论文部分内容阅读
Nitrogen was the main limiting nutrient of net primary pro-duction in the southeast Keerqin Sandy Lands, Northeast China. Spe-cies richness declined and biomass increased after five consecutive years of nitrogen fertilization of these sandy grasslands (2004-2008). After fertilization had been stopped for three years (2009-2011), we surveyed vegetation on previously fertilized plots to quantify changes in commu-nity composition. Respect species richness showed an increasing trend over time since the cessation of fertilization. Respect vegetation height and coverage showed decreasing trends over time since the cessation of fertilization. Species composition changed after fertilization ceased, the dominant species shifting from Cannabis sativa, Phragmites communis and Chenopodium acuminatum in 2008 to Cannabis sativa, Phragmites communis and Artemisia scoparia in 2011. Dominance of dominant species declined from 66.2%in 2008 to 57.5%in 2011. The importance value of annual plants in the earlier nitrogen addition plots was higher than in control plots, but the differences were not significant in 2011. The importance value of perennial plants differed significantly between treatments from 2009 to 2011. The reversion rate not only differed be-tween community characteristics, but also between functional groups in the same community characteristic. Although the residual effect of nitro-gen addition on vegetation was still observed three years after fertiliza-tion ceased, the vegetation showed signs of recovery.