Responses of symbiotic N2 fixation in Alnus species to the projected elevated CO2 environment

Abstract

Nitrogen fixation in Alnus species in response to elevated CO 2 may depend on the presence of non-N 2 -fixing tree species in addition to soil conditions. Alnus is a major genus of actinorhizal plants. Symbiosis with Frankia allows the Alnus species to fix nitrogen (N) at the rate of several to 320 kg N ha−1 year−1 with a nodule biomass of 16–480 kg ha−1. Alnus species ensures an effective supply of N to soils because of the high N content of leaf litter, rapid decomposition rate, and the influx of herbivorous insects. In addition, the association between regenerated endozoochorous species and Alnus hirsuta suggests that N2 fixation in Alnus species influences the distribution patterns of regenerated plants as well as improve soil fertility. N2 fixation by the Alnus–Frankia symbiotic relationship may be positively associated with elevated carbon dioxide (CO2) levels. Nodule biomass increased under elevated CO2 due to enhanced plant growth, rather than changes in biomass allocation. The inhibitory effect of high soil N on nodulation was retained under elevated CO2, and the effects of elevated CO2 on N2 fixation depended on soil P availability, drought, and many other abiotic and biotic factors. Recent free-air CO2 enrichment experiments have demonstrated increased N2 fixation in A. glutinosa exposed to elevated CO2 in mixed-species stands containing non-N2-fixers but not in monocultures, suggesting that N2 fixation depends on an association with non-N2-fixing tree species. Because elevated CO2 can alter the N and P contents and stoichiometry of plants, it will be necessary to evaluate N allocation and accumulation of biomass when investigating the response of Alnus species to future global climate change.