water
Article
Soil CO2 Uptake in Deserts and Its Implications to
the Groundwater Environment
Wenfeng Wang 1 , Xi Chen 1, *, Hongwei Zheng 1 , Ruide Yu 1 , Jing Qian 2 , Yifan Zhang 3
and Jianjun Yu 4
1
2
3
4
*
State Key Laboratory of Desert and Oasis Ecology, Xinjiang Institute of Ecology and Geography,
Chinese Academy of Sciences, Urumqi 830011, China; [email protected] (W.W.);
[email protected] (H.Z.); [email protected] (R.Y.)
Center for Geo-Spatial Information, Shenzhen Institutes of Advanced Technology,
Chinese Academy of Sciences, Shenzhen 518055, China; [email protected]
Department of Chemistry, Inha University, 100 Inharo, Incheon 402-751, Korea; [email protected]
Environmental Change Institute, University of Oxford, Oxford OX1 3QY, UK; [email protected]
Correspondence: [email protected]; Tel.: +86-0991-7885-303
Academic Editors: Y. Jun Xu, Guangxin Zhang and Hongyan Li
Received: 30 June 2016; Accepted: 30 August 2016; Published: 5 September 2016
Abstract: Recent studies of soil carbon cycle in arid and semi-arid ecosystems demonstrated that
there exists an abiotic CO2 absorption by saline-alkali soils (Aa ) at desert ecosystems and suggested
potential contributions of CO2 dissolution beneath deserts to the terrestrial ecosystems carbon balance.
However, the overall importance of such soil CO2 uptake is still undetermined and its implications to
the groundwater environment remain unaddressed. In this manuscript, a simple method is proposed
for the direct computation of Aa from the total soil CO2 flux (Fa ) as well as for the evaluation
of Aa importance to Fa . An artificial soil-groundwater system was employed to investigate the
implications to groundwater environment and it was found that soil CO2 uptake in deserts can
contribute a possible influence on the evolution of the groundwater environment, providing that the
absorbed CO2 largely remained in the soil-groundwater system.
Keywords: abiotic CO2 absorption; soil CO2 flux; the soil-groundwater system
1. Introduction
Global soil CO2 releases account for 20%–38% of the annual input of CO2 from terrestrial and
marine sources to the atmosphere [1–11]. Therefore, soil CO2 flux (Fa ) is a significant determinant
of the ecosystem carbon balance. Among the components of Fa , the biotic processes (including the
root and microbial respiratory components) were thought to be significantly contributing to it [2–6],
while the abiotic processes were always neglected [12–20]. However, a series of recent studies of deserts
ecosystems have highlighted that the abiotic processes (including subterranean CO2 dissolution and
other abiotic carbon sink beneath deserts) must be taken into account in the budget of Fa [7–10].
Some recent studies attributed the abiotic processes to the effects of the soil water dynamics
on subterranean carbon cycle and presented theoretical and experimental analyses based on the
biogeochemical reactive transport modeling [21,22]. This not only developed the methodology of
the partition of Fa into the biotic and abiotic components across some special environments, but also
highlighted that the subterranean dissolution of CO2 has been involved in the ecosystem carbon
balance in deserts, and can even temporally dominate [7–11]. However, soil CO2 dissolution and
effusion, as abiotic processes in deserts, are different from the considered processes in these previous
studies. Especially in some experimental contrast, only the top few cm of the soil was considered.
It became much more impressive after the abiotic CO2 absorption (Aa ) by the soil collected from
a saline desert was directly observed after a minimizing-disturbance sterilization treatment on the
Water 2016, 8, 379; doi:10.3390/w8090379
www.mdpi.com/journal/water
Water 2016, 8, 379
2 of 12
soil [8]. This demonstrated that there exists an abiotic CO2 uptake by saline-alkali soils (Aa ) at desert
ecosystems and suggested a potential contribution of such soil CO2 uptake to the ecosystem carbon
balance in deserts [8,10].
There is strong evidence suggesting that Aa can contribute significantly to Fa and the net ecosystem
carbon balance, but the overall importance of the soil CO2 uptake to Fa is still undetermined. Presently,
both the field observations and the laboratory experiments are still limited on site scales. Because of
the lack of large-scale experiments, the magnitude of such CO2 uptake is still a matter of controversy.
Some recent publications have further presented the laboratory data on the soil abiotic CO2 absorption
and the field data on the net CO2 exchange in desert ecosystems [23,24], whereas it is still imperative
to have a simple method for a direct computation of the abiotic soil CO2 uptake from the total soil CO2
flux and for quantifying its overall importance to Fa .
The minimizing-disturbance sterilization treatment on soils has been widely recognized as
an experimental method for separating Aa [10]. Since the significance of CO2 dissolution and effusion as
the abiotic processes has become much more impressive after the soil CO2 uptake was directly observed
after such a sterilization treatment on the soil [8,10], another subsequent unresolved issue is whether
the beneath CO2 dissolution has a non-negligible implication to the groundwater environment beneath
deserts, providing that the dissolved and absorbed CO2 largely remained in the soil-groundwater
system. This certainly desires a further investigation but remains unaddressed.
