Potential of paddy drainage optimization to water and food security in China

Highlights

• Characteristics of water and fertilizer use, and nitrogen loss of paddy fields were investigated.

• Optimal drainage management was determined considering rice growth and precipitation conditions.

• Drainage optimization ensured rice yields and mitigated nitrogen runoff loss.

Abstract

Rice production not only consumes large amounts of irrigation water and fertilizer, but also poses a high risk of water pollution by delivering nitrogen (N) through surface runoff. To ensure sustainable rice production, many water-saving irrigation managements have been proposed and implemented, but drainage water managements receive far less attention and need to be further explored. This study aimed to determine the paddy drainage optimization management and assess its potential to water and food security in China via different scale methods (from pot and field experiments to national assessment). The national investigation of water and N fertilizer use in paddy fields implied that diffuse N pollution was expected to continue increasing, especially in the Yangtze river basin. Two-years field experiments at typical sites identified that the tillering and jointing–booting stages were critical risk stages for N runoff loss, and pot experiments on the critical stages were conducted to determine the optimal drainage water level without yield reduction. Then, the applicability of paddy drainage optimization was verified and evaluated by drainage optimization field experiment and precipitation characteristics analysis. Finally, the potential of drainage optimization on mitigating N runoff loss was estimated by scenario analysis at the national scale. After implementing paddy drainage optimization in field experiments, surface runoff and nitrogen runoff loss decreased by 27.97–78.94% and 35.17–67.95%, respectively, without affecting rice yield. By full implementation of the optimal drainage and fertilization management, N runoff loss could be reduced by 0.19 Tg yr⁻¹ at the national scale. These results suggest that paddy drainage optimization is an agro-ecosystems friendly water management for sustainable rice production, and has notable potential to ensure water and food security in China.

Introduction

Ensuring water and food security is the central concern of sustainable agricultural development across the world (Tian et al., 2021). Tensions between water resources shortage and water quality deterioration foster the urgent demand for effective agricultural water managements (Huang et al., 2020; Zhang et al., 2020; Wang et al., 2019). As an important crop that feeds more than 50% the world's population, paddy rice is excessively irrigated and fertilized, which leads to low water and fertilizer use efficiency and serious nutrient losses through surface runoff (Mekonnen et al., 2015; Liu et al., 2020). Specially, nitrogen (N) runoff loss has been highly concerned because N is a vital nutrient for rice growth and a critical cause of water deterioration (Fu et al., 2021; Bowles et al., 2018). Paddy fields are usually blocked by levees to maintain a ponding water level (i.e., waterlogging depth) at 20–50 mm during most rice growing stages (Maruyama et al., 2017; June et al., 2012). Surface runoff happens when rainfall exceeds the field water storage capacity, which depends on the elevation difference between the ponding and drainage water levels (Hitomi et al., 2010; Liu et al., 2021).The most common approach to increase the field water storage capacity is to lower the ponding water level by water-saving irrigation managements, which have been extensively studied and adopted in rice producing countries (Folberth et al., 2020; Zhuang et al., 2019; Sriphirom et al., 2019; Samoy-Pascual et al., 2019). However, the other approach, namely, improving the drainage water level by drainage water management, receives far less attention. Therefore, appropriate drainage optimization management needs to be further explored because it can not only store rainfall water in paddy fields to save irrigation water but also alleviate surface drainage to reduce N runoff loss (Lu et al., 2016; June et al., 2012).

Compared with conventional drainage, total N (TN) loss can be reduced by 59.6–95.6% after increasing the drainage water level (Lu et al., 2016). Surface runoff and phosphorus loss can be reduced by 8.1–8.7 mm and 1.2–3.4 kg/ha, respectively, for every 10 mm increase in the drainage water level (Hitomi et al., 2010). Zero-drainage management, which means that no artificial or natural drainage occurs during rice-growing season, is suggested to avoid water and nutrient loss (Hua et al., 2019; Zhang et al., 2007). One the one hand, increasing the drainage water level can improve water use efficiency by storing irrigation water and rainfall in paddy fields. On the other hand, increasing the drainage water level inappropriately may potentially pose risks to rice growth. Because when the ponding water level is too high, waterlogging disasters may happen, which can lead to plant growth retardation and yield reduction (Chen et al., 2020). Previous studies reported that long-duration waterlogging or complete submergence at depths greater than 250–300 mm adversely affected rice yield by reducing the tillers number and increasing plant lodging (Singh et al., 2017; Ismail et al., 2013). Therefore, identifying the optimal drainage water level that does not cause rice yield reduction is critical to determine the appropriate drainage optimization management.

The optimal drainage water level is mainly dependent on the waterlogging tolerance of rice, which is affected by the waterlogging depth (i.e., the ponding water level), waterlogging duration and waterlogging stage (i.e. rice growing stages; Anbumozhi et al., 1988). The influences of waterlogging depth, waterlogging duration and the combination of these two factors at different rice growing stages have been assessed (Singh et al., 2011; Gautam et al., 2014; Kato et al., 2014; Kotera and Nawata., 2007). However, previous studies mainly focus on the physiological and morphological responses of rice (e.g., plant height, tillers and yields) under a large range of waterlogging depths (5 –100 cm) and durations (2 days–the entire growth period). The suggested drainage water level was based on a relatively high yield reduction (10–20%; Wang et al., 2014; Yan et al., 2017). Few researchers have identified the optimal drainage water level for different rice growing stages without significantly affecting rice yield and comprehensively analyzed its potential for mitigating N runoff loss (Wang et al., 2020; Shao et al., 2014).

Rice production consumes 65% of total agricultural water use and 14% of the total fertilizer use in China, which poses a large risk on water pollution (Zhuang et al., 2019; Fu et al., 2021). Therefore, identifying the optimal drainage water level for paddy fields would be very valuable for the sustainable rice production in China. Given this foundation, the goal of this study was to determine the paddy drainage optimization management and assess its potential to water and food security in China via different scale methods (from pot and field experiments to national assessment). First, we investigated the long-term water and fertilizer use in paddy fields at the national scale and chose the most important rice area, the Yangtze river basin, as the main study area. Then, a series of pot and field experiments, and historical precipitation analysis were conducted at four typical sites to identify the optimal drainage water level for critical risk stages without yield reduction and assess the applicability of paddy drainage optimization. Finally, the experiments results were extrapolated to the main rice planting regions in China based on the consideration of the response of rice yield to waterlogging at the critical risk stages and precipitation characteristics. The mitigation potential of paddy drainage optimization for mitigating N runoff loss was estimated at the national scale. Research results are expected to provide significant guidelines for drainage water management for sustainable rice production.