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).