1 Introduction
Soil acidity or alkalinity, described by soil pH, is strongly associated
with soil biogeochemical cycles and biodiversity in terrestrial
ecosystems. Controlled experiments and field monitoring have revealed
that changes in
soil
pH have strong effects on organic matter accumulation (Evans et
al. , 2012), microbial activity (Lauber et al. , 2009; Li et
al. , 2017) and
greenhouse
gas emissions (Wu et al. , 2018). In addition, soil acidification
has cascading effects on structures and functions of land ecosystems
(Hong et al. , 2018). In particular, decrease in pH is a major
threat to species diversity both at the regional and continental scale
with increasing nitrogen deposition (Azevedo et al. , 2013).
Further
soil
acidification could result in a large amount of cation nutrient loss
(Haynes & Swift, 1986),
intensifying
the
nutrient limitation for
plant
growth (Weaver & Hamill, 1985), reproduction (Roem et al. , 2002;
Basto et al. , 2015) and productivity (Kallenbach et al. ,
1996). Accordingly, the changes of soil pH need to be systematically
evaluated under anthropogenic changes (Berthrong et al. , 2009;
Kirk et al. , 2010; Riofrío‐Dillon et al. , 2012; Honget al. , 2018).
GE is considered to be an effective revegetation practice for overgrazed
grasslands. Previous studies have mainly focused on restoring the
degraded grassland through its self-healing capacities as well as to
promoting carbon sequestration (Piñeiro et al. , 2009; Wu et
al. , 2010; Hu et al. , 2016; Deng et al. , 2017). However,
despite the importance of GE in vegetation recovery and soil nutrient
accumulation, the effects of GE on soil pH has received less attention
and considerable uncertainty remains. For example, previous in
situ studies have found mixed results of GE effects on soil pH, with
studies showing negative (Wu et al. , 2009; Raiesi & Riahi,
2014), neutral (Shang et al. , 2017; Sigcha et al. , 2018)
or positive (Cheng et al. , 2016) effects of GE. Because of this
lack of consensus on the effects of GE on soil pH dynamics, there is
little knowledge about the trajectory of soil pH dynamics along the age
gradient of GE. In addition, most individual studies have focused on
comparison of soil pH between the GE and the grazing plots. Therefore,
limited knowledge about the effects of GE on soil pH under different
climatic conditions restricts our ability to explore the effects of
possible anthropogenic changes on soil biogeochemical processes on a
broader scale.
Grasslands
are an important part of the Eurasian steppe and cover approximately
40%
of the total territorial area of China (Wang et al. , 2018).
Because of the intense grazing
pressure,
a markedly increasing percentage of grasslands is displaying degradation
across China. The Chinese government has invested heavily to restore
degraded grasslands over the past several decades. For example, the
‘Returning
Grazing Land to Grassland’ project was implemented to protect grasslands
through GE from heavy grazing pressures (Xiong et al. , 2016).
Through compilation of published data in addition to field samples from
grasslands in China, this study aimed to examine the dynamics of soil pH
during the ecosystem recovery processes following GE. Our specific
objectives were to: 1) explore the effects of
GE on soil pH dynamics, and 2)
identify factors influencing the effects of GE on soil pH changes.
Understanding the soil pH dynamics resulting from GE is crucial for
developing control measures to maintain soil quality and enhance the
stability of terrestrial ecosystems.