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\begin{document}

\title{The effect of afforestation on inorganic carbon in soils of arid and
semi-arid lands of northwest China}



\author[1]{xinglin zhu}%
\author[2]{Si J. H.}%
\author[1]{bin jia}%
\author[2]{Xiaohui HE}%
\author[1]{Dongmeng Zhou}%
\author[1]{chunlin wang}%
\author[1]{jie qin}%
\author[2]{Zijin Liu}%
\affil[1]{Northwest Institute of Eco-Environment and  Resources, Chinese Academy of Sciences}%
\affil[2]{Northwest Institute of Eco-Environment and Resources}%


\vspace{-1em}



  
  \date{June 05, 2024}


\begingroup
\let\center\flushleft
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\selectlanguage{english}
\begin{abstract}
Alxa is a region with severe land desertification and extreme ecological
fragility in China. The plantation in the area has effectively curbed
the desertification of the local land. However, Studies on soil
inorganic carbon(SIC)dynamics after sandy land afforestation are still
relatively few. Understanding SIC profiles' distribution and stock
changes after afforestation is essential for assessing regional,
continental, and global soil carbon sink potential. Using 5, 11, 22, and
46 years of Haloxylon ammodendron (H. ammodendron) plantations and
control sands (MS) in the Alxa region, we studied the variation
characteristics of SIC with increasing stand age from 0 to 300 cm.
Within the 0-300 cm soil layer, SIC storage increased significantly
after afforestation, with 46yrs increasing by 6.52 kg m-2 compared to
MS. SIC sequestration rate (CSR) decreased with increasing stand age, in
the order of 5yrs(0.054 kg m-2 yr-1)>11yrs(0.025 kg m-2
yr-1)>22yrs(0.016 kg m-2 yr-1)>46yrs(0.009 kg m-2 yr-1). The
100\textasciitilde{}300 cm SIC storage accounts for over 60\% of the SIC
pool. Soil carbon pool estimation will be largely underestimated if only
the shallow SIC is considered.%
\end{abstract}%



\sloppy


\textbf{The effect of afforestation on inorganic carbon in soils of arid
and semi-arid lands of northwest China}

Xinglin Zhu \textsuperscript{1,2}, Jianhua Si \textsuperscript{1*}, Bing
Jia \textsuperscript{1}, Xiaohui He \textsuperscript{1,2,3}, Dongmeng
Zhou \textsuperscript{1,2}, Chunlin Wang \textsuperscript{1,2}, Jie
Qin\textsuperscript{1,2}, and Zijin Liu \textsuperscript{1,2}

\textsuperscript{1} Key Laboratory of Eco-Hydrology of Inland River
Basin, Northwest Institute of Eco-Environment and Resources, Chinese
Academy of Sciences, Lanzhou, China, \textsuperscript{2} University of
Chinese Academy of Sciences, Beijing, China, \textsuperscript{3} Faculty
of Resources and Environment, Baotou Teachers' College, Inner Mongolia
University of Science and Technology, Baotou, China

\textbf{Abstract:} Alxa is a region with severe land desertification and
extreme ecological fragility in China. The plantation in the area has
effectively curbed the desertification of the local land. However,
Studies on soil inorganic carbon(SIC)dynamics after sandy land
afforestation are still relatively few. Understanding SIC profiles'
distribution and stock changes after afforestation is essential for
assessing regional, continental, and global soil carbon sink potential.
Using 5, 11, 22, and 46 years of Haloxylon ammodendron (H. ammodendron)
plantations and control sands (MS) in the Alxa region, we studied the
variation characteristics of SIC with increasing stand age from 0 to 300
cm. Within the 0-300 cm soil layer, SIC storage increased significantly
after afforestation, with 46yrs increasing by 6.52 kg
m\textsuperscript{-2} compared to MS. SIC sequestration rate (CSR)
decreased with increasing stand age, in the order of 5yrs(0.054 kg
m\textsuperscript{-2} yr\textsuperscript{-1})>11yrs(0.025 kg
m\textsuperscript{-2} yr\textsuperscript{-1})>22yrs(0.016 kg
m\textsuperscript{-2} yr\textsuperscript{-1})>46yrs(0.009 kg
m\textsuperscript{-2} yr\textsuperscript{-1}). The
100\textasciitilde{}300 cm SIC storage accounts for over 60\% of the SIC
pool. Soil carbon pool estimation will be largely underestimated if only
the shallow SIC is considered.

