Effects of exotic plant invasion on soil nitrogen availability.
Aims: Exotic plant invasion has been a global eco-environmental issue, which declines biodiversity and influences ecosystem structure and function. Over the past decades, more and more studies showed that influences of exotic plant invasion on soil nitrogen (N) cycles, and soil N availabilities can facilitate the success and aggravation of invading plants. Methods: Based on differences in soil N contents between invaded and uninvaded areas in natural ecosystems at the same study sites, this study explored magnitudes and ecophysiological mechanisms of soil N variations under exotic plant invasion. Important findings: Based on the data integrated from 107 papers, we found that contents of soil total N, ammonium-N, nitrate-N, inorganic N, microbial biomass N under exotic plant invasion were increased by (50±14)%, (60±24)%, (470±115)%, (69±25)%, (54±20)% respectively relative to those under no invasion. The increment in the soil nitrate-N pool was highest, suggesting higher nitrification rate, which potentially promoted plant nitrate-N utilization and the coexistence of nitrate-preferring species. The increment of soil nitrate-N pool under invasion was higher in the temperate zone than the subtropical zones significantly. Invasion of N2-fixing plants caused obviously larger increments of soil total N and nitrate-N contents compared to invasion of non-N2-fixing plants. The invasion of woody and evergreen invasive plants caused larger increments of soil total N than herbaceous and deciduous plants, respectively. The increases in soil ammonium-N under invasion did not differ substantially among different life forms and showed no clear relationship with the percentage of N2-fixing plants. Differently, soil nitrate-N showed much larger increments under invasion and showed positive linear relationship with the percentage of N2-fixing invasive plants. The N2-fixing function, litter quality and quantity of invasive plants are important factors regulating soil N mineralization and nitrification under invasion. This study provides novel insights into the mechanisms underlying the success and aggravation of plant invasion and into the relationships between soil N dynamics and plant functional traits in ecosystems under exotic plant invasion.