Characterisation of hydrogen isotope pro®les in an
agroforestry system: implications for tracing
water sources of trees
Stephen S.O. Burgess
a,*, Mark A. Adams
a,
Neil C. Turner
b, Brett Ward
caDepartment of Botany, University of Western Australia, Nedlands, WA 6907, Australia bCSIRO Plant Industry, Private Bag, Wembley, WA 6014, Australia cWater and Rivers Commission, 5 Bevan Street, Albany 6330, Australia
Accepted 26 April 2000
Abstract
Tracing sources of water utilised by plants is important to understand species interactions in intercropping/agroforestry systems, particularly where species vary greatly in life-form. Isotopic techniques are an increasingly common means to trace water sources. The distribution of stable isotopes of water within the soil±plant±atmosphere continuum is indicative of a range of hydrologic processes and plant functions. Before we can infer plant or other biological effects on the distribution of isotopes we require a thorough characterisation of the environmental isotope distribution as well as an understanding of the physical processes that determine this distribution. Unless distinct features are recognisable in the isotopic `landscape' surrounding a plant, links cannot be made between plant function and environment. As a means to trace water acquisition by fourEucalyptusspecies in an agroforestry planting, we tested a simple `end member' approach and a more extensive characterisation of hydrogen and deuterium distribution pro®les within an agricultural soil in south-western Australia. Hydrogen isotope distribution within the soil was highly uniform; an apparent result of climatic factors, soil physical properties and the ability of tree roots to redistribute soil water. Such ®ndings have important implications for measurement strategies and experimental design when attempting to quantify plant use of water from differing soil sources. In the absence of large isotopic discrimination among water sources, simple `end member' models are likely to be misleading and isotopic labelling techniques may be more appropriate in some environments.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Stable isotopes; Agroforestry; Water use; Resource partitioning
*Corresponding author. Present address: Department of Integrative Biology, University of California,
Berkeley, CA 94720, USA. Tel.:1-510-642-1054; fax:1-510-643-6264.
E-mail address: sburgess@socrates.berkeley.edu (S.S.O. Burgess)
1. Introduction
The practice of combining functionally distinct plant types on agricultural land has been the subject of considerable research over the past two decades (Akinnifesi et al., 1998). Termed `agroforestry' and generally based around a combination of woody perennials (trees and shrubs) with herbaceous annuals (crops and pasture), such agricultural systems are increasingly being adopted to optimise resource utilisation by cultivated plants.
Both agronomic and ecological principles have been employed to investigate whether agroforestry systems might be more productive and sustainable than monocultures as a result of closer coupling between the use and availability of resources (Sanchez, 1995). The role of water is of key importance. In terms of sustainability, Sadler and Turner (1994) argued that if amounts of rainfall, irrigation and water stored in soil exceed that used by plants, surplus water will lead to land degradation by erosion, waterlogging and salinisation. A clear example can be seen in regions of southern Australia, where the replacement of functionally complex native vegetation with agricultural crops has increased waterlogging (Cox and McFarlane, 1995) and groundwater recharge with resultant salinisation of soils and waterways (Walker et al., 1993). If plants are able to utilise a larger fraction of available water, particularly in environments where water is limited, greater yield and less recharge should result (McIntyre et al., 1997; Wallace et al., 1999).
The role of agroforestry in increasing the amount of water used for plant growth and reducing environmentally damaging, surplus water, hinges on a central concept that functionally distinct plant types differ in their resource requirements. If resource requirements among plant types are complementary, the set of resources used in a functionally complex agricultural system will be greater than that used by a monocrop (Cannell et al., 1996; Narain et al., 1998). There is evidence that resource use is increased under agroforestry systems (Vandermeer et al., 1998), however more research on the basic biophysical processes that dictate resource partitioning among, and acquisition by, different plant types is needed (Rao et al., 1997). For this, both the availability of water resources and the ability of plants to acquire them need to be measured.