This manuscript aims to design a simple method for a direct computation of the abiotic soil
CO2 uptake from the total soil CO2 flux and proposed a simple approach to quantify its overall
importance to Fa . After a special treatment, Fa is reconciled as a simple function of Aa , which is
implemented in two different approaches. In the experimental approach, all Aa data are collected after
the minimizing-disturbance sterilization treatment on the soils, while in the other semi-experimental
approach, most of the Aa data are directly calculated from a simple regressive model. The potential
implications of Aa and the beneath CO2 dissolution to the groundwater environment are also
preliminarily discussed.
2. Materials and Methods
2.1. Data Sources
Sampling data of soil CO2 fluxes were collected from Xie et al. [8]. The measurements were
conducted in the Gurbantunggut Desert, which is located at hinterland of the Eurasian Continent
(87◦ 560 E, 44◦ 170 N; elevation: 461 m), with an arid, windy climate (sunshine: 3079 h; precipitation:
144.7 mm; evapotranspiration: 2020 mm; radiation: 5439 MJ/m2 ; velocity: 2.6 m/s). Note that two
LI-8100s (LI-COR, Lincoln, Nebraska, NE, USA) have been employed for the valid comparison between
the flux data measured at the sterilized and unsterilized samples. It should be noted that the soil was
smashed, roots-sieved, air-dried and the many sinks and sources of CO2 in the subsurface environment
have been excluded. After sterilization, the net soil CO2 fluxes were negative and the CO2 exchange
was abiotic. In order to highlight the soil CO2 uptake, such abiotic exchange is hence defined as
“soil abiotic CO2 absorption” throughout this manuscript (Figure 1). Consequently, the variations of Fa
were largely determined by Aa and the soil microbial respiration (Rm ).
With the purpose of a preliminary discussion on the potential implications of the beneath CO2
dissolution to the groundwater environment, an artificial soil-groundwater system was employed
and the groundwater were sampled with six replications (Figure 2). The groundwater pH and the
concentration of dissolved inorganic carbon (DIC) in the artificial soil-groundwater system were
measured, where the concentration of DIC (symbolized as (DIC) was defined as the sum of the
concentration of the dissolved carbonate ion and bicarbonate ion.
Water 2016, 8, 379
Water 2016, 8, 379 3 of 12
3 of 12 Water 2016, 8, 379 3 of 12 CO2 flux averaged from sampled days[ mol m-2 s-1]
CO2 flux averaged from sampled days[ mol m-2 s-1]
2.5
flux data measured before sterilization
flux data measured after sterilization
flux data measured before sterilization
flux data measured after sterilization
2.5
2
1.5
2
1.5
1
1
0.5
0.5
0
0
-0.5
-0.5
-1
-1
-1.5
0
-1.5
0
1
1
2
2
3
3
4
4
5
5
Time [d]
Time [d]
6
7
6
7
8
8
9
9
Figure 1. Contrast of mean CO
Figure
1. Contrast of mean CO222 flux before/after sterilization. flux before/after sterilization.
Figure 1. Contrast of mean CO
flux before/after sterilization. Figure
2.
diagram theartificial
artificial soil-groundwater
soil‐groundwater system. collected from #1, #1,
Figure
2. A A A diagram
ofof the
system.The Thesamples sampleswere were
collected
from
Figure
2.
diagram of the
artificial
soil‐groundwater system. The samples were collected from #1, with 6 replications. with
6 replications.
with 6 replications. 2.2. Modeling Approach 2.2.
Modeling Approach
2.2. Modeling Approach Neglecting Aa, it is well‐known that Fa can be formulated as an exponential function of the soil Neglecting
Aaa, it is well‐known that F
, it is well-known that Faa can be formulated as an exponential function of the soil can be formulated as an exponential function of the soil
Neglecting A
surface temperature or the ambient air temperature [19]: surface
temperature or the ambient air temperature [19]:
surface temperature or the ambient air temperature [19]: Fa  Fa 20  Q10（TS  20）/10 (1) （
 20
(TTSS−
20）
)/10
/10

F

Q
FaF=
F
×
Q
(1)
(1) a20
10
a
a 20
10
where Fa20 is measured Fa at 20 °C, TS is the soil temperature (here it is measured at 0–10 cm), and Q10 is the relative change in F
a20 is measured F
10 where F
where
Fa20
is measured Faa at 20 °C, T
ata with 10 °C increases. 20 ◦ C, TSS is the soil temperature (here it is measured at 0–10 cm), and Q
is the soil temperature (here it is measured at 0–10 cm), and Q10
R
m
is formulated as in [20]: is the relative change in F
is
the relative change in Faa with 10 °C increases. with 10 ◦ C increases.
