Rice Genomics and Genetics 2024, Vol.15, No.4, 203-211 http://cropscipublisher.com/index.php/rgg 205 Soil organic nitrogen was fractionated using the modified Bremner method, which separates SON into different fractions such as amino acid N, ammonium N, amino sugar N, and hydrolyzable unknown (Wang et al., 2017). The soil aggregate was classified using the wet-sieving method, and light fraction (LF) and heavy fraction (HF) were classified according to density fractionation (Mukhi et al., 2022). This study used the flooded culture method to determine nitrogen mineralization, which helps to understand the availability of nitrogen for plant uptake over time (Wang et al., 2017) and measured soil total nitrogen (TN), organic carbon (C), and other chemical properties using standard laboratory techniques. For instance, the Elementar Vario ISOTOPE elemental analyzer was used to measure aggregate organic C (AC) and total N (AN) concentrations (Mukhi et al., 2022). Soil pH, organic matter, and alkali-hydrolyzed nitrogen concentrations were also determined (Wei et al., 2022). Microbial biomass C (MBC) and N (MBN) were measured to assess the microbial activity in the soil. Phospholipid fatty acids (PLFAs) were analyzed to determine the microbial community structure, focusing on fungi and bacteria (Schmidt-Rohr et al., 2004). Advanced solid-state NMR spectroscopy was used to detect nitrogen-bonded aromatics in soil organic matter (Wang et al., 2018). Non-parametric methods and principal component analysis (PCA) were used to compare carbon metabolism characteristics and distinguish soil chemical properties among different soil samples across seasons (Wei et al., 2022). Correlation analysis was performed to understand the relationships between organic C content and total N content in soil profiles. By employing these comprehensive methods, the study aimed to elucidate the long-term effects of rice cultivation on soil organic nitrogen dynamics, providing valuable insights into soil fertility and sustainable agricultural practices. 3 Effects of Long-term Rice Cultivation on Soil 3.1 Effects of rice cultivation on soil organic nitrogen content Long-term rice cultivation has been shown to significantly influence soil organic nitrogen (SON) content. Studies indicate that soil total nitrogen (TN) increases with the duration of rice cultivation, stabilizing after approximately 100 years (Wang et al., 2017). This increase in TN is attributed to the accumulation of various nitrogen fractions, including amino acid N, ammonium N, amino sugar N, and hydrolyzable unknown N, which exhibit exponential growth over time. Continuous rice-wheat cultivation without fertilization, however, results in a decrease in total soil nitrogen and its fractions, highlighting the importance of nutrient management practices (Kaur and Jp, 2018). The dynamic changes in organic nitrogen in paddy soils are influenced by both biotic and abiotic factors. Long-term application of organic manures combined with inorganic fertilizers has been shown to sustain high rice yields and improve soil chemical properties, including organic nitrogen content (Chen et al., 2017). The integration of organic and inorganic fertilizers enhances soil properties such as bulk density, soil porosity, soil organic carbon, and total nitrogen, which in turn improve nitrogen use efficiency and grain yield (Iqbal et al., 2019). 3.2 Effects of seasonal changes on soil organic nitrogen Seasonal variations also play a crucial role in the dynamics of soil organic nitrogen. During the rice growth stages, the response of nitrogen runoff loss and nitrogen variation in standing water is significantly influenced by soil properties and bacterial communities (Zhang et al., 2021). The tillering stage, in particular, shows notable differences in nitrogen loss due to variations in soil organic carbon, pH, and bacterial composition (Zhang et al., 2021). Interannual changes in soil organic nitrogen are evident in long-term studies. For instance, a 27-year field experiment demonstrated that partial replacement of mineral fertilizers with in situ crop residues maintained soil fertility and rice yields comparable to those achieved with full mineral fertilization (Chen et al., 2021). This approach not only sustained soil organic carbon and total nitrogen levels but also improved nutrient use efficiency and yield stability over the years (Chen et al., 2021).
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