One way to identify different water resources and investigate their availability to plants is to measure the variation in the relative abundance of the stable isotopic species that comprise water (hydrogen, deuterium16O and18O) (Smith et al., 1997). The distribution of these isotopes in soil is influenced by the numerous physical and chemical processes that govern the hydrologic cycle. Temperature, altitude and distance from the ocean/site of evaporation alter the isotopic composition of rainfall (Dawson, 1993b). Rainfall reaching land surfaces is subject to immediate evaporation and the remaining water percolates through bulk soil in response to gravity, where soil texture influences the rates of diffusion and convection of the water and its constituent isotopes (Barnes and Allison, 1984). Evaporation of soil water enriches the remaining water in heavier isotopic forms since light fractions evaporate most readily. The influence of evaporation is greatest close to the soil surface, thus creating a profile of declining enrichment with depth. However, migration of water vapour (depleted in heavy isotopes) through the soil atmosphere via diffusion partially offsets the enrichment caused by evaporation and may actually deplete
water of heavy isotopes at the soil surface. The net result of these opposing processes is that the maximum enrichment of water with heavy isotopes may be some distance below the evaporation front (Barnes and Allison, 1984). In contrast to the physical processes governing the distribution of isotopes of water within soil, mechanisms of water uptake by plants are non-fractionating (see Turner et al. (1987) and references therein). Hence, uptake of water by plant roots will alter the amount, but not the isotopic composition, of remaining water in the soil. As a result of these processes, spatial and temporal variations in isotope abundance are common throughout most soil profiles. Once characterised, these variations can be used to infer the contribution of particular water sources to plant hydrology (Dawson et al., 1998).
We explored the utility of isotope (H/D) analyses to indirectly determine if tree roots reached groundwater or exploited only soil water. Since trees at our site exhibited `hydraulic redistribution' of soil water (Burgess et al., 1998), we also aimed to test whether this process influenced the isotopic composition of water in surface layers, thus indicating the lateral extent of tree roots into the cropping zone.
2. Methods
2.1. Site description
The site was located 11 km west of Katanning, 280 km south east of Perth in south-western Australia (338450
S, 1178270
E, see Fig. 1). The area has a Mediterranean-type climate characterised by cool wet winters and hot dry summers. Annual rainfall is 485 mm (116 days of rain), whilst class A pan evaporation is 1826 mm. Seasonal rainfall distribution at Katanning is shown in Fig. 2. Amount-weight isotopic composition (dD) of
rainfall for three sites in south-western Australia (marked in Fig. 1) is shown in Table 1. The soil profile at Katanning is duplex (Typic Palexerult, USDA Soil Taxonomy) with an A horizon of loamy medium sand to clayey coarse sand (generally to 50 cm) underlain by a B horizon of medium to light clay with 10 to 20% smooth faced lateritic gravel at depth. The textural/permeability contrast between upper and lower soil horizons causes discontinuous hydraulic properties that can lead to transient waterlogging. Water tends to pond at the base of the sandy A horizon, since the clay B horizon has extremely low permeability that restricts water entry (Tennant et al., 1992; Cox and McFarlane, 1995). In an attempt to reduce episodic waterlogging as well as groundwater recharge, mixed 8 m wide belts ofEucalyptustrees (Eucalyptus salignaSmith,Eucalyptus camaldulensis
Table 1
Summary of isotopic data collected for rainfall at three sites (see Fig. 1) in south-western Australia
Sample dD (%)
Amount weighted monthly rainfall (Perth 1983±1995) (J. Turner, personal communication, 1999) ÿ18.1 Amount-weighted rainfall (Salmon catchment, Collie WA, 1985, Turner et al., 1987) ÿ19.5 Amount weighted rainfall (Susannah Brook catchment, Gidgegannup WA, 1987)
(Turner and Macpherson, 1990)
ÿ24.5
Dehnh.,Eucalyptus leucoxylonF. Muell andEucalyptus platypusHook) were planted in 1986. Tree belts were planted immediately downslope from interceptor drains spaced 100±200 m along contour lines to intercept subsurface water flow. The trees had an average height of 5.5 m in September 1997 (White et al., 2000). Between tree belts,
Fig. 1. The location of the research site in relation to sites for which the isotopic composition of rainfall has been measured.
Fig. 2. The pattern of rainfall distribution of Katanning.
canola and cereals were cropped in rotation with lucerne (Medicago sativa L.) and subterranean clover (Trifolium subterraneanL.) pastures.
2.2. Sample collection
2.2.1. `End member' study
The first study was performed in November 1997, when the predawn leaf water potentials in E. camaldulensis, E. leucoxylon and E. platypus were ÿ0.7, ÿ1.6 and ÿ2.2 MPa, respectively (White et al., 2000). Under the `end member' model (Dawson, 1993a), groundwater and either rainwater or shallow soil water are considered as two potential and discrete `end member' sources of which a plant may use as a mixture. If the isotopic signature (dD) of the water sources differ, the contribution of each source to the dD of plant xylem sap can be calculated. This approach has been used successfully in a
number of studies, mainly in North America (Dawson and Ehleringer, 1991; Ehleringer et al., 1991; Thorburn and Ehleringer, 1995; Dawson, 1998; Dodd et al., 1998). Samples of groundwater (at approximately 5±7 m depth) were taken from dip-wells beneath trees at four positions along a tree belt. Dip-well positions were 20 m apart along the contour line on which the tree belt was planted. Perched water (at 0.5 m depth) was collected at two positions from shallow piezometers adjacent to the dip-wells. Topsoil (0±20 cm depth) was collected directly beneath trees at positions coinciding with the dip-wells. A30 g soil sub-sample was sealed in an airtight vial. ThedD of water in
topsoil was presumed to represent the integrated dD signals of recent rain events
(modified partly by evaporation) and the source of water available to fine roots in the topsoil.
