Q. 2009. Effects of increased residue biomass under elevated CO 2 on carbon and nitrogen in soil aggregate size classes (rice-wheat rotation system, China). Can. J. Soil Sci. 89: 567Á577. In order to study how soil carbon and nitrogen contents of different aggregate size fractions are affected by an increase in crop biomass and additional carbon inputs to soil due to elevated [CO 2 ], a field experiment under a free-air CO 2 enrichment (FACE) system was conducted. The experiment was set up with two CO 2 levels [ambient CO 2 and elevated CO 2 (ambient'200 mol mol , and high N (HN), 350 kg N ha (1 and 250 kg N ha (1 , during the rice season and the wheat season, respectively] and straw was added at the same soil:biomass ratio as in the field during the rice and wheat seasons. Compared with ambient CO 2 , little change was observed in the percentage distribution of soil fractions, carbon and nitrogen content, and C:N ratio in each soil fraction under elevated CO 2 from 2001 to 2003. However, after the soil was cultivated with straw at two CO 2 levels, as a ratio of biomass to field area for 1 yr, elevated CO 2 decreased the percentage distribution of the macroaggregate (250 mm) and microaggregate (53Á250 mm) fractions by 28.9% (PB0.01) and 27.8% (PB0.01), respectively, and increased that of the clay-and silt-sized (B53 mm) fraction by 38.2% (PB0.01). Elevated CO 2 increased the carbon concentration by 4Á41% and increased the nitrogen concentration by 0Á30% compared with ambient CO 2 , with the largest increases of 41.2% (P B0.01) and 30.2% (PB0.05), respectively, in the macroaggregate fraction with NN. Elevated CO 2 decreased the contributions of soil carbon and nitrogen contents, respectively, in each fraction, to the whole soil by 13.1 and 17.2% in macroaggregates and by 12.9% (PB0.05) and 16.9% (PB0.01) in microaggregate, but increased them, on average, by 47.6% (P B0.01) and 44.4% (PB0.01), respectively, in the clay-and silt-sized fractions. The changes in soil C:N due to elevated CO 2 were largest in the B53 mm fraction with LN and in the 250 mm fraction with NN and HN. These results suggest that adding straw is an important factor for soil structure and function with regard to soil carbon and nitrogen storage and cycling under FACE. For personal use only.