# Effect of Precision Nitrogen Fertilization of Grassland on Soil Microbial Structure
## 1 Introduction
Approaches to soil quality assessment can be either qualitative or quantitative. Soil quality indicators include a combination of physical, chemical, and biological properties [25]. Among the physical indicators, texture, bulk density, water retention capacity, retention characteristics, porosity, aggregate stability, soil depth, pH, electrical conductivity, and cation exchange capacity are distinguished. Biological indicators, on the other hand, include the abundance, activity, and diversity of soil organisms, as well as enzyme activity [26; 27; 28]. Microorganisms are a particularly sensitive measure of soil quality, which cannot always be assessed based on physical or chemical indicators. They play a crucial role in biogeochemical cycles, soil structure formation, and pollutant degradation [29]. Due to their short generation time and close interaction with the environment, microorganisms respond quickly to environmental changes and disturbances. Changes in their abundance and activity can be used as indicators of soil condition and the qualitative changes occurring within it [30; 31; 32]. Plants obtain nutrients from the products of microbial activity because soil microorganisms constantly oxidize dead plant residues, converting them into nitrogen and mineral compounds that are available to plants and essential for their growth. Soil microorganisms are divided into main groups such as bacteria, actinomycetes, fungi, and algae. Actinomycetes are a large and widely distributed group of microorganisms in the soil, with their population estimated at 10\({}^{4}\)-10\({}^{8}\) individuals per gram of soil. They are particularly sensitive to low pH (optimal pH ranges from 6.5 to 8.0) and excessive soil moisture. The highest population of actinomycetes occurs in the surface layer of the soil, decreasing with depth. However, individual strains can be found in all soil layers [33]. Typical filamentous actinomycetes primarily belong to the genera Streptomyces and Micromonospora. They are responsible for recycling agricultural waste, utilizing carbon and nitrogen compounds such as cellulose, hemicellulose, proteins, and lignin [34]. Most actinomycetes are aerobic organisms, although some exhibit limited nitrate-reducing capabilities. In the process of organic matter decomposition, actinomycetes act at a later stage, after bacteria and fungi, which are the first to attack fresh organic residues. Actinomycetes are involved in the breakdown of more resistant, harder-to-decompose material, producing dark pigments that affect the color of humus. They are also responsible for the further decomposition of humus, contributing to the formation of more stable forms of organic matter in the soil. The Green Revolution contributed to the widespread use of mineral fertilizers, which significantly increased crop yields but also led to changes in soil microorganism communities. In the short term, the added nutrients stimulate microbial activity, particularly of bacteria, which quickly utilize them [35]. However, the intensive use of these resources can lead to rapid depletion of organic carbon in the soil, and consequently, a reduction in microbial diversity over the long term [36]. Such changes have significant implications for soil health. To reconcile the short-term benefits of fertilizer application with their long-term impact, it is necessary to precisely adjust nutrient doses and develop new technologies that support sustainable agriculture. Long-term use of mineral fertilizers significantly alters the structure of soil bacterial communities, affecting both their abundance and species diversity. Although initial fertilizer applications increase bacterial populations due to the added nutrients [37], species diversity often decreases. This reduction in diversity limits the soil's resilience to disturbances [38]. Changes also occur in the functional groups of bacteria.