Drainage, no-tillage and crop rotation decreases annual cumulative emissions of methane and nitrous oxide from a rice field in Southwest China

Permanently flooded rice fields, a special kind of all year-round flooded rice fields in China, where the crop system is summer rice ( Oryza sativa ‘Q You 6′) with winter fallow, contribute to both CH 4 and N 2 O emissions. To investigate their CH 4 and N 2 O emissions over a whole year (November 2009 to October 2010) and responses to long-term tillage-cropping systems, four treatments after the conversion of such rice fields were examined: conventional tillage with a single summer rice and floodwater winter fallow (CTRF) or drained winter rapeseed ( Brassica napus ‘W You 25′) (CTRR), no-tillage narrow- or wide-ridge with a rice and rapeseed rotation (NTNRR or NTWRR). Results showed that CTRF emitted the highest CH 4 owing to permanently flooding water layer and higher soil organic carbon concentrations. Compared to CTRF, CH 4 emissions under other three tillage-cropping systems were decreased not only in the winter season but also in the rice-growing season. In contrast, N 2 O emissions over a whole one-year rice-rapeseed rotation cycle were almost equivalent to each other under these four tillage-cropping systems. Also compared to CTRR, the two no-tillage-cropping systems tended to enhance CH 4 while decrease N 2 O emissions, though with insignificant effects. The annual cumulative emissions of CH 4 and N 2 O were highest under CTRF (1.07 ± 0.20 kg CO 2 -eq ha −1 kg −1 yield) and significantly decreased under CTRR, NTNRR and NTWRR (0.59 ± 0.10, 0.67 ± 0.05 and 0.58 ± 0.09 kg CO 2 -eq ha −1 kg −1 yield, respectively), indicating that the summer rice-winter rapeseed rotation system, irrespective of tillage management, rather than the summer rice-winter fallow system, had achieved the objective of higher yields with less greenhouse gas emissions. These results demonstrate that the no-tillage wide-ridge with a rice and rapeseed rotation (NTWRR) is the most efficient management in terms of decreasing CH 4 and N 2 O emissions in Southwest China.