Land use differentially alters microbial interactions and detritivore feeding during leaf decomposition in headwater streams

Human-driven land conversion from forest to agriculture introduces nutrients, like nitrogen (N) and phosphorus (P), which upon entering nearby streams can stimulate growth and metabolism of microbial autotrophs (algae) and heterotrophs (fungi and bacteria). Recent evidence indicates that algae also can stimulate microbial heterotrophs associated with decaying plant litter by exuding labile carbon (C), defined as the priming effect on decomposition. These autotroph-heterotroph interactions may further alter detritivore feeding ecology, suggesting many complex mechanisms of land-use change on stream ecosystem functions. In order to investigate the effects of land use on autotroph-heterotroph interactions, we used a full-factorial design manipulating labile C (glucose and acetate) and nutrients (N and P) to test their effects during red maple litter decomposition within four forested and four pasture agriculture streams in the Point Remove catchment, Arkansas. We deployed nutrient-diffusing substrates (NDS) containing litter and either agar, agar + labile C, agar + NP, or agar + CNP for 20 days and measured microbial biomass and metabolism, and detrital mass loss. Additionally, we fed litter to Tipula sp. to assess shifts in consumption and egestion rates. Both microbial heterotrophs and autotrophs responded to NDS manipulations, but not land use. Heterotrophic microbes showed evidence of nutrient limitation and negative priming, while algal biomass was negatively associated with heterotrophs. Tipula consumption and egestion rates were both elevated when feeding on litter incubated in agriculture streams, but only egestion responded to NDS manipulations, suggesting a change in assimilation efficiencies. Cumulatively, we suggest that altered stream conditions, such as those resulting from land-use change, can alter food webs and detritivore feeding more heavily than microbial interactions during litter decomposition in stream ecosystems. These results add to the literature demonstrating shifts in autotroph-heterotroph interactions with anthropogenic nutrient pollution, and the implications for detritivore feeding and overall C cycling in streams.