The breakdown of allochthonous organic matter is considered to be the main source of energy and nutrients for the majority of first-order streams. Thus, land-use change and riparian vegetation, such as deforestation and conversion of native forest to pasture lands, will lead to unwanted changes of the structure and function of aquatic ecosystems due to the disturbance of organic-matter supply. The C4 grasses, extensively used as forage in tropical regions, are poorly studied as important sources of allochthonous material because they are usually considered as a poor source of nutrients. Because the effects of land-use change on ecosystem functions are not fully known, we aimed to evaluate how such changes in riparian vegetation can affect nutrient cycling by means of measuring the decomposition rate of an abundant native C3 species and an exotic C4 grass species in first-order streams of the Atlantic Forest. Our results showed that C4 detritus decomposed faster than did C3 detritus, despite its lower nutrient concentration. This was likely to be due to the lower lignin concentration of the C4 species than the native C3 species. Lignin also influenced nutrient-loss dynamics of the C3 species, because it can interact with other cellular constituents and prevent the decomposition of most labile compounds. Our results supported the observation that the replacement of riparian vegetation alters breakdown rates and nutrient distributions, which may disrupt aquatic food webs.