R
m
is formulated as in [20]: Rm is formulated as in [20]:
R m  R m20  Q10(TS 20)/10 (2) )/10
S −20
Rm = Rm20 · Q10 (T(T
(2)
S 20)/10
R m  R m20  Q10
(2) where Rm20 is the measured value of Rm at 20 °C. where RTaking into account A
of Rm at 20 ◦ C.
m20 is the measured value
a, Fa is reconciled as, m at 20 °C. where Rm20 is the measured value of R
Taking into account Aa , Fa is reconciled as,
Taking into account Aa, Fa is reconciled as, Fa  R m  A a (3) FFa =
Rm + Aa
(3)
(3) a  R m  Aa Water 2016, 8, 379
4 of 12
The experimental method separates Aa by the sterilization treatment on the soils and hence Rm20
can be estimated as follows:
Rm20 = Fa20 − Aa20
(4)
where Aa20 are measured Aa at 20 ◦ C.
Substituting Equation (2) into Equation (4), Rm is reconciled as:
Rm = (Fa20 − A20 ) · Q10 (TS −20)/10
(5)
Correspondingly, Fa can be finally reconciled as:
Fa = (Fa20 − A20 ) · Q10 (TS −20)/10 + Aa
(6)
Unfortunately, extensive time is required during the minimizing-disturbance sterilization
treatment on the soil. At large scales, it is necessary to couple with other methodologies. Development
of a semi-experimental method for the rough partition of Aa is thus necessary.
Similar to semi-experimental approaches for the partition of the biotic components of Fa [12],
the relationship between Aa and Fa could be assumed to be represented by a simple linear regressive
model and written as:
Aa = β0 Fa + β1
(7)
where β0 and β1 are determined by the contributions of Aa to Fa .
Under this assumption, Rm is reconciled as:
Rm = (1 − β0 )Fa − β1
(8)
Rm10 = (1 − β0 )Fa10 − β1
(9)
and correspondingly
By Equation (2), the Q10 value for Rm can be estimated by
Rm = [(1 − β0 )Fa10 − β1 ] · Q10 (TS −10)/10
(10)
Combining Equations (8) and (10), the soil CO2 flux could be finally reconciled as
Fa = [Fa10 −
β1
β1
] · Q10 (TS −10)/10 +
1 − β0
1 − β0
(11)
The parameters β0 and β1 can be estimated by Equation (7) after a few experiments and a large
magnitude of time will be saved in the partition of Aa .
Regarding the first approach, the relationship between the groundwater pH and (DIC) are
analyzed by utilizing an exponentially regressive model:
pH = λ0 eλ1 (DIC)
(12)
where λ0 and λ1 are determined by the contributions of (DIC) to the groundwater pH.
3. Results and Discussion
3.1. Separated Biotic and Abiotic Components of Soil CO2 Fluxes
The aforementioned parameters of β0 and β1 are estimated using the measured values of Aa and
Fa on two clear days in two typical desert ecosystems, saline desert and oasis farmland (Figure 3).
Water 2016, 8, x 5 of 13 The aforementioned parameters of β
0 and β1 are estimated using the measured values of Aa and Water 2016,
8, 379
5 of 12
Fa on two clear days in two typical desert ecosystems, saline desert and oasis farmland (Figure 3). Water 2016, 8, 379 5 of 12 0
0.2
Data
Fit
Confidence bounds
-0.2
a
Data
Fit
Confidence bounds
0.1
0
-0.4
b
-0.1
-0.2
y
y
-0.6
-0.8
-0.3
-0.4
-0.5
-1
-0.6
-1.2
-0.7
-1.4
-1
-0.5
x
0
-0.8
0.5
0
0.5
1
x
1.5
2
2.5
Figure 3. Parameters β0 and β1 estimated at desert ecosystems (a) and oasis farmland (b).
Figure 3. Parameters β0 and β1 estimated at desert ecosystems (a) and oasis farmland (b). Figure 3. Parameters β0 and β1 estimated at desert ecosystems (a) and oasis farmland (b). The
estimated parameters of β00 and β
and β11 are then applied to the semi‐experimental separation of are then applied to the semi-experimental separation of
The estimated parameters of β
The estimated parameters of β
0 and β1 are then applied to the semi‐experimental separation of AaAata at the corresponding ecosystems in a drought period of one growing season in 2006, taking into the corresponding ecosystems in a drought period of one growing season in 2006, taking into
Aa at the corresponding ecosystems in a drought period of one growing season in 2006, taking into account
their difference for different ecosystems (the estimated β00 and β
and β1 from saline desert are also account their difference for different ecosystems (the estimated β
1 from saline desert are also
account their difference for different ecosystems (the estimated β0 and β1 from saline desert are also applied to its neighbor abandoned farmland, as provided in Table 1). applied
to its neighbor abandoned farmland, as provided in Table 1).
applied to its neighbor abandoned farmland, as provided in Table 1). Table
1. Suitable parameters for the isolation of Aa. a .
Table 1. Suitable parameters for the isolation of A
Table 1. Suitable parameters for the isolation of Aa. Study Sites Parameters Applied in Semi‐Experimental Method Study Sites
Parameters Applied in Semi-Experimental Method
Study Sites Parameters Applied in Semi‐Experimental Method Saline desert β0 = 0.5284 β1 = −0.8975 Saline desert
β0 = 0.5284
= −0.8975
Saline desert β0 β= 0.5284 βββ1 1 1= −0.8975 Abandoned farmland 0 = 0.5284 = −0.8975 Abandoned farmland
β0 = 0.5284
β1 = −0.8975
Abandoned farmland β0 β= 0.5284 ββ1 1 = −0.8975 0 = 0.4266 = −0.8330 Oasis farmland Oasis farmland
β = 0.4266
β = −0.8330
0
1
Oasis farmland β0 = 0.4266 β1 = −0.8330 It is obvious that Aa separated in both the experimental and semi‐experimental approaches acts It is
obvious
that Aa aseparated
in both
the experimental
and semi-experimental approaches acts
It is obvious that A
separated in both the experimental and semi‐experimental approaches acts as a CO
2 sink. Aa separated into two approaches varies in a similar pattern during the considered as drought period. Through comparison with the total soil CO
a
CO
sink.
