Nitrogen stable isotopes reveal enhanced nitrogen losses and altered plant nitrogen uptake strategies in degraded alpine peatlands

Ten sites along a peatland degradation gradient in Zoige were investigated in July 2022 . The ten sites were classified into four degradation levels, including flooded wetlands (FW), wet meadows (WM), moderately-degraded meadows (DM), and severely-degraded meadows (SM) . We set up three 1 m × 1 m quadrats that were at least 50 m apart from each other at each site, and recorded total plant cover and the relative cover of four functional types (i.e., sedge, grass, non-leguminous and leguminous forb) in each quadrat. Aboveground biomass was harvested and placed in paper bags. Five soil cores of 30 cm depth were collected in each quadrat using an auger of 38 mm in diameter. One core was used to measure belowground biomass, and the remaining cores were used for measuring soil chemical properties, including soil δ¹⁵N (‰) (δ¹⁵N_soil), total nitrogen content (STN, g kg^-1), organic carbon content (SOC, g kg^-1), total phosphorus content (STP, g kg^-1), and pH. Soil cores were separated into three layers, i.e., 0–10 cm, 10–20 cm, and 20–30 cm. Samples for measuring belowground biomass were rinsed with running water before placing in paper bags. Samples for measuring other soil properties were homogenized, with plant debris and rocks removed, and were stored at ambient temperature. Additionally, soil samples for measuring soil water content (SWC, g g^-1) and bulk density (BD, g cm^-3) were collected using 100 cm³ cutting rings.