Abstract
Some free-living microorganisms can fix atmospheric nitrogen (N) to meet their nutritional demands. Resource optimization predicts that free-living N fixation should decrease with N addition and increase with phosphorus (P) addition. Substrates with higher labile carbon concentrations should fix more N. This study investigated the influence of N and P addition on free-living N fixation rates in coarse woody debris, leaf litter, and Oa horizon soil in a study of Multiple Element Limitation in Northern Hardwood Ecosystems (MELNHE). Nitrogen fixation rates decreased by 42% in coarse woody debris, 79% in leaf litter, and 27% in Oa horizon soil with N addition. Fixation rates were inconsistent under P addition. Scaled up to ecosystem fluxes, Oa horizon soil accounted for 51.08 ± 11.64 g N fixed ha-1 yr-1 , while litter contributed 0.85 ± 0.21 g N fixed ha-1 yr-1, and CWD contributed 2.79 ± 0.80 g N fixed ha-1 yr-1.
Key Words: free-living nitrogen fixation, nitrogen, phosphorus, MELNHE, fertilization study, forest floor, coarse woody debris, leaf litter, organic soil