\textbf{Keywords:} Soil inorganic carbon; Haloxylon ammodendron
plantations; Stand age; Afforestation

\textbf{1 Introduction}

Land sanding and desertification are the most threatening environmental
problems in arid desert areas. More than 70\% of arid desert lands
suffer from long-term sanding or desertification, and land degradation
has resulted in a loss of 19-29 Pg of soil carbon (Lal, 2004; Li et al.,
2015). Vegetation construction is one of the main ways to improve land
degradation in these areas. Related studies have shown that revegetation
of sandy areas leads to changes in soil physicochemical properties, and
makes it possible for the soil to capture and sequester atmospheric
CO\textsubscript{2}, increasing the soil carbon sink potential (Huang et
al., 2012). Soil carbon pools, including SOC and SIC, are terrestrial
ecosystems' largest carbon pools. Due to the potential rapid response of
soil carbon pools to vegetation type changes, SOC has been widely and
intensively studied by scholars in China and abroad (Deng et al., 2014;
Jackson et al., 2002). Compared to studies on SOC, there are few studies
on the dynamics of SIC after revegetation. Since the storage of SIC in
arid desert areas far exceeds that of SOC, small revegetation-induced
changes in SIC may exert a stronger effect than major changes in SOC in
changing the terrestrial carbon balance (Gao et al., 2017; Wang et al.,
2016). Therefore, the study of SIC changes after revegetation in arid
desert areas is crucial for finding ''lost carbon sinks'' in terrestrial
ecosystems.

In arid desert areas, changes in SIC after revegetation are highly
uncertain. The Colorado Plateaus Desert, USA, showed a 53.8\% reduction
in SIC content on the soil surface after 10 years of poplar planting
(Sartori et al., 2007). In the Badangirin Desert, China, the SIC content
at the soil surface increased by 45.7\% after 7 years of planting poplar
trees (Su et al., 2010). In the Loess Plateau of China, studies have
shown that the SIC content of cultivated land at 0-100 cm soil depth is
significantly lower than that of restored plantation forests (Wang et
al., 2016). In addition, other studies in the region showed that
reforestation only affected the distribution characteristics of SIC in
different soil layers and did not change the amount of SIC storage
(Chang et al., 2012). Inorganic carbon after revegetation showed
different or even contradictory findings in the context of similar stand
types, climatic conditions, and plant species. These results suggest
that the response of SIC content to afforestation in arid desert areas
needs further study. In addition, studies on SIC in arid desert areas
have mainly focused on soil profiles of 100 cm or even shallower (Shi et
al., 2012), while the changing pattern of SIC below 100 cm is still a
big question. Recent studies have found that deeper SIC levels in arid
desert areas are more than twice as high as those in shallow, In
contrast, within 1 to 3 m of the shrub deserts and desert soil profiles,
the soil carbon pool is composed mainly of SIC (Naorem et al., 2022).
Therefore, the change pattern of SIC after revegetation in arid desert
areas needs to be urgently studied.

Located in the interior of northwestern China, Alxa League is a typical
arid desert region with the distribution of Ulaanbaatar, Tengger, and
Badangirin Deserts. The region's chronic drought and lack of water,
combined with sparse vegetation, make it one of the most active areas of
wind-sand in China. The region began vegetating sandy areas in the
middle of the last century. As a result, it has extensive artificial
sand-fixing forests. To a certain extent, land sanding and sand, and
afforestation have typical characteristics of arid desert areas. H.
ammodendron belongs to the small tree Quinoa family, which is
characterized by its resistance to drought and barrenness. It is the
largest sand-fixing tree in the desert area of northwest China, and is
called a ''desert plantation''. In this study, we hypothesized that
sandy vegetation restoration could increase SIC storage. To test this
hypothesis, we selected four stands (5, 11, 22, and 46 years old,
respectively) of the H. ammodendron plantation at the edge of the
Badangirin Desert in the Alxa League, and used nearby moving sand (MS)
as a control. By collecting soils from 0 to 300 cm profiles in the above
sample sites, we focused on (1) the characteristics of soil profile
changes in SIC along the age sequence of H. ammodendron plantation; (2)
the effects of soil moisture(SM) and pH on SIC; and (3) the importance
of deep soil SIC in soil carbon storage estimation.

\textbf{2 Materials and Methods}

\textbf{2.1 Site Description}

The study was conducted in Alxa League, the westernmost part of the
Inner Mongolia Autonomous Region. It is located at 37\selectlanguage{ngerman}°24\selectlanguage{english}'
\textasciitilde{} 42deg47\selectlanguage{english}'N, 97deg10\selectlanguage{english}' \textasciitilde{} 106deg53\selectlanguage{english}'E. The area
is dominated by high plains, with large desert and Gobi areas, followed
by grassland areas, and smaller mountain, forest, and arable areas. It
has an altitude range of 820 \textasciitilde{} 1 400 m. The region has
an arid climate and strong winds, and is a typical arid desert area. The
average annual temperature is about 8.3 \selectlanguage{english}, the average annual
precipitation is 40-180 mm, mainly in July and August, and the average
annual evaporation is 2,400-4,200 mm. Soil types have obvious zonal
characteristics, from northwest to southeast, gray-brown desert soil,
gray-desert soil and gray-calcium soil are distributed in order, and in
the lake basin area there are saline soils. The vegetation types are
mainly desert vegetation, oasis vegetation and mountain vegetation.
Natural poplar forests grow in the Ejin Oasis area, while the presence
of the Helan Mountains enriches the biodiversity and complexity of the
Alxa Plateau system. Alxa region is an ecologically fragile area, which
is one of the birthplaces of sandstorms in China. In recent years,
around the three major deserts, a large area of artificial afforestation
has been carried out, and artificial H. ammodendron plantations have
become an important artificial barrier to curb land desertification and
promote ecological restoration in the region. Fig. 1 shows the
geographical location and experimental survey area of Alxa.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

