4.3 Relationships among GRSP, SOC, and soil variables
GRSP content was significantly positively correlated with SOC and TN. GRSP comprises carbon and nitrogen and is highly correlated with the carbon and nitrogen content in the soil (Vasconcellos et al., 2016). Other studies have also shown a positive relationship between GRSP with SOC and TN (Zhang et al., 2015; Wang et al., 2018; Qiao et al., 2019). C and N contents in glomalin represent 4–5% of the total soil C and N (Rillig et al., 2001). GRSP content was associated with SOC probably because GRSP is also a component of soil organic matter (Rillig et al., 2001). Besides, GRSP and SOC depend on similar ecosystem component (photosynthetic productivity) and contribute to similar ecosystem functions (feeding decomposer community and formation of soil aggregates) (Kumar et al., 2018). Moreover, GRSP promotes soil C pools in native grasslands for a soil binding agent (Purin et al. 2006). In summary, GRSP regulates the accumulation and circulation of SOC in alpine grassland soils. Meanwhile, GRSP is an N-linked glycoprotein that increases soil N levels (Wright and Upadhyaya, 1998).
GRSP content was negatively correlated with pH, consistent with other studies (Wang et al., 2014; Wang et al., 2017). Antibus et al. (2006) also indicated that EE-GRSP is negatively correlated with pH. Previous studies have also shown that soil pH regulates GRSP accumulation, precipitated under acidic conditions, and soluble under alkaline pH conditions (Singh et al., 2017). Decreased soil pH facilitates GRSP accumulation (Wang et al., 2017), explaining the negative correlation between GRSP and pH.
5. CONCLUSION
This study demonstrated that livestock grazing can change GRSP stability in alpine grassland. Also, T-GRSP and EE-GRSP contents were gradually decreased as grazing intensity increased. Grazing also affects SOC fixation. The succession process of alpine grassland also causes various soil organic carbon succession. GRSP is strong positive correlated with SOC and TN, and GRSP contributing 3.7-14.18% of SOC. N and P nutrient addition for five years increases the available N and P content, but it does not affect SOC and GRSP contents. Therefore, GRSP is a stable organic carbon and regulates SOC fixation. Moreover, short-term grazing cannot affect GRSP stability, but long-term overgrazing can gradually reduce GRSP stability. This study showed the impact of grazing and fertilization on GRSP and another indicator of stable soil carbon. Therefore, it provides insights into the changes of soil carbon pool and ecological stoichiometry during grassland succession on the Qinghai-Tibet plateau.
ACKNOWLEDGEMENTS
This work was financially supported by the National Natural Science Foundation of China (32171650, 31400483, 41730752), the National Program on Key Basic Research Project (2014CB954002). We thank Xun ke, Yanan Sun, Shuli Liu, Jingzheng Ouyang from the Northwest Institute of Plateau Biology, Chinese Academy of Sciences, for assistance in soil sampling. We also heartily acknowledge the input from anonymous reviewers and editors that improved earlier versions of this manuscript.