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 CH4 and N2O emissions. To investigate their CH4 and N2O 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 CH4 owing to permanently flooding water layer and higher soil organic carbon concentrations. Compared to CTRF, CH4 emissions under other three tillage-cropping systems were decreased not only in the winter season but also in the rice-growing season. In contrast, N2O 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 CH4 while decrease N2O emissions, though with insignificant effects. The annual cumulative emissions of CH4 and N2O were highest under CTRF (1.07 ± 0.20 kg CO2-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 CO2-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 CH4 and N2O emissions in Southwest China.