Interactions between litter lignin and soil nitrogen availability during leaf litter decomposition in a Hawaiian montane forest

Previous work in a young Hawaiian forest has shown that nitrogen (N) limits aboveground net primary production (ANPP) more strongly than it does decomposition, despite low soil N availability. In this study, I determined whether (a) poor litter C quality (that is, high litter lignin) poses an overriding constraint on decomposition, preventing decomposers from responding to added N, or (b) high N levels inhibit lignin degradation, lessening the effects of added N on decomposition overall. I obtained leaf litter from one species, Metrosideros polymorpha, which dominates a range of sites in the Hawaiian Islands and whose litter lignin concentration declines with decreasing precipitation. Litter from three dry sites had lignin concentrations of 12% or less, whereas litter from two wet sites, including the study site, had lignin concentrations of more than 18%. This litter was deployed 2.5 years in a common site in control plots (receiving no added nutrients) and in N-fertilized plots. Nitrogen fertilization stimulated decomposition of the low-lignin litter types more than that of the high-lignin litter types. However, in contrast to results from temperate forests, N did not inhibit lignin decomposition. Rather, lignin decay increased with added N, suggesting that the small effect of N on decomposition at this site results from limitation of decomposition by poor C quality rather than from N inhibition of lignin decay. Even though ANPP is limited by N, decomposers are strongly limited by C quality. My results suggest that anthropogenic N deposition may increase leaf litter decomposition more in ecosystems characterized by low-lignin litter than in those characterized by high-lignin litter.