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

Abstract

Abstract. 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 soil-mediated 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 yr N 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 g N m−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.

Processes at the soil–root interface determine the different responses of nutrient limitation and metal toxicity in forbs and grasses to nitrogen enrichment

“…Nitrogen (urea) was applied by hand annually in July every year since 2004 when maximal precipitation occurs. Our previous results showed that the applications of urea caused a significant increase in soil nitrate concentrations (Tian et al., 2015), because urea is successively transformed into ammonia/ammonium and nitrate by urease and nitrification. Therefore, urea addition can simulate soil N enrichment caused by atmospheric N deposition.…”

“…The atmospheric bulk N deposition in this area is approximately 15.1 kg N ha −1 year −1 (Xu et al., 2015). Detailed soil properties in this area were described in our previous study (Tian et al., 2015). The vegetation comprises common forbs ( Artemisia frigida , Potentilla acaulis , Potentilla bifurca , Potentilla tanacetifolia , Allium bidentatum ), sedges ( Carex korshinskyi ) and common grasses ( Stipa krylovii , Leymus chinesis , Agropyron cristatum , Cleistogenes squarrosa ).…”

“…Therefore, urea addition can simulate soil N enrichment caused by atmospheric N deposition. In our previous study, we measured the element concentrations in soil and plants after 9 years of N addition, especially Mn and Fe (Tian et al., 2015, 2016). In the present study, we collected plant and soil samples from 24 plots (six replicates) supplemented with four levels of N addition (0, 2, 8, 16 g N/m 2 ) after 10 years of N addition.…”

“…Soil pH, available soil P and exchangeable Ca 2+ and Mn 2+ in soil were determined according to the methods used in our previous studies (Tian et al., 2015). After determination of above‐ground biomass, leaf samples were ground to a powder.…”

“…Our previous studies revealed that long‐term and high levels of N addition decreased the concentrations of exchangeable cations (i.e. Ca 2+ , Mg 2+ , K + ) in soil, and increased the availability of P and Mn 2+ , but it had little effects on soil Al (Al 3+ ), copper (Cu 2+ ) and zinc (Zn 2+ ) concentrations (Tian et al., 2015). However, it remains unknown whether this pattern also occurs under short‐term N enrichment, and how the soil chemistry‐related traits are affected by short‐term and long‐term N enrichment.…”