Optimizing Irrigation and Nitrogen Regimes in Rice Plants Can Contribute to Achieving Sustainable Rice Productivity

1. Introduction

Ensuring Food security is increasingly challenging each year as the global population is projected to be 9.7 billion in 2050 and 10.4 billion by 2100 [1]. The situation has been worsened by urbanization, land degradation, and the rising scarcity of water resources and environmental risks. Rice (Oryza sativa L.), a vital food crop worldwide and a staple in Asia, holds immense significance. In order to meet the increasing food demands of a growing global population, it will be crucial to enhance rice production by 70–90% by 2050 compared to the levels in 2010 [2,3]. The grain yield of rice in China has experienced significant growth, rising from 2.1 t ha⁻¹ in 1950 to 6.8 t ha⁻¹ in 2020. This increase can largely be attributed to breeding techniques and the adoption of dwarf, lodging-resistant, and fertilizer-tolerant varieties, as well as the increased utilization of chemical fertilizers, pesticides, and irrigation systems. However, it is important to note that this growth has been accompanied by the prevalence of high-input-intensive agriculture, characterized by high input, output, and pollution but low efficiency [4].

Rice is the biggest user of water in agriculture, specifically in Asia, where it consumes approximately 80% of the total freshwater resources allocated for irrigation [5,6,7]. Furthermore, water scarcity and low water use efficiency (WUE) pose significant challenges in rice production [6,8]. Over the past two decades, an estimated 1.6–2.0 million hectares of rice cultivation in China have been affected by seasonal droughts each year due to inadequate water resources [9,10]. The situation is expected to worsen in the future, particularly with the increasing global demand for food. Enhancing water productivity through technological advancements is crucial in tackling the challenges of food security and water shortage.

Nitrogen (N) is a crucial nutrient for plants, needed in large quantities for their growth and development. However, excessive application of nitrogen by producers worldwide has resulted in low nitrogen use efficiency (NUE). Studies showed that the nitrogen use efficiency for cereals was 35% globally, 41% in the United States, 30% in China, and 21% in India [11]. This suggests that a significant portion of the applied nitrogen is not effectively utilized and can potentially be lost within the ecosystem through leaching or in gaseous forms. Such losses can have detrimental effects on the environment and human health [12,13].

Water and nitrogen are the two key factors that significantly impact crop productivity [14]. The main questions that arise are whether an increase in rice yield is solely dependent on higher amounts of water and fertilizers and if it is possible to enhance crop yield while simultaneously improving water and fertilizer use efficiencies.

The yield of rice is the product of two factors: harvest index (HI) and aboveground biomass (B). This can be represented by the equation: grain yield (Y) = B × HI [15]. HI is not only an independent variable in the equation, but it also indicates the efficiency of converting dry matter into grain production. HI is a variable factor in crop production, influenced by crop management practices [16,17]. Effective water and nitrogen management can boost the growth rate and translocation of nutrients to grains during grain development, leading to a higher HI [18,19,20]. By enhancing HI, it is possible to simultaneously improve crop yield and the efficient utilization of water and nutrients.

2. Water Use Efficiency in Rice

Water is crucial for many physiological processes in plants. It helps to maintain cell turgor, ensure proper solute concentrations for metabolic activity, transport nutrients and phytohormones from roots to shoots, cool leaves through evaporation, and facilitate the movement of sucrose to non-photosynthetic organs. However, most of the water absorbed by plants is lost through transpiration and evaporation, with only a small portion utilized for carbohydrate synthesis and plant tissue composition. Water use efficiency (WUE) refers to the ratio of grain yield (Y) to the amount of water transpired (Tr); that is, WUE = Y/Tr. As Y = HI × B, the water productivity can be further defined as WUE = HI × B/Tr. The B/Tr is commonly referred to as transpiration efficiency. To enhance water productivity, there are two main approaches: increasing transpiration efficiency or increasing the harvest index. For a specific species in a particular climate, B/Tr tends to remain relatively constant [21]. For rice, the value of B/Tr is ~1.5 g kg⁻¹ [15].

There is a generally observed linear relationship between plant biomass production and the amount of water that transpired. WUE could enhanced, but it often results in a trade-off with lower biomass production. The question that arises is the possibility of simultaneously increasing food production and conserving water by manipulating HI in crop production. Water-saving regimes have been implemented to reduce water usage. These include techniques such as aerobic rice systems, the system of rice intensification (SRI), alternate wetting and soil drying (AWD) irrigation, controlled soil drying during grain filling, and non-flooded mulching cultivation. These techniques have the potential to significantly improve WUE and maintain or even increase rice grain yield. These approaches mainly work by improving canopy structure, optimizing source and sink activities, and enhancing carbon remobilization, ultimately leading to an improved HI [22].

