Shrub encroachment decreases soil inorganic carbon stocks in Mongolian grasslands

Abstract

Widespread shrub encroachment in global drylands may increase plant biomass and change soil organic carbon stocks of grassland ecosystems. However, the response of soil inorganic carbon (SIC), which is a major component of dryland carbon pools, to this vegetation shift remains unknown. We conducted a systematic field survey in 75 pairs of shrub‐encroached grassland (SEG) and control plots at 25 sites in the grasslands of the Inner Mongolia Plateau to evaluate how shrub encroachment affects SIC density (SICD) in these ecosystems. We found that shrub encroachment significantly reduced SICD in the upper 100 cm (3.85 vs. 4.74 kg C m−2, p < .05), especially in the subsurface soil (20–50 cm layer). The magnitude of SICD changes was related to the change in soil pH, shrub patch size and initial SICD, reflecting that the reduction in SICD might be attributed to the shrub encroachment‐related soil acidification. Our results also revealed that the lost SIC was mainly released into the atmosphere rather than redistributed into deeper soil layers. Synthesis. We provide the first evidence for the soil acidification‐induced SIC loss caused by shrub encroachment. Our findings highlight the non‐negligible role of SIC dynamics in the C budget of SEG ecosystems and the need to consider these dynamics in terrestrial C cycle research.

Contrasting Responses of Soil Inorganic Carbon to Afforestation in Acidic Versus Alkaline Soils

“…In explaining the relative change in SIC with afforestation, we found that RR_SIC is well correlated with ΔpH, indicating the sensitivity of SIC to changes in soil pH (S. Liu et al, 2020). Given that pH value is the negative logarithm of hydrogen ion concentration, this correlation between RR_SIC and ΔpH indicates that the dynamics of SIC and hydrogen ions are synergetic, confirming our earlier finding based on a spatial analysis (Hong et al, 2019).…”

“…Moreover, afforestation can increase the capture of N deposition due to increased surface roughness (Högberg et al, 2006), or decrease the amount of N reaching soils due to canopy interception (Wang et al, 2021), which would result in complex effects on soil pH and SIC. As the hydrological effects of afforestation could also affect carbonate leaching in soils and lead to vertical redistribution of SIC (S. Liu et al, 2020), SIC loss is generally observed in arid areas, where the leaching effect is weak (Chang et al, 2012) and may result in limited vertical reallocation (S. Liu et al, 2020). Surprisingly, we also observed non-significant effects of OVLUT on SIC responses to afforestation.…”

“…Considering the high risk of soil acidification caused by increasing nitrogen and sulfur deposition in these regions, this reduction in soil buffering capacity may lead to positive feedbacks between soil acidification and carbonate loss (Ito et al, 2018;. Carbonate dissolution also drives the losses of base cations such as Ca 2+ , Mg 2+ , and K + , and could further decrease soil fertility (S. Liu et al, 2020). A global meta-analysis suggested that afforestation resulted in a significant loss of base cations, especially for pine plantations (Berthrong et al, 2009).…”

Controls on variation of soil organic carbon concentration in the shrublands of the north‐eastern Tibetan Plateau

“…Similar results from a structural equation model also indicated that high soil acidity can increase SOC across shrublands in China (Chen et al, 2018). In addition, it has been demonstrated that shrubland encroachment can decrease soil pH (Liu et al, 2020), which has significantly caused an increase in SOC accumulation (Li et al, 2019).…”

Rhizosphere Effects of Maize and Wheat Increase Soil Organic and Inorganic Carbon Release in Carbonate-Rich Soils: A Three-Source 13C Partitioning Study

“…In the past, most of the studies on soil C emissions focused on SOC decomposition, but little attention was given to the role of SIC. Recent studies found that SIC can be considered a C source (carbonate dissolution and release; Zamanian et al, 2018;Liu et al, 2020) or C sink (secondary carbonate formation; Wang et al, 2014;Dong et al, 2019). Therefore, the stability of the SIC pool directly affects atmospheric CO 2 concentrations and the soil C balance.…”