Journal of Integrative Agriculture 2016, 15(11): 2507–2514

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RESEARCH ARTICLE

Non-leguminous winter cover crop and nitrogen rate in relation to double rice grain yield and nitrogen uptake in Dongting Lake Plain, Hunan Province, China

ZHU Bo1, 2, YI Li-xia1, XU He-shui2, GUO Li-mei2, HU Yue-gao2, ZENG Zhao-hai2, CHEN Fu1, 2, LIU Zhang-yong1

1 College of Agriculture, Yangtze University, Jingzhou 434025, P.R.China 2 College of Agronomy and Biotechnology, China Agricultural University, Beijing 100193, P.R.China

Abstract Annual ryegrass (Lolium multiflorum Lam.), a non-leguminous winter cover crop, has been adopted to absorb soil native N to minimize N loss from an intensive double rice cropping system in southern China, but a little is known about its effects on rice grain yield and rice N use efficiency. In this study, effects of ryegrass on double rice yield, N uptake and use efficiency were measured under different fertilizer N rates. A 3-year (2009–2011) field experiment arranged in a split-plot design was undertaken. Main plots were ryegrass (RG) as a winter cover crop and winter fallow (WF) without weed. Subplots were –1 three N treatments for each rice season: 0 (N0), 100 (N100) and 200 kg N ha (N200). In the 3-year experiment, RG reduced grain yield and plant N uptake for early rice (0.4–1.7 t ha–1 for grain yield and 4.6–20.3 kg ha–1 for N uptake) and double rice (0.6–2.0 t ha–1 for grain yield and 6.3–27.0 kg ha–1 for N uptake) when compared with WF among different N rates. Yield and

N uptake decrease due to RG was smaller in N100 and N200 plots than in N0 plots. The reduction in early rice grain yield in RG plots was associated with decrease number of panicles. Agronomic N use efficiency and fertilizer N recovery efficiency were higher in RG plots than winter fallow for early rice and double rice among different N rates and experimental years. RG tended to have little effect on grain yield, N uptake, agronomic N use efficiency, and fertilizer N recovery efficiency in the late rice season. These results suggest that ryegrass may reduce grain yield while it improves rice N use efficiency in a double rice cropping system.

Keywords: double rice, grain yield, N uptake, N use efficiency, winter cover crop

1. Introduction

In most rice producing countries in Asia, double cropped early and late rice (double rice) plays an important role in Received 7 December, 2015 Accepted 9 March, 2016 ZHU Bo, E-mail: [email protected]; Correspondence ZENG guaranteeing national food security (Cassman and Pingali Zhao-hai, Tel: +86-10-62734884, Fax: +86-10-62732441, 1995). This intensive cropping system is usually featured E-mail: [email protected] with high chemical fertilizer N input but relative low N use © 2016, CAAS. Published by Elsevier Ltd. This is an open efficiency. In double rice cropping system in China, farmers access article under the CC BY-NC-ND license (http:// –1 creativecommons.org/licenses/by-nc-nd/4.0/) generally applied N fertilizer as much as 200 kg N ha per doi: 10.1016/S2095-3119(16)61331-X planting season (Peng et al. 2002), with less than 36% of 2508 ZHU Bo et al. Journal of Integrative Agriculture 2016, 15(11): 2507–2514

