Disruption of metal ion homeostasis in soils is associated with nitrogen deposition-induced species loss in an Inner Mongolia steppe.

Enhanced deposition of atmospheric nitrogen (N) resulting from anthropogenic activities has negative impacts on plant diversity in grassland ecosystems globally. Several mechanisms have been proposed to explain the species loss. Ion toxicity due to N deposition-induced soil acidification has been suggested to be responsible for species loss in acidic grasslands, while few studies have evaluated the role of soilmediated homeostasis of ions in species loss under elevated N deposition in alkaline grasslands. To determine whether soil-mediated processes are involved in changes in species composition by N deposition, the effects of 9 yrN addition on soil properties, aboveground biomass (AGB) and species composition were investigated in an Inner Mongolia steppe. Low to moderate N addition rate (2, 4, 8 g N m^-2 yr^-1) significantly enhanced AGB of grasses, while high N addition rate (> 16 g N m^-2 yr^-1) reduced AGB of forbs, leading to an overall increase in AGB of the community under low to moderate N addition rates. Forb richness was significantly reduced by N addition at rates greater than 8 gNm^-2 yr^-1, while no effect of N addition on grass richness was observed, resulting in decline in total species richness. N addition depleted base cations (Ca²⁺, Mg²⁺ and K⁺) in soils, reduced soil pH and mobilized Mn²⁺, Fe³⁺ and Cu²⁺ ions in soils. Soil inorganic-N concentration was negatively correlated with forb richness, explaining 27.2% variation of forb richness. The concentrations of base cations (Ca²⁺ and Mg²⁺) and metal ions (Mn²⁺ and Cu²⁺) showed positively and negatively linear correlation with forb richness, accounting for 25.9 and 41.4% variation of forb richness, respectively. These results reveal that disruption of metal ion homeostasis in soils by N addition, particularly enhanced release of soil Mn²⁺ and Cu²⁺ may be associated with reduction in forb richness in temperate steppe of Inner Mongolia. [ABSTRACT FROM AUTHOR]