Soil water is an important driving force of the
ecosystems, especially in the semiarid hill and gully region of the
northwestern Loess Plateau in China. The mechanism of soil water migration
in the reconstruction and restoration of Loess Plateau is a key scientific
problem that must be solved. Isotopic tracers can provide valuable
information associated with complex hydrological problems,
difficult to obtain using other methods. In this study, the oxygen and hydrogen
isotopes are used as tracers to investigate the migration processes of soil
water in the unsaturated zone in an arid region of China's Loess Plateau.
Samples of precipitation, soil water, plant xylems and plant roots are
collected and analysed. The conservative elements deuterium (D) and
oxygen (
Water in the soil environment plays a crucial role as a carrier of dissolved and solid species and as a reservoir in the hydrological cycle. Soil water represents a small proportion (only 0.05 %) of the hydrological cycle, but it is vital for ecosystems and affects spatial and temporal processes at different scales (Koeniger et al., 2016; Busari et al., 2013). Understanding soil water migration in the unsaturated zone is essential to describe the movement of salt, carbon, nitrogen and other nutrients. Soil water migration plays an important role in the processes of infiltration, evaporation, transpiration and percolation, hydraulic conductivity and water uptake capacity of soils in the unsaturated zone. The traditional methods have been carried out to study the movement of soil water such as hydrologic experiments, intensive observations, modeling and remote sensing (Luo et al., 2013; Yang et al., 2013; Carucci et al., 2012). However, soil water migration is a complex nonlinear and inhomogeneous flow process. It is difficult to model on the basis of Darcy's law exclusively or using other techniques. Climatic conditions, soil texture and structure, antecedent moisture and vegetation cover exert influence on the movement of soil water (Bose et al., 2016; Kidron and Gutschick, 2013).
Isotopic tracers can provide valuable information on complex
hydrological problems, such as the runoff processes, residence time, runoff
pathway and the origin and contribution of each runoff component (Arny et al.,
2013; Ohlanders et al., 2013; Yang et al., 2012a; McInerney et al., 2011;
Maurya et al., 2011; Kevin et al., 2010). The isotopes of D and
Different flow mechanisms result in different isotopic profiles. Stable
isotope compositions of soil water, plant xylems and precipitation can be
obtained to identify soil water migration processes such as infiltration,
evaporation, transpiration and percolation (Caley and Roche, 2013; Yang et al.,
2012b; Catherne et al., 2010; Stumpp et al., 2009). Zimmerman et al. (1967)
first applied stable isotopes to study the soil water profile, showing
that evaporation at the surface of a saturated soil column causes deuterium
enrichment near the surface that decreases exponentially with depth.
Robertson and Gazis (2006) studied the seasonal trends of oxygen isotope
composition in soil water fluxes at two sites along the climate gradient.
The hydrogen and oxygen isotopes are used to study the transforming of
precipitation, soil water and groundwater of typical vegetation in area of the Taihang
Mountains (Song et al., 2009). Gazis and Feng (2004) compared the oxygen
isotope compositions of precipitation and soil water from profiles at six
sites with different soil textures.
Stable isotopes provide evidence for assessing plant water sources according
to their variations caused by equilibrium and kinetic isotopic fractionation
mechanisms (Orlowski et al., 2016a; Haverd et al., 2011; Zhao et al.,
2013; Bhatia et al., 2011). Several studies (Liu et al., 2013; Mathieu
and Bariac, 1996; Allison and Barnes, 1985) have shown that transpiration do not cause
isotopic fractionation of soil water. The isotopes
Soil moisture is an important driving force of ecosystems, especially in the northwestern Loess Plateau. Characterized by dry climate, less precipitation, more evaporation and thicker soil layers, groundwater in this region is difficult to use due to the depth of water table. Thus, soil water is almost the only water resource in the study area, and has become the only factor controlling agricultural production and ecological restoration. It is necessary to investigate the mechanism of soil water migration in the Loess Plateau. Stable isotopes can provide valuable information on the mechanism of soil water migration. At present, much research focuses on the soil water recharge by precipitation and its isotope variation characteristics. There is some research on the interaction of soil water, plant water and precipitation. These studies are mostly on the individual scale in one specific region (Lin and Horita, 2016; Heathman et al., 2012; William and Eric, 2010; Ferretti et al., 2003). However, the research on the migration process of soil water, plant water, precipitation and groundwater in the unsaturated zone based on the isotopic technique is still rare. At present, the mechanism of soil water migration in the Loess Plateau is a key scientific problem to be solved.
