Abstract: Biochar is a highly effective soil amendment that has been widely used for ecological remediation and has shown great potential in promoting plant nutrient acquisition and growth. However, it remains unclear whether biochar addition influences competition between invasive and native plants and thus is suitable for restoration of communities invaded by exotic plants. We conducted a field experiment and a ¹⁵N labelling test to investigate the impact of biochar addition on N uptake of invasive Spartina alterniflora and native Phragmites australis under both intra- and interspecific competition. Biochar addition significantly promoted the NO₃⁻-N uptake rate of both P. australis and S. alterniflora under interspecific competition, and promoted the NH₄⁺-N uptake rate of both P. australis and S. alterniflora under both intra- and interspecific competition. However, biochar addition did not influence the competitive balance between S. alterniflora and P. australis. We conclude that biochar addition can enhance N uptake of both native and invasive plants, but cannot alter their competitive superiority in N acquisition or shift their N form preferences. The findings suggest that biochar application will not be useful if we target at restoring wetlands due to exotic plant invasions, as it cannot enhance the competitive advantages of natives over invasives. However, biochar may be applied if we target at restoring degraded wetlands caused by other disturbances such as such as salinization and nutrient impoverishment, as it will not enhance the competitive advantage of invasives over natives.

Key words: Coastal wetland, Interspecific competition, Invasive plant, Nitrogen form, Nitrogen uptake