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).