Biological Nitrogen Fixation: A Key Process for the Response of Grassland Ecosystems to Elevated Atmospheric [CO2

Abstract

Data from our 10 years of studies done at the Swiss FACE site, along with numerous process studies done by ourselves and by others, clarifies the role that biological (symbiotic) N2 fixation plays in a CO2-rich world. Under e[CO2], symbiotic N2 fixation increases as a result of increased plant growth (N demand) and not due to direct CO2 stimulation leading to greater photosynthate availability. Under fertile soil conditions, e[CO2] apparently caused changes in soil processes and nitrogen status; and, as a result, increases in total symbiotic N2 fixation (N sink-driven) and changes in the population structure of N2-fixing soil micro-organisms were detected. Under fertile soil conditions and ample water availability, e[CO2] apparently caused a pronounced N limitation in the initial periods of CO2 enhancement. However, within a few years, a new N balance was apparently reached (progressive N saturation); but only under conditions of high-N input and not under a low-N input. This was accompanied by a readjustment of symbiotic N2 fixation capacity in legumes and by further shifts in the population of N2-fixing soil micro-organisms to a structure seen previous to CO2 enrichment. The integrated nature and interdependence of photosynthesis and biological (symbiotic) N2 fixation was confirmed from the Swiss FACE experiment.