Greenhouse gas emissions during the seedling stage of rice agriculture as affected by cultivar type and crop density

Methane (CH4) and nitrous oxide (N2O) emissions from a paddy nursery at the rice seedling stage were measured on a daily basis by using the conventional rice cultivar Nangeng 56 under both conventional (NG-C) and reduced (NG-R) sowing density, and the hybrid rice Changyou 3 under both conventional (CY-C) and reduced (CY-R) sowing density. High N2O and CH4 emissions were observed during the first and last 2 weeks, respectively. Cumulative CH4 emissions were significantly (P < 0.001) affected by sowing density rather than by the rice cultivar. Cumulative CH4 emissions reached 68.2 kg C ha(-1) in the CY-C treatment and 121.6 kg C ha(-1) in the NG-C treatment, which were significantly (P < 0.001) higher than the emissions at reduced sowing densities (15.9 kg C ha(-1) in the CY-R treatment and 20.9 kg C ha(-1) in the NG-R treatment). Under the conventional sowing density, cumulative CH4 emissions during the seedling stage were comparable to data of rice-growing season. Both the rice cultivar and the sowing density significantly (P < 0.05-0.01) affected cumulative N2O emissions. Relative to the CY cultivar, the NG cultivar increased global warming potential (GWP) over a 100-year horizon by 62.1% and 70.7% under the reduced and conventional sowing densities, respectively. The GWP of N2O and CH4 during the seedling stage was equivalent to the GWP of the entire rice-growing season in this region, indicating that the seedling stage is an important greenhouse gas emission source of rice agriculture.