Effects of increased residue biomass under elevated CO2 on carbon and nitrogen in soil aggregate size classes (rice-wheat rotation system, China)

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 [CO2], a field experiment under a free-air CO2 enrichment (FACE) system was conducted. The experiment was set up with two CO2 levels [ambient CO2 and elevated CO2 (ambient + 200 ?mol mol-1)] and three N levels [low N (LN), 150 kg N ha-1 and 90 kg N ha-1, normal N (NN), 250 kg N ha-1 and 180 kg N ha-1, 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 CO2, 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 CO2 from 2001 to 2003. However, after the soil was cultivated with straw at two CO2 levels, as a ratio of biomass to field area for 1 yr, elevated CO2 decreased the percentage distribution of the macroaggregate (> 250 µm) and microaggregate (53-250 µm) fractions by 28.9% (P < 0.01) and 27.8% (P < 0.01), respectively, and increased that of the clay- and silt-sized (< 53 µm) fraction by 38.2% (P < 0.01). Elevated CO2 increased the carbon concentration by 4-41% and increased the nitrogen concentration by 0-30% compared with ambient CO2, with the largest increases of 41.2% (P < 0.01) and 30.2% (P < 0.05), respectively, in the macroaggregate fraction with NN. Elevated CO2 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, by 12.9% (P < 0.05) and 16.9% (P < 0.01) in microaggregate, but increased them, on average, by 47.6% (P < 0.01) and 44.4% (P < 0.01), respectively, in the clay- and silt-sized fractions. The changes in soil C:N due to elevated CO2 were largest in the <53 µm fraction with LN and in the >250 µm 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.Key words: Elevated CO2, nitrogen levels, aggregate turnover, carbon and nitrogen fractions, rice-wheat cropping, addition of straw