外来种互花米草和土著种芦苇空中凋落物氮动态的比较研究

土壤氮含量是限制植物生长的重要因素, 所以入侵植物要入侵成功必须突破土壤氮限制的瓶颈。近年来, 外来种互花米草(Spartina alterniflora)在中国海岸带盐沼中快速取代土著种芦苇(Phragmites australis), 引起了多方面的生态学后果。为了解互花米草与本地种芦苇空中凋落物的氮含量是否存在差异及产生这种差异的机制, 2003年11月至2004年4月, 作者在长江口九段沙湿地对互花米草与芦苇空中凋落物氮含量(单位面积凋落物的总氮量, N g/m²)进行了测定, 结果表明互花米草的氮含量比芦苇高。在空中分解过程中, 互花米草茎(包括叶鞘与秆)凋落物的氮含量显著上升, 但芦苇茎凋落物的氮含量显著降低。2006年1月, 又对中国海岸带6个地点的盐沼中互花米草的凋落物进行取样和氮浓度测定, 发现互花米草空中叶鞘与秆的老凋落物(2004年冬季产生)的氮浓度均显著高于其新产生的凋落物(2005年冬季产生), 表明在空中分解过程中, 互花米草叶鞘与秆凋落物氮含量增加具有普遍性。进一步的温室受控实验结果表明, 互花米草凋落物氮含量增加可能是由腐生固氮微生物引起的。以上结果表明,互花米草取代芦苇后, 改变了空中凋落物的氮动态, 增加了生态系统中氮的输入, 可能有利于互花米草的快速扩张。

Nitrogen dynamics of aerial litter of exotic Spartina alterniflora and native Phragmites australis

Plant invasion is a significant element of global change. Recently, exotic Spartina alterniflora have rapidly displaced native Phragmites australis on China’s East Coast. In the Yangtze Estuary’s Jiuduansha wetlands, nitrogen content in aerial standing litter for both Spartina and Phragmites was measured from November 2003 to April 2004. Spartina had larger nitrogen content (N g/m²) in aerial litter than Phragmites. Nitrogen content in aerial litter of Spartina sheathes and stems increased during our study period; this trend was not observed for Phragmites. In January 2006, new (produced in winter of 2005) and old (produced in winter of 2004) Spartina aerial sheath and stem litter was collected from six sites on China’s East Coast. Aerial litter of old sheathes and stems had higher nitrogen concentrations than new sheathes and stems, suggesting that the increase of nitrogen content in Spartina sheath and stem aerial litter was consistent across sites. Further greenhouse experiments showed that the increase of nitrogen content in Spartina aerial litter was associated with microbial epiphytic nitrogen fixation. Our results suggest that Spartina invasion alters aerial litter nitrogen dynamics, which might increase nitrogen input into invaded ecosystems and facilitate rapid expansion of Spartina into additional habitats.