A
separated
into
two
approaches
varies
in
a
similar pattern during the considered
a a separated into two approaches varies in a similar pattern during the considered 2 2 sink. A
as a CO
2 flux Fa, it was found that most Fa data drought
period. Through comparison with the total soil COa, the microbial soil respiration R
flux Faa, it was found that most F
, it was found that most
a data
drought period. Through comparison with the total soil CO
22 flux F
aF
data are positive. This implies that the other flux component of F
m, plays area dominant role within this period (Figure 4) positive.
This
implies
that
the
other
flux
component
of
F
,
the
microbial
soil
respiration
R
,
m plays
are positive. This implies that the other flux component of Fa, the microbial soil respiration Rm, plays a dominant
role within this period (Figure 4)
a dominant role within this period (Figure 4) 4
-2 -1
-2 m
hourly
CO2[ flux
mol
mol[ m
hourly
meanmean
CO2 flux
s-1] s ]
24
Total soil CO2 flux Fa
02 Total soil CO2 flux Fa
-20
0
-2
10
01
-10
-2
-1
0
-2
10
01
-10
-2
-1
0
-2
0
50
50
Experimental isolation of Aa
100
150
200
250
100
150
200
250
100
150
200
250
100
150
200
250
150
200
250
150
200
250
Experimental isolation of Aa
50
50
Semi-experimental isolation of Aa
Semi-experimental isolation of Aa
50
100
50
100
Time [h]
Time [h]
Figure 4. Isolated Aa by experimental/semi‐experimental method. Figure 4. Isolated Aa by experimental/semi‐experimental method. Figure 4. Isolated Aa by experimental/semi-experimental method.
Considering that β0 and β1 are determined by the contributions of Aa to Fa, they might be very Considering that β0 and β1 area is not very different, but F
determined by the contributions
of Aa to Fa , they might be very
different in various ecosystems (A
a can be very different). Choosing soil different
in various ecosystems (Aa is not very different, but Fa can be very different). Choosing soil
sites for the few sampled experiments should be done cautiously. sites forThe the values few sampled
shouldsoil be respiration done cautiously.
of Q10 experiments
for the microbial Rm might also be different in various The
values
of
Q
for
the
microbial
soil
respiration
also be
different
in various
m might
10 visualization of the separated RmR
ecosystems. From the in considered drought period by a ecosystems. From the visualization of the separated Rm in considered drought period by a continuous
Water 2016, 8, 379
Water 2016, 8, 379 Water 2016, 8, 379 6 of 12
6 of 12 6 of 12 -2 -2-1 -1
Estimated
Rm
byby
experimental
method
[ [mol
mm
s s] ]
mol
Estimated
Rm
experimental
method
curve,
the gradient of Rm in some sites
and sampling days might not be evident. For to this reason, only
continuous curve, the gradient of R
m in some sites and sampling days might not be evident. For to continuous curve, the gradient of R
m in some sites and sampling days might not be evident. For to the
separated
results
of
R
on
nine
sampled
presented
here
andare three
kinds ofhere ecosystems
are
this reason, only the separated results of Rdays
m on are
nine sampled days presented and three m
this reason, only the separated results of Rm on nine sampled days are presented here and three selected
the comparison,
the saline
desert,
abandoned
farmland
and oasis
farmland.farmland It could also
kinds of for
ecosystems are selected for the comparison, the saline desert, abandoned and kinds of ecosystems are selected for the comparison, the saline desert, abandoned farmland and facilitate
the validIt and
explicit
between
the separated
results of Rmbetween from thethe experimental
oasis farmland. could also comparison
facilitate the valid and explicit comparison separated oasis farmland. It could also facilitate the valid and explicit comparison between the separated method
and
semi-experimental
method.
Obviously,
the separated Rm inmethod. the semi-experimental
results of Rmthe
from the experimental method and the semi‐experimental Obviously, the results of Rm from the experimental method and the semi‐experimental method. Obviously, the and
experimental
approaches
show similar
patterns.
