Temporal Effects of Elevated Carbon Dioxide and Ozone on Soil Carbon and Nitrogen Stoichiometry in a No-till Soybean-Wheat Agro-ecosystem

The effects of elevated CO 2 and O 3 on plants have been studied widely, but their possible effects on carbon: nitrogen stoichiometry of soil organic matter (SOM) have received much less attention. We conducted a five-year field experiment to address such effects on SOM. We used open-top chambers to expose a no-till (NT) soybean-wheat system to a factorial combinations of two CO 2 (360 & 500 µmol mol1) and two O 3 (25 & 70 nmol mol -1) treatments daily from April to October under ambient light and temperature conditions. Geo-referenced composite soil samples were collected annually and analyzed for soil total organic C (TOC), N (TN), hydrogen (H) concentrations, and bulk density. TOC increased through time under both CO 2 levels, but the increase was significantly greater under elevated CO 2 (585-kg ha-1 y-1) compared with the ambient CO 2 (18-kg ha-1 y-1). TN decreased over time under both levels of the CO 2 and O 3 treatments. The rate of decrease in TN was greater under elevated CO 2 (216-kg N ha-1 y-1) compared with to ambient CO 2 (152-kg N ha-1 y-1), but it was smaller under elevated O 3 (172-kg N ha-1 y-1) as compared to low O 3 (197kg N ha-1 y-1). The C:N was increased significantly over time under both levels of CO 2 or O 3 treatments. Results suggest that increased TOC sequestration at elevated CO 2 could elicit a progressive soil N deficiency. In contrast, the impact of elevated O 3 (+O 3 and O 3 +CO 2 ) may cause a small accumulation of recalcitrant C, which in the longterm could affect SOM labiality. Increased soil N under elevated O 3 may enhance N leachate from soil.