Limitations to Symbiotic Nitrogen Fixation in Primary Succession on the Tanana River Floodplain

Constraints on nitrogen fixation are the ultimate causes of N limitation of primary production, but hypotheses concerning limitations to N2 fixation remain largely untested in natural terrestrial ecosystems. We examined limitations to N2 fixation by thinleaf alder (Alnus tenuifolia) in two stages of primary forest succession on the Tanana River floodplain (interior Alaska, USA) and focused on the hypothesis that N, fixation was limited by low soil P availability. Paired control and P fertilized plots were established at four replicate early successional alder stands and four later successional poplar (Populus balsanzifera) stands (dense alder understories with mature poplar overstories) and N2 fixation was estimated with an acetylene reduction assay. In alder stands, P fertilization increased total nodule dry biomass and increased total ecosystem N inputs, but it had little effect on nitrogenase activity per unit nodule dry mass (specific acetylene reduction activity, ARA). Specific ARA increased only in late July when soil temperature and ARA were at their maximum values. In contrast, fertilization had no effect on these measures in poplar stands where reduced soil moisture may have superseded limitation by P. We detected no differences in specific ARA, total nodule biomass, or N inputs, between alder and poplar stands but all of these measures were highly variable. Leaf area of the alder canopy emerged as the best predictor of ecosystem inputs of fixed N among control plots. Alders resorbed high amounts of P but little N (consistent with low P availability and a high P demand and a high N availability in alder), and P fertilization reduced P resorption but had no effect on N resorption. The timing of N, fixation and N resorption indicate that late-season increases in leaf N, following a midseason reduction in leaf N, were driven by N2 fixation in excess of plant N demands as nodules continued fixing N while alder leaves senesced. These results have shown that P limits N2 fixation in alder stands in this nitrogen-limited sere, but that factors limiting N2 fixation can change over short successional time scales.