Soil microbes weaken the positive effect of an aquatic-terrestrial subsidy on plant performance

Abstract

Aims: Linkages formed through aquatic-terrestrial subsidies can play an important role in structuring communities and mediating ecosystem functions. Aquatic-terrestrial subsidies may be especially important in nutrient-poor ecosystems, such as the freshwater sand dunes surrounding Lake Michigan. Adult midges emerge from Lake Michigan in the spring, swarm to mate and die. Their carcasses form mounds at the base of plants, where they may increase plant productivity through their nutrient inputs. However, the effect of aquatic-terrestrial subsidies on plant productivity could depend on other biotic interactions. In particular, soil microbes might play a key role in facilitating the conversion of nutrients to plant-available forms or competing for the nutrients with plants. Methods: In a greenhouse experiment, we tested how carcasses from lake emergent midges (Chironomidae) and soil microbes independently and interactively influenced the performance of a common dune grass, Calamovilfa longifolia. To determine whether midges influenced abiotic soil properties, we measured how midge additions influenced soil nutrients and soil moisture. Important Findings: Midges greatly increased plant biomass, while soil microbes influenced the magnitude of this effect. In the absence of soil microbes plant biomass was seven times greater with midges than without midges. However, in the presence of soil microbes, plant biomass was only three times greater. The effect of midges might be driven by their nutrient inputs into the soil, as midges contained 100 times more N, 10 times more P and 150 times more K than dune soils did. Our results suggest that soil microbes may be competing with plants for these nutrients. In sum, we found that midges can be an important aquatic-terrestrial subsidy that produces strong, positive effects on plant productivity along the shorelines of Lake Michigan, but that the impact of aquatic-terrestrial subsidies must be considered within the context of the complex interactions that take place within ecological communities.