Heterotrophic nitrogen fixation in decomposing litter: Patterns and regulation

Metrosideros polymorpha is the dominant tree in a wide range of Hawaiian forests, and it produces litter that varies widely in structure and chemistry. Long-term fertilizer experiments have further expanded the range of litter characteristics and have also provided plots that differ substantially in nutrient availability. We used natural and experimentally induced variation in litter characteristics and site nutrient availabilities to determine controls on heterotrophic N₂ fixation (acetylene reduction or AR, calibrated with ¹⁵N₂) in decomposing litter. In a large-scale experiment that evaluated 28 litter types, we found that low-lignin litter fixed substantially more N than did higher-lignin litter (0.9-1.3 vs. 0.06-0.26 mg N/gram initial litter mass, integrated over two years of decomposition). The supply of nutrients other than N had small and inconsistent effects on N₂ fixation. A second experiment then focused on the importance of non-N nutrients, using litter collected from control plots and plots fertilized with P and with all other essential elements except N (abbreviated 'T'), and decomposed in plots that had been fertilized with P, T, and P + T for 12 yr. Litter from P-fertilized plots supported significantly greater AR activity; however, the increase could be explained by decreased lignin concentrations in litter produced in P-fertilized plots. Related studies on controls of decomposition rates in Hawaiian montane forests show that the decomposition of low-lignin litter is increased substantially by added N, while that of high-lignin litter is increased less or not at all. The extent to which decomposition is N-limited correlates strongly with the integrated quantity of N fixed during decomposition. We conclude that low carbon quality of litter, indicated by high lignin concentrations, constrains decomposition and heterotrophic N₂ fixation in Hawaiian montane forests, even where N supply limits net primary production. This disconnection in resource limitation between producers and decomposers contributes to the maintenance of N limitation in terrestrial ecosystems.