Effects of Straw-returning on N₂O Emission and Nitrifying/Denitrifying Microbes in Paddy Soil in Southern China

  Objective   Effects of returning spent straws to soil after crop harvest as a conditioner/fertilizer on the N₂O emission and nitrifying/denitrifying microbes on a double-cropping rice field in late season were studied.

  Method   Based on a positioning experiment set up in 2015, the field soil was treated with (1) chemical fertilizer without straw-returning (CK), (2) chemical fertilizer + 100% straw-returning in same season (CKS), (3) CKS + straws to replace 10% potassium fertilizer (S10), (4) CKS + straws to replace 20% potassium fertilizer (S20) or (5) CKS + straws to replace 30% potassium fertilizer (S30). N₂O emitted and microorganisms in the soils were detected using the closed static dark box-gas chromatography and metagenomic sequencing technique.

  Result   N₂O was released from the paddy soils basically during the rice tillering stage. Compared with CK, returning spent straws to the ground significantly reduced the cumulative gas emission from the soil under the S30 treatment, which rendered the lowest rate at 0.09 kg·hm^-2, and thus, the least contribution to global warming. Of the nitrifying microflora in soil, the dominant genus of ammonia-oxidizing bacteria (i.e., amoA and amoB) at the tillering and mature stages was Methylospora. Among the denitrifying microbes, the nirK-type Gemmatimonadetes, Rhodobacter,and Opitutus and the nirS-type Methylobacillus and Methylotenera were dominantly present. At the rice tillering stage, the N₂O emissions were significantly inversely correlated with Methylotenera but positively with Ramlibacter. At the mature stage, a significant correlation between the soil N₂O emission and Aquabacterium population was observed.

  Conclusion   By returning the spent straws to the field, a significant reduction on the N₂O emissions from paddy soil was resulted. And Methylospora was the dominant genus involved in the ammonia oxidation, whereas the nirK-type Gemmatimonadetes,Rhodobacter and Opitutus and the nirS-type Methylobacillus and Methylotenera were the dominant genera of denitrifying bacteria.