Methane and nitrous oxide emission under different paddy-upland crop rotation systems during rice growth season in Taihu Lake Region

Methane (CH₄) and nitrous oxide (N₂O) are tow potent greenhouse gases (GHGs) that have contributed to global warming. The emissions of these gases from rice paddies have been affected by several factors, including climate, soil property, water regime, fertilizers, etc. Although research has focused on CH4 and N₂O emissions in rice paddies under various agro-management systems, little has been available on CH4 and N2O emissions from rice paddies under different paddy-upland crop rotation systems. To that end, a field experiment was conducted in the 2011 rice growth season to benefit scientific evaluation of GHG emissions and provide scientific strategies for developing rational measures to reduce GHGs emissions in paddy fields across China. Using static chamber/gas chromatographic techniques, the field experiment was conducted in Suzhou, Jiangsu Province simultaneously measured CH4 and N₂O emissions under five paddy-upland crop rotation systems. The five systems included fallow-rice (control, CK), Chinese milk vetch-rice (T1), ryegrass-rice (T2), winter wheat-rice (T3) and rape-rice (T4). The results showed characteristics seasonal variations in CH₄ emission under different paddy-upland crop rotation systems during rice growth season. Although CH₄ emission initially increased, it eventually declined in the course of the rice growth season. Peak CH₄ flux was during the early stage of rice growth. CH₄ cumulative emission from transplanting to the critical stage of productive tillering accounted for 65%~81% of the total emission during the rice growth season. Peak N₂O flux was only observed during midseason drainage period. Total CH₄ and N₂O emissions during the rice growth season were significantly influenced by paddy-upland crop rotation systems (P < 0.01). Total CH₄ emission under different paddy-upland crop rotation systems was in following order: T1 (283.2 kg·hm ²) > CK (139.5 kg·hm²) > T3 (123.4 kg·hm²) > T4 (114.7 kg·hm²) > T2 (100.8 kg·hm²). Total N₂O emission was in order of: T1 > T4 > T3 > T2 > CK, with respective means of 1.06 kg·hm², 0.87 kg·hm², 0.81 kg·hm², 0.72 kg·hm² and 0.53 kg·hm ². Combined global warming potential (GWP) of CH₄ and N₂O under T1 was 7 396 kg(CO₂)·hm², significantly higher than CK, T2, T3 or T4. Compared with CK 3 646 kg(CO₂)·hm², T1 increased GWP by 103% while that of T2 2 735 kg(CO₂)·hm² decreased by 25%. The pilot study suggested that Chinese milk vetch-rice rotation intensified greenhouse effects during rice growth season in the Taihu Lake Region.