Does Grazing Exclusion Improve Soil Carbon and Nitrogen Stocks in Alpine Grasslands on the Qinghai-Tibetan Plateau? A Meta-Analysis

Abstract

Grazing exclusion has been widely used to restore the degraded alpine grasslands on the Qinghai-Tibetan Plateau (QTP). However, the dynamics of soil organic carbon (SOC) and soil total nitrogen (STN) pools after grazing exclusion and their controlling factors are currently less understood in this region. Here, a meta-analysis was conducted to quantitatively assess the changes in SOC and STN stocks in topsoil (0–30 cm) following grazing exclusion in three major grassland types (alpine meadow, alpine steppe, and alpine desert steppe) on the QTP and to explore the potential factors controlling the effects of grazing exclusion on SOC and STN stocks. The results showed that overall, grazing exclusion significantly increased SOC stock by 16.5% and STN stock by 11.2%. Significant increases in both SOC and STN stocks were observed after grazing exclusion of alpine meadow. In contrast, grazing exclusion did not improve SOC and STN stocks in the other two grassland types. The difference in mean annual precipitation among grassland types was a likely reason for the different dynamics of SOC and STN stocks after grazing exclusion. The effect sizes of both SOC and STN stocks were positively related to the duration of grazing exclusion, and a positive relationship was detected between the effect size of SOC stock and that of STN stock, demonstrating that the dynamics of SOC and STN were closely coupled during the period of grazing exclusion. However, grazing exclusion had no impact on soil C:N ratio for all grassland types, indicating that soil C:N ratio was generally stable after grazing exclusion. Therefore, it is suggested that the increase in STN can support continuous SOC accumulation following grazing exclusion. In conclusion, the findings suggest that the effects of grazing exclusion on SOC and STN stocks differ among grassland types on the QTP, and grazing exclusion of alpine meadows may provide substantial opportunities for improving SOC and STN stocks in this region.

Changes in soil carbon and nitrogen stocks following degradation of alpine grasslands on the Qinghai‐Tibetan Plateau: A meta‐analysis

“…For example, a recent synthesis reported that the accretion of STN could not meet the demand of SOC increase at the later recovery stage of degraded grasslands which then limited SOC sequestration (Deng et al, 2017). Hence, STN may play a key role in determining the capacity of SOC sequestration during the restoration of degraded alpine grasslands on the QTP (Liu, Sheng, et al, 2020; Yu, Chen, Sun, & Huang, 2019).…”

“…The degradation stages of grassland were divided into five groups including non‐degradation (ND), light degradation (LD), moderate degradation (MD), heavy degradation (HD), and extreme degradation (ED) to assess the effects of grassland degradation stage on SOC and STN stocks (Liu et al, 2018; Ma et al, 2002; Zhang, Xue, Peng, You, & Hao, 2019). The raw data in the selected studies were extracted from graphs using GetData Graph Digitizer v.2.25 (http://www.getdata-graph-digitizer.com/index.php) or directly obtained from tables (Liu et al, 2020). Information on the location, mean annual temperature (°C), mean annual precipitation (mm), and grassland type of each study site was also gathered when it was available (Table S2).…”

“…This method adjusts soil mass to the lightest soil mass across study sites in individual studies and has been proved to be a better choice than the maximum equivalent soil mass method when estimating the changes in SOC and STN stocks in grassland ecosystems (Lee et al, 2009). The detailed procedure of the minimum equivalent soil mass method was described in Bárcena et al (2014) and Liu, Sheng, et al (2020). In brief, the first step of this method was to calculate the mass per unit area of the soil according to the fixed depth at non‐degradation and degradation sites for each study: Where: M f (Mg ha −1 ) is the dry mass of soil to a fixed depth; BD (g cm −3 ) is the bulk density; and h (cm) is the thickness of soil layer.…”

“…In the present meta‐analysis, the natural log‐transformed response ratio (lnRR) was used as the effect size to evaluate the differences in soil BD and stocks of SOC and STN between non‐degraded and degraded grasslands (Dlamini, Chivenge, & Chaplot, 2016; Lam, Chen, Norton, Armstrong, & Mosier, 2012): Where: X nd and X d indicate the mean values of soil BD, SOC stock and STN stock at non‐degraded grasslands and degraded grasslands, respectively. The response ratios were transformed to percent changes ([RR − 1] × 100) to present the dynamics of soil BD, SOC stock, and STN stock after grassland degradation (Liu, Sheng, et al, 2020; Zhang et al, 2019). Positive values implied increases in soil BD, SOC stock, and STN stock after grassland degradation, whereas negative values indicated that grassland degradation decreased soil BD, SOC stock, and STN stock.…”

“…Where: X nd and X d indicate the mean values of soil BD, SOC stock and STN stock at non-degraded grasslands and degraded grasslands, respectively. The response ratios were transformed to percent changes ([RR − 1] × 100) to present the dynamics of soil BD, SOC stock, and STN stock after grassland degradation (Liu, Sheng, et al, 2020;Zhang et al, 2019). Positive values implied increases in soil BD, SOC stock, and STN stock after grassland degradation, whereas negative values indicated that grassland degradation decreased soil BD, SOC stock, and STN stock.…”