Overuse of nitrogen (N) fertilizer has led to low N use efficiency (NUE) and high N loss in single rice cropping systems in southeast China. Application of controlled-release urea (CRU) is

considered as an effective N fertilizer practice for improving crop yields and NUE. Here, field experiments were conducted during 2015–2017 to assess the effects of two CRUs (resin-coated

urea (RCU) and polyurethane-coated urea (PCU)) on rice yields, NUE and soil fertility at two sites (Lincheng town (LC) and Xintang town (XT)). Four treatments were established at each site:

(1) control with no N application (CK), (2) split application of conventional urea (U, 270 kg N ha−1), (3) single basal application of RCU (RCU, 216 kg N ha−1), and (4) single basal

observed between the U and CRU treatments at either site. There were no significant differences in the N apparent recovery efficiency (NARE) among the U, RCU and PCU treatments, with the

exception of that in XT in 2015. Compared to application of U, application of CRU increased the N agronomic efficiency (NAE) and N partial factor productivity (NPFP) by 17.4–52.6% and

23.4–29.8% at the LC site, and 15.0–84.1% and 23.2–33.4% at the XT site, respectively, during 2015–2017. Yield component analysis revealed that greater rice grain yield in response to N

fertilizer was attributed mainly to the number of panicles per m2, which increased in the fertilized treatments compared to the CK treatment. The application of CRU did not affect the soil

fertility after rice harvest in 2016. Overall, these results suggest that single basal application of CRU constitutes a promising alternative N management practice for reducing N application

rates, time- and labor-consuming in rice production in southeast China.

Rice (Oryza sativa L.) is one of the most important food crops in China and plays a vital role in guaranteeing national food security1,2,3. China is the largest rice producer worldwide and

produces nearly 30% of the rice produced globally4,5. In 2016, 207 million tons of rice were produced in China, accounting for more than 30% of total food production6. Nitrogen (N)

fertilizer plays an indispensable role in improving rice yield and quality7. More than 4 Tg per year of N fertilizer was applied for rice production in China from 2001 to 20108. However,

increases in crop yields are not linearly correlated with increases in the application rate of N fertilizer, which inevitably results in deceased the N use efficiency (NUE) and increased N

losses9,10. Excessive use of N fertilizer has resulted in a series of environmental issues, such as surface water eutrophication, groundwater pollution, greenhouse gas emission and soil

acidification11,12,13,14. Therefore, efficient N management is a crucial approach for increasing the NUE while minimizing environmental pollution in rice agro-ecosystems.

Various approaches to N management, such as multiple split application and deep placement, can improve both rice yields and NUE and reduce N losses10,15,16,17. However, these practices

require more labor and knowledge of N management than do conventional practices or are limited by technology lag. Application of controlled-release urea (CRU) constitutes an effective

practice for increasing crop yields and NUE while reducing N loss via ammonia volatilization (AV), nitrous oxide (N2O) production, surface runoff and leaching12,18,19,20. Numerous studies

have shown that the application of CRU significantly increased rice grain yield and NUE compared to the application of traditional N fertilizer7,21,22,23. The results of a field experiment

conducted by Li et al.20 showed that CRU significantly increased the grain yield of late rice and the apparent N recovery by 6–18% and 3–17%, respectively, compared to urea application at

the same N rate. A review by Chalk et al.18 showed that the recovery of fertilizer N was higher, and the unaccounted N loss lower in response to a 15N-labeled CRU treatment than in response

to a 15N-labeled urea treatment. In addition, several studies have reported that CRU could significantly reduce N2O, nitric oxide (NO) and methane (CH4) emissions from paddy soils compared

to conventional urea20,24,25. Resin-coated urea (RCU) and polyurethane-coated urea (PCU) are two kinds of CRU that can increase both crop yields and NUE and reduce N loss7,20,23,26,27.

With respect to rice production, split application of N fertilizer is usually recommended for improving NUE and crop yields28. Compared to single application of urea, split application can

significantly increase rice grain yields and NUE, but other studies have reported no significant difference in rice grain yield between a one-time application of urea and a split application

of urea in central China20. Split application of N is more time consuming and labor intensive than is a single basal application28,29. In China, many rice farmers have part-time jobs in

cities, which leads to limited time and labor for agriculture30,31. Furthermore, it is difficult for farmers to master the proper time of application and amount of topdressing of N

fertilizer32. CRU is helpful for decreasing the use of N fertilizer and saving time and labor inputs20,29. Many studies have shown that CRU can be applied once as basal fertilizer with no

effect on rice grain yields7.