Fig. 1 The geographical location and experimental survey area of Alxa
League

\textbf{2.2 Experimental design}

In September 2021, the H. ammodendron plantation at the southeastern
edge of the Badangirin Desert in the Alxa League was used as the study
object. Using the method of spatial substitution of time, we
sequentially selected the H. ammodendron plantation in 2016, 2010, 1999,
and 1975 (5, 11, 22, and 46yrs, respectively) for the study, and these
four vegetation construction sites were referred to as 5yrs, 11yrs,
22yrs, and 46yrs, respectively, with nearby moving sands (MS) as
controls. Because all eras of the H. ammodendron plantation were in
moving sands when planted and in the same location, soil texture,
standing conditions, and meteorological conditions, satisfying the
comparative study of pike soil samples from different eras. The
information of each stand age sample site is shown in Table 1.

Tab. 1 The basic description of the studied sites\selectlanguage{english}
\begin{longtable}[]{@{}lllllll@{}}
\toprule
\begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
Stand Age (a)\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
Stand Age (a)\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
MS\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
5yrs\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
11yrs\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
22yrs\strut
\end{minipage} & \begin{minipage}[b]{0.12\columnwidth}\raggedright\strut
46yrs\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
Year of planting\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
Year of planting\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
/\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
2016\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
2010\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1999\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1975\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
Altitude (m)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
Altitude (m)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1203.97\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1204.86\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1204.95\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1203.69\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1204.37\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
Diameter at breast height (cm)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
Diameter at breast height (cm)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
/\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
3.5 \selectlanguage{ngerman}± 1.2\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
8.9 \selectlanguage{ngerman}± 3.8\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
12.4\selectlanguage{ngerman}± 5.5\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
22.1 \selectlanguage{ngerman}± 7.5\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
Mean tree height (cm)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
Mean tree height (cm)\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
/\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
55\selectlanguage{ngerman}± 12\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
140 \selectlanguage{ngerman}± 28\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
220 \selectlanguage{ngerman}± 46\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
420 \selectlanguage{ngerman}± 81\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
Bulk Density (g cm\textsuperscript{-3} )\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0 \textasciitilde{} 100cm\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.49\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.49\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.51\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.52\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.52\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
100 \textasciitilde{} 200cm\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.49\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.52\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.52\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.52\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.53\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
200 \textasciitilde{} 300cm\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.5\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.51\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.52\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.53\strut
\end{minipage} & \begin{minipage}[t]{0.12\columnwidth}\raggedright\strut
1.53\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

\textbf{2.3 Soil collection and indoor analysis}

In this study, we first selected three sample sites with approximately
the same plant community composition within each of the five sample
plots of MS, 5yrs, 11yrs, 22yrs, and 46yrs, and sampled the same soil
stratum in three replicates at each sample site in 20 cm strata. The
soil collected in each stratum was mixed thoroughly and divided into two
groups of samples within the three sample points of the five forest age
sample plots. One group measured the SIC content and the other group
tested soil moisture. To determine the soil volume, we collected
complete soil samples using a stainless steel soil sampler with a volume
of 100 cm\textsuperscript{3}. After collecting the soil, we air dried it
and passed it through a 2 mm sieve to remove any rocks or root systems
larger than 2 mm. To analyze these samples, we ground them with a ball
mill and passed them through a 100 mesh soil sieve. Soil inorganic
carbon was determined by the air volumetric method, soil pH (water to
soil ratio of 2.5:1) was determined by the electrode method, and soil
bulk weight was the dry mass per unit volume of soil at 105\selectlanguage{ngerman}°C. Soil
moisture was determined by weighing the fresh weight of the soil, then
drying it at 105°C and weighing the dry weight.

\textbf{2.4 Data analysis}

SIC density and storage\selectlanguage{english}(kg m\textsuperscript{-2}\selectlanguage{english}) were calculated from
soil bulk weight and inorganic carbon content according to the following
equation:

\selectlanguage{english}(1\selectlanguage{english})

\selectlanguage{english}(2\selectlanguage{english})

where SIC\emph{\textsubscript{i}} (g\selectlanguage{english}[?]kg\textsuperscript{-1}) is the SIC
contents of the \emph{i} -cm soil profile,
BD\emph{\textsubscript{i}}(g\selectlanguage{english}[?]cm\textsuperscript{-3}) is the soil bulk
density of the \emph{i} -cm soil profile, and D\emph{i} (cm) is the soil
layer thickness.

