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.