Rice is a typical ammonium-loving crop. Lots of studies had demonstrated that rice could grow better and gain higher yield under ammonium-source fertilizer application under conventional flood environment (Britto et al. 2001; Guo et al. 2007a; Tabuchi et al. 2007; Kronzucker et al. 2010). In our study, the rice also gained a higher grain yield under urea application compared with nitrate application under flood treatment (Table 1). However, under drought treatment, grain yield was higher for nitrate application compared with urea application. As biomass was invariable between nitrate and urea as sources of N under drought treatment, partitioning of dry matter appeared to contribute to higher grain yield when nitrate was used as a source of N (Table 1), which meant the nitrate application improved the assimilate productivity of the rice plant under drought treatment compared with urea application. This is mainly due to the interactions between N sources and water treatments.
Both N sources and soil water status would affect the root growth of the plant. In our study, after the two-stage progressive drought treatment, we found the rice plant had a larger root system for nitrate application compared with urea application (Table 3). Some studies also indicated that nitrate nutrition would stimulate root growth of the plant under different soil water status. For example, enriched supply of nitrate was found to promote the lateral root elongation (Zhang and Forde. 1998; Zhang and Forde. 2000). And Ogawa et al. (2014) and Li et al. (2007) found that some rice lines had higher root length and higher root biomass under nitrate nutrition than under ammonium nutrition. Besides, Li et al. (2008) reported that the nitrate-ammonium mixed nutrition increased the root length, root surface area and root density compared with ammonium nutrition. However, different results were found when scientists subjected rice seedlings to polyethylene glycol-induced drought stress using hydroponic culture. Gao et al. (2010) demonstrated that rice seedlings supplied with ammonium nutrition had increased numbers of root tips and a larger root surface area compared with nitrate nutrition. And Ding et al. (2015) found that drought stress decreased the root elongation rate in rice seedlings when they were supplied with nitrate nutrition. These results indicated that the effect of N sources on rice root phenotypic traits were different, which depending on the methods of N sources and water treatments. Although various effects of N sources on root phenotypic traits were observed, the consensus in previous studies was that the nitrate nutrition would decrease the root activity of rice plant compared with ammonium nutrition (Li et al. 2009; Gao et al. 2010; Yang et al. 2012; Ding et al. 2015). Rice is an ammonia-loving crop that easily absorbs ammonia-nitrogen, and nitrate will reduce its root activity. Recent studies showed that the regulation mechanism of ammonia and nitrate nitrogen on rice root activity is closely related to pH and microorganism in the rhizosphere (Chen et al. 2017; Oldroy et al. 2020). In our study, to get as close to the actual condition as possible, we set the water disruption-induced drought treatment using soil culture and took the urea application as the control. What is more, instead of a period N sources treatment, multiple nitrate and urea applications during the rice growth duration were proceed in our study. Consistent with the previous studies, the root activity of rice plant for nitrate application was lower compared with urea application in our study (Fig. 5). Our results indicated that the nitrate application in the paddy soil obviously stimulated root growth of the rice plant compared with urea application, but did not improve the root activity.
Compared with the various results of effects of N sources on root growth of rice plant in the previous studies, the effects of N sources on abovegroud plant growth were relatively consistent. All the previous evidences pointed that nitrate nutrition would inhibite the aboveground plant growth of rice compared with ammonium nutrition (Lin et al. 2005; Guo et al. 2007a; Li et al. 2009). Similar phenomena was observed in our study. Whether under flood or drought treatment, the aboveground rice plant for nitrate application was smaller compared with urea application (Table 2). Moreover, our results also showed that the aboveground plant was not only smaller but also more compact for nitrate application, which was reflected by the angle between the canopy leaves and the stem became smaller (Fig. 3). A smaller and more compact plant was proved to have lower water demand and transpiration (Kondo et al. 2004; Giuliani and Edwards. 2013). Our results further revealed the rice plant for nitrate application had higher leaf water potential after the two-stage progressive drought treatment.
Compared with solution, soil has a stronger buffering effect on fertilizer application (Nye. 1966; Mendoza. 1989). In our study, nitrate application significantly increased the concentration of NO3−-N in the soil (Fig. 1). However, there was still a certain amount of NH4+-N in the soil. Our result also showed that the drought treatment would also increase the ratio of NO3−-N to NH4+-N in the soil, which is the same as previous studies (Hartmann et al. 2013; Tran et al. 2015). The available N nutrition in the soil was a nitrate–ammonium mixed nutrition. Some studies had checked the effects of nitrate–ammonium mixed nutrition (different ratios of nitrate to ammonium) on rice plant. Other previous studies indicated that the rice plant usually grew better under nitrate–ammonium mixed nutrition than under sole nitrate nutrition (Guo et al. 2007a; Guo et al. 2008). In our study, we did not combine the nitrate and urea nutrition source by ratios. Therefore, investigating the combined effects of nitrate nitrogen and urea on rice plants under drought stress is an important direction of the next step. Beyond that, the nitrate used in our study was Ca(NO3)2, which could increase the Ca2+ concentration in the soil. The intracellular Ca2+ has been found to regulate the responses of the plant to drought stress (Sanders et al. 1999; Saijo et al. 2000). Therefore, we could not rule out the possibility that both N source and Ca2+ affected the plant growth under drought stress in our study. For the soil water status, though the water supply was suspended at the same time and lasted for the same time for nitrate application and urea application under drought treatment. The soil water potential decreased slower for nitrate application compared with urea applicattion (Fig. 2). These indicated that the plants for nitrate application and urea application under drought treatment were actually facing different ratios of NO3−-N to NH4+-N nutrition and different degrees of drought stresses.