Influence of Azolla incorporation and/or dual cropping on CH4 and N2O emission from a paddy field

Our previous pot experiments showed that using Azolla either or both as dual and green manure with rice increases its yield or significantly reduces either or both methane (CH4) and nitrous oxide (N2O) emissions. To confirm these findings in an actual field, Azolla was either grown as a dual crop (herein Cover) or incorporated as green manure plus dual cropping (herein AGM + Cover) at the beginning of the experiment along with rice. Compared with the control (chemical fertilizer; herein NPK), NPK + Cover and AGM + Cover treatments did not influence cumulative CH4 emissions throughout the rice growth period. However, AGM + Cover treatment affected significant CH4 emissions at early, middle, and later rice growth stages by 140.6%, 24.6%, and 33.1%, respectively, compared with NPK + Cover treatment. These emissions were attributed to the readily available carbon substrate for methanogens following the incorporation of Azolla as green manure. Compared with NPK, NPK + Cover and AGM + Cover significantly increased N2O emissions by 645.9% and 816.2%, respectively, during the middle rice growth stage. No significant N2O emission differences were observed in the three treatments in the early or later rice growth stages. The higher N2O emissions from the middle rice growth stage were ascribed to high substrate availability from the dead Azolla by higher summer air temperature in the 2019 season. AGM + Cover significantly decreased rice yield by 37.5% (NPK) and 35.3% (NPK + Cover), with no significant differences between NPK and NPK + Cover. This reduction was attributed to nitrogen immobilization from the incorporated Azolla during the early stage. Therefore, to ascertain the potential of Azolla in paddy fields to address environmental safety while sustaining yield, emphasis on the interaction of different application methods with various management practices is necessary.

我们此前的盆栽试验显示，将满江红（Azolla）单独作为间作作物、单独作为绿肥，或两者兼具与水稻配施，既可提升水稻产量，又能显著降低甲烷（CH4）与一氧化二氮（N2O）的排放量。为在实际大田环境中验证上述发现，本试验于实验初期将满江红分别以间作模式（下文简称Cover）、绿肥翻压还田+间作模式（下文简称AGM + Cover）与水稻共植。与对照组（施用氮磷钾化肥，下文简称NPK）相比，NPK + Cover与AGM + Cover处理下水稻全生育期的甲烷累计排放量无显著变化。但相较于NPK + Cover处理，AGM + Cover处理在水稻生育前期、中期与后期的甲烷排放量分别显著升高140.6%、24.6%与33.1%。上述排放差异可归因于满江红作为绿肥翻压还田后，为产甲烷菌（methanogens）提供了易利用的碳底物。与NPK组相比，NPK + Cover与AGM + Cover处理在水稻生育中期的一氧化二氮排放量分别显著提升645.9%与816.2%。而在水稻生育前期与后期，三种处理间的一氧化二氮排放量无显著差异。水稻生育中期一氧化二氮排放量升高的原因，在于2019年季夏季气温偏高，使得死亡满江红分解释放的底物可用性提升。AGM + Cover处理下水稻产量较NPK组与NPK + Cover组分别显著降低37.5%与35.3%，而NPK组与NPK + Cover组的水稻产量无显著差异。该产量降低现象可归因于试验初期翻压还田的满江红发生了氮固持（nitrogen immobilization）作用。因此，若要明确满江红在稻田中兼顾环境安全与产量维持的应用潜力，需重点关注不同施用方式与各类田间管理措施间的交互作用。