Molecular Soil Biology 2025, Vol.16, No.4, 162-174 http://bioscipublisher.com/index.php/msb 164 In contrast, developing countries often face challenges related to over-reliance on conventional nitrogen fertilizers and lack of access to advanced technologies. In regions like China, where rice is a staple crop, farmers tend to apply excessive amounts of nitrogen fertilizers to ensure high yields, leading to significant environmental pollution and economic inefficiencies (Hu et al., 2023). Efforts are being made to promote balanced fertilization and integrated nutrient management practices, but widespread adoption remains limited due to socio-economic barriers and lack of awareness (Qiu et al., 2022; Shrestha et al., 2022). Addressing these challenges requires concerted efforts to educate farmers, improve access to advanced fertilization technologies, and implement policies that encourage sustainable nitrogen use. 2.3 Environmental impacts of excess nitrogen The excessive use of nitrogen fertilizers in rice farming has profound environmental impacts, including soil degradation, water contamination, and greenhouse gas emissions. High rates of nitrogen application can lead to the accumulation of nitrates in the soil, which can subsequently leach into groundwater, posing risks to human health and aquatic ecosystems. Additionally, nitrogen runoff from rice fields can contribute to the eutrophication of water bodies, leading to algal blooms and the depletion of oxygen levels, which adversely affect aquatic life (Qiu et al., 2022; Shrestha et al., 2022). Moreover, the volatilization of ammonia and the emission of nitrous oxide (N2O) from rice fields are significant contributors to air pollution and climate change. Nitrous oxide is a potent greenhouse gas with a global warming potential approximately 300 times that of carbon dioxide. Studies have shown that conventional nitrogen fertilization methods, particularly surface broadcasting, result in higher N2O emissions compared to more efficient practices such as deep placement of nitrogen fertilizers (Baral et al., 2020; Li et al., 2021). Therefore, improving NUE and adopting environmentally friendly fertilization practices are critical for mitigating the adverse environmental impacts of nitrogen use in rice farming. 3 Genetic Approaches to Enhancing NUE in Rice 3.1 Genetic variability for NUE Identification of genetic variation in rice varieties for nitrogen uptake and utilization is crucial for improving nitrogen use efficiency (NUE). Studies have shown significant genetic variability in NUE among different rice varieties, particularly in rainfed upland conditions. For instance, research conducted in Madagascar with 13 tropical japonica rice varieties revealed substantial genetic variability for NUE, nitrogen uptake efficiency (NUPE), and nitrogen utilization efficiency (NUTE) under both high and low nitrogen conditions (Rakotoson et al., 2017). This variability is essential for breeding programs aimed at enhancing NUE, as it provides a pool of genetic resources that can be exploited to develop rice varieties with superior nitrogen uptake and utilization capabilities. Moreover, the genetic variability in NUE is influenced by environmental factors such as rainfall distribution, which affects nitrogen availability and uptake. The study in Madagascar highlighted the significant Year ×N and Year × G interactions due to varying rainfall patterns across different cropping seasons, further emphasizing the complexity of NUE as a trait (Rakotoson et al., 2017). Understanding these interactions is vital for developing rice varieties that can maintain high NUE under diverse environmental conditions. 3.2 Key genes and pathways involved in NUE The role of nitrate transporters, nitrogen assimilation enzymes, and regulatory genes is pivotal in enhancing NUE in rice. Nitrate transporters such as OsNPF6.1 have been identified as key players in nitrate uptake and NUE. A genome-wide association study identified an elite haplotype of OsNPF6.1, which enhances nitrate uptake and confers high NUE by increasing yield under low nitrogen supply (Tang et al., 2019). This transporter is differentially trans-activated by the transcription factor OsNAC42, highlighting the importance of regulatory networks in NUE. Additionally, nitrogen assimilation enzymes and regulatory genes such as OsNLP1 and OsNLP3 play crucial roles in NUE. OsNLP1 rapidly responds to nitrogen deficiency and improves yield and NUE by regulating multiple
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