Linking N2O Emission with AOB and nirK-Denitrifier in Paddy Fields of Karst and Non-Karst Areas

Denitrification and nitrification are two pivotal microbial processes relating to N₂O emissions. However, the difference in N₂O emission fluxes and N₂O-producing bacteria between a karst (KA) and non-karst area (NKA) remains unclear. The objective of this study is to compare the differences in soil N₂O emissions, nitrifying bacteria, and denitrifying bacteria during the growth period of rice in KA and NKA, and to explore the mechanisms by which microorganisms and environmental factors drive N₂O emissions. Here, N₂O emission fluxes of paddy fields were collected using the static dark chamber and measured using gas chromatography at KA and NKA in the Maocun Karst Experimental Site in Guilin, China. The nitrifying bacteria (ammonia-oxidizing bacteria, AOB) and denitrifying bacteria (<i>nirK</i>-denitrifier) were determined using real-time PCR and high-throughput sequencing, respectively. Results showed that during the rice growth period, the N₂O emission fluxes in KA was generally lower than that in NKA, with cumulative N₂O emissions of -0.054 and 0.229 kg·hm^-2 in KA and NKA, respectively. The absolute abundance of AOB in KA (8.91 × 10⁶-2.68 × 10⁷ copies·g^-1) was significantly higher than that in NKA (1.57 × 10⁶-6.48 × 10⁶ copies·g^-1), while the absolute abundance of <i>nirK</i>-denitrifier had no significant difference between the two areas. The composition and diversity of AOB and <i>nirK</i>-denitrifier differed significantly between KA and NKA. Results from partial least squares structural equation modeling (PLS-SEM) indicated that soil properties, carbon sources, and nitrogen sources had positive effects on AOB and <i>nirK</i>-denitrifier, while <i>nirK</i>-denitrifier had a negative effect on N₂O emissions. Partial least squares regression (PLSR) predictions revealed that NO₃^--N, SOC, TN, Mg²⁺, Ca²⁺, and pH were the most important factors influencing N₂O emission fluxes. This study highlights the critical role of the typical characteristics of KA soils in reducing N₂O emissions from paddy fields by driving the evolution of AOB and <i>nirK</i>-denitrifier.