CO2 and soil water potential as regulators of the growth and N fraction derived from fixation of a legume in tallgrass prairie communities

Authors: Polley, Herbert; Collins, Harold; REICHMANN, LARA - University Of Texas; Fay, Philip

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/23/2016
Publication Date: 12/12/2016
Publication URL: http://handle.nal.usda.gov/10113/63335
Citation: Polley, H.W., Collins, H.P., Reichmann, L.G., Fay, P.A. 2016. CO2 and soil water potential as regulators of the growth and N fraction derived from fixation of a legume in tallgrass prairie communities. Plant and Soil. 409:361-370.

Interpretive Summary: The concentration of carbon dioxide (CO2) gas in the atmosphere has increased by 40% since industrialization and is expected to reach double the pre-industrial concentration by mid-century. CO2 is a required resource of plants, consequently higher CO2 often increases plant growth. By stimulating the growth of leguminous plants (legumes), rising CO2 also may increase the amount of gaseous nitrogen (N) in the atmosphere that is converted to plant-available forms, the process of N fixation. The greater N input to grassland ecosystems that results as legumes fix more N may ultimately stimulate growth of neighboring plants. We measured N fixation by the legume Illinois bundle flower following 2-8 years of growth along an experimental CO2 gradient that spanned pre-industrial to anticipated concentrations. Bundle flower was grown in mixed plantings with other tallgrass prairie species on three soils of differing texture. Contrary to expectation, increased CO2 reduced the fraction of legume N that was derived from fixation by 20% on a clay soil. This negative effect of CO2 on N fixation was offset by a greater growth response of the legume to CO2 when prairie communities as a whole responded to CO2 with greater growth. Bundle flower was too rare to reliably determine CO2 effects on fixation on the other soils. Across the three soils, legume growth increased as water availability declined indicating that potential N fixation also increased as soils dried. Our results imply that the amount of N that legumes add to grasslands depends on both soil water availability as affected by precipitation patterns and soil properties and the relative expression of potentially opposing effects of CO2 on legume growth and the fraction of legume N that is derived from fixation.

Technical Abstract: CO2 enrichment may increase N input to ecosystems by increasing N2 fixation, but the fixation-CO2 response depends on factors such as soil water availability that are influenced by both CO2 and soil properties. We used the d15N natural abundance method to determine N2 fixation by the legume Desmanthus illinoensis following 2-8 years of growth along a subambient to elevated CO2 gradient. Desmanthus was grown in tallgrass prairie communities on each of three soils of differing texture. Only on a clay soil was it possible to calculate fixation (Nfix; g N m-2). CO2 reduced the fraction of legume N derived from fixation (Ndfa) by 20%. The negative effect of reduced Ndfa on Nfix was offset by variation in Desmanthus production along the CO2 gradient that was positively linked to the ANPP-CO2 response of communities. Across soils, legume production and, thus capacity for Nfix, was negatively correlated to soil water potential to 0.3 m depth ('soil). Nfix in grasslands may depend primarily on 'soil as affected by precipitation patterns and soil hydrological properties. CO2 may reduce Nfix during years in which the legume-CO2 and related ANPP-CO2 response is small by depressing Ndfa.