Denitrifying communities differentially respond to flooding drying cycles in paddy soils

Abstract

Paddy soils are an important source of nitrous oxide (N2O) emission, especially during frequent flooding and drying cycles. The N2O flux from paddy soils is mainly driven by denitrifying microorganisms, but the response of denitrifying communities to flooding drying cycles has been little studied. N2O emission was monitored under laboratory conditions in two paddy soils. Quantitative PCR (qPCR) and terminal restriction fragment length polymorphism (T-RFLP) were used to determine the abundance and community composition of narG- and nosZ-genes in denitrifiers. The N2O emission was more significantly related to soil Eh than soil water content during the drying process. Significant increases in the copy number and obvious alterations in the community composition of both narG- and nosZ-containing denitrifiers were detected after only one day of drying. Among the two denitrifying communities, narG gene abundance was significantly correlated to both Eh and water content, whereas nosZ was only significantly correlated to water content during the flooding drying process, indicating that different responses among various denitrifiers occurred in flooding and drying cycles. Furthermore, narG copy number varied following the flooding drying cycles, where drying caused an obvious increase and flooding caused a decrease in numbers. However, despite the first drying phase resulting in a significant increase in nosZ copy number, further flooding and drying cycles did not cause any remarkable changes compared to the first drying. The narG-containing denitrifiers were much more closely correlated with the N2O flux than nosZ-containing communities in the flooding drying cycles in the paddy soils studied here.