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.