Will elevated carbon dioxide concentration amplify the benefits of nitrogen fixation in legumes?

Response of archaeal communities in the rhizosphere of maize and soybean to elevated atmospheric CO2 concentrations

BACKGROUND

Archaea are important to the carbon and nitrogen cycles, but it remains uncertain how rising atmospheric carbon dioxide concentrations ([CO(2)]) will influence the structure and function of soil archaeal communities.

METHODOLOGY/PRINCIPAL FINDINGS

We measured abundances of archaeal and bacterial 16S rRNA and amoA genes, phylogenies of archaeal 16S rRNA and amoA genes, concentrations of KCl-extractable soil ammonium and nitrite, and potential ammonia oxidation rates in rhizosphere soil samples from maize and soybean exposed to ambient (∼385 ppm) and elevated (550 ppm) [CO(2)] in a replicated and field-based study. There was no influence of elevated [CO(2)] on copy numbers of archaeal or bacterial 16S rRNA or amoA genes, archaeal community composition, KCl-extractable soil ammonium or nitrite, or potential ammonia oxidation rates for samples from maize, a model C(4) plant. Phylogenetic evidence indicated decreased relative abundance of crenarchaeal sequences in the rhizosphere of soybean, a model leguminous-C(3) plant, at elevated [CO(2)], whereas quantitative PCR data indicated no changes in the absolute abundance of archaea. There were no changes in potential ammonia oxidation rates at elevated [CO(2)] for soybean. Ammonia oxidation rates were lower in the rhizosphere of maize than soybean, likely because of lower soil pH and/or abundance of archaea. KCl-extractable ammonium and nitrite concentrations were lower at elevated than ambient [CO(2)] for soybean.

CONCLUSION

Plant-driven shifts in soil biogeochemical processes in response to elevated [CO(2)] affected archaeal community composition, but not copy numbers of archaeal genes, in the rhizosphere of soybean. The lack of a treatment effect for maize is consistent with the fact that the photosynthesis and productivity of maize are not stimulated by elevated [CO(2)] in the absence of drought.

Evaluating the nodulation status of leguminous species from the Amazonian forest of Brazil

Numerous leguminous species are used or have potential uses for timber production, pharmacological products, or land reclamation. Through N(2)-fixation, many leguminous trees contribute to the N-balance of tropical wetlands and rainforests. Therefore, studies of the N(2)-fixation ability of leguminous species appear to be crucial for the better use and conservation of these resources. The global nodulation inventory in the Leguminosae family is constantly being enriched with new records, suggesting the existence of undiscovered nodulated species, especially in tropical natural ecosystems and other hot spots of biodiversity. In this respect, the nodulation of leguminous species from the Amazonian forest of Porto Trombetas (Brazil) was surveyed. Overall, 199 leguminous species from flooded and non-flooded areas, were examined for their nodulation status by combining field observations, seedling inoculations, and screening of N(2)-fixing bacterial strains from the collected nodules. The results revealed a tendency for a higher relative frequency of nodulation in the species from the flooded areas (74%) compared with those from the non-flooded areas (67%). Nodulation was observed in the Caesalpinioideae, Mimosoideae, and Papilionoideae, with 25, 88, and 84% of the examined species in each subfamily, respectively. Of the 137 nodulated leguminous species, 32 including three Caesalpinoideae, 19 Mimosoideae, and 10 Papilionoideae are new records. One new nodulated genus (Cymbosema) was found in the Papilionoideae. Twelve non-nodulating leguminous species were also observed for the first time. The results are discussed based on the systematics of the Leguminosae family and the influence of available nutrients to the legume-bacteria symbiosis.