Influence of elevated CO2 and nitrogen nutrition on rice plant growth, soil microbial biomass, dissolved organic carbon and dissolved CH4

Rice (Oryza sativa) was grown in six sunlit, semi-closed growth chambers for two seasons at 350 μL L−1 (ambient) and 650 μL L−1 (elevated) CO2 and different levels of nitrogen (N) supplement. The objective of this research was to study the influence of CO2 enrichment and N nutrition on rice plant growth, soil microbial biomass, dissolved organic carbon (DOC) and dissolved CH4. Elevated CO2 concentration ([CO2]) demonstrated a wide range of enhancement to both above- and below-ground plant biomass, in particular to stems and roots (for roots when N was not limiting) in the mid-season (80 days after transplanting) and stems/ears at the final harvest, depending on season and the level of N supplement. Elevated [CO2] significantly increased microbial biomass carbon in the surface 5 cm soil when N (90 kg ha−1) was in sufficient supply. Low N supplement (30 kg ha−1) limited the enhancement of root growth by elevated [CO2], leading consequently to diminished response of soil microbial biomass carbon to CO2 enrichment. The concentration of dissolved CH4 (as well as soil DOC, but to a lesser degree) was observed to be positively related to elevated [CO2], especially at high rate of N application (120 kg ha−1) or at 10 cm depth (versus 5 cm depth) in the later half of the growing season (at 80 kg N ha−1). Root senescence in the late season complicated the assessment of the effect of elevated [CO2] on root growth and soil organic carbon turnover and thus caution should be taken when interpreting respective high CO2 results.