Contrasting growth response of an N₂-fixing and non-fixing forb to elevated CO₂: dependence on soil N supply

With the ability to symbiotically fix atmospheric N2, legumes may lack the N-limitations thought to constrain plant response to elevated concentrations of atmospheric CO2. The growth and photosynthetic responses of two perennial grassland species were compared to test the hypotheses that (1) the CO2 response of wild species is limited at low N availability, (2) legumes respond to a greater extent than non-fixing forbs to elevated CO2, and (3) elevated CO2 stimulates symbiotic N2 fixation, resulting in an increased amount of N derived from the atmosphere. This study investigated the effects of atmospheric CO2 concentration (365 and 700 micromol mol-1) and N addition on whole plant growth and C and N acquisition in an N2-fixing legume (Lupinus perennis) and a non-fixing forb (Achillea millefolium) in controlled-chamber environments. To evaluate the effects of a wide range of N availability on the CO2 response, we incorporated six levels of soil N addition starting with native field soil inherently low in N (field soil + 0, 4, 8, 12, 16, or 20 g N m-2 yr-1). Whole plant growth, leaf net photosynthetic rates (A), and the proportion of N derived from N2 fixation were determined in plants grown from seed over one growing season. Both species increased growth with CO2enrichment, but this response was mediated by N supply only for the non-fixer, Achillea. Its response depended on mineral N supply as growth enhancements under elevated CO2 increased from 0% in low N soil to +25% at the higher levels of N addition. In contrast, Lupinus plants had 80% greater biomass under elevated CO2 regardless of N treatment. Although partial photosynthetic acclimation to CO2 enrichment occurred, both species maintained comparably higher A in elevated compared to ambient CO2 (+38%). N addition facilitated increased A in Achillea, however, in neither species did additional N availability affect the acclimation response of A to CO2. Elevated CO2 increased plant total N yield by 57% in Lupinus but had no effect on Achillea. The increased N in Lupinus came from symbiotic N2 fixation, which resulted in a 47% greater proportion of N derived from fixation relative to other sources of N. These results suggest that compared to non-fixing forbs, N2-fixers exhibit positive photosynthetic and growth responses to increased atmospheric CO2 that are independent of soil N supply. The enhanced amount of N derived from N2 fixation under elevated CO2 presumably helps meet the increased N demand in N2