A possible plant-mediated feedback between elevated CO2, denitrification and the enhanced greenhouse effect

Natural abundances (δ) of 15N were used to detect effects of elevated atmospheric CO2 concentration ([CO2]) and soil wetness on soil N transformations in the presence or absence of plants. An elevated [CO2] of 1000 μl l−1 reduced water use by the perennial C4 grass Panicum coloratum and stimulated root and whole-plant growth. Soil remained wetter between infrequent irrigations than in soil supporting P. coloratum grown in an ambient [CO2] (350 μl l−1). The δ15N value of soil nitrate increased from −2.4 to +9.6‰ as nitrate was depleted from the soil, but remained unchanged in unplanted soil. The change in δ15N of soil nitrate was greatest in frequently watered soil regardless of [CO2], and in infrequently watered soil only in elevated [CO2]. It was least in the infrequently watered, ambient [CO2] treatment. Isotope mass balances and 15N/14N fractionation theory identified denitrification as the most probable cause of this effect, through the effect of elevated [CO2] on soil wetness. Nitrification, nitrogen assimilation, leaching or ammonia volatilisation were unlikely causes. The data suggest a positive, plant-induced effect of elevated atmospheric [CO2] on denitrification. The possibility exists, therefore, for a positive feedback between elevated atmospheric [CO2], a greater soil-to-atmosphere N2O flux and an exacerbation of the enhanced greenhouse effect.