NUE can be sub-classified in terms of N apparent recovery efficiency (NARE), N agronomic efficiency (NAE) and N partial factor productivity (NPFP)33. The NARE is used to describe the uptake

efficiency of N fertilizer. The grain yield increase per unit N fertilizer applied is expressed by the NAE, and the NPFP represents the use efficiency of soil N and fertilizer N. Dobermann34

proposed that the recommended NARE, NAE and NPFP for good management are 50–80%, 25–30 kg kg−1 and 60–80 kg kg−1, respectively. The average values of NARE, NAE and NPFP of rice in China are

39.3%, 12.6 kg kg−1 and 48.6 kg kg−1, respectively1. Many studies have shown that NUE is much lower in China than in developed countries35,36,37.

The middle and lower Yangtze River (MLYR) basin is one of the most important agricultural regions in China38. Rice–wheat rotation, rice–rapeseed rotation and early rice–late rice rotation

compose the main cropping systems in the middle and lower reaches of the Yangtze River38. In this region, the application rate of N fertilizer ranges from 200 to 300 kg ha−1, whereas the

rice grain yield is only 6.7–7.6 t ha−1 39. A review by Che et al.1 demonstrated that the NARE, NAE and NPFP of rice were 35–44%, 10–15 kg kg−1 and 29–68 kg kg−1 in the MLYR basin,

respectively.

Thus, the objective of the present study was to compare the effects of CRU and U on rice yield, NUE and soil fertility from 2015 to 2017 in the MLYR basin, China. We hypothesized that

application of CRU would increase the grain yield and NUE of rice and would not affect soil fertility.

The monthly temperature and rainfall were recorded from rice transplantation to harvest during the 2015 to 2017 rice growing seasons at the two sites (Fig. 1). The average temperatures were

24.2 (19.2–26.3 °C), 25.5 °C (20.7–28.9 °C) and 25.3 °C (17.6–31.4 °C) in LC and 24.1 (18.9–27.4 °C), 25.4 °C (20.5–29.6 °C) and 25.4 °C (18.1–31.6 °C) in XT during the 2015 to 2017 rice

growing seasons, respectively. The total rainfall during the rice growing season was 820, 1207 and 570 mm at the LC site, and 871, 1237 and 595 mm at the XT site, in 2015, 2016 and 2017,

respectively (Fig. 1). The rainfall data during the rice growing season was 710 mm in long-term averages at both sites, accounting for 55% of total rainfall of whole year, respectively. The

rainfall distributions were different among the three years at both sites. The total rainfall during the rice growing season was greater in 2016 than in 2015 and 2017 at both sites.

Monthly mean rainfall and temperature at the two experimental sites during the 2015 to 2017 rice growing season.

The rice grain yield was significantly affected by treatment (T) but was not influenced by year (Y) or their interaction at both sites (Table S1). The rice grain yield in the CK treatment

was 6.7–7.2 t ha−1 in LC and ranged from 5.6 to 6.5 t ha−1 in XT from 2015 to 2017 (Fig. 2a,c). The grain yield of rice in the N fertilizer treatments (U, RCU and PCU) significantly

increased by 21.1–23.1%, 21.2–25.8% and 19.6–23.3% at the LC site and by 16.5–24.3%, 28.4–35.6% and 24.2–30.3% at the XT site compared to those in CK in 2015, 2016 and 2017, respectively.

The N rate in the CRU treatments was reduced by 20% relative to that in the U treatment from the 2015 to 2017 rice season (Table 1). However, no significant differences in rice grain yield

was observed between the U and CRU treatments at either site (Fig. 2a,c). Similarly, applications of RCU and PCU led to similar grain yields at both sites. Moreover, single CRU application

reduced the N fertilization time and reduced the work force needed for rice production relative to the spilt application of urea in the paddy fields.

The grain yield and aboveground biomass of rice were affected by the different treatments at the two experimental sites during the 2015 to 2017 rice growing season. The values are presented

as the mean ± standard deviation (n = 3). The different letters represent significant differences at the level of P