Do reduced water and nitrogen input in rice production necessarily reduce yield?

Rice is an essential staple food crop for more than half of the world’s population (Xiong et al., 2013). China is the leading rice producer worldwide, and rice plays an important role in China’s grain production. Moreover, over 65% of China’s population consumes rice as their staple food (Zhang et al., 2005). Nitrogen (N) is an essential nutrient in crop growth and plays a decisive role in ensuring a high and stable crop yield (Erisman et al., 2008). Currently, the average N application rate for rice in China is 180 kg ha (Peng et al., 2010; Zhang et al., 2013; Fu et al., 2021). However, the N application rate reaches 350 kg ha in the high-yield Taihu Lake area (Jiao et al., 2018). The past two decades have witnessed increased N fertilizer use, promoting essential rice yield growth in China. Unfortunately, the excessive N input has also caused water eutrophication, soil acidification, reduced rice production efficiency, and other adverse effects (Xia et al., 2016; Townsend et al., 2003). Minimizing N application while avoiding yield reduction is thus a research hot spot in China. Taking into account the disadvantages of predominantly applying base fertilizer and low-efficiency tiller fertilizer in traditional rice production (Ling et al., 2014), most agricultural scientists and technological workers promote the split application of N fertilizer based on leaf age (Ling et al., 1983), site-specific N management based on soil testing (Roland et al., 2019; Ling et al., 2005), real-time N management based on the © 2022 Institute of Agrophysics, Polish Academy of Sciences CHUANHAI SHU et al. 48 relationship between leaf colour and N content (Mohanty et al., 2021), and computer-assisted model optimization to guide fertilization policy (Baral et al., 2021; Sharma S., 2019; Pan et al., 2017; Peng et al., 2002; Angus et al., 1996). These systems have greatly contributed to reduced N use and increased rice yield. Existing studies have shown that improvements in N application methods have more potential than the optimization of the N application rate to further increase rice yield and nitrogen use efficiency (NUE) (Yang et al., 2020; Chen et al., 2015). Increasing the number of N applications in paddy fields from 3 to 4 and 6 to 7 times can notably enhance NUE and achieve the goal of reducing N without affecting the yield. Nevertheless, increasing the frequency of N application can also add to operational costs and induce high water supply requirements, this has limited the promotion of low-intensity/high-frequency N application (Yang et al., 2020; Ohnishi et al., 1999). Rice requires more water than any other cereal grain, it accounts for approximately 60-70% of agricultural water use (Pan et al., 2017) and 50% of domestic water consumption. With increasing demands for industrial water and water for both urban and rural residents, the proportion of water allocated to rice production decreases each year. Therefore, researchers have also conducted many water-saving irrigation studies (Tabbal et al., 2002; Belder et al., 2004), including the application of alternate wetting and drying irrigation (AWD) and controlled irrigation (CI) technologies. AWD is a water management technique that employs periodic drying and rehydration to reduce water consumption during the rice-growing season (Wang et al., 2016). Most studies propose that AWD can be used to enhance rice yield (Christy et al., 2018; Pan et al., 2017; Carrijo et al., 2018); however, a reduced yield has also been reported. These inconsistent findings may be related to soil water potential, quality, and pH (Carrijo et al., 2017). Compared with AWD, CI involves the application of more rigid water management techniques (Peng, 2009; Yu et al., 2002). After the rice seedlings have been transplanted, the field surface retains a thin 5-25 mm layer of water to allow the rice seedlings to recover from transplantation stress. However, there is no water layer on the field surface during the stages of growth after recovery. The effectiveness of irrigation is determined by taking the soil moisture of the root layer as the control index. The lower limit of soil moisture at different rice growth stages is 60-80% of the saturated moisture content of the soil, while the upper limit is at the point of soil saturation. This technique, which can be used to promote the migration of N from surface water to soil and increase the water and nutrient uptake of rice plants (Peng et al., 2009), is popular in areas prone to water shortages such as Ningxia, Jiangsu, and Heilongjiang (Peng et al., 2011). Due to its huge water requirements, rice has a more significant water-fertilizer coupling effect than that of other crops, which necessitates human regulation. Hence, research concerning water-fertilizer coupling in paddy fields has attracted much academic attention (Liu, 2019; Lin et al., 2016). With the large-scale construction of highstandard farmlands