At four positions coinciding with dip-wells, xylem sap was extracted from suberised stems (Dawson and Ehleringer, 1993) of two specimens of each of the fourEucalyptus species. Stems were harvested with pruning shears and the cut end of each stem placed in the open mouth of a plastic vial. A hypodermic needle attached to a vacuum hose was also placed into the vial. Mouldable putty was used to seal the stem and needle around the mouth of the vial. A hand vacuum was used to create a mild vacuum in the vial. To aid displacement of sap by vacuum, the length of the stem section was progressively pruned to reduce capillary tension in the xylem (Bollard, 1960).
Root bases of the lucerne plants were also harvested to provide an indication of topsoil waterdD in the field adjacent to the tree belt.
2.2.2. Detailed soil analysis
A second study involving more detailed soil analysis was then performed in February 1998, when the predawn leaf water potentials inE. camaldulensis,E. leucoxylonandE. platypus were ÿ0.9, ÿ2.8 and ÿ4.1 MPa, respectively (White et al., 2000). Vertical profiles were sampled at the positions beneath the belt of trees using a rotary, air-blast drilling rig. A third profile was sampled approximately 50 m distant to the tree belt in the adjacent pasture. Heavy clay sub-soils prevented the use of other means of soil sampling. Care was taken to immediately collect samples from within clumps of spoil. Soil was collected at depth intervals of 20 cm from 0±200 cm and at 50 cm intervals from thereon until groundwater was reached.
Soil was also collected from three horizontal transects at a depth of 20 cm using a hand auger. Samples were taken every 30 cm along each transect to a distance of 3 m from the tree belt. From thereon, soil was sampled every 60 cm to a distance of 9 m (approximately 1(1/2) tree heights).
Care was taken with all samples to avoid isotopic fractionation by evaporation during collection and all samples were stored frozen.
2.3. Extraction of water from soil samples
Soil water was extracted from soil by cryogenic vacuum distillation (Ehleringer and Osmond, 1989). Two Vycor glass tubes were attached to a vacuum pump in Y-shape configuration. Approximately 15 g of soil was placed in one tube and frozen by submerging the tube in liquid nitrogen. Both tubes were evacuated and then isolated from the vacuum line to create a closed U-shape configuration. The tube containing the sample was placed in boiling water, whilst the second tube was placed in liquid nitrogen to `trap' water evaporating from the heated sample. After 1 h, the collection tube was removed and sealed. After thawing, the collected water was decanted into an airtight vessel.
2.4. Preparation of samples for mass spectrometry
Water samples were reduced to hydrogen gas for analysis using a gas phase mass spectrometer (VG Isogas SIRA 10). Granulated zinc reactant (0.4 g) was dried by heating under vacuum and placed in a Vycor reaction tube back-filled with nitrogen gas. A 0.2ml aliquot of sample was added and then frozen with the catalyst by immersing the tube in liquid nitrogen. The nitrogen gas was then evacuated from the reaction tube and the tube sealed before heating at 5008C for 1 h.
Hydrogen/deuterium ratios (dD) were expressed according to standard notation in parts
per thousand relative to the VSMOW (Vienna Standard Mean Ocean Water) standard.
dD D=Hsample D=Hstandardÿ1
1000
3. Results
3.1. `End member' study
Comparison of the dD values of water from topsoil beneath the trees and those of
groundwater indicated that water from topsoil was enriched in deuterium relative to groundwater by8%(Table 2). Perched groundwaterdD was intermediate to that of soil
water or deeper groundwater (see Table 2). Water extracted from the root base of lucerne plants showed greater enrichment in deuterium than that of topsoil collected beneath the trees by10%(Table 3).
At the time of the study (November 1997), the trees were moderately short of water (White et al., 2000). ThedD of xylem sap ofE. camaldulensis,E. saligna,E. leucoxylon
Table 2
ThedD (standard error) of different water sources at different depths within a duplex soil pro®le at Katanning, November 1997
Sample n Depth of source (m) dD (%)
Topsoil water 4 0.2 ÿ21.111.00
Perched water 2 1 ÿ24.262.19
Groundwater 7 3.5±7.6 ÿ29.530.53
Fig. 3. ThedD of water from soil collected at different depths in the soil pro®le beneath either a belt of eucalypt trees (A and B) or lucerne pasture (C) during February 1998. Grey bars indicated the range of groundwaterdD values measured during November 1997.