the applied N absorbed by the rice plant within the year (Zhu of cover crop and N application rate on main crop yields 2008; Zhu et al. 2012a). The excessive applied fertilizer N and N dynamics. not only increased cost of production, but also resulted in In the present study, we measured double rice grain many unintended environmental consequences such as eu- yields and N dynamics for continuous three years in a field trophication in rivers and lakes (Ju et al. 2009), greenhouse experiment. There were two main objectives of this study: gas emissions (Xing and Yan 1999; Liu et al. 2016) and (1) to evaluate the combined effects of annual ryegrass as a atmospheric deposition (Liu et al. 2013). Thus, extensive winter cover crop and N application rate on grain yields and efforts have been devoted to improve N use efficiency and N use efficiencies of the succeeding double rice crops; (2) to minimize N loss into the environment by scientists, environ- measure the impact of different N application rates on annal mental groups and governments worldwide. ryegrass winter cover crop, as compared with bare fallow. Replacing bare fallow with cover crops has been mainly used as an effective tool to increase retention of post-harvest 2. Materials and methods surplus soil N in intensive cropping systems and conse- quently reduce the risk of N introduced into the environment 2.1. Experimental site (Hooker et al. 2008; Zhu et al. 2012b; Ramírez-García et al. 2015). In many countries and regions such as USA, A field experiment was conducted from 2009 to 2011 at the Germany and Mediterranean areas, cover crops have been experimental station of Soil and Fertilizer Institute located in successfully established to intercept residual soil N other- Huarong County in Dongting Lake Plain (29°52´N, 12°55´E), wise be lost into rivers and into groundwater (Möller and Hunan Province, China. The climate in this region is sub- Reents 2009; Campiglia et al. 2011; Gabriel and Quemada tropical, monsoonal and humid, with a long hot period in 2011). However, the choice of a cover crop depends largely the summer and a short cold period in the winter. Monthly on its effects on yields and N dynamics of the following main mean temperatures and precipitations from 2009 to 2011 in crops. An ideal cover crop in a sustainable agricultural prac- Huarong County is shown in Fig. 1. Double rice production tice should prevent reactive N running into the environment is mostly widely practiced cropping system in this region. from intensive agriculture systems and maintain the main Soil properties from the upper 20 cm layer were sand 24.5%, crop yields (Tonitto et al. 2006; Singer et al. 2007). Studies silt 62.1%, clay 13.4%, pH 6.0, soil organic matter 2.55%, concerning the effects of cover crops on main crop yields total N 0.18%, Olsen extractable phosphorus 12.4 mg kg–1, have got inconsistent results. Some reported increased and available potassium 117.5 mg kg–1. yields favored by several factors such as weed suppression (Samarajeewa et al. 2005; Bernstein et al. 2014) and ad- 2.2. Treatments and crop management ditional N supply from the decomposition of the cover crop residue (Baijukya et al. 2006). Some reported yield declines Treatments were arranged in a split-plot design. Main plots after cover crop planting (Zhu et al. 2012c; Nascente et al. were ryegrass (RG) as a winter cover crop and winter fallow 2013) or no difference between cover crop management (WF) without weed. Subplots were three N treatments: –1 and bare fallow systems (Tonitto et al. 2006). 0 (N0), 100 (N100, 70 kg N ha urea as basal fertilizer to In double rice cropping systems in southern China, winter early and late rice, 30 kg N ha–1 urea as top dressing at –1 fallow duration is nearly six months, lasting from late rice tillering stage for early and late rice) and 200 kg N ha (N200, harvest in October to early rice transplanting in April in the 140 kg N ha–1 urea as basal fertilizer to early and late rice, following year. This allows farmers to establish such cover 60 kg N ha–1 urea as top dressing at tillering stage for early crops. Annual ryegrass (Lolium multiflorum Lam.), a fast and late rice). The size of subplot was 5 m×6 m. The ex- growing grass which is usually broadcasted into paddy fields periments were replicated three times. The treatments were approximately a half month before late rice harvest when assigned in the same experimental plots in the continuous used as a winter cover crop, had been demonstrated as an three studying years. effective tool for accumulating soil residual N and reducing Before the experiment, the field was planted with con- soil inorganic N contents in our previous studies (Zhu et al. tinuous double rice for at least five years. Before the initi- 2012b). Ryegrass decreased double rice grain yields and ation of the field experiment, subplots were separated by N uptakes in the case of zero N application during double 0.5 m wide ridges and plots were separated by 1 m wide rice seasons, possibly due to the lower soil N availability irrigation ditches. The ridges were covered with plastic film compared with bare fallow. Thus, planting ryegrass as a to prevent runoff of nutrients from one plot to another. In cover crop in the double rice cropping system may alter RG plots, ryegrass seeds were hand broadcast in late rice the rate of N fertilization for an optimal supply of N to rice. field approximately 10 d before late rice harvest. Details of However, few studies have examined the combined effects ryegrass variety, seeding rates, sowing dates, and harvest ZHU Bo et al. Journal of Integrative Agriculture 2016, 15(11): 2507–2514 2509 dates are given in Table 1. Ryegrass received no fertilizer 2.3. Sample collection and analysis application during the whole growing season. Ryegrass was cut twice each year. After cutting, the above-ground At maturity, two 1-m2 areas were sampled diagonally from part was removed from the plots. In WF plots, gramoxone each subplot to determine rice yield. Six representative hills (paraquat) was used to control weed to keep bare fallow in of rice plant were collected to measure yield components winter seasons. in each subplot. Plant samples were separated into straw Soil plowing with a depth of 20 cm was done before the and panicles. The dry weights of straw and grain were transplanting of early rice. Early rice seedlings (30-d-old) measured after oven drying at 60°C to constant weight. were transplanted at a spacing of 15 cm×20 cm with 2–3 Yield components including panicles m–2, spikelets panicle–1, plants per hill after the field preparation and basal fertiliza- grain filling percentage, and grain weight were thereafter tion. Details of early and late rice varieties, seeding rates, measured. Tissue N concentration was analyzed by micro transplanting dates, and harvest dates are given in Table 1. Kjeldahl (Bremner and Mulvaney 1982) to calculate rice