The diurnal
dynamics
on nine
sampled
days also
separated Rm in the semi‐experimental and experimental approaches show similar patterns. The separated Rm in the semi‐experimental and experimental approaches show similar patterns. The reveal
a duplicated variability (Figures 5 and 6).
diurnal dynamics on nine sampled days also reveal a duplicated variability (Figures 5 and 6). diurnal dynamics on nine sampled days also reveal a duplicated variability (Figures 5 and 6). 1
1
0.8
0.8
0.6
0.6
0.4
0.4 0
0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6 0
0
2.2
2.2
2
2
1.8
1.8
1.6
1.6 0
0
a
a
10
10
20
20
30
30
d
d
10
10
20
20
30
30
g
g
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0
0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0
0
b
b
10
10
20
20
30
30
e
e
10
10
20
20
2.2
2.2
30
30
h
h
2
2
10
10
20
20
30
30
1.8
1.8 0
0
10
20
10Time [h]20
Time [h]
30
30
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0
0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
0
0
2.2
2.2
2.1
2.1
2
2
1.9
1.9
0
0
c
c
10
10
20
20
30
30
f
f
10
10
20
20
30
30
i
i
10
10
20
20
30
30
-2 -2-1 -1
Estimated
bybysemi-experimental
[ [mol
mm
s s] ]
mol
EstimatedRm
Rm
semi-experimentalmethod
method
Figure 5. Isolated RR
m by experimental method (a–c: saline desert; d–f: abandoned farmland; g–i: Figure
5. Isolated IsolatedR
experimental
method
(a–c:
saline
desert;
abandoned
farmland;
Figure 5. m m
by by
experimental method (a–c: saline desert; d–f: d–f:
abandoned farmland; g–i: oasis farmland). g–i: oasis farmland).
oasis farmland). 1
1
0.8
0.8
0.6
0.6
0
0
1
1
0.9
0.9
0.8
0.8
0.7
0.7
0
0
2
2
1.8
1.8
1.6
1.6
1.4
1.4 0
0
1
1
0.8
0.8
0.6
0.6
a
a
10
10
20
20
30
30
d
d
10
10
20
20
20
20
10
10
20
20
30
30
0.6
0.6
0
0
2
2
10
10
20
20
1.6
1.6
0
0
10
20
10Time [h]20
Time [h]
1
1
0.8
0.8
0.6
0.6
0.4
0.4 0
0
1
1
c
c
10
10
20
20
30
30
0.6
0.6
0
0
2
2
10
10
20
20
1.6
1.6
0
0
30
30
i
i
1.8
1.8
30
30
30
30
f
f
0.8
0.8
h
h
1.8
1.8
30
30
30
30
e
e
0.8
0.8
g
g
10
10
0
0
1
1
b
b
10
10
20
20
30
30
Figure 6. Isolated Rm by semi‐experimental method (a–c: saline desert; d–f: abandoned farmland; g–i: Figure 6. Isolated R
by semi‐experimental method (a–c: saline desert; d–f: abandoned farmland; g–i: Figure
6. Isolated Rmm
by semi-experimental method (a–c: saline desert; d–f: abandoned farmland;
oasis farmland). oasis farmland). g–i: oasis farmland).
The separated results of Rm in the former two ecosystems are almost the same (Table 2), which The separated results of Rm in the former two ecosystems are almost the same (Table 2), which The
separated results of Rm in the former two ecosystems are almost the same (Table 2), which are
are all different with the oasis farmland. This might be the reason that the soil microbial activity in are all different with the oasis farmland. This might be the reason that the soil microbial activity in all
different with the oasis farmland. This might be the reason that the soil microbial activity in the
the saline desert is very different to that in oasis farmland. the saline desert is very different to that in oasis farmland. salineThe desert
is very different
to that in oasis farmland.
separated A
a from the semi‐experimental approach represents about 80% of that from the The separated Aa from the semi‐experimental approach represents about 80% of that from the The separated
Aa from
the semi-experimental
approach
80%7). of that
from the
experimental approach, which indicates a reliable cross represents
validation about
(Figure In fact, the experimental approach, which indicates a reliable cross validation (Figure 7). In fact, the experimental
approach, which indicates a reliable cross validation (Figure 7). In fact, the experimental
experimental method should still be used with much caution since it indicates a large magnitude of experimental method should still be used with much caution since it indicates a large magnitude of method
should still
used
with much
caution[10,14]. Thus, since it indicates
large validations magnitude of
carbon sink
intwo the
carbon sink in the be
most barren ecosystems the across between these carbon sink in the most barren ecosystems [10,14]. Thus, the cross validations between these two most
barren ecosystems [10,14]. Thus, the cross validations between these two methods are necessary.
methods are necessary. methods are necessary. Water 2016, 8, 379
7 of 12
Water 2016, 8, 379 Water 2016, 8, 379 7 of 12 7 of 12 -2 -1
isolated
by semi-experimental
method
[ mol
isolated
Aa A
bya semi-experimental
method
[ mol
m-2ms-1]s ]
Table 2. Q10 values for isolated Rm by two methods.