Table 3
Average (standard error)dD values of xylem sap extracted from stems of fourEucalyptspecies and roots of lucerne (M. sativa) at Katanning, November 1997
Specimen Sample n dD (%)
E. camaldulensis Stem xylem sap 6 ÿ23.081.92
E. saligna Stem xylem sap 7 ÿ23.891.26
E. leucoxylon Stem xylem sap 8 ÿ22.911.94
E. platypus Stem xylem sap 7 ÿ20.271.82
M. sativa Root xylem sap 2 ÿ11.451.11
Fig. 4. ThedD of water from soil collected at 20 cm depth along three transects (A±C) perpendicular to a belt of eucalypt trees during February 1998.
andE. platypusranged from ÿ20 toÿ24%(Table 3). These values were significantly
different (p<0.05 Mann±Whitney U-test) fromdD of groundwater (ÿ29.53%), but were
not significantly different (p>0.05) fromdD of topsoil water (ÿ21.11%) or perched water
(ÿ24.26%).
3.2. Detailed soil analysis
At the time of the detailed soil analysis, the predawn water potentials in leaves of the trees at the site were at their lowest in 3 years of study (White et al., 2000). Thus the soils were extremely dry. Data from the two vertical profiles beneath the belt of trees and one profile beneath pasture are shown in Fig. 3A±C. ThedD profiles showed little trend with
depth, centering on a value of approximatelyÿ30%, with the exception of the surface
(20 cm) values, which had a value of approximatelyÿ50%. The three horizontal profiles
(Fig. 4A±C) showed no clear trend with distance from the tree belt and there was considerable spatial variation (e.g. see especially Fig. 4B).
4. Discussion
4.1. `End member' study
In isolation from the data collected during the detailed soil analysis, the initial `end member' data suggested that groundwater and soil water had a different isotopic composition. In addition, water from topsoil collected beneath trees showed greater similarity to groundwater than did topsoil water under pasture as indicated by lucerne xylem sap (Table 3). These data were consistent with the hypothesis that some water in the surface soil under the trees had been `lifted' by the trees. Dawson (1993a) drew such a conclusion on the basis of similar measurements of topsoil underAcer saccharum. During the dry conditions of November, tree hydration appeared to be at least partly due to sources of water other than groundwater. However, Brunel et al. (1995) state that the error fordD techniques (including sample extraction, analysis, etc.) is5%, hence an attempt
to distinguish between `end members' that differ less than 10% may be beyond the
sensitivity of the technique.
4.2. Detailed soil analysis
The uniformity ofdD values for water from soil at different depths within the profile was not expected, either from the results of the first study or from theoretical predictions. For example, the predicted enrichment of deuterium in water near the soil surface (Barnes and Allison, 1984) was not observed. Our chosen sample interval (every 20 cm to a depth of 2 m) may have been too coarse to elucidate the enrichment profile. However, Turner et al. (1987) sampled at less frequent intervals than in our study and demonstrated the predicted surface enrichment. Our results, however, indicate a depletion maximum (as predicted by Barnes and Allison (1984)) near the soil surface (see Fig. 3A and C) in contrast to the results of Turner et al. (1987). A feature common to our data and that of
Turner et al. (1987) was the nearly uniformdD values of the unsaturated zone at depth.
These values were also similar to those of groundwater, making inferences regarding the source of water used by plants difficult.
At sites where there are large variations in thedD values (in some cases, >100%) of
potential sources of water (including snowmelt and fog water as well as rainfall, soil water and groundwater), researchers have been able to apportion the water taken up by plants among the various sources (Dawson and Ehleringer, 1991; Ehleringer et al., 1991; Thorburn and Ehleringer, 1995; Dawson, 1998). Comparable studies in southern Australia have typically found much smaller ranges ofdD (Thorburn and Walker, 1994;
Thorburn and Ehleringer, 1995; Dawson and Pate, 1996; Bleby et al., 1997) reducing the scope within which to discriminate differing water sources. Turner et al. (1987), for example, found the averagedD of shallow groundwater in the Salmon catchment (Collie, see Fig. 1) showed little variation at ÿ21.01.0 (n70). Furthermore, at a number of sites in south-western Australia,dD values of soil water are uniform throughout the soil
profile and differ little from groundwater (Turner et al., 1987; Farrington et al., 1996). Turner et al. (1987) suggested that the isotopically uniform profiles of lateritic soils in south-western Australia result from good mixing of isotopically variable rainfall during recharge. In view of the findings of Burgess et al. (1998) that root systems of trees transfer water between soil layers in response to gradients in water potential, we suggest that plant roots contribute to the mixing of water within the profile by redistributing water. For example, measurements of soil moisture at Katanning demonstrated that water redistributed from shallow soil layers to deeper layers (1.8±2.6 m) by tree roots increased soil moisture content by up to 2.8%, 1 month after break of season in 1998 (Burgess et al., 2000). On a v/v basis, up to 14% of the water contained in deep soil layers was water from the topsoil transferred by plants. Lucerne roots would probably also redistribute water to some extent since previous studies have shown that this species also exhibits hydraulic lift (Caldwell et al., 1998). Clearly, extantdD discrimination between subsoil and surface soil water would have been reduced by up to 14%, by mixing resulting from water transfer by plant roots.