3. Nitrogen Use Efficiency in Rice

3.1. Nitrogen Transformation and Nitrogen Losses in the Paddy Rice Field

In paddy rice fields, nitrogen-based fertilizers, mainly urea, are commonly used to fertilize rice plants. Upon application, urea undergoes hydrolysis, resulting in the formation of ammonium (NH₄⁺), which is then converted to nitrate (NO₃⁻) in the oxygen-rich soil surrounding the rice roots. Soil particle surfaces typically carry a negative charge, which allows them to attract cations such as NH₄⁺ through electrostatic forces. NO₃⁻ is an anion and generally does not bind to soil particles, allowing it to easily leach through soil layers and reach groundwater. Its diffusion capacity is significantly higher (10–100 times) compared to NH₄⁺ in the soil. This mobility allows nitrate to move downwards into oxygen-depleted soil, where it can undergo processes such as denitrification.

The process of nitrification converts NH₄⁺ into hydroxylamine (NH₂OH), nitrite (NO₂⁻), and NO₃⁻. Denitrification, on the other hand, involves the reduction of NO₃⁻ back to NO₂⁻, nitric oxide (NO), nitrous oxide (N₂O), and finally N₂ by various microorganisms like fungi, bacteria, and archaea. It is important to note that the partial reduction of NO₃⁻ to N₂O can have adverse effects on the environment as N₂O is a potent greenhouse gas. Additionally, N₂O is a significant contributor to ozone depletion. Under anaerobic or low-oxygen conditions, there are alternative processes, such as dissimilatory nitrate reduction to ammonia (DNRA), where NO₃⁻ is reduced to NH₃ via NO₂⁻. In anammox, bacteria convert NO₂⁻ and NH₃ into N₂ through intermediates like NO and N₂H₄ [23,24].

Nitrogen mineralization refers to the process where microbial activities convert organic nitrogen into inorganic nitrogen form, primarily NH₄⁺. The rate of mineralization is limited by the depolymerization of organic nitrogen, which releases biologically available organic nitrogen compounds like amino acids. Both plants and microorganisms can take up amino acids [25], but the majority of the amino acid nitrogen is subsequently converted into inorganic nitrogen forms and absorbed by plants [26]. This process is essential for making organic nitrogen available for plant growth and nutrient cycling in ecosystems.

Apart from nitrogen being taken up by crop plants, a significant amount of nitrogen is immobilized by soil microorganisms. This process leads to the storage of nitrogen in soil organic matter pools, effectively retaining otherwise mobile nitrogen within soil particles. This retained nitrogen can later become available through mineralization in subsequent crop cycles. This immobilization and subsequent mineralization play a crucial role in the long-term cycling and availability of nitrogen in agricultural systems [27].

Nitrogen losses from paddy rice fields include volatilization of ammonia gas (10–50% of applied fertilizer-N). Nitrification, leaching, and runoff of NO₃⁻ contribute to a similar extent of losses (5.6–50%). Smaller losses (0.03–0.68%) occur through denitrification as N₂, N₂O, and NO_x emissions [28].

3.2. The Physiological Basis of Nitrogen Use Efficiency

The nitrogen use efficiency (NUE) refers to the amount of grain yield obtained per unit of nitrogen applied. Enhancing NUE can reduce excess nitrogen application and limit environmental pollution [29]. Strategies like agronomic, physiological, and crop management approaches can be employed to improve NUE, minimizing nitrogen losses while maximizing crop yield. Generally, NUE is constrained by two physiological processes: the plant’s ability to uptake nitrogen and the physiological efficiency of the absorbed nitrogen.

The plant’s nitrogen uptake ability, known as nitrogen recovery efficiency, refers to the percentage of applied nitrogen fertilizer absorbed by plants. Various approaches to enhance apparent recovery efficiency have been extensively discussed in the literature [12,30]. These approaches include the following: (a) Selecting genotypes with enhanced nitrogen uptake capabilities; (b) Developing decision support tools to provide accurate information on crop and soil nitrogen status; (c) Optimizing the placement, timing, and formulation of nitrogen fertilizer in paddy rice fields to ensure proper temporal and spatial delivery of nitrogen; (d) Improving soil quality, such as increasing organic matter content to enhance the soil’s nutrient retention capacity.