Therefore, this study investigates soil water migration processes in the unsaturated zone in the hill and gully region of Loess Plateau using isotopes, integrated with sampling in the field, experimental observations and laboratory analysis. Samples include the soil water, plant xylem and root, etc. The objectives of this research are (1) to probe the migration process and variation of plant water, soil water and precipitation and (2) to identify each potential water source uptake by plant, and evaluate their contributions of each potential water source in the unsaturated zone. It can provide a scientific basis for ecological water demand, ecological construction, and management of water resources.
The study area is located in the Anjiagou River basin in area of the city of Dingxi,
China, at 34
Location of the sampling sites in the study area.
The watershed area is 8.91 km
The study area has broken terrain and serious soil erosion, with an overall gully- and valley-filled landscape. Geological
structure is the uplift zone between the eastern part of Qilian fold system
and the western Qinling fold system, at an altitude of 1700–2580 m,
with the gully density of 3–5 km km
The vegetation type belongs to arid grassland vegetation type, with infrequent natural tree coverage. The grassland and shrubland ecosystems were
the most extensive dominant ecosystems. Woodland area is minimal, most
is open forest land. Areas with crown density of greater than 0.2
have only
The research was carried out in the Anjiagou River basin in the hill and gully
region of Loess Plateau from May 2013 to October 2015. Samples of
precipitation, soil, xylem and root were collected in the study area. The
locations of the sampling sites are shown in Fig. 1. Precipitation was
collected after each rainfall event. Precipitation was filtered and
transferred to sealed glass vials prior to analysis. Plant xylem and root of
Soil was sampled at 10 cm intervals for the first 40 cm, 20 cm intervals from 40 to 100 cm and 30 cm intervals from 100 to 130 cm. Maximum depths of sampling ranged up to 130 cm (plant root is rarely found below 100 cm in the study area). At each sampling site, soil moisture (volumetric soil water content) was obtained with time domain reflectometry (TDR) in the field manually at 0–10, 10–20, 20–30, 30–40, 40–60, 60–80, 80–100 and 100–130 cm. Soil moisture content was determined by oven drying simultaneously.
Water was extracted from soil, xylem, root, stem and leaf by cryogenic
vacuum distillation method, and the extracted water (2 to 10 mL) was trapped
at liquid nitrogen temperature. Vacuum distillation was considered a
reliable and acceptable method. The moisture in the soil or plants under the
condition of vacuum (vacuum below 60 mTorr), was heated to
105
When
Yurtsever and Gat (1981) modified Craig's global water line, and made a more
accurate global precipitation linear relationship between
Statistic characteristics of isotopic compositions for precipitation and soil water in the study area.
Stable isotope compositions of soil water are presented in Table 1. The
measured
Vertical profiles of soil water
Plot of
Isotope profiles of
The
However, the values of
Isotope profiles of
Values of soil moisture obtained by TDR in the Anjiagou River basin are shown in Fig. 5. The variation of soil moisture content in the shallow soil layer (0–40 cm) is extremely large, ranging from 5.94 to 17.51 %, which belongs to the highly variable layer. The variation of soil moisture content at the 40–60 cm soil depth is relatively small, which belongs to the less active layer. The variation of soil moisture content is relatively stable at 60–100 cm soil depth, ranging from 17.78 to 29.47 %, which belongs to the relatively stable layer. Soil water content also exhibits depth variation. Variability of water content is larger in the surface horizons more than 40 cm depth in the soil profile. The shallow soil layers are impacted by evaporation and precipitation recharge more than the deeper soil layer. Therefore, the high evapotranspiration rate and precipitation recharge are the main factors controlling soil moisture, especially at the surface horizon.
