Availability of soil nitrogen and phosphorus in a typical rice–wheat rotation system under elevated atmospheric [CO2]

An experiment with the free-air carbon dioxide enrichment (FACE) method was conducted in a paddy field at Wuxi (Jiangsu Province, China) to study effects of elevated atmospheric [CO2] on availability of soil nitrogen and phosphorus. Rice (Oryza sativa L.) and winter wheat (Triticum aestivum L.) were grown under ambient CO2 or FACE (ambient + 200 μmol mol−1 CO2) conditions throughout the growth season. Low N (LN) and normal N (NN) were applied, LN being 150 kg ha−1 for rice and 125 kg ha−1 for wheat and NN being 250 kg ha−1 for both rice and wheat.Compared with the ambient [CO2] condition, elevated [CO2] significantly increased crop biomass and P uptake for both rice and wheat and N uptake only for wheat at several main growth stages. The positive effects of elevated [CO2] on biomass, N and P uptake of wheat were greater than of rice.Soil available N was decreased by elevated [CO2] by 47% in LN and 29% in NN at the rice tillering stage at jointing stage and decreased by 25.4% and 28.3% in LN and by 33.3% and 53.1% in NN at the wheat seeding and heading stages, respectively. Soil available P was decreased by elevated [CO2] in rice by 32.0% in LN and by 29.6% in NN at the jointing stage, but increased by 22.4% and 20.8% at the heading stage and by 33.8% and 30.7% at the ripening stage in LN and NN, respectively. While in the wheat season, soil available P was not affected significantly by elevated [CO2] at each stage.These results suggest that under elevated [CO2] availability of soil N and P increased, particularly P and application of N and P should be adjusted to need for rice at tillering and jointing and for wheat at whole growth stages.