In each year, both early and late rice received basal fertilizer plant total N uptake. Agronomic N use efficiency (AEN) in –1 as 75 kg ha P2O5 (calcium super phosphate) and 100 kg fertilized WF plots was calculated as the increase relative –1 ha K2O (potassium chloride). to the unfertilized WF plots in grain yield per kg N applied.

Precipitation Air tempratue 35 2009 2010 2011 400 30

25 300

20

200 15

10

Month precipitation (mm) 100 5 Monthly mean air temperature (°C)

0 0 1 2 3 4 5 6 7 8 9 1011 12 1 2 3 4 5 6 7 8 9 1011 12 1 2 3 4 5 6 7 8 9 10 Month

Fig. 1 Total monthly precipitation and mean monthly air temperature from 2009 to 2011 in Huarong County, Hunan Province, China.

Table 1 Ryegrass (RG), early and late rice varieties used including their seeding rate, seeding date and the harvesting date in 2009, 2010 and 2011 in Huarong County, Hunan Province, China Crop Variety Seeding rate (ha–1) Transplanting/Sowing date Harvest date 2009 RG Tetragold 30 kg 6 October 10 March, 16 April Early rice Luliangyou 996 200 000 seedling 29 April 16 July Late rice Yueyou 712 200 000 seedling 20 July 17 October 2010 RG Tetragold 30 kg 8 October 11 March, 15 April Early rice Luliangyou 996 200 000 seedling 6 May 16 July Late rice Yueyou 712 200 000 seedling 27 July 17 October 2011 RG Tetragold 30 kg 18 October 14 March, 15 April Early rice Luliangyou 996 200 000 seedling 1 May 20 July Late rice Yueyou 712 200 000 seedling 24 July 26 October 2510 ZHU Bo et al. Journal of Integrative Agriculture 2016, 15(11): 2507–2514

Fertilizer N recovery efficiency (REN) in fertilized WF plots 2011 indicated that the effects of year, N rate, winter cover, was calculated as the ratio of the increase relative to the and interactions of winter cover and year were significant unfertilized WF plots in total N uptake of rice to the amount for grain yield of early rice and double rice (Table 2). The of applied N (Xu et al. 2010). AEN in fertilized RG plots effects of year and N rate were significant for late rice yield. was calculated as the increase relative to the unfertilized N application increased grain yield significantly both for early

RG plots in grain yield per kg N applied. REN in fertilized and late rice in the three experimental years. Yield increase RG plots was calculated as the ratio of the increase relative due to N application was 1.8–2.1, 0.9–2.1 and 2.7–3.4 t ha–1 to the unfertilized RG plots in total N uptake of rice to the for early rice, late rice and double rice, respectively (Table 3). amount of applied N. Calculations of AEN and REN were With the exception of N200 plots in 2009, ryegrass reduced done based on values in each replicate block. early and double rice grain yields significantly. Grain yield

reduction due to ryegrass in N0 plots were 0.5–1.7 and 2.4. Statistical analyses 0.8–2.0 t ha–1 for early rice and double rice, respectively, which were greater than yield reduction in N application plots Data were analyzed following analysis of variance (SAS In- (approximately 0.4–0.9 t ha–1 for early rice and approximately stitute 2003) and means were compared based on the least 0.6–1.5 t ha–1 for double rice). Late rice grain yields were significant difference (LSD) test at the 0.05 probability level. comparable between ryegrass and winter fallow treatments

except for N0 and N200 plots in 2011. 3. Results When compared with WF, RG decreased panicles m–2 for early rice during the three experimental years and spikelets 3.1. Rice grain yield and yield components panicle–1 in 2011 (Table 4). No significant difference was observed between RG and WF for grain filling, grain weight Combined analysis of variance using data from 2009 to and harvest index for early rice and panicles m–2, spikelets