SIC sequestration rate (CRS, kg
m\textsuperscript{-2}yr\textsuperscript{-1}) is calculated as follows:

\selectlanguage{english}(3\selectlanguage{english})

where CS\emph{\textsubscript{f}} is the SIC storage in the final year,
CS\emph{\textsubscript{i}} is SIC storage in the initial year,
and\emph{t} (years) is the interval between year \emph{i} and
year\emph{f} . A positive value indicates an increase in the SIC pool,
while a negative value indicates a decrease.

All statistical analyses were performed using Origin 2022 and SPSS
23.0.One-way ANOVA followed by the Tukey's HSD test (P\selectlanguage{english}<0.05) was used
to compare the effects of vegetation restoration on SIC in the study
sites.

\textbf{3 Results}

\textbf{3.1 Profile changes in soil pH, soil moisture, and SIC}

The profile changes of soil pH and soil moisture d during the growth of
H. ammodendron plantation from MS to 46yrs are shown in Figs. 2a and 2b.
The mean soil pH increased and then decreased along the depth of the
profile as the age of the H. ammodendron plantation increased. In the 0
\textasciitilde{} 240 cm soil layer, the pH showed an overall increasing
trend, and within the 240 \textasciitilde{} 300 cm soil layer, the pH
showed a decreasing trend. The soil pH varied from 7.07 to 9.0
throughout the stand age stage, with an overall alkaline soil. Soil
moisture, on the other hand, showed a trend of decreasing and then
increasing. In the 0 \textasciitilde{} 200 cm soil layer, the soil
moisture showed a decreasing trend with a varied range of 3.99
\textasciitilde{} 9.24 cm\textsuperscript{3} cm\textsuperscript{-3}, and
in the 200 \textasciitilde{} 300 cm soil layer, it showed an increasing
trend with a varied range of 4.78 \textasciitilde{} 8.09
cm\textsuperscript{3} cm\textsuperscript{-3}.

The profile variation of the SIC content is shown in Fig. 2c. With
increasing soil depth, the SIC content showed an overall trend of first
increasing and then decreasing. The SIC content first decreased in the
surface layer from 0 to 40 cm and then increased again in the soil layer
from 40 to 100 cm. Within the 100-260 cm soil layer, the SIC content
first decreased and then increased, followed by a continuous decrease in
the 260-300 cm soil layer. In the 0 \textasciitilde{} 300 cm soil
profile, the SIC content varied from 0.751 to 4.413 g
kg\textsuperscript{-1}.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

Fig. 2 Variation of pH\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}SM and SIC along the soil profile

\textbf{3.2 Variation of SIC density with stand age}

The statistical results of SIC density in different stand ages are shown
in Table 2. The variation of SIC density for MS, 5yrs, 11yrs, 22yrs, and
46yrs ranged from 0.37 \textasciitilde{} 0.57 kg
m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}0.51 \textasciitilde{} 1.05 kg
m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}0.59 \textasciitilde{} 1.08 kg
m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}0.60 \textasciitilde{} 1.28 kg
m\textsuperscript{-2}, and 0.68 \textasciitilde{} 1.35 kg
m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}。\end{CJK}\selectlanguage{english}It can be seen from the mean values that SIC
density showed an overall increasing trend with stand age.

Tab. 2 Descriptive statistics of SIC density at 0 \textasciitilde{} 300
cm depth\selectlanguage{english}
\begin{longtable}[]{@{}llllll@{}}
\toprule
\begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Planting Age\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Min (kg m\textsuperscript{-2})\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Max (kg m\textsuperscript{-2})\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
Mean (kg m\textsuperscript{-2})\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
SD (kg m\textsuperscript{-2})\strut
\end{minipage} & \begin{minipage}[b]{0.16\columnwidth}\raggedright\strut
CV (\%)\strut
\end{minipage}\tabularnewline
\midrule
\endhead
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
MS\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.37\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.57\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.46\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.50\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
28.70\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
5yrs\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.51\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.05\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.74\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.16\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
21.62\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
11yrs\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.59\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.08\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.74\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.12\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
16.22\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
22yrs\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.60\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.28\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.81\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.17\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
20.99\strut
\end{minipage}\tabularnewline
\begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
46yrs\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.68\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
1.35\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.90\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
0.19\strut
\end{minipage} & \begin{minipage}[t]{0.14\columnwidth}\raggedright\strut
21.11\strut
\end{minipage}\tabularnewline
\bottomrule
\end{longtable}

