Greenhouse gas emissions of rice season soils under different paddy-upland rotation systems

Objective The research investigated the greenhouse gas emissions in the subsequent rice season under different paddy-upland rotation systems and straw (green manure) return condition, to propose an environmental friendly rotation mode in the middle reaches of the Yangtze River.

Method A field experiment was conducted in Jingzhou of Hubei Province. The rice-wheat (RW), rice-rape (RR) and rice - Chinese milk vetch (RC) rotation systems were arranged, and under each rotation system, the upland crop straws were returned to or removed out of the field. In the following rice season, the methane (CH₄) and nitrous oxide (N₂O) emissions in paddy fields were continuously monitored using static dark box-gas chromatography. At the tillering, heading and maturing stages of rice, soil samples were collected at 0−5 cm, 5−10 cm, and 10−20 cm layers for the determination of microbial biomass carbon (MBC), dissolved organic carbon (DOC), NH₄⁺-N and NO₃⁻-N content, and rice yield were investigated at harvest.

Results Straw return and rotation patterns did not affect rice yield significantly, but did in CH₄ and N₂O emissions. Compared with no straw return, the CH₄ and N₂O emissions in the three rotation systems under straw return increased by 26%−39% and 20%−29% on average, respectively. Compared with RC rotation, the CH₄ and N₂O emissions in RR and RW rotation systems were increased by 44%−58% and 17%−30% under straw return, and by 58%−72% and 15%−22% under no straw return, respectively. The global warming potential (GWP) and greenhouse gas emission intensity (GHGI) of RR and RW rotation systems were 14%–19% and 12%–19% higher than those of RC rotation system, respectively. Regardless of the effect of rotation system, the GWP and GHGI of subsequent rice under straw returning were 31% and 32% higher than those under no straw returning, respectively. On average, the contents of NO₃⁻-N, MBC and DOC in three rotation systems in different soil layers under no straw returning were higher than those under no straw returning. The MBC and DOC in RW were higher than in RC and RR. Correlation analysis showed that the emissions of CH₄ and N₂O were positively correlated with NO₃⁻-N under straw return (P<0.01). While the CH₄ emissions were positively correlated with NO₃⁻-N (P<0.01), and N₂O emissions were positively correlated with NH₄⁺-N (P<0.01) under no straw returning.

Conclusions The field CH₄ and N₂O emissions in rice season are related to the increased soil available carbon and nitrogen concentration. Straw return could significantly increase the greenhouse gas emission as the increased MBC and DOC content. Among the three rotation modes, rice-milk vetch rotation exhibits the lowest greenhouse gas emission, so is an ecologically friendly paddy-upland rotation system in the middle reaches of Yangtze River.