Much of the annual rainfall at Katanning would probably have adD similar to the
mean weighted values recorded at other sites in south-western Australia (Table 1), but perhaps further depleted due to the inland position of Katanning (Dawson, 1993b). The importance of large winter rain events (see Fig. 2) to soil recharge probably contributed to the uniformity of soil water dD within the profile. Mixing due to the action of root
systems would further homogenise thedD values of water in the soil profile. Exchange
among water sources by hydraulic redistribution should have most impact where sources within the soil profile have markedly different water potentials. The strongly seasonal rainfall distribution of Katanning's Mediterranean-type climate favours the establishment of strong gradients in water potential between water sources. A further possibility is that the hydraulic discontinuity between the A horizon (sand) and subsequent horizons (clay) isolates these parts of the soil profile (>50 cm deep) from the fractionating process of evaporation, but not to extraction of water by plants (which is non-fractionating).
Horizontal transects showed no apparent trend indD of topsoil water with distance from trees.dD of topsoil water were both more enriched and more depleted in deuterium
enrichment may be found in the upper few centimetres of the soil profile (Barnes and Allison, 1984). Spatial heterogeneity of soil characteristics and microclimate add to this complexity (Le Roux et al., 1995). Duplex soils have a characteristically variable subsurface relief, leading to significant spatial variation in the depth boundaries between horizons (Tennant et al., 1992). Such variability will increase the difficulty of measuring the influence of `hydraulic lift' on soil waterdD. ThedD of topsoil water collected during
the November study (Table 2) was more uniform than that collected in February, probably as a result of rainfall (5 mm) in November, and suggests that further drying serves to increase heterogeneity in topsoildD.
Unlike the results of Dawson (1993a), which clearly demonstrated that water exudation from roots ofAcer saccharummodified thedD of topsoil water as a function of distance from tree (and decreasing root length density), our data were inconclusive. The absence of a cleardD discrimination among water sources, coupled with variable dD of topsoil water in the sand horizon of the duplex soils at our site, prevented the tracing of the acquisition and redistribution of water sources by the trees.
5. Conclusions
Non-destructive methods for assessing potential complementarity in patterns of water extraction among species are limited. Isotopic techniques have the potential to indirectly assess resource access by plant roots. However, we conclude that these techniques are difficult to apply at sites where uniformity ofdD with soil depth, heterogeneity at the
surface level or limited range ofdD among differing water sources exist. Measurements
of `end member' water sources alone provide insufficient information regarding the spatial variability of dD in the soil profile and are potentially misleading. Robust characterisation of spatial variation in the natural abundance of stable isotopes must be undertaken when using environmental isotope techniques. In environments where insufficient isotopic discrimination exists among water sources, labelling techniques such the application of deuterium enriched water either by irrigation or direct injection into plant tissues may be more appropriate.
Clearly, much research effort will be required to characterise the belowground functioning of woody perennials so as to predict their effects on the productivity and sustainability of agroforestry systems. Until new methods are developed, it is likely that root excavations and measurements of soil moisture will remain the basic tools to investigate the belowground resource acquisition by different plant functional types.
Acknowledgements
This work was supported by the Australian Centre for International Agricultural Research. Support for S.S.O.B. was provided by the Western Australian Department of Conservation and Land Management. We thank Drs Jeff Turner and Don White (CSIRO) for reviewing an earlier version of this paper and suggesting improvements. We are grateful to Todd Dawson for advice and assistance with sample collection and analysis
and Lidia Bednarek performed the mass spectrometry. Frank Dunin, Phil Ward, Shane Micin and Gerald Watson assisted with sample collection. The Grains Research and Development Corporation and Agriculture Western Australia contributed to site establishment and maintenance.
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