The invasion of native riparian forests by exotic tree species can lead to profound changes in the ecological integrity of freshwater ecosystems. We assessed litter decomposition of native (Alnus glutinosa and Quercus robur) and invasive (Acacia melanoxylon and Acacia dealbata) tree species, and associated microbial activity and community structure, after being immersed for conditioning in 3 reference and 3 “invaded” streams in Serra da Lousã (central Portugal) and used in microcosms simulating stream conditions. Litter decomposition differed among species, in the order: Al. glutinosa > Q. robur > (Ac. melanoxylon ~ Ac. dealbata). Alnus glutinosa litter decomposed faster probably because it was soft and had high nitrogen concentration for decomposers. Quercus robur litter decomposed slower most likely because it was tough and had high polyphenol and low nitrogen concentrations. Acacia melanoxylon litter was the toughest and had a thick cuticle that likely acted as a physical barrier for microbial colonization. In Ac. dealbata, the small-sized leaflets and high lignin concentration may have limited microbial litter decomposition. Litter decomposition was faster in “invaded” streams, probably because they were N-limited and increases in nitrogen concentration in water, promoted by Acacia species invasion, stimulated microbial activity on litter. The aquatic hyphomycete community structure differed among litter species and between stream types, further suggesting that microbes were sensitive to litter characteristics and water nutrient concentrations. Overall, the invasion of native riparian forests by Acacia species may affect microbial decomposer activity, thus altering important stream ecosystem processes, such as litter decomposition and nutrient cycles.

外来树种入侵原生河岸森林可导致淡水生态系统生态完整性的深刻变化。我们在塞拉达的3个参考和3个“入侵”溪流中进行调理后，评估了原生树种（Alnus glutinosa和Quercus robur）和入侵树种（Acacia melanoxylon和Acacia Dealbata）的凋落物分解以及相关的微生物活性和群落结构。 Lousã（葡萄牙中部），用于模拟河流状况的缩影。不同种类的凋落物分解顺序不同：Al。黄> Q.栎>（AC。黑木〜AC。银荆）。nu木凋落物分解得更快，可能是因为它柔软并且分解剂中的氮含量很高。刺槐凋落物分解较慢的可能性最大，因为它坚韧且具有高多酚和低氮浓度。黑合欢（Acacia melanoxylon）垃圾最坚硬，表皮较厚，可能是微生物定植的物理屏障。在交流中。竹叶小叶和木质素浓度高可能限制了微生物凋落物的分解。在“入侵”的溪流中，凋落物分解更快，可能是因为它们受到N限制并由相思树促进了水中的氮浓度增加物种入侵，刺激了垃圾的微生物活动。凋落物物种之间和溪流类型之间的水生菌丝群落结构不同，这进一步表明微生物对凋落物特征和水养分浓度敏感。总体而言，相思树种对原生河岸森林的入侵可能会影响微生物分解物的活动，从而改变重要的河流生态系统过程，例如凋落物分解和养分循环。