## 4 Discussion
The direct relationships between the fertilizer input and soil chemical properties and the composition and size of microbial functional guilds have been observed in relation to nitrogen cycling [2, 31] and other important soil-based biogeochemical processes [9, 11, 20, 26, 28]. Their relationships under long-term compost fertilizer amendment, even during the rice fallow period, should be assessed, especially in terms of their potential impact, particularly greenhouse gas (GHG) emissions. Under compost and NPK + compost fertilization, the abundances of the nitrogen cycling-related gene bacterial ammonia monooxygenase (_Bamo_) and nitrous oxide reductase genes (_nosZI_, _nosZII_) were significantly greater compared to the unfertilized paddy fields. In addition, the archaeal ammonia monooxygenase (_Aamo_) gene was also significantly more abundant in the compost amended paddy fields. The nitrite reductase gene (_nirK_) abundance does not seem to be affected by any fertilization amendment. Correlation and multiple regression analyses pointed to some soil chemical properties attributed to these observations. The gene abundance of _Aamo_ is correlated to DOC; _Bamo_ to SOM, TN, and K; _nosZI_ to SOM, TN, K, Ca, and DOC; and _nosZII_ to SOM, TN, P, K, and DOC. Studies with long-term fertilizer amendment showed differentiated soil properties, altering microbial communities, which consequently led to changes in the microbial activity [31, 32]. The bacterial abundance under long-term fertilization was also positively correlated with the soil nutrient content [3]. Bacterial communities dynamically change in terms of abundance or composition due to alteration in the soil nutrient status under short-term fertilization [33, 34, 35]. This is most probably due to a change in the metabolic lifestyle involving a more active microbial community adapted to higher nutrient input [36]. As observed in the current study, a stable and higher nutrient status in paddy fields even in the fallow period could have also maintained more abundant microbial guilds related to nitrogen cycling due to decades of the standardized application of NPK + compost and compost. Organic fertilization becomes a prominent source of dissolved or labile carbon, with higher nutrient content similarly observed in previous studies [20, 37, 38]. It was also observed that the input of livestock manure restored the decreased microbial community composition and population, stably maintaining the microbial diversity of soils under long-term fertilization with inorganic fertilizer [3]. The effects of compost and NPK + compost long-term amendment on specific microbial guilds associated with nitrogen cycling may have different trends compared to the overall microbial diversity and abundance. However, similar to the observation by [PERSON] et al. [2], most nitrogen cycling-related genes had an elevated abundance due to long-term N fertilization, suggesting paddy field agroecosystem succession where nitrification and denitrification microbial guilds have a higher abundance, which was also observed in the current study. This was particularly true for the _Bamo_, _Aamo_, _nosZI_, and _nosZII_ genes. On the other hand, a correlation between SOM and the denitrifier abundance has been reported [39]. This is due to organic carbon donating electrons for denitrification, and, thus,its accessibility greatly controls denitrification [1]. However, taxon-specific responses of microbial guilds related to nitrogen cycling to a particular type of fertilizer amendment may also occur. For instance, in our study, the _nirK_ gene, which encodes the nitrate reductases, was non-responsive to compost and NPK + compost amendment. The _nirS_ gene, another functionally equivalent nitrate reductase, was not amplified using the primer set we employed, which potentially indicates a minor undetected abundance of this microbial guild in the studied paddy fields or that other factors were involved.