Nitrospira are more sensitive than Nitrobacter to land management in acid, fertilized soils of a rapeseed-rice rotation field trial

Abstract

Nitrite oxidation is recognized as an essential process of biogeochemical nitrogen cycling in agricultural ecosystems. How nitrite-oxidizing bacteria (NOB) respond to land managements (the effect from the long-term straw incorporation and environmental variability caused by the shift from the upland stage to the paddy stage) in a rapeseed-rice rotation field remains unclear. We found the nitrite oxidation (NO) in soils increased from the upland stage to the paddy stage. An inhibitory effect of the long-term straw incorporation on NO was detectable in the upland stage. The abundance of Nitrospira was always greater than Nitrobacter, and it was affected by the rice-growing and straw incorporation while Nitrobacter was not. NO correlated positively with the abundance of Nitrospira and with soluble sulfate (SO42−), soil moisture, pH and NH4+. The high-throughput sequencing analysis of the nitrite oxidoreductase nxrA and nxrB genes for Nitrobacter- and Nitrospira-like NOB was performed respectively. The dominating (relative abundance>1%) operational taxonomic units (OTUs) from Nitrobacter were closely related to Nitrobacter hamburgensis, whereas those from Nitrospira were affiliated with or related to lineage II, lineage V and several unknown groups. Heatmap analysis showed that a few dominant Nitrobacter OTUs were affected by the straw treatment or the rice-growing, and half of the dominant Nitrospira ones were explained by at least one of the variables. Multi-response permutation procedure (MRPP) and redundancy analyses showed that the Nitrospira-like NOB community changes were significantly shaped by the land managements and the soil chemical properties, including pH, moisture and NH4+, whereas that of the Nitrobacter-like NOB community was not. These results suggested that Nitrospira are more sensitive than Nitrobacter to land management in acid and fertilized soils of a rapeseed-rice rotation field trial.