4.1 Change in of soil pH following GE
Mixed results regarding the effects of GE on soil pH have been found in previous studies, with negative (Wu et al. , 2009; Raiesi & Riahi, 2014), neutral (Shang et al. , 2017; Sigcha et al. , 2018) or positive (Cheng et al. , 2016) effects. In our study, we found soil pH decreased with GE at most grassland sites across China. This result is consistent with other studies on grasslands of the world (Anderson et al. , 2008; Raiesi & Riahi, 2014). GE could lead to soil pH acidification by altering the balance between soil hydrogen ion generation and consumption during the nutrient cycle. GE-induced changes in litter decomposition and rhizospheric processes may play a major role in soil pH dynamics. Firstly, organic matter inputs from aboveground biomass, litter biomass, and root biomass all increased because of the removal of grazing pressure and the amelioration of soil water content (Hu et al. , 2016; Xiong et al. , 2016; Deng et al. , 2017). The addition of plant residues can decrease soil pH through N nitrification in the residue (Binkley & Richter, 1987; Rukshanaet al. , 2014). Secondly, root exudates (e.g., H+, OH and HCO3) can modify rhizospheric pH to enhance nutrient uptake by plant roots (Dakora & Phillips, 2002). Plants take up more cations than anions in calcareous grassland soils, which reduces the rhizospheric pH as they release H+from their roots to maintain charge balance (Dakora & Phillips, 2002). Thus, the decrease of effective cation exchange capacity in soil and the depletion of soil exchangeable base ions (e.g., calcium ions) accelerate the acidification of grassland soil. Thirdly, higher root biomass was observed in GE grasslands. Consequently, root respiration in the GE grasslands could be expected to exhibit an increasing trend and subsequently the increase in H+ion inputs from carbonic acid could lead to lower soil pH (Ji et al. , 2014).
Overall, surface soil showed greater soil acidification rates than deep soil. The soil pH decreased by 0.18 and 0.13 at 0–10 and 10–20 cm soil depth for China’s grassland, respectively. However, GE had limited impacts on soil pH dynamics for 20–30 cm and 30–100 cm soil layers. The vertical differentiations of RPC may arise from the plant–soil biological processes, such as litter decomposition, root distribution, rhizospheric processes and microbe activities, are mainly concentrated in topsoil, not in the deep soil. This result is consistent with those of Talore et al. (2015), which showed that soil pH was decreased by 0.27 in the top 10 cm following long-term GE in South Africa. Conversely, the pH of deeper soils in GE grasslands increased slightly or remained unchanged.
The largest decrease of soil pH was in the medium-term period after GE. The trends were inconsistent with AB and BB dynamics following GE because the highest biomass accumulation occurred after a short period of GE (≤5 years) (Hu et al. , 2016; Deng et al. , 2017), whereas the strongest soil acidification occurred after a medium-term period of GE (5–15 years). The changes in soil pH lag behind changes in vegetation biomass variation. This could be because the decomposition of plant litter takes a certain period of time (Parton et al. , 2007), which in turn affects soil microbial activity, N transformation and the proportion of cations and organic anions. GE leads to a limited effect on the decrease of RPC in long-term period (≥15 years). This result is consistent with the ‘dynamic disequilibrium’ theory, which states that GE leads to temporal changes in many biogeochemical cycles (e.g., vegetation biomass and soil C and N accumulation) in the short term, but does not affect long-term dynamics (Luo & Weng, 2011). To our knowledge, this is the first study that simultaneously quantifies the large-scale reduction of soil pH in medium-term GE grasslands. Based on this information relating to the relation between the length of GE and soil pH dynamics, the potential soil acidity for GE in grassland ecosystems can be evaluated, which has important implications for soil biogeochemical processes and predicting ecosystem structure and functions in grasslands.