2.1 Study area and marsh degradation gradient
The study area is located in the Huahu Science Spot, Haqingqiao, and Heihe Village on the Zoige Plateau, China, northeast edge of the Qinghai-Tibet Plateau (between 102°25′–103°0′ E and 33°30′–33°55′ N) (Figure 1). The area is at an average altitude of ~3450 m and subjected to an alpine cold temperate monsoon humid climate, the wet season between May and September, and the dry season between October and April. The mean annual temperature is 0.4–1.1 °C, with a minimum mean monthly temperature of −10.8 °C in January and a maximum of 10.9 °C in July. The annual precipitation is 647–753 mm, with a relative humidity range of 64~73% and a potential evapotranspiration of 1100–1274 mm. The landform is characterized by a combination of broad valleys, low mounds, lake depressions, terraces and river flood plains that are associated with the Black River system (a tributary of the Yellow River).
In this region, large marsh areas distribute on lowlands, terraces, and some underground stream gully zones due to the flat, low-lying ground and poor drainage of surface water. These marsh soils have the similar pedogenic conditions, such as soil parent materials of the Quaternary loose sediments from rivers and lakes, perennial waterlogging with water table depth greater than 10 cm, and the dominant hygrophytes ofCarex L . (Carex muliensis , Carex lasiocarpa andCarex meyeriana ). However, since the early 1960s, these marshes have occurred a regressive succession from original marsh to marsh meadow, meadow, desertified meadow with “black beach” due to climate change, rodents and human activities (Li et al., 2015, 2019; Huo et al. 2013; Zhao et al., 2017b). Taking comprehensively into account the diagnostic indices of the hydrologic conditions, dominant plant community and external disturbance (Huo et al., 2013; Wu et al., 2015), we divided degraded marshes into lightly degraded marsh (LDM), moderately degraded marsh (MDM) and heavily degraded marsh (HDM) according to the grading criteria of the degradation of relatively pristine marsh (RPM) (Table 1). Furthermore, the moderately and heavily degraded marshes are eroded by wind and easy to be sandy due to an increase in the area of surface soil exposed to sunlight. The soils of RPM, LDM, MDM and HDM are classified as Histi-Orthic Gleyosols, Molli-Orthic Gleyosols, Dark-Aquic Cambosols, Ochri-Aquic Cambosols in the Chinese soil taxonomy, respectively (Cooperative research group on Chinese soil taxonomy, 2001).