The release of soluble compounds (i.e., leachates) from allochthonous coarse particulate organic matter (CPOM) into stream water can be an important source of dissolved organic matter (DOM) and nutrients for instream microbial activity. Here, we assessed variability in the chemistry and the composition of DOM of leachates from different CPOM sources and examined how these characteristics influenced the activity of microbial assemblages in streams. Specifically, we considered leaf litter from 6 tree species that are broadly distributed in riparian zones of the Mediterranean region, a mixture of fruits from some of these species, and a mixture of twigs. We analyzed the leachates from each CPOM source for the concentration of dissolved forms of carbon, nitrogen, and phosphorus, as well as for the composition of DOM based on optical indices associated with the degree of aromaticity and molecular weight. Under laboratory conditions, we used the Resazurin–Resorufin system to estimate the microbial metabolic activity associated with each leachate type. Microbial metabolic activity varied among leachates from different CPOM sources and was positively related to the degree of aromaticity and the NO3– concentration of leachates. Hence, certain types of riparian CPOM inputs can constitute sources of high-quality DOM and dissolved nutrients for instream microbial assemblages. Thus, management of riparian vegetation should consider variation in leachate properties among plant species, as they can influence dissolved organic carbon and nutrient dynamics and heterotrophic activity in stream ecosystems.