Table 2 Analysis of variance (F values) for grain yield, total N uptake, agronomic N use efficiency (AEN), and fertilizer N recovery efficiency (REN) of early rice, late rice and double rice in 2009, 2010 and 2011 in Huarong County Early rice Late rice Double rice Source of Grain Total N Grain Total N Grain Total N variation AE RE AE RE AE RE yield uptake N N yield uptake N N yield uptake N N Year (Y) 26.2** 131.0** 16.8** 4.4* 191.0** 160.3** 61.9** 24.3** 48.2** 216.6** 39.4** 5.3* Winter cover (W) 163.5** 223.7** 54.1** 78.0** NS 19.2** NS NS 162.7** 143.6** 23.2** 41.1** N rate (N) 334.8** 588.1** 59.6** 30.7** 191.2** 303.4** NS NS 447.0** 707.4** 24.9** 15.5** W×Y 6.8** 6.4* 22.2** NS NS 7.2* NS NS 7.3** NS 6.8** NS N×Y NS NS 7.2** NS NS 4.9* NS NS NS NS 4.1* NS W×N NS 12.4** NS NS NS NS NS NS NS NS NS NS W×N×Y NS NS NS NS NS NS NS NS NS NS NS NS * and **, significant at P≤0.05 and P≤0.01, respectively; NS, non-significant.

Table 3 Effect of N rate and winter cover treatment on grain yield (t ha–1) of early rice, late rice and double rice in 2009, 2010 and 2011 in Huarong County Winter 2009 2010 2011 N rate cover1) Early rice Late rice Double rice Early rice Late rice Double rice Early rice Late rice Double rice 0 kg ha–1 WF 5.2 a 4.9 a 10.1 a 4.6 a 4.6 a 9.2 a 4.5 a 5.6 a 10.1 a RG 3.5 b 4.6 a 8.1 b 4.1 b 4.4 a 8.5 b 3.4 b 4.8 b 8.2 b Mean 4.3 B 4.8 C 9.1 C 4.3 C 4.5 C 8.9 C 4.0 B 5.2 C 9.1 C

100 kg ha–1 WF 6.4 a 5.4 a 11.8 a 5.6 a 5.1 a 10.7 a 5.8 a 6.8 a 12.5 a RG 5.5 b 5.1 a 10.6 b 5.1 b 4.9 a 10.0 b 4.9 b 6.3 a 11.2 b Mean 5.9 A 5.3 B 11.2 B 5.4 B 5.0 B 10.4 B 5.3 A 6.6 B 11.9 B

200 kg ha–1 WF 6.6 a 6.1 a 12.8 a 6.4 a 5.5 a 11.9 a 6.2 a 7.7 a 13.9 a RG 6.2 a 6.0 a 12.2 a 6.0 b 5.2 a 11.2 b 5.4 b 6.9 b 12.3 b Mean 6.4 A 6.1 A 12.5 A 6.2 A 5.4 A 11.6 A 5.8 A 7.3 A 13.1 A 1) WF, winter fallow; RG, ryegrass as winter cover crop. Within a column, values followed by different capital letters mean significant difference caused by N rate according to LSD (P=0.05). Within a particular N rate in a column, values followed by different lower case letters mean significant difference caused by winter cover according to LSD (P=0.05). The same as below. ZHU Bo et al. Journal of Integrative Agriculture 2016, 15(11): 2507–2514 2511 panicle–1, grain filling, grain weight, and harvest index for 3.3. N use efficiency late rice. The effects of year, winter cover and N rate were significant

3.2. N uptake for AEN for early rice and double rice (Table 2). Interactions of winter cover and year, N rate and year had significant

As shown in Table 2, the effects of year, N rate and winter effect for AEN for early rice and double rice, whereas win- cover were significant for total N uptake by early rice, late rice ter cover, N rate and the interactions among year, N rate and double rice. The interactions between winter cover and and winter cover were not significant for AEN for late rice year, winter cover and N rate were significant for early rice N (Table 2). N200 plots had lower AEN values than N100 plots in uptake while the interactions between winter cover and year, early rice season during the three studying years (Table 6).

