1. INTRODUCTION
Glomalin is a glycoprotein mainly produced by arbuscular mycorrhizal fungi (AMF) (Wright & Upadhyaya, 1996). It is a hyphal wall component that accumulates in soils and has a slow turnover (Rillig et al., 2001). Glomalin is composed of carbon (C) (36-59%), nitrogen (N) (3-5%), and iron (Fe) (1-9%). Glomalin mainly increases SOC pools and improves soil aggregates (Rillig et al., 2001; Wang et al., 2018). C content in glomalin represents 4~5% of the total soil C, which is much higher than soil microbial biomass C (Zhu and Miller, 2003). Glomalin in soils is called glomalin-related soil protein (GRSP) (Rillig, 2004), which is classified into two fractions: easily extractable GRSP (EE-GRSP) and total GRSP (T-GRSP).
GRSP, as an insoluble and hydrophobic proteinaceous substance, can act as long as 6~42a in soil (Rillig et al., 2001). However, it is unclear whether GRSP changes with the environment and human activities. Grazing via domestic ungulates is the main human activity on grassland ecosystems that affects ecosystem processes and functions (Ebrahimi et al., 2016). The alpine grassland ecosystem dominates the Qinghai-Tibet Plateau, covering over 1.28×106km2 (50.9% of the entire plateau area) and contains enormous organic carbon. In recent decades, alpine grasslands have experienced rapid degradation (Xiong et al., 2014) mainly due to livestock overgrazing and dry climate (Dong et al., 2013). Excessive livestock grazing affects nutrient pool and cycling, net primary productivity, vegetation composition, belowground biomass productivity, and associated changes in the soil microbial community (Cao et al., 2004; Veen et al., 2014). Chemical fertilizers (nitrogen (N) and phosphorus (P)) are widely used to enhance grassland productivity yields and soil fertility for grazing (Hacker et al., 2011; Zhou et al., 2015). N and P are the main limiting elements controlling the productivity and functioning of the Tibetan alpine grassland ecosystems (Yang et al., 2020; Dong et al., 2020).
Grazing and fertilization can change plant productivity, soil carbon pool, and microbial community of grassland ecosystem, thus affecting the protein content secreted by soil microorganisms. Wang et al. (2014) showed that overgrazing significantly decreases the total mycorrhizal colonization and GRSP content in grassland. Moreover, nitrogen fertilizer (10 g/m2) can significantly increase T-GRSP content but has no significant effect on the EE-GRSP in the Prairies (Wilson et al., 2009). Long-term phosphate fertilizer application significantly increases GRSP content compared with the control (Johnson et al., 2003; Chagnon and Bradley, 2013). Various components of GRSP have different responses to fertilization (Turgay et al., 2014; Zhang et al., 2015), possibly because different soil nutrient contents have different plant productivity and decomposition rates of GRSP (Rillig et al., 2002; Steinberg and Rillig, 2003).
The alpine grassland ecosystem of the Qinghai-Tibet Plateau is extremely fragile, slow to recover, and greatly affected by external disturbance. No study has reported on the effect of human activities on GRSP. Besides, the influence of the interaction between environmental factors and human activities on GRSP is unknown. The spatial and temporal variation and the mechanistic of stable structural GRSP caused by grazing and fertilization are also unclear. Herein, GRSP response to grazing intensity and fertilization was studied.
Herein, an alpine meadow with grazing intensity gradients and different fertilization practices were used. The study aimed to: (1) determine GRSP content variation in response to different gradients of grazing intensity and different fertilization (N and P) treatments; (2) Identify interrelationships among GRSP, SOC, and edaphic properties in response to human activities. This study provides insights into the stable organic carbon function of GRSP for improving rangeland management practices on the Qinghai-Tibet plateau.