Mediterranean and dryland ecosystems are home to an incredible diversity of legumes, including hundreds of annual herbaceous plants. Annual legumes are found in almost any herbaceous plant community, often in diverse composition but seldom as dominants. Most of these legumes potentially fix dinitrogen (N2), through root symbiosis with N2-fixing bacteria. However, the persistence of ephemeral nitrogen-fixing legumes in Mediterranean ecosystems is puzzling, given the combination of high cost of fixation and the potential for legumes to increase soil nitrogen availability over time and therefore create conditions that allow their competitive exclusion by non-fixing plants. We examined the strategies by which legume plants regulate symbiotic nitrogen fixation in response to soil nutrient availability in a suite of herbaceous species. Plant investment in nitrogen fixation was evaluated in both in controlled and field experiments. We also assessed nitrogen allocation in these plants and how it influences inputs of new nitrogen to the soil over time.
Results/Conclusions
Three legume species showed three different strategies in controlled experiments – obligate strategy of nitrogen fixation (constant fixation rate regardless of soil nitrogen availability), facultative fixation (downregulation when nitrogen is available), and up-regulation of fixation when soil nitrogen was available. Similar results were found in field conditions, but with the added complexity of limitation by other nutrients (mostly phosphorus), and the influence of diverse bacteria communities. Nitrogen-fixing plants allocated only a small portion of their fixed nitrogen to seeds, therefore leaving much of this nitrogen in the decomposing litter. The species richness and abundance of legumes in the herbaceous plant community was strongly affected by both nitrogen- and phosphorus-limitation, potentially explaining the coexistence of legumes with diverse regulation strategies. Our findings indicate that seasonal nitrogen fixation, regulation of fixation, and nitrogen recycling, are key adaptations influencing the role of annual herbaceous legumes in these hyper-diverse plant communities.