## 3 Soil Features as a Consequence of Reduced Fertilization  
N fertilization plays a fundamental role in shaping soil properties, particularly its structure, chemistry, and biological activity. N is an essential element within soil systems, as it drives the processes of OM mineralization, accelerates the decomposition of plant residues, and enhances microbial activity [29]. Microorganisms in the soil convert organic N into plant-available forms (Figure 3), such as NO3- and ammonia [NH3], which are crucial not only for plant nutrition but also for the physical and chemical stability of the soil itself [29; 30]. The conversion and enhancement of N bioavailability for plants and other organisms is facilitated by a diverse array of microorganisms through three interrelated processes: N fixation, nitrification, and denitrification [31]. N fixation, the first step in the N cycle (Figure 3), involves the conversion of atmospheric nitrogen (N2) into NH3 by bacteria such as _Azotobacter_, making N available for plant and microbial use [32]. The NO3- produced during this process is subsequently available in the soil as NH4+, a form that can be readily absorbed by plants, thus contributing to their nutrient uptake [33]. Following N fixation, the process of nitrification occurs. This aerobic process involves the oxidation of NH3 to nitrite [NO2-] by nitriifying bacteria such as _Nitrosomonas_, followed by the further oxidation of NO2- to NO3- by _Nitrobacter_. NO3- is the primary form of N that plants uptake, making nitrification essential for ensuring adequate N availability in agricultural and natural ecosystems [31; 32]. The final stage in the N cycle is denitrification, wherein denitrifying bacteria, such as _Pseudomonas_ and _Clostidium_, convert NO3- back into atmospheric N2 (Figure 3). This process is vital for regulating N levels in the soil and preventing the accumulation of excess NO3-, which can lead to environmental issues such as water pollution [32; 33]. Together, these processes ensure the continuous cycling of N in nature, thereby allowing plants to efficiently utilize this essential element for growth and development [32]. This dynamic interplay among microorganisms not only supports plant health but also plays a crucial role in maintaining ecosystem balance and soil fertility [33]. [PERSON] et al. [34] employed high-throughput sequencing technology to analyze the taxonomic composition of soil N-transforming microbial communities (N-fixing bacteria, NH3-oxidizing archaea, NH3-oxidizing bacteria, and denitrifying bacteria) under varying N sources and levels in grassland soils. They identified the following N-fixing bacterial community: _Azohydromonas_, _Bradyrhizobium_, and _Azospirillum_, while the NH3-oxidizing archaea community was mainly composed of _Candidatus Nitroscosomicus_, _Nitrososphaera_, and _Nitrosopumilus_[34]. _Nitrosospira_, _Nitro  
Figure 3: The N cycle in agricultural soils, highlighting processes like N fixation, nitrification, and denitrification (own work by [PERSON]). somonas_, and _Nitrosovibrio_ were the major genera in the NH\({}_{3}\)-oxidizing bacterial community, whereas _Rhodpseudomonas_, _Mesorhizobium_, and _Bradyrhizobium_ were the major genera in the denitrifying bacterial community [34]. Interestingly, the application of NO\({}_{3}\)\({}^{-}\)-source N increased the relative abundance of _Candidatus_ and _Nitrosocosmicus_, while organic-source-N application led to a significant increase in the relative abundance of _Nitrosospira_[34]. NH\({}_{4}\)\({}^{+}\)- and NO\({}_{3}\)\({}^{-}\) source-N applications caused a decrease in the relative abundance of _Rhodpseudomonas_, while the application of organic- and NO\({}_{3}\)\({}^{-}\)-source N was shown to reduce the relative abundance of _Bradyrhizobium_[34]. N fertilization has a profound impact on soil structure, depending on its intensity and the balance of nutrient inputs [23]. When N is deficient, microbial activity in the soil declines, slowing the decomposition of OM.