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 CO2, 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
CO2 from the atmosphere. Sufficient CO2can 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 CO2into the soil by decomposing SOC, producing large amounts of free
HCO3−和H+ (Zamanian
et al., 2016). The accumulated HCO3−can drive reaction (5) to the right, and when Ca2+ 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 ~ 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 CO2 concentration. Subsequently,
under adequate soil moisture conditions, driving equations (4) and (5)
to the right, more abundant HCO3− is
formed to bind with Ca2+ 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 ~ 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
CO2 in the soil is dissolved at shallow depths and
follows the effective precipitation downward. This process continuously
reacts chemically with shallow Ca2+ and
Mg2+, which causes the migration and precipitation of
carbonates. In the 120 ~ 240 cm soil profile, the
variation of SIC content was mainly influenced by root biomass,
CO2 partial pressure Ca2+ and
HCO3−. 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
CO2, driving equation (4) to the right, further
dissolving in the soil solution to form HCO3-, which
subsequently combines with Ca2+ released from
decomposing litter to precipitate as CaCO3 (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 Ca2+ and a
large amount of HCO3− accumulates in
the mucilaginous sheath due to respiration, allowing the mucilaginous
sheath to provide a unique environment for Ca2+ and
HCO3− 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 CO2 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
Ca2+ and
HCO3− content, the large enrichment of
CO2 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 CO2 partial pressure in the
shallow layer; and by soil moisture, root litter and pH value in the
deep layer.