Effect Of Free-Air CO₂ Enrichment (FACE) On Availability Of Mineral Elements In Paddy Soil

As a result of fossil fuel combustion and land use change, the concentration of carbon dioxide (CO₂) in the atmosphere is rising rapidly, with the potential to alter many ecosystem growth and biomass accumulation. Soil processes are tightly coupled with plant activity. The responses of the below-ground to elevated CO₂, however, are not well understood. Effect of elevated pCO₂ on the absorption of mineral elements of rice and the change of mineral elements in paddy soil were studied under both ambient CO₂ and elevated atmospheric CO₂ (Ambient +200μmol·mol^-1) by using the China FACE (Free-air CO₂ Enrichment) system.Increasing atmospheric CO₂ concentrations are generally expected to enhance photosynthesis and growth of agricultural C₃ cereal crops and as a result substantially increase yields. However, little is known about the combined effect of elevated CO₂ and nitrogen (N) supply on mineral elements. To better understand the interactive effects of these factors on the mineral elements of rice (Oryza sativa L.), we conducted the experiment of the xylem exudates in Yangzhou, Jiangsu under China FACE (Free Air Carbon dioxide Enrichment) system. Results showed that elevated CO₂ increased the xylem exudates at the tillering stage and the heading stage, decreased it at the milk ripening stage, but they showed no significance. At the three stage elevated CO₂ decreased the concentration of the Ca,Mg and Si in rice xylem sap, but increased the P concentration. Elevated CO₂ increased the absorption of the Ca,Mg,P,K and Si in rice xylem sap at the heading stage. At the milk ripening stage elevated CO₂ tended to decrease the absorption of Ca,Mg and Si, but increased the P absorption significantly. The different treatments of N fertilizer have insignificant effect on the concentrations and the absorption. The treatment of FACE and N fertilization have no significant effect on the xylem exudates.Soil solution is the active part in the soil three phases, its change can bring the imbalance of the solution, solid phase and liquid phase, then have an effect on kinds of chemistry processes in soil. During the rice-growing season, elevated CO₂ decreased soil redox potential and led to a differently increase in the concentrations of Fe²⁺, Mn²⁺, Ca²⁺, Mg²⁺ and Si²⁺ in the soil solution. In addition, enriched CO₂ decreased the concentrations of K and Na in the soil solution. No nitrogen supply effects were found on soil solution chemistry.After a rotation of rice and wheat, separately in the tillering stage, the heading stage, the milk ripening stage and the mature stage the soil samples were collected for availability of microelements determinations in 0-5 and 5-15cm soil depth. The DTPA extracting reagent had a fine repeatability and stability of the test values to value availability of microelements in paddy soil. Though DTPA extraction, soil available Fe, Mn, Cu and Zn was measured with Inductively Coupled Plasma Atomic Emission Spectrometer method. Results showed that elevated CO₂ increased the content of DTPA-extractable microelements of soils at different development stages of rice. The content of available Zn riched significant levels in addition to the increase in the heading stage and milk ripening stage of the 5-15cm soil. There were insignificant effect on the content of available Fe, Mn and Cu. The different treatments of N fertilizer have insignificant effect on the concentrations of DTPA-extractable microelements of soils. There was also no significant effect on availability of microelements under the treatment of FACE and N fertilization.