2.2 Soil sampling
To ensure degraded marshes from the degradation of RPM, we compared the remote sensing images recorded by the 1977 Landsat 2 MSS and 2015 Google Earth, and established sampling plots (100 m × 100 m) from differently degraded marshes following grading criteria of marsh degradation (Table 1) within the Huahu Science Spot, Haqingqiao, and Heihe Village in August 2017. After three subplots (1 m × 1 m) in each sampling plot were randomly selected, we investigated the main habitat conditions (Table 2) and then clipped the aboveground parts of all standing plants (dead and alive) to measure the aboveground biomass. Subsequently, soil samples were taken at depths of 0–10, 10–20, 20–30, 30–40, 40–60, 60–80, and 80–100 cm in each subplot using a soil sampler (Eijkelkamp 07.53. SE, Netherlands) to determine soil bulk density and other properties. Three soil cores from the same soil depth in each plot were pooled to form a composite sample. 84 mixed soil samples were collected from 12 plots. Root biomass estimated from soil bulk density samples at a depth of 0–60 cm was washed through a 0.5 mm sieve to remove the soil, and all roots with a diameter of > 0.5 mm were collected and oven-dried at 65 °C (Table 2).