Microbial Enzyme Shifts Explain Litter Decay Responses to Simulated Nitrogen Deposition

Some natural ecosystems near industrialized and agricultural areas receive atmospheric nitrogen inputs that are an order of magnitude greater than those presumed for preindustrial times. Because nitrogen (N) often limits microbial growth on dead vegetation, increased N input can be expected to affect the ecosystem process of decomposition. We found that extracellular enzyme responses of a forest-floor microbial community to chronically applied aqueous NH4NO3 can explain both increased and decreased litter decomposition rates caused by added N. Microbes responded to N by increasing cellulase activity in decaying leaf litter of flowering dogwood, red maple, and red oak, but in high-lignin oak litter, the activity of lignin-degrading phenol oxidase declined substantially. We believe this is the first report of reduced ligninolytic enzyme activity caused by chronic N addition in an ecosystem. This result provides evidence that ligninolytic enzyme suppression can be an important mechanism explaining decreased decay rates of plant matter seen in this and other N-addition experiments. Since lignin and cellulose are the two most abundant organic resources on earth, these altered enzyme responses signal that atmospheric N deposition may be affecting the global carbon cycle by influencing the activities of microbes and their carbon-acquiring enzymes—especially the unique ligninolytic enzymes produced by white-rot fungi—over broad geographic areas.