analysis. Ecohydrology, 9, 3-10. DOI:10.1002/eco.1722

Orlowski N., Pratt D.L., McDonnell J.J., 2016b. Intercomparison of soil pore water extraction methods for stable isotope analysis. Hydrol. Process., 30, 3434-3449. DOI:10.1002/hyp.10870

Orlowski N., Breuer L., Angeli N., Boeckx P., Brumbt C., Cook C.S., Dubbert M., Dyckmans J., Gallagher B., Gralher B., Herbstritt B., Hervé-Fernández P., Hissler C., Koeniger P., Legout A., Macdonald C.J., Oyarzún C., Redelstein R., Seidler C., Siegwolf R., Stumpp C., Thomsen S., Weiler M., Werner C., McDonnell J.J., 2018. Inter-laboratory comparison of cryogenic water extraction systems for stable isotope analysis of soil water. Hydrol. Earth Syst. Sci., 22, 3619-3637. DOI:10.5194/hess-22-3619-2018

Poca M., Coomans O., Urcelay C., Zeballos S.R., Bodé S., Boeckx P., 2019. Isotope fractionation during root water uptake by Acacia caven is enhanced by arbuscular mycorrhizas. Plant and Soil, 441, 485-497. DOI:10.1007/s11104-019-04139-1

Vargas A.I., Schaffer B., Yuhong L., da Silveira Lobo Sternberg L., 2017. Testing plant use of mobile vs immobile soil water sources using stable isotope experiments. New Phytol., 215, 582-594. DOI:10.1111/nph.14616

Zhao L., Wang L., Cernusak L.A., Liu X., Xiao H., Zhou M., Zhang S., 2016. Significant difference in hydrogen isotope composition between xylem and tissue water in Populus Euphratica. Plant Cell Environ., 39, 1848-1857. DOI:10.1111/pce.12753
Citation: https://doi.org/10.5194/hess-2022-142-RC2
- AC2: 'Reply on RC2', Xiaodong Gao, 12 Sep 2022
  
  Thank you for your constructive and encouraging comments and giving us an opportunity to revise this paper. Corrections have been made based on the recommendations. The detailed response to each comment was uploaded in the form of a supplement.
  
  Citation: https://doi.org/10.5194/hess-2022-142-AC2

Shaofei Wang et al.

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https://doi.org/10.5194/hess-2022-142-supplement

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Short summary

Water uptake depth of 11-year-old apple trees reached 300 cm in the blossom and young fruit stage and only 100 cm in the fruit swelling stage, whereas 17-year-old trees always consumed water from 0–320 cm soil layer. Overall, the natural abundance of stable water isotopes method overestimated the contribution of deep soil water, especially in the 320–500 cm soils. Our findings highlight that determining the occurrence of root water uptake in deep soils helps quantify trees water use strategy.


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