Table 2. Q10 values for isolated Rm by two methods. Table 2. Q10 values for isolated Rm by two methods. Study
Sites
Semi-Experimental
Method
Experimental Method
Study Sites Semi‐Experimental Method
Experimental Method Study Sites Semi‐Experimental Method
Experimental Method Saline
desert
0.53200
0.58711
Saline desert 0.53200 0.58711 Saline desert 0.53200 0.58711 Abandoned
farmland
0.81835
0.77012
Abandoned farmland 0.81835 0.77012 Abandoned farmland 0.81835 0.77012 Oasis
farmland
1.21009
1.19123
Oasis farmland 1.21009 1.19123 Oasis farmland 1.21009 1.19123 1.5
1.5
1
1
Slope =0.81
Slope =0.81
RMSE =0.29
RMSE =0.29
0.5
0.5
0
0
-0.5
-0.5
-1
-1
-1.5
-1.5
-2
-2
-2
-2
-1.5
-1.5
-1
-0.5
0
0.5
-1
-1
-0.5
0 [ mol m-2 s0.5
method
]
isolated
Aa by experimental
isolated Aa by experimental method [ mol m-2 s-1]
1
1
1.5
1.5
Figure 7. Reliability of semi‐experimental partition method Figure 7. Reliability of semi-experimental partition method(the (thegreen green line line is is the the linear linear regression regression
Figure 7. Reliability of semi‐experimental partition method (the green line is the linear regression line determined by the observed values and the simulated values) utilizing a linear regression curve line determined by the observed values and the simulated values) utilizing a linear regression curve
line determined by the observed values and the simulated values) utilizing a linear regression curve (the green line). The diagonal blue line is taken as a reference. (the green line). The diagonal blue line is taken as a reference.
(the green line). The diagonal blue line is taken as a reference. -1 s-1]
isolated
by two methods [ mol-2m-2
isolated
RmRby
m two methods [ mol m s ]
Employing the estimated β0 and β1 in Equation (10) and fitted Q10 provided in Table 2, the Employing
the estimated estimated ββ00 and andββ
Equation
(10)
and
fitted
in Table
2,
1 in
10 provided
Employing the 1 in Equation (10) and fitted Q10 Qprovided in Table 2, the separated R
m by the semi‐experimental method explain well the variability of separated Rm by the the
separated
R
by
the
semi-experimental
method
explain
well
the
variability
of
separated
R
m
m
separated Rm by the semi‐experimental method explain well the variability of separated R
m by the experimental method (Figure 8). Both semi‐experimental and experimental methods reconcile F
a as a by
the experimental method (Figure 8). Both semi-experimental and experimental methods reconcile
experimental method (Figure 8). Both semi‐experimental and experimental methods reconcile F
a as a direct sum of Aa and Rm, rendering the cross validation of reconciled Fa using these two methods Fdirect sum of A
of Ama , rendering and Rm , rendering
the cross validation of reconciled
a as a direct sum
a using these two
a and R
the cross validation of reconciled F
a using F
these two methods necessary. In Figure 9, it is also found that F
a reconciled by Equation (11) using the semi‐experimental methods
necessary.
In
Figure
9,
it
is
also
found
that
F
reconciled
by
Equation
(11) using the
a
necessary. In Figure 9, it is also found that F
a reconciled by Equation (11) using the semi‐experimental approach almost coincides with that
reconciled by Equation (6) in the experimental approach. The semi-experimental
approach almost coincides
with that reconciled bythe experimental approach. The Equation (6) in the experimental
approach almost coincides with that
reconciled by Equation (6) in aforementioned results of cross validation also show that the semi‐experimental separations of Rm approach.
The
aforementioned
results
of
cross
validation
also
show
that the semi-experimentalm aforementioned results of cross validation also show that the semi‐experimental separations of R
a proposed in Section 2 are both feasible and reliable (Table 3). and A
separations
of Rm and Aa proposed in Section 2 are both feasible and reliable (Table 3).
and Aa proposed in Section 2 are both feasible and reliable (Table 3). 1.5
1.5
saline desert:
saline desert:
experimental method
experimental method
semi-experimental method
semi-experimental method
1
1
0.5
0.5
0
0
0
0
1
1
R2=0.99973
R2=0.99973
10
10
RMSE=0.077922
RMSE=0.077922
20
30
20
30
abandoned farmland:
abandoned farmland:
0.8
0.8
0.6
0.6
0
0
R2=0.99969
R2=0.99969
20
20
RMSE=0.081291
RMSE=0.081291
40
60
40
60
40
40
3
3
2.5
2.5
2
2
1.5
1.5
1
80
0
1
80 Time [h]
0
Time [h]
50
50
60
60
70
70
80
80
oasis farmland:
oasis farmland:
R2=0.99998
R2=0.99998
20
20
RMSE=0.22938
RMSE=0.22938
40
60
40
Figure 8. Cross validation of isolated Rm by two methods. Figure 8. Cross validation of isolated R
Figure
8. Cross validation of isolated Rmm by two methods. by two methods.
60
80
80
reconciled Fa by semi-experimental method [ mol m-2 s-1]
Water 2016, 8, 379
Water 2016, 8, 379 8 of 12
8 of 12 2.5
Slope =0.91
RMSE =0.25
2
1.5
1
0.5
0
-0.5
-1
-1
-0.5
0
0.5
1
1.5
reconciled Fa by experimental method [ mol m-2 s-1]
2
2.5
Figure 9. Cross validation of reconciled Fa by two methods, utilizing a linear regression curve (the Figure 9. Cross validation of reconciled Fa by two methods, utilizing a linear regression curve (the green
green line). The diagonal blue line is taken as a reference. line). The diagonal blue line is taken as a reference.
Table 3. Fitted relationships between isolated components by two methods. Table 3. Fitted relationships between isolated components by two methods.
Flux Components
Slope
Flux
Components
Alkaline absorption Slope
0.80630 Alkaline
absorption
Microbial respiration Microbial
respiration 2 flux Reconciled soil CO
Reconciled soil CO2 flux
0.80630
0.71842 0.71842
0.91428 0.91428
RMSE
RMSE
0.29089 0.29089
0.14753 0.14753
0.25061 0.25061
3.2. Implications of Soil CO2 Uptake to the Groundwater Environment 3.2. Implications of Soil CO2 Uptake to the Groundwater Environment
Although soil CO2 uptake in deserts has been investigated in many previous studies, its Although soil CO2 uptake in deserts has been investigated in many previous studies, its potential
potential influences on the groundwater environment remain unaddressed [25–39]. This is partially influences
on the groundwater environment remain unaddressed [25–39]. This is partially
due to the
due to the lack of a simple method for the separation and quantification of such CO
2 uptake. This lack
of
a
simple
method
for
the
separation
and
quantification
of
such
CO
uptake.
This
study
presents
2
study presents a simple yet efficient method for partitioning CO2 influxes to and efflux from bare asaline‐alkali soils, implying a time to consider the implications of such CO
simple yet efficient method for partitioning CO2 influxes to and efflux from
bare saline-alkali soils,
2 uptake beneath deserts implying
a
time
to
consider
the
implications
of
such
CO
uptake
beneath
deserts
to the
groundwater
2 where the absorbed CO
to the groundwater environment. An unresolved issue, 2 has gone, still environment.
An
unresolved
issue,
where
the
absorbed
CO
has
gone,
still
remains.
Most
studies
2
remains. Most studies assumed that these absorbed CO2 are partially dissolved in the soil‐groundwater assumed
that these absorbed CO2 are partially dissolved
in the soil-groundwater system. However,
system. However, the magnitude of the beneath CO
2 dissolution remains undetermined [25–30]. the magnitude
of
the
beneath
CO
dissolution
remains
undetermined
[25–30].
2
Furthermore, the experimental partition method is still desired for further improvement. Based Furthermore, the experimental partition method is still desired for further2 influx and efflux when improvement. Based on
on some recent studies, there exists little difference between abiotic soil CO
some
recent
studies,
there
exists
little
difference
between
abiotic
soil
CO
influx
and efflux
when
2
excluding the role of groundwater and soil is left to absorb/release CO2 naturally [28]. The excluding
the
role
of
groundwater
and
soil
is
left
to
absorb/release
CO
naturally
[28].
The
difference
2
difference might be attributed to the extent of the sterilization treatment on the soil and the might
be attributed to the extent of the sterilization
treatment on the soil and the existence of msome
soila. existence of some soil microbial respiration (R
m), or the interaction between the existing R
and A
microbial
respiration
(R
),
or
the
interaction
between
the
existing
R
and
A
.
Future
studies
must
m
m
a
Future studies must keep a most cautious mind in extending the results from the laboratory keep
a most cautious mind in extending
the results from the laboratory experiments tothe field
research.
experiments to field research. In reality, the conditions are much more complex and additional In
reality, the conditions are much more complex and the additional confounding factors for many flux
confounding factors for many flux components exist and might interact with each other. components
existwidely and might
interact that withthe each
other. magnitude of CO2 dissolution beneath deserts It has been recognized overall It has been widely recognized that the overall magnitude of CO2 dissolution beneath deserts
and its contributions to the ecosystem carbon balance can be huge [30–39], but its effect on the local and
its
contributions
to the
ecosystem
balanceto can
be huge
but its
on CO
the2 environment are seldom reported. As a carbon
first attempt analyze the [30–39],
implications of effect
the soil local
environment
are
seldom
reported.
As
a
first
attempt
to
analyze
the
implications
of
the
soil
uptake to the groundwater environment, the present study simply assumes that the absorbed CO2 CO
uptakelargely to the groundwater
the present
study simply
that the
absorbed
has 2 been dissolved in environment,
the soil‐groundwater system beneath assumes
the deserts. Under this CO
has
been
largely
dissolved
in
the
soil-groundwater
system
beneath
the
deserts.
Under
this
2
assumption, an artificial soil‐groundwater system has been incubated for a long period in the assumption,
an artificial soil-groundwater system has been incubated for a long period in the laboratory.
laboratory. Collected data at the first stage confirm that the implications to the water environment Collected
data at the first stage confirm that the implications to the water environment should be taken
should be taken into account (Figure 10). into account (Figure 10).
Water 2016, 8, 379
9 of 12
Water 2016, 8, 379 9 of 12 The concentration of DIC (μmol/kg)
4000
y = 0.0008e1.3841x
R² = 0.3664
3500
3000
2500
2000
1500
1000
500
0
9.60
9.80
10.00
10.20
10.40
10.60
10.80
11.00
The groundwater pH
Figure 10.
10. The
The relationship between the groundwater pH the concentration of dissolved Figure
relationship
between
the groundwater
pH and
theand concentration
of dissolved
inorganic
inorganic carbon (DIC) in the artificial soil‐groundwater system. carbon (DIC) in the artificial soil-groundwater system.
Since the potential contributions of these abiotic components to the total soil CO2 fluxes were Since the potential contributions of these abiotic components to the total soil CO2 fluxes
non‐negligible, the implication of the biotic CO2 effluxes and abiotic CO2 influxes to the were non-negligible, the implication of the biotic CO2 effluxes and abiotic CO2 influxes to the
soil‐groundwater environment cycle is a subsequent problem. Despite the recent studies of soil soil-groundwater environment cycle is a subsequent problem. Despite the recent studies of soil carbon
carbon cycle in the desert ecosystems and the biogeochemical investigations in the arid ecosystems cycle in the desert ecosystems and the biogeochemical investigations in the arid ecosystems [40–49],
[40–49], the dynamics of beneath CO2 dissolution [7,10] is still not well‐understood. Furthermore, the dynamics of beneath CO2 dissolution [7,10] is still not well-understood. Furthermore, even the
even the mechanisms of subterranean abiotic processes are still not wholly known. It is worth mechanisms of subterranean abiotic processes are still not wholly known. It is worth noting that the
noting that the groundwater pH is essentially significant for the local water environment and has groundwater pH is essentially significant for the local water environment and has attracted a wide
attracted a wide attention [50–60]. Attaining a reliable quantification for the contributions of soil attention [50–60]. Attaining a reliable quantification for the contributions of soil CO2 uptake to the
CO2 uptake to the groundwater environment remains one major challenge and hence should be a groundwater environment remains one major challenge and hence should be a research priority.
research priority. Because the mechanisms for the abiotic CO2 absorption and the overall magnitude of the soil CO2
Because the mechanisms for the abiotic CO2 absorption and the overall magnitude of the soil uptake are still not well-understood, there are still considerable uncertainties in more explicit analyses
CO2 uptake are still not well‐understood, there are still considerable uncertainties in more explicit of the contributions of such soil CO2 uptake to the groundwater environment [8,10,14]. Additionally,
analyses of the contributions of such soil CO2 uptake to the groundwater environment [8,10,14]. the data required by the present experimental method can only be collected from the top few cm of the
Additionally, the data required by the present experimental method can only be collected from the soil. The whole atmospheric exchange of CO2 requires some further investigations towards better and
top few cm of the soil. The whole atmospheric exchange of CO2 requires some further investigations more suitable experimental and non-experimental methodologies.
towards better and more suitable experimental and non‐experimental methodologies. 4. Conclusions
4. Conclusions This study presented a simple yet efficient method to work out the magnitude of soil abiotic CO2
This study presented a simple yet efficient method to work out the magnitude of soil abiotic absorption
in deserts from the data set of soil CO2 fluxes, along with a first approach to illustrate its
CO
2 absorption in deserts from the data set of soil CO2 fluxes, along with a first approach to potential role in influencing environments in an artificial soil-groundwater system. The first approach
illustrate its potential role in influencing environments in an artificial soil‐groundwater system. The highlighted
significant events where an evident variation of the groundwater pH, provided that
first approach highlighted significant events where an evident variation of the groundwater pH, the absorbed CO
2 largely remained in the soil-groundwater system. Considering the CO2 sink size
provided that the absorbed CO
2 largely remained in the soil‐groundwater system. Considering the across different environments remains
unknown, there are still considerable uncertainties and further
CO
2 sink size across different environments remains unknown, there are still considerable investigations are necessary.
uncertainties and further investigations are necessary. Acknowledgments: This research was financially supported by the National Natural Science Foundation of China
Acknowledgments: This research was financially supported by the National Natural Science Foundation of (41571299), the “Thousand Talents” plan (Y474161), the CAS “Light of West China” Program (XBBS-2014-16)
China the “Thousand Talents” plan (Y474161), the CAS “Light of West China” Program and the(41571299), Shenzhen Basic
Research Project
(JCYJ20150630114942260).
Essential
geodata
were
collected
from
(XBBS‐2014‐16) and the Shenzhen Basic Research Project (JCYJ20150630114942260). Essential geodata were National Science and Technology Infrastructure Center-Earth System Science Data Sharing Infrastructure
of
(www.geodata.cn).
collected from National Science and Technology Infrastructure Center‐Earth System Science Data Sharing Infrastructure of (www.geodata.cn). Author Contributions: Xi Chen and Wenfeng Wang conceived and designed the experiments; Wenfeng Wang performed the experiments; Xi Chen, Wenfeng Wang and Jing Qian analyzed the data; Hongwei Zheng, Ruide Yu, Yifan Zhang and Jianjun Yu contributed analysis tools; and Wenfeng Wang wrote the paper. Water 2016, 8, 379
10 of 12
Author Contributions: Xi Chen and Wenfeng Wang conceived and designed the experiments; Wenfeng Wang
performed the experiments; Xi Chen, Wenfeng Wang and Jing Qian analyzed the data; Hongwei Zheng, Ruide Yu,
Yifan Zhang and Jianjun Yu contributed analysis tools; and Wenfeng Wang wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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