The distribution characteristics of SIC density at different stand ages
from 0 to 100 cm, 100 to 200 cm, and 200 to 300 cm are shown in Fig. 3.
In the comparison of SIC density of MS, 5yrs, 11yrs, 22yrs, and 46yrs,
it was found that SIC density gradually increased with the increase of
stand age in 0-100cm soil layer, and the difference of SIC density
between MS and other stand age stages was significant, and SIC density
of 46yrs was significantly higher than that of 5yrs and 11yrs. In the
100-200 cm soil layer, the SIC density with increasing stand age was
similar to that of 0-100 cm. In the 200-300 cm soil layer, the change in
SIC density with increasing stand age was not significant, and only the
difference in inorganic carbon density between MS and other stand age
stages was significant, while the difference in inorganic carbon density
between other stand ages was not significant (P \textless{} 0.05). From
0 to 300 cm soil layer, the SIC density values of MS and other stand
ages were 46yrs (13.57 kg m\textsuperscript{-2}) \textgreater{} 22yrs
(12.16 kg m\textsuperscript{-2}) \textgreater{} 11yrs (11.16 kg
m\textsuperscript{-2}) \textgreater{} 5yrs (11.07 kg
m\textsuperscript{-2}) \textgreater{} MS (7.04 kg m\textsuperscript{-2})
in order.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

Fig. 3 Variation of SIC density with stand age in different soil layers

Figure 4 reflects the linear fit trend of SIC density with increasing
stand age. Within the 0-100 cm soil layer, soil inorganic carbon density
in the top 0-40 cm layer increased significantly (P \textless{} 0.05)
with increasing stand age, and within the 100-200 cm soil layer, the
120-140 cm and 160-180 cm layers increased significantly (P \textless{}
0.05) with increasing stand age, while the rest of the soil layers did
not change significantly with stand age. As can be seen from Figure 4,
soil inorganic carbon density increased overall with increasing stand
age (not significant).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

Fig. 4 Correlation between SIC and Stand age

\textbf{3.3 Effect of soil moisture and pH on SIC content}

The relationship between SIC content and pH and soil moisture was
evaluated by the linear regression method, as shown in Figure 5. In the
0-300 cm soil layer, SIC content showed a significant negative
correlation with soil moisture (P \textless{} 0.001), while a
significant positive correlation with pH (P \textless{} 0.05) was
observed only in the deeper 200-300 cm soil layer. It showed that the
degree of influence of soil pH on SIC varied with increasing soil depth,
while soil moisture had a significant effect on SIC in all the different
soil layers.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image8/image8}

\end{center}
\end{figure}

Fig. 5 Correlation of SIC with soil pH and soil moisture

\textbf{3.4 Variation in SIC storage}

The SIC storage in the soil layers of 0 \textasciitilde{} 100 cm and 100
\textasciitilde{} 300 cm at different stand ages are shown in Fig. 6.
The SIC storage of MS, 5yrs, 11yrs, 22yrs, and 46yrs in 0
\textasciitilde{} 100 cm soils was 2.15 kg m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}3.30
kg m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}3.47 kg m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}3.77 kg
m\textsuperscript{-2}, and 4.14 kg m\textsuperscript{-2}, respectively.
The SIC storage in the 100 \textasciitilde{} 300 cm soil layer was 4.89
kg m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}7.76 kg m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}7.68 kg
m\textsuperscript{-2}\begin{CJK}{UTF8}{gbsn}、\end{CJK}\selectlanguage{english}8.39 kg m\textsuperscript{-2}, and 9.43 kg
m\textsuperscript{-2}, respectively. Figure 6b indicates that there is
still a large amount of SIC storage in the soil layer below 100 cm, and
its proportion accounts for more than 60\% of the SIC storage in the
soil layer from 0 to 300 cm. This result indicates that in arid and
semi-arid regions, SIC below 1 m is not negligible in soil carbon stock
estimation.

As can be seen from Figure 7, CSR was greatest at 5yrs and least at
46yrs during the MS to 46yrs stand age growth stage. That is, the
proportion of the net increase in inorganic carbon stock from MS to
46yrs decreases with increasing stand age. Throughout the study period,
CSR in the 0-300 cm soil layer was in the following order: 5yrs (0.054
kg m\textsuperscript{-2} yr\textsuperscript{-1}) \textgreater{} 11yrs
(0.025 kg m\textsuperscript{-2} yr\textsuperscript{-1}) \textgreater{}
22yrs (0.016 kg m\textsuperscript{-2} yr\textsuperscript{-1})
\textgreater{} 46yrs (0.009 kg
m\textsuperscript{-2}yr\textsuperscript{-1}). The results of this study
indicate that CSR decreases continuously with increasing stand age.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

Fig. 6 Distribution characteristics of SIC storage in different soil
layers\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image10/image10}

\end{center}
\end{figure}

Fig. 7 Variation of SIC sequestration rate with stand age in different
soil layers

