4.2 Factors affecting soil pH
Climate may affect soil pH through physicochemical and biological
processes associated with water permeability and leaching processes,
productivity of vegetation and decomposition of organic matter, and soil
buffering capacity in GE grasslands. This study showed significant
negative RPC correlations with the MAP, but there were no significant
correlations between RPC and MAT. This indicates that GE can cause soil
acidification under wetter climatic conditions and soil pH dynamics are
mainly determined by MAP rather than the MAT in China’s grasslands. The
decrease in RPC responses to GE with MAP may be attributed to the
following causes. Firstly, GE decreased soil bulk density and increased
the porosity, which can increase the soil water permeability coefficient
and leaching effect, thereby resulting in the dissolution of carbonates
and larger loss of base cations and nitrate in humid grasslands (Brady
& Weil, 2008). Furthermore, increased rainfall may introduce a large
amount of acidic substances into the GE soil, such as nitrogenous
compounds in humid regions (Guo et al. , 2010). Secondly, Huet al. (2016) and Deng et al. (2017) found that grassland
restoration projects could be more beneficial for biomass accumulation
in humid regions. Those stronger biological responses would result in a
greater decrease in soil pH by altering litter decomposition, and root
exudates and respiration. Thirdly, under humid climate conditions, most
soil pH was below 7.0 and such a neutral or slightly acidic environment
are not suited to the accumulation of soil carbonates (Yang et
al. , 2010; Yang et al. , 2012).
The low amount of carbonates stored
in wetter grassland ecosystems may in turn provide a weak buffering
capacity for increases in soil acidity (Kirk et al. , 2010).
Variations of SOC and SN concentrations were the main factors
influencing
RPC
following GE. The RCC was negatively correlated with RPC, which
indicated faster accumulation of SOM leading to
lower
pH values. This was mainly because the composition of
H+ and Al3+ adsorbed to SOM is
dependent on various processes such as humification of SOM and microbial
decomposition, physical incorporation of SOM and chemical reactions
between mineral and organic matter (Skyllberg et al. , 2001). In
particular, SOM contains a number of acid functional groups from which
H+ ions can dissociate, and thus it is an important
source of H+ ions in soil, and is favorable for
maintaining soil acidity (Brady & Weil, 2008). The mechanisms of soil
N-induced soil acidification involved the adsorption and desorption of
exchangeable basic/acidic cations. Improved soil N significantly reduced
the exchangeable basic cations of Ca2+,
K+, Mg2+ and Na+,
while increasing free Al3+ and Mn2+in soils (Tian & Niu, 2015).
Plant species is a key factor affecting RPCs of grassland ecosystems.
Hong et al. (2018) reported that the effects of afforestation on
soil pH are species-specific through different rhizospheric processes.
For example, they found Pinus tabuliformis and Pinus
koraiensis significantly reduced soil pH by 0.18–0.20, whereas other
tree species had no detectable effects on soil pH. In our study, we
found the grassland dominated by sedge (e.g., Heleocharis
uniglumis , Carex enervis and Kobresia humilis ) had a
higher decrease in soil pH than the grassland dominated by grass species
(e.g., Leymus chinensis , Stipa grandis and Stipa
purpurea ) at 0–10 cm soil depth. This is because the root biomass of
sedge species is mainly concentrated in the surface soil and generally
shows relatively high absorption rates for
NH4+ (Jiang et al., 2016), which could
be replaced by H+, thus decreasing soil pH (Berthronget al. , 2009). In addition, GE had significant decreases in deep
soil pH for forb and shrub species (e.g., Artemisia desertorumand Astragalus adsurgens ) in grasslands arise from the deep root
allocation. The absorption of a large amount of cations reduced the soil
pH by releasing rhizospheric H+ in deep soil (Dakora
& Phillips, 2002).