and the rapidly increasing availability of low-cost water and fertilizer integration facilities, lowintensity/high-frequency N application in paddy fields has overcome its previous limitations. The three “uniform” technique is an integrated water and fertilizer technology developed to meet the water and N requirements of rice in paddy fields, with “uniform nitrogen application (UN)” and “uniform water with fertilizer application” at its core (Yang et al., 2020). This technology allows for greatly reduced N and water use in rice production, but few studies have focused on the mechanism of action underlying these savings. In order to fill this gap, UN was studied (low-intensity/ high-frequency N application) which employed integrated water and fertilizer technology to explore the impact of water and N management on rice yield and the utilization of water and N. This study aimed to provide theoretical and technical support for high-yield practices and the efficient utilization of resources in rice production. MATERIALS AND METHODS The experimental sites were located at the experimental farm of the Rice Research Institute of Sichuan Agricultural University (Wenjiang District, Chengdu City, Sichuan Province; 30°43’N, 103°47’E) and also at the experimental farm of the Southwest University of Science and Technology (Fucheng District, Mianyang City, Sichuan Province; 31°32’N, 104°41’E). The Wenjiang site was located in the Chengdu Plain, a subtropical humid monsoon climate zone, with abundant precipitation, less sunshine, and relatively minor diurnal temperature differences. The Fucheng site was located west of the Sichuan Basin, in the north subtropical humid monsoon mountain climate zone, with uneven precipitation distribution, sufficient sunshine, and large temperature differences occurring between day and night. In addition, drought frequently occurs during the rice season of this region. In 2016, field experiments were conducted at both sites (experiments 1 and 2). A third field experiment (experiment 3) was performed at the Wenjiang site in 2017. The nutrient content of the soil at the two experimental sites is listed in Table 1. The Wenjiang site had fine sandy loam soil, whereas the Fucheng site had clay loam soil. Indica hybrid rice F-you 498 was used as the testing material. This variety is a three-line super hybrid Indica rice planted in a large area in Sichuan in the middle and lower reaches of the Yangtze River. REDUCED WATER AND NITROGEN INPUT IN RICE PRODUCTION NOT REDUCE YIELD 49 All three experimental designs were identical: a randomized block experiment involving two factors. The primary block was water management, which was divided into flooding irrigation (FI) and controlled irrigation (CI). In FI, after the rice was transplanted, a 1-3 cm water layer was always maintained above the surface of the paddy fields and dried naturally a week before harvesting. In CI, transplantation was conducted in shallow water (~1 cm), a 2 cm water layer was maintained in the fields for 5-7 days after transplanting to ensure that the seedlings turned green and survived. Subsequently, the surface water was drained and soil moisture of 70-80% was maintained before the booting stage. The fields were dried during the ineffective tillering stage, a 1-3 cm water layer was maintained above the soil surface during the booting stage, and alternate wetting and drying irrigation was implemented from heading to maturity (i.e., irrigated with 1-3 cm layer of water and dried naturally to achieve a soil water potential of -25 kPa). The secondary block was N management, which was divided into CK, farmers’ usual nitrogen management (FU), optimized nitrogen treatment (ONT), and uniform nitrogen application (UN). In FU, 150 kg ha of N fertilizer was applied according to the ratio of base fertilizer:tillering fertilizer = 7:3, one day before and seven days after transplanting. In ONT, 150 kg ha of N fertilizer was applied according to the ratio of base fertilizer:tillering fertilizer:panicle fertilizer = 3:3:4, one day before and seven days after transplanting, and at the reciprocal fourth and second leaf stages (the panicle fertilizer was divided into two equal portions). In UN, 15, 15, 30, 15, 15, 15, and 15 kg (total 120 kg ha) of N fertilizer were applied at 7, 14, 35, 49, 56, 70, and 77 days after transplanting. There were 24 blocks in total, and each treatment was repeated three times. The plot area was 12 m (3 × 4 m), and the seedlings were transplanted at a hill spacing of 33.3 × 16.7 cm. There were 216 seedlings (12 rows and 18 seedlings per row) in each plot, and the planting density was 18 plants m. The extent of the irrigation was measured using a water meter, and all other field management practices were identical. Light interception (LI): During the heading stage and 10, 20, and 30 days after the heading stage, the effective solar radiation at the top (30 cm above the flag leaf tip) and base (10 cm from the ground) of the plant were measured using an Li-191 lig