\textbf{4 Discussions}

\textbf{4.1 Reasons for profile variation of SIC content}

In general, the variation of SOC content with soil depth after
afforestation of arid desert areas mainly depends on the distribution of
below-ground biomass (Schlesinger and Pilmanis, 1998). In contrast, the
variation of SIC content is controlled by a variety of complex factors,
such as soil properties, effective precipitation, soil moisture, and
partial pressure of CO\textsubscript{2}, thus forming a complex profile
distribution characteristic (Diaz-Hernandez et al., 2003). (Chang et
al., 2012) reported that reforestation of the central Loess Plateau
resulted in a redistribution of SIC along different soil layers. This
study found that revegetation in sandy areas also caused redistribution
of SIC content along the soil profile. The stratification
characteristics of the SIC content with increasing stand age further
corroborate this view. The accumulation process of carbonate, the main
component of inorganic carbon, involves two main chemical reactions
(Eqs. 4 and 5). On the one hand, there is an abiotic ''inorganic
respiration'' process in arid sandy soils, which absorbs
CO\textsubscript{2} from the atmosphere. Sufficient
CO\textsubscript{2}can advance reactions (4) and (5) to the right to
form loam-forming carbonate rocks. On the other hand, after planting
vegetation in sandy areas, the root system releases large amounts of
CO\textsubscript{2}into the soil by decomposing SOC, producing large
amounts of free
HCO\textsubscript{3}\textsuperscript{-}\begin{CJK}{UTF8}{gbsn}和\end{CJK}\selectlanguage{english}H\textsuperscript{+} (Zamanian
et al., 2016). The accumulated
HCO\textsubscript{3}\textsuperscript{-}can drive reaction (5) to the
right, and when Ca\textsuperscript{2+} is sufficient, carbonate can be
precipitated to increase the accumulation of SIC (Li et al., 2012; Wang
et al., 2015).

(4)

(5)

In this study, the SIC content of the 0 \textasciitilde{} 20 cm surface
layer increased after vegetation restoration. The first reason is the
increase in soil organic matter and soil microorganisms originating from
dead fallen matter in the topsoil layer. During biological processes,
the decomposition of soil organic matter and microbial activity
increases the soil CO\textsubscript{2} concentration. Subsequently,
under adequate soil moisture conditions, driving equations (4) and (5)
to the right, more abundant HCO\textsubscript{3}\textsuperscript{-} is
formed to bind with Ca\textsuperscript{2+} in the soil, facilitating the
formation and precipitation process of carbonate. The second reason is
the possible existence of the process of ''inorganic respiration'' of
the soil as described above, which leads to an increase in the SIC
content of the soil surface. In the 20 \textasciitilde{} 100 cm soil
layer, the SIC content further increased with the profile and showed a
significant negative correlation with soil moisture. This result is
consistent with the study on the SIC content of saline soils in the
southern Gurbantunggut Desert (Wang et al., 2013). The possible reason
for this is that in extremely arid areas, effective precipitation is
more likely to affect the shallow soil SIC content. The increased
CO\textsubscript{2} in the soil is dissolved at shallow depths and
follows the effective precipitation downward. This process continuously
reacts chemically with shallow Ca\textsuperscript{2+} and
Mg\textsuperscript{2+}, which causes the migration and precipitation of
carbonates. In the 120 \textasciitilde{} 240 cm soil profile, the
variation of SIC content was mainly influenced by root biomass,
CO\textsubscript{2} partial pressure Ca\textsuperscript{2+} and
HCO\textsubscript{3}\textsuperscript{-}. With the increase of stand age,
the deep soil root biomass is also increasing and growing deeper into
the soil. On the one hand, the increased input of root litter stimulates
the activity of soil microorganisms and accelerates the decomposition of
the litter. During decomposition SOC is mineralized to produce more
CO\textsubscript{2}, driving equation (4) to the right, further
dissolving in the soil solution to form HCO3\textsuperscript{-}, which
subsequently combines with Ca\textsuperscript{2+} released from
decomposing litter to precipitate as CaCO\textsubscript{3} (Zhao et al.,
2016). On the other hand, in extremely arid soil environments, secondary
carbonates can form in the mucilaginous sheaths around root hairs. The
root system is enriched with unused excess Ca\textsuperscript{2+} and a
large amount of HCO\textsubscript{3}\textsuperscript{-} accumulates in
the mucilaginous sheath due to respiration, allowing the mucilaginous
sheath to provide a unique environment for Ca\textsuperscript{2+} and
HCO\textsubscript{3}\textsuperscript{-} binding. Under these two
effects, the formation of secondary carbonates in deep soils is promoted
(Monger et al., 2015). (Liu et al., 2014)studied the profile change
characteristics of SIC in agricultural fields, and grasslands restored
for 12 years and 22 years of restoration, and finally found that SIC
storage decreases with revegetation. This is in contrast to the results
of this study where SIC content increased with the vegetation
restoration sequence. Some soil carbonates may be temporarily decomposed
to CO\textsubscript{2} due to the decrease in soil pH and the increase
in soil moisture after the restoration of grassland on agricultural
land. In deep soils from 240 to 300 cm, the SIC content was
significantly and positively correlated with soil pH. Increasing stand
age causes decreasing SIC content, which is due to the large amount of
organic acid secreted by the deep root system, which makes the pH
decrease. Under the conditions of relatively low
Ca\textsubscript{2}\textsuperscript{+} and
HCO\textsubscript{3}\textsuperscript{-} content, the large enrichment of
CO\textsubscript{2} from SOC decomposition and root respiration drives
equation (5) to the left, leading to carbonate dissolution and lower
inorganic carbon content (Jin et al., 2018). In previous studies, soil
samples were collected at depths ranging from 20 cm (Li et al., 2021) to
300 cm (Wang et al., 2010), and SIC profile changes exhibited different
responses to different soil layer combinations. These results indicated
that after afforestation of sandy areas in arid and semi-arid regions,
SIC content was mainly influenced by aboveground litter decomposition,
effective precipitation, and CO\textsubscript{2} partial pressure in the
shallow layer; and by soil moisture, root litter and pH value in the
deep layer.