As the soil moisture profile shows (Fig. 4), soil moisture content is low in the shallow layer (0–20 cm) and relatively high at the active layer (40–60 cm), and then tends to be stable. Figure 4 shows that soil moisture content is different in different sample sites, but soil moisture content is low under 60 cm soil depths in all the sample sites. Soil moisture content generally increases with the increase of soil depth in the variable greatly layer. Soil moisture content decreases with the increase of soil depth in the active layer, and then tends to be stable.
As Fig. 4 shows, water content increases and soil water
The variation of soil moisture content at different soil depths.
Precipitation infiltration and evapotranspiration in the vertical direction
are the main processes of the water cycle, playing an important role in
the transformation process of the water cycle in the unsaturated zone. Different
migration mechanisms of soil water result in different isotopic profiles.
Therefore, isotope
The
At the sample sites on 20 July 2014 and 24 September 2014, the vertical trend of
The second peak of
Statistic characteristics of isotopic compositions for root water and xylem water in the study area.
The contributions of each potential water source to plants.
Zimmermann et al. (1967) studied the effect of transpiration with an
experiment in which the root water uptake of various plant species is
monitored, and no fractionation is found. Allison and Barnes (1985) tested a
much greater fraction of root water uptake and found no conclusive proof of
fractionation by root water uptake. Isotope fractionation is
not caused when soil water is taken up by plant roots. By comparing the
The isotopic composition of the soil layer is similar to the xylem water,
confirmed by comparing
Hooper (2003) and Christophersen and Hooper (1992) introduced the
end-member mixing analysis (EMMA), which is an often used method for
analyzing possible source area contributions to flow. Multiple-source mixing
models (Parnell et al., 2010; Phillips et al., 2013) account for water uptake
from more than one discrete soil layer and weigh the importance of certain
layers for water uptake by incorporating soil water potentials into the
calculation. In this study, the mixing model is used to identify potential
source areas and mixing processes, and to quantify the contribution of each
end member using mixture fractions. The conservative elements
It is more likely that plant xylem water is a mixture of soil water from several soil depths. Multiple-source mass-balance mixing models of proportional contributions (%) of plant xylem water (Parnell et al., 2010) is used to evaluate the contributions of each potential water source (Fig. 6). The results show that soil water from the surface horizons (20–40 cm) comprised 8–21 % of plant xylem water, while soil water at 40–60 cm soil depth comprised the largest portion of plant xylem water (ranging from 68 to 83 %). Soil water below 60 cm depth comprised 10–26 % of plant xylem water and only 0–18 % comes directly from precipitation. Water source is dominated by soil water at a depth of 40–60 cm.
The following conclusions can be drawn from the present study:
A sharp isotopic front at approximately 40 cm depth observed shortly
after an isotopic distinct rainfall event suggests that infiltration into
soil occurred as piston-type flow with newer water pushing older water
downward in the soil profile. Soil waters are recharged from precipitation.
The soil water migration is dominated by piston-type flow in the study area and rarely preferential flow, except where there are macrospores in the Loess
Plateau, caused by plant root or animal invasion, etc. Water
migration exhibited a transformation pathway from precipitation to soil
water to plant water. Soil water from the surface horizons (20–40 cm) comprised
8–21 % of plant xylem water, while soil water at 40–60 cm soil depth was the is the largest component of plant xylem water (ranging from 68 to 83 %).
Soil water below 60 cm depth comprised 10–26 % of plant xylem water
and only 0–18 % comes directly from precipitation. Water
source is dominated by soil water at depth of 40–60 cm.
The authors declare that they have no conflict of interest.
This research is supported by National Natural Science Foundation of China (41390464;41201043), and China Postdoctoral Science Foundation Funded Project (2014M550095). The authors are grateful to the experimental research station staff and all participants in the field for their contributions to the progress of this study. We also express our appreciation to the anonymous reviewers of the manuscript. Edited by: B. Hu Reviewed by: L. Bing and four anonymous referees