N rate and year were significant for late rice. Plant N uptake In late rice and double rice seasons, N200 plots had lower by early rice, late rice and double rice increased significantly AEN values than N100 plots in 2011, while no significant with the amount of N fertilizer application (Table 5). Except difference was observed in 2009 and 2010. In 2009 and for N200 plots in 2011, RG decreased N uptake by early rice 2011, ryegrass treatments had higher AEN values than among N rates and experimental years when compared WF for early rice and double rice, while no difference was with winter fallow. Double rice N uptake was decreased found for late rice. In 2010, RG increased AEN for early by RG in N0 and N100 plots, but not in N200 plots. N uptake rice only in N200 plots. reduction due to ryegrass in N0 plots were 15.4–20.3 kg The effects of year, N rate and winter cover were sig- –1 –1 ha for early rice and 22.3–27.0 kg ha for double rice, nificant for REN for early rice and double rice. In late rice which were greater than N uptake reduction in N application seasons, year had a significant effect onN RE (Table 2). –1 –1 plots (4.6–13.8 kg ha for early rice and 6.3–16.9 kg ha N200 plots had lower REN values than N100 plots for early rice for double rice). No difference was observed for late rice N and double rice with the exception of double rice in 2009 uptake between RG treatments and WF treatments in 2009 (Table 7). Except for double rice in N100 plots in 2010, RG and 2010. However, late rice N uptake in RG plots in N0 and treatments had higher REN values than WF for early rice and

N100 was significantly lower than that in WF plots in 2011. double rice among different N application rates and years.

Table 4 Effect of winter cover treatment on yield components of early rice and late rice in 2009, 2010 and 2011 in Huarong County Early rice Late rice Winter Panicles Spikelets Grain filling Grain weight Harvest Panicles Spikelets Grain filling Grain weight Harvest cover (m–2) panicle–1 (%) (mg) index (%) (m–2) panicle–1 (%) (mg) index (%) 2009 WF 254 a 103 a 84.1 a 27.8 a 56.5 a 211 a 126 a 84.3 a 24.1 a 45.0 a RG 227 b 97 a 86.0 a 27.7 a 57.3 a 199 a 127 a 83.9 a 24.0 a 44.1 a 2010 WF 188 a 121 a 85.6 a 28.2 a 57.0 a 278 a 107 a 63.5 a 24.5 a 49.8 a RG 174 b 119 a 85.6 a 28.1 a 57.1 a 269 a 107 a 63.7 a 24.1 a 48.8 a 2011 WF 365 a 75 a 85.1 a 26.8 a 57.8 a 191 a 136 a 77.2 a 24.4 a 46.5 a RG 341 b 61 b 86.2 a 25.8 a 57.6 a 184 a 138 a 75.6 a 24.5 a 47.4 a

Table 5 Effect of N management and winter cover treatment on total N uptake (kg ha–1) of early rice, late rice and double rice in 2009, 2010 and 2011 in Huarong County Winter 2009 2010 2011 N rate (kg ha–1) cover Early rice Late rice Double rice Early rice Late rice Double rice Early rice Late rice Double rice 0 WF 70.7 a 90.3 a 161.0 a 66.9 a 73.8 a 140.7 a 82.7 a 92.3 a 175.0 a RG 50.4 b 87.4 a 137.8 b 47.0 b 71.4 a 118.4 b 67.3 b 80.7 b 148.0 b Mean 60.6 C 88.9 C 149.4 C 56.9 C 72.6 C 129.6 C 75.0 C 86.5 C 161.5 C 100 WF 87.3 a 103.3 a 190.7 a 87.1 a 87.5 a 174.5 a 97.2 a 111.9 a 209.0 a RG 78.3 a 97.3 a 175.6 b 73.3 b 84.4 a 157.6 b 92.1 b 103.9 b 196.0 b Mean 82.8 B 100.3 B 183.1 B 80.2 B 85.9 B 166.1 B 94.7 B 107.9 B 202.5 B 200 WF 103.3 a 114.6 b 217.8 a 98.9 a 98.9 a 197.8 a 110.1 a 125.9 a 236.0 a RG 90.0 b 121.5 a 211.5 a 87.0 b 96.1 a 183.1 a 105.5 a 117.1 a 222.7 a Mean 96.6 A 118.0 A 214.7 