\textbf{4.2 Effect of vegetation restoration on inorganic carbon stock}

Although there are more studies on the effects of revegetation on SIC
(He et al., 2016; Li et al., 2016; Wang et al., 2013), there are no
clear conclusions yet. A part of the study found that SIC reserves
remained unchanged after revegetation, which was attributed to carbonate
leaching in the upper part of the profile and precipitation in the
deeper part of the profile (Jin et al., 2014; Monger et al., 2015; Wang
et al., 2016). Another part of the study found an increase in SIC
attributed to the involvement of exotic Ca\textsuperscript{2+} and
Mg\textsuperscript{2+} in the formation of SIC (BOETTINGER and SOUTHARD,
1991; Zhao et al., 2016) , the formation of carbonate from biogenic
CO\textsubscript{2} (He et al., 2016), or a decrease in soil erosion (Fu
et al., 2009). The study also found a decrease in SIC, which was
attributed to the transfer of SIC into groundwater with water
infiltration (Kessler and Harvey, 2001), or in low pH soil environments,
SIC is released into the atmosphere as CO\textsubscript{2} (Liu et al.,
2014). The seemingly contradictory results reflect the uncertainty of
the changes in SIC storage after revegetation.

In this study, the SIC storage in the sand showed an increasing trend
after afforestation. (Su et al., 2010) reported that planting camphor
pine and aspen in the Badangirin sands promoted the accumulation of SIC.
(Gao et al., 2017) found that SIC storage also increased with stand age
sequence after planting aspen (8 \textasciitilde{} 30 years old aspen)
in Mauwusu sands, with significantly higher SIC content after vegetation
restoration than in moving sands. The findings of this study are
consistent with those of the aforementioned scholars on SIC storage
after the afforestation of sandy areas. The increase in SIC after the
afforestation of sandy areas is due to the high input of litter and the
high rate of decomposition of organic matter, resulting in the release
of large amounts of CO\textsubscript{2} into the soil (Zhang et al.,
2013). A considerable amount of CO\textsubscript{2} is involved in the
chemical reaction of formula (4), forming a large amount of
HCO\textsubscript{3}\textsuperscript{-}, which leads to the
precipitation of loam-forming carbonates (Zamanian et al., 2016). In
addition, the trapping and deposition of fine particles in the wind and
sand streams by the plant canopy, after the afforestation of sandy areas
also contributes to the accumulation of SIC. Soiling fine particles
contain a large amount of carbonate, which leads to rapid accumulation
of SIC (Chang et al., 2012). However, our results differ from earlier
ones on the Loess Plateau of China and the Colorado Plateau Desert, USA
(Chang et al., 2012; Sartori et al., 2007). In their previous findings,
vegetation restoration reduced SIC storage. The reduction in SIC storage
may be caused by surface runoff, and irrigation in these areas, which
could reduce dissolved SIC content from the restored vegetation-soil
ecosystem. In arid semi-arid sands are not affected by irrigation or
heavy precipitation. Therefore, our results suggest that vegetation
construction has the potential to increase SIC storage in arid semi-arid
sands.

The establishment of vegetation on shifting sands at different ages also
provided us with a good-time series to study CSR. This study shows that
CSR is not a constant value; instead, it tends to decrease with
increasing stand age. The CSR of short afforestation time (\textless{}5
years) is greater than that of a long time (\textgreater{}5 years). If
we classify the H. ammodendron plantation from 5 to 46 years old as
young, near-mature, mature, and over-mature forests, respectively. We
found that the CSR was greatest in young stands and least in over-mature
stands after moving sand afforestation. Therefore, if the change of CSR
over time after vegetation restoration is neglected, the magnitude of
long-term changes in SIC storage after sand afforestation will be
incorrectly estimated, which will greatly increase the uncertainty of
soil carbon estimation. Currently, many studies have only estimated the
increase or decrease in SIC storage after afforestation (An et al.,
2019; Raza et al., 2020), while few studies have quantified how the
change of CSR with afforestation time (Chen et al., 2018; Wang et al.,
2019)(Chen et al.,2018; Wang et
al.,2019)\textsuperscript{{[}34-35{]}{[}34,35{]}{[}34, 35{]}}.
Therefore, the CSR in these studies was assumed to be the same over 30
years, with the result that SIC storage increments with stand age were
overestimated. To improve the accuracy of CSR calculations, we strongly
recommend long-term monitoring of SIC at different stages in long-term
studies or using space-for-time methods.