The physiological efficiency of absorbed nitrogen (Np), also known as internal nitrogen use efficiency, is the grain yield (Y) divided by the amount of nitrogen uptaken by above-ground plants (Nr), expressed as Np = Y/Nr = HI × B/Nr. The ratio of B/Nr represents nutrient production efficiency, indicating the dry biomass produced per absorbed nitrogen. It tends to be relatively consistent when N rates are similar for a specific rice variety in the same climatic environment [15]. For example, the B/Nr is around 100 g g⁻¹ and 110 g g⁻¹ for the japonica and indica rice variety, respectively, when the N rate is 180–300 kg ha⁻¹ in the rice fields of East China [31]. Increasing NUE (NUEg, internal N use efficiency) in cereals requires varieties with high HI, which is commonly recognized by agronomists [15].

3.3. The Approaches to Increase NUE

Genetic variations in nitrogen use efficiency (NUE) within rice germplasms offer a promising solution for breeding high NUE varieties. Notably, indica rice subspecies exhibit 30–40% higher NUE compared to japonica rice. Recent research has identified specific genes, such as NRT1.1B, ARE1, and NR2, that contribute to the divergence in NUE between these two rice subspecies [13]. However, further studies are necessary to explore and understand the candidate genes responsible for this NUE variation.

The mismatch between nitrogen availability in the soil and the nitrogen needs of rice plants is a major contributing factor to nitrogen losses in cropping systems. Better prediction of rice nitrogen needs is crucial, as it helps assist farmers with better decisions regarding the timing and quantity of nitrogen fertilizer application. Multiple methods, such as the use of chlorophyll meters (SPAD), have been developed for this purpose. For example, in site-specific nutrient management (SSNM), the topdressing N rate is adjusted either upward or downward based on the SPAD threshold values [32]. However, SPAD value varies with genotypes, environmental factors, and leaf characteristics [33,34]. An optimized strategy of nitrogen-split application based on relative SPAD value (RSPAD) due to the leaf positional differences in chlorophyll meter readings was developed and significantly improved NUE [34].

Modern agricultural technologies can reduce the mismatch between nitrogen availability and crop needs by applying fertilizer at variable rates based on crop growth. On-the-go fertilization, which adjusts nitrogen application rates in real-time using plant spectral reflectance, is one such technology that enables precise nitrogen management in agriculture [35].

Fertilizer placement, application method, and formulation are important factors [30,36]. Traditional broadcast application of nitrogen fertilizer can cause environmental losses. Deep placement fertilization conserves nitrogen in the rhizosphere of crops [37]. Controlled-release fertilizers, in pelletized form with a coating, slowly release nitrogen, reducing losses and labor costs compared to conventional urea-N fertilizer splitting [38].

In addition to N management, water management, such as AWD irrigation in rice cultivation, has a significant impact on NUE. Moderate AWD regimes increase total nitrogen uptake and NUE by improving HI and photosynthetic NUE. Higher HI can save N for grain yield rather than investing in vegetative growth, while higher photosynthetic NUE reduces nitrogen investment in non-photosynthetic components, leading to improved NUE [22,39].

4. The Interactions of Water and Nitrogen in Plants: New Paths toward Sustainable Crop Production

Increasing global scarcity of water will profoundly compromise crop productivity and also impact the way in which N fertilizer is accessed and taken by plants [14]. The presence of water in the soil directly impacts the availability of nitrogen and their efficiency. It affects nitrogen loss and transformation, including volatilization, nitrification, and/or urease hydrolysis. The plant’s access to nitrate and ammonium is also affected by the amount of water available in the soil, as soil water contents play a role in altering the soil’s anaerobic and aerobic conditions, which affect the conversion between nitrate (NO₃⁻) and ammonium (NH₄⁺). This, in turn, has an impact on plant growth. The coexistence of NO₃⁻ and NH₄⁺ is widely recognized to stimulate plant development and growth. Nitrate not only serves as a nitrogen nutrient for plants but also functions as a crucial signal for optimizing rice plant development [40].

It is hypothesized that proper management of soil moisture and nitrogen fertilizer can potentially enhance crop growth, yield, WUE, and NUE through a synergistic effect [41]. For example, synergistic water–N interaction could be achieved by implementing an AWD irrigation regime (plants were rewatered at soil water potential of −15 kPa) with a moderate N rate (200 kg N ha⁻¹) [42]. Combining SSNM and AWD techniques, known as SSNM-AWD, considerably boosts grain yield, NUE, and WUE compared to using SSNM or AWD alone [43,44]. Fertigation is a modern agricultural practice that combines irrigation water and fertilizer, delivering it directly to the crop roots. This technique saves water, reduces fertilizer loss, and improves crop yield and quality [45]. However, the evidence is still very scarce, and the ways and the underlying mechanisms to realize the synergistic interaction between water and N on crop growth are yet to be explored.