Nitrogen cycling during seven years of atmospheric CO2 enrichment in a scrub oak woodland

Experimentally increasing atmospheric CO₂ often stimulates plant growth and ecosystem carbon (C) uptake. Biogeochemical theory predicts that these initial responses will immobilize nitrogen (N) in plant biomass and soil organic matter, causing N availability to plants to decline, and reducing the long-term CO₂-stimulation of C storage in N limited ecosystems. While many experiments have examined changes in N cycling in response to elevated CO₂, empirical tests of this theoretical prediction are scarce. During seven years of postfire recovery in a scrub oak ecosystem, elevated CO₂ initially increased plant N accumulation and plant uptake of tracer ¹⁵N, peaking after four years of CO₂ enrichment. Between years four and seven, these responses to CO₂ declined. Elevated CO₂ also increased N and tracer ¹⁵N accumulation in the O horizon, and reduced ¹⁵N recovery in underlying mineral soil. These responses are consistent with progressive N limitation: the initial CO₂ stimulation of plant growth immobilized N in plant biomass and in the O horizon, progressively reducing N availability to plants. Litterfall production (one measure of aboveground primary productivity) increased initially in response to elevated CO₂, but the CO₂ stimulation declined during years five through seven, concurrent with the accumulation of N in the O horizon and the apparent restriction of plant N availability. Yet, at the level of aboveground plant biomass (estimated by allometry), progressive N limitation was less apparent, initially because of increased N acquisition from soil and later because of reduced N concentration in biomass as N availability declined. Over this seven-year period, elevated CO₂ caused a redistribution of N within the ecosystem, from mineral soils, to plants, to surface organic matter. In N limited ecosystems, such changes in N cycling are likely to reduce the response of plant production to elevated CO₂.