Cover crop cultivation during the cold fallow season has been strongly recommended in mono-rice cultivation
system to improve soil quality. Its addition as green manure could stimulate methane (CH4)
emission in the submerged paddy soil during rice cultivation, but its effect has not been evaluated
well. Leguminous Chinese milk vetch (hereafter milk vetch) and non-leguminous rye were seeded
after rice harvesting in 2008 and 2009, and total above-ground biomasses were incorporated with
the recommended mineral fertilizers (NPK + milk vetch, and NPK + rye, respectively) before rice transplanting
in 2009 and 2010. No fertilization (control) and mineral fertilization alone (NPK) plots were
installed for comparison. Methane flux which was investigated by the closed chamber method during
rice cultivation was significantly increased by chemical fertilization to ca. 80–250% over the control
(138–141 kg CH4 ha−1), but increased more markedly by cover crop biomass addition to ca. 28–121% over
the NPK treatment. Milk vetch was more effective on stimulating less CH4 emission (28–61% increase
over NPK) and improving rice productivity (18–31% increase over NPK) than rye (86–121% increase of
CH4 emission and
−3–6% of yield increase over NPK). As a result, the total CH4 flux per grain yield was significantly
increased by rye addition, but were similar between the NPK + milk vetch and NPK treatments.
Conclusively, low C/N ratio cover crop like milk vetch might be more recommendable green manure to
minimize CH4 emission impact and increase rice productivity than high C/N ratio cover crop like rye in
paddy soil.