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.