## 4 Discussion
However, microbial guilds which have the nitrite copper-containing reductase gene (_nirK_) are generally more dominant than the microbial guilds which have the nitrite cytochrome _cd1_-containing reductase gene (_nirS_) in rice paddy soils [1; 40; 41; 42]. The _nirS_ gene was not affected by long-term urea application as observed by [PERSON] et al. [2]. The _nirA_ gene, a usual molecular marker for DNRA, was also not amplified in our samples. Under regular conditions, dissimilatory nitrate reduction to ammonia had lesser activity in rice agroecosystems [1]. The implication of the changed abundance of molecular markers associated with microbial guilds related to nitrogen cycling, particularly nitrification and denitrification, may only by assumed in our study. The paddy soils were also collected during the fallow period before soil tilling, fertilization amendment, water submersion, and rice seedling transplantation. It is possible that, under our experimental conditions, increases in both N\({}_{2}\)O and NO emissions occur in paddy soils with higher rather than lower nutrient content soils, as observed by [PERSON] et al. [10]. During the fallow period, a potentially higher than expected emission of nitrous oxide may occur from drained paddy fields [43; 44]. This is similar to the dry season, where populations of ammonia oxidizers, nitrite oxidizers, and denittifiers were significantly higher compared to the wet season, which may also lead to increased activities under long-term fertilization, although the fallow periods have a lower amount of N\({}_{2}\)O-N flux compared to the planting season [20]. Characteristic Paddy Fields Associated with Nitrogen Cycling-Related Microbial Guilds and Soil Chemical Fertilizers under Long-Term Compost Fertilization  
The principal component analysis clearly supported the clustering of paddy fields according to treatment amendments. A clear separation of three distinct groups according to the different fertilizer input could be observed in the current study, mainly influenced by soil edaphic (chemical) factors and the population density of microbial guilds measured through the abundance of denitrification and nitrification genes. The effects of PC1 and PC2 in the PCA analysis explained 67% of the overall soil variabilities, incorporating both soil chemical properties and gene abundances. This implies that the paddy fields receiving different fertilization treatments had characteristic soil variabilities exemplified through long-term fertilizer amendments. Long-term fertilization regimes have changed multiple physical, chemical, and biological properties of paddy fields [20; 28]. Groups of microbial guilds associated with nitrogen cycling have also responded differently to long-term fertilization, as evidenced by the distinct microbial guilds, in terms of diversity and abundance, particularly associated to the different fertilization treatments. These are observed in long-term amendments with compost and NPK + compost [20; 42; 45], and inorganic or chemical fertilizer including urea [6; 20; 42; 45]. Differences in the fertilization application in terms of the amount of fertilization over long-term fertilization also show group-specific changes in microbial communities associated to nitrogen cycling [2; 46], creating paddy fields with distinct microbial guilds that are directly affected by fertilization treatments and soil properties. These results were also reflected in our study, where long-term NPK + compost and compost fertilization created characteristic paddy soils. The unfertilized paddy fields were characterized with soil containing a lower nutrient content and lower population of nitrifying and denittifying microbial guilds. The NPK + compost- and the compost-fertilized paddy fields had soils containing a higher nutrient status with more abundant nitrifying and denittifying microbial guilds.