Effect of Atmospheric CO2 Enrichment on Soil Respiration in Winter Wheat Growing Seasons of a Rice-Wheat Rotation System

Abstract

Studies on the effect of elevated CO2 on C dynamics in cultivated croplands are critical to a better understanding of the C cycling in response to climate change in agroecosystems. To evaluate the effects of elevated CO2 and different N fertilizer application levels on soil respiration, winter wheat (Triticum aestivum L. cv. Yangmai 14) plants were exposed to either ambient CO2 or elevated CO2 (ambient [CO2] + 200 μmol mol−1), under N fertilizer application levels of 112.5 and 225 kg N ha−1 (as low N and normal N subtreatments, respectively), for two growing seasons (2006–2007 and 2007–2008) in a rice-winter wheat rotation system typical in China. A split-plot design was adopted. A root exclusion method was used to partition soil respiration (RS) into heterotrophic respiration (RH) and autotrophic respiration (RA).Atmospheric CO2 enrichment increased seasonal cumulative RS by 11.8% at low N and 5.6% at normal N when averaged over two growing seasons. Elevated CO2 significantly enhanced (P < 0.05) RS (12.7%), mainly due to the increase in RH (caused by decomposition of larger amounts of rice residue under elevated CO2) during a relative dry season in 2007–2008. Higher N supply also enhanced RS under ambient and elevated CO2. In the 2007–2008 season, normal N treatment had a significant positive effect (P < 0.01) on seasonal cumulative RS relative to low N treatment when averaged across CO2 levels (16.3%). A significant increase in RA was mainly responsible for the enhanced RS under higher N supply. The correlation (r2) between RH and soil temperature was stronger (P < 0.001) than that between RS and soil temperature when averaged across all treatments in both seasons. Seasonal patterns of RA may be more closely related to the plant phenology than soil temperature. The Q10 (the multiplier to the respiration rate for a 10 °C increase in soil temperature) values of RS and RH were not affected by elevated CO2 or higher N supply. These results mainly suggested that the increase in RS at elevated CO2 depended on the input of rice residue, and the increase in RS at higher N supply was due to stimulated root growth and concomitant increase in RA during the wheat growing portion of a rice-winter wheat rotation system.