Heavy metal concentrations differ along wetland-to-grassland soils: a case study in an ecological transition zone in Hulunbuir, Inner Mongolia

Purpose Ecosystem changes occurring against a backdrop of climate changes have set off a chain reaction through the cycle of matter and energy affecting water, soils, and plants. Heavy metal elements have long been research hotspots due to their unique impacts on the composition and function of soils and plants. Most studies of heavy metals are conducted in contaminated industrial and residential areas, but more studies need to be conducted on well-protected nature reserves undergoing ecological transition. Method This study was conducted in a wetland-to-grassland ecotone in the Huihe National Nature Reserve, Inner Mongolia, where wetland has been decreasing and turning into grassland in the past few decades. To compare associations between the grasslandification of wetland and the presence of heavy metals, concentration of 11 heavy metals [iron (Fe), copper (Cu), zinc (Zn), manganese (Mn), nickel (Ni), vanadium (V), antimony (Sb), arsenic (As), chromium (Cr), cadmium (Cd), and lead (Pb)], vegetation characteristics (above and belowground plant biomass, height, and species), and topsoil (0–20 cm) characteristics [pH, electrical conductivity (EC), soil moisture, bulk density, and concentration of P, K, Ca, Na, and Mg] were measured at four independent sites, each of which contained three kinds of ecosystems (i.e., treatments): wetlands, wetland-grassland junctions, and grasslands. Results Soil Fe, Cr, and Pb concentrations increased as the ecotype shifted from wetland to grassland, whereas Mn and Cd decreased. SW (soil water) and pH levels also decreased; the SW value was the highest in the wetlands (32.8 ± 3.63%) and decreased from the junction lands (30.6 ± 4.7%) to the grasslands (22.1 ± 2.85%) ( p < 0.01); and pH decreased from 8.9 to 7.8 from the wetlands to the adjacent wetlands ( p < 0.01). Plant coverage and aboveground biomass levels were highest in the wetland plots, whereas belowground biomass levels were highest in the grassland plots. Conclusions Even though little human disturbance occurs in the grassland-to-wetland ecosystems in the nature reserves of Huihe Hulunbuir, Inner Mongolia, trace amounts of heavy metals were still detected (Fe, Cr, Pb, Mn, and Cd). Shifts in soil water and pH may be responsible for this phenomenon.