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 tho se presumed for preindustrial rimes. Because nitrogen (N) often limits micr obial growth on dead vegetation, increased N input can be expected to affec t the ecosystem process of decomposition. We found that extracellular enzym e responses of a forest-floor microbial community to chronically applied aq ueous NH4NO3 can explain both increased and decreased litter decomposition rates caused by added N. Microbes responded to N by increasing cellulase ac tivity in decaying leaf litter of flowering dogwood, red maple, and red oak , but in high-lignin oak litter, the activity of lignin-degrading phenol ox idase declined substantially. We believe this is the first report of reduce d 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 se en in this and other N-addition experiments. Since lignin and cellulose are the two most abundant organic resources on earth, these altered enzyme res ponses signal that atmospheric N deposition may be affecting the global car bon cycle by influencing the activities of microbes and their carbon-acquir ing enzymes-especially the unique ligninolytic enzymes produced by white-ro t fungi-over broad geographic areas.