Nitrogen (N) fixation is that largest natural input of new nitrogen into ecosystems and drives many critical ecosystem functions such as carbon storage. However, we do not yet have a good explanation for why N-fixing trees are distributed across latitudes as they are, nor have we quantified how the amount of N fixed by these trees will change with ongoing anthropogenic changes to climate and environmental conditions. Existing observational and experimental evidence suggests that many N-fixing legumes will down regulate how much N they fixed when more N is available from the soil, suggesting that legumes will fix less N under N-rich conditions. Many further questions remain, though: Do legumes and actinorhizal N-fixers down regulate N fixation similarly? Do patterns of regulation differ across latitudinal (tropics to temperate) or environmental (e.g., dry to wet) gradients? We used a fine-scale 15N-labeled fertilizer gradient to quantify rates of N fixation of 12 N-fixing species (6 legumes and 6 actinorhizal species) in a greenhouse experiment. We then compared nodule biomass and the percentage of total biomass comprised of nodules (%Biomassnodule) across species to identify fixation strategy.
Results/Conclusions
Our results demonstrate that rhizobial (legume) trees generally turn down relative rates of N fixation when more N is available from the soil (%Biomassnodule). While %Biomassnodule decreased with increasing fertilization, legumes did not turn off fixation: %Biomassnoduleranged from 3.5% to 12% at low N levels, but did not decreased below 3% (range 3-4%) even at the highest N fertilization levels. These same species also show increasing total nodule biomass as N fertilization increases, suggesting that more total N is fixed by legumes under high N conditions. The actinorhizal species showed more variability: two actinorhizal species showed no relationship between %Biomassnoduleand N fertilization level, demonstrating no down regulation of fixation, while several actinorhizal species did show down regulation. Interestingly, there was also more variation in overall %Biomassnodule in actinorhizal species (ranging from 0.6-12% at low N levels to 0.6-5% at high N levels) than in legume species. These data suggest that legumes rely relatively less on N fixation for their N needs at higher soil N availability, though this is not always true for actinorhizal species. In addition, total amount of fixed N (using nodule biomass as a proxy) entering the system increases with increasing N fertilization levels across all species, with implications for understanding N fixation rates under N deposition scenarios.