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LIMITATIONS TO SYMBIOTIC NITROGEN FIXATION IN PRIMARY SUCCESSION ON THE TANANA RIVER FLOODPLAIN
Author(s) -
Uliassi Daniel D.,
Ruess Roger W.
Publication year - 2002
Publication title -
ecology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.144
H-Index - 294
eISSN - 1939-9170
pISSN - 0012-9658
DOI - 10.1890/0012-9658(2002)083[0088:ltsnfi]2.0.co;2
Subject(s) - ecological succession , floodplain , ecology , primary succession , nitrogen fixation , environmental science , geography , nitrogen , biology , chemistry , organic chemistry
Constraints on nitrogen fixation are the ultimate causes of N limitation of primary production, but hypotheses concerning limitations to N 2 fixation remain largely untested in natural terrestrial ecosystems. We examined limitations to N 2 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 2 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 balsamifera ) stands (dense alder understories with mature poplar overstories) and N 2 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 In contrast, fertilization had no effect on these measures in poplar stands where reduced soil moisture may have superseded limitation by P. We detected no differences in specific ARA, total nodule biomass, or N inputs, between alder and poplar stands but all of these measures were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 2 fixation and N resorption indicate that late‐season increases in leaf N, following a midseason reduction in leaf N, were driven by N 2 fixation in excess of plant N demands as nodules continued fixing N while alder leaves senesced. These results have shown that P limits N 2 fixation in alder stands in this nitrogen‐limited sere, but that factors limiting N 2 fixation can change over short successional time scales.