\textbf{4.3 The importance of deep SIC}

Most previous studies on SIC have focused on the upper layers,
especially the top 1.0 m. However, the excavation of deeper soil
profiles becomes more important for the study of SIC storage (Mi et al.,
2008; NEPSTAD et al., 1994; Veldkamp et al., 2003). (Li et al., 2007)
estimated that the SIC storage from 1 m depth to intact soil is about
57\% in sandy soils and 61\% in gray desert soils. (Wang et al., 2010)
reported that more than 50\% of the SIC in forests, scrub, grasslands,
and sandy areas in temperate regions of northern China were located at a
depth of 100 \textasciitilde{} 300 cm. (Wang et al., 2013) found
stratified storage of SIC at 0 \textasciitilde{} 1 m, 1
\textasciitilde{} 3 m, 3 \textasciitilde{} 6 m, and 6 \textasciitilde{}
9 m depth in saline and alkaline soils in arid northwest China. SIC
storage below 1 m is above 80\%, and SIC storage below 3 m is above
50\%. The values estimated in this study were higher than theirs, and we
found more than 60\% of SIC storage in the 1m to 3m soil profiles in
sandy and post-afforestation areas. The differences between these values
may be due to different soil sampling depths and vegetation types.
Compared to the estimated total SIC storage in the top 1 m of the soil
of 53.3 Pg, soils below 1 m in the arid desert areas of northwest China
still contain 6.2 Pg or more of SIC storage (Mi et al., 2008). Recent
studies have shown that carbon uptake in saline soils in arid zones can
be as high as 62-622 g C m \textsuperscript{-2} y\textsuperscript{-1}.
Thus, on a global scale, the deep soil SIC pool and its variability may
be more critical than we have recognized. In China, 47\% of the land is
arid and semi-arid, and the deeper layers of the soil are rich in SIC.
The contribution of deep soil SIC storage to total soil carbon storage
after vegetation construction has not been well studied for these areas.

Understanding the changes in deep-section SIC storage is essential for
assessing the climate change mitigation potential of soil carbon
pools(George et al., 2012; Jobbagy and Jackson, 2000). In this study, we
analyzed the soil profile variation of SIC storage along the age of
artificial H. ammodendron plantation and its influencing factors. On the
one hand, the variation characteristics of SIC storage in different
forest age stages provide important data for regional carbon estimation.
On the other hand, understanding the effect of vegetation restoration on
changes in SIC storage under different soil depth conditions helps us to
understand the importance of deep SIC for soil carbon pool estimation.
Our results suggest that considering only shallow SIC when assessing the
impact of regional vegetation restoration on SIC reserves will result in
a significant underestimation of the SIC pool. However, all soil samples
are difficult to reach down to 300 cm due to a large amount of labor and
cost consumption. In this context, the combination of remote sensing
techniques and model simulations for in-depth soil sample collection in
typical areas is a feasible method for estimating the soil inorganic
carbon pool.

\textbf{5 Conclusion}

In this study, we found that SIC stocks increased from 0 to 300 cm after
afforestation in arid semi-arid sandy land, indicating that the
afforestation of sandy land increased the soil carbon sequestration
capacity. 0-300 cm SIC increased continuously (insignificantly) with
stand age, while the difference between MS and other stand age stages of
soil inorganic carbon was significant. Afforestation in semi-arid sandy
areas not only increased SIC storage but also significantly changed the
profile distribution characteristics of SIC. Soil moisture had a
significant effect on SIC in all layers, while pH had a significant
effect on SIC in deep layers. The decreasing rate of inorganic carbon
fixation with increasing stand age implies that the rate of soil
inorganic carbon fixation is not a constant value, and it is necessary
to quantify the temporal pattern of SIC changes to accurately estimate
the dynamics of regional SIC stocks. Our study also highlights the
importance of deep SIC stocks in semi-arid regions in soil carbon sink
estimation.

\textbf{Funding:} This study was supported by the Major Science and
Technology Project in Inner Mongolia Autonomous region of China (No.
Zdzx2018057), the Innovation Cross Team Project of Chinese Academy of
Sciences, CAS (No. JCTD-2019-19), Transformation Projects of Scientific
and Technological Achievements in Inner Mongolia Autonomous region of
China (No. 2021CG0046), and Groundwater environmental evolution in
Minqin Basin under the influence of climate change and human activities
(No. 23JRRA611).

\textbf{Data availability statement:} The original contributions
presented in the study are 404 included in the article/Supplementary
Material. Further inquiries can be directed to the 405 corresponding
author.

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