Legume presence increases photosynthesis and N concentrations of co-occurring non-fixers but does not modulate their responsiveness to carbon dioxide enrichment

摘要

Legumes, with the ability to fix atmospheric nitrogen (N), may help alleviate the N limitations thought to constrain plant community response to elevated concentrations of atmospheric carbon dioxide (CO2). To address this issue we assessed: (1) the effects of the presence of the perennial grassland N-2 fixer, Lupinus perennis, on biomass accumulation and plant N concentrations of nine-species plots of differing plant composition; (2) leaf-level physiology of co-occurring non-fixing species (Achillea millefolium, Agropyron repens, Koeleria cristata) in these assemblages with and without Lupinus; (3) the effects of elevated CO2 on Lupinus growth and symbiotic N-2 fixation in both monoculture and the nine-species assemblages; and (4) whether assemblages containing Lupinus exhibit larger physiological and growth responses to elevated CO2 than those without. This study was part of a long-term grassland field experiment (BioCON) that controls atmospheric CO2 at current ambient and elevated (560 mumol mol(-1)) concentrations using free-air CO2 enrichment. Nine-species plots with Lupinus had 32% higher whole plot plant N concentrations and 26% higher total plant N pools than those without Lupinus, based on both above and belowground measurements. Co-occurring non-fixer leaf N concentrations increased 22% and mass-based net photosynthetic rates increased 41% in plots containing Lupinus compared to those without. With CO2 enrichment, Lupinus monocultures accumulated 32% more biomass and increased the proportion of N derived from fixation from 44% to 57%. In nine-species assemblages, Lupinus N derived from fixation increased similarly from 43% to 54%. Although Lupinus presence enhanced photosynthetic rates and leaf N concentrations of co-occurring non-fixers, and increased overall plant N pools, Lupinus presence did not facilitate stronger photosynthetic responses of non-fixing species or larger growth responses of overall plant communities to elevated CO2. Non-fixer leaf N concentrations declined similarly in response to elevated CO2 with and without Lupinus present and the relationship between net photosynthesis and leaf N was not affected by Lupinus presence. Regardless of the presence or absence of Lupinus, CO2 enrichment resulted in reduced leaf N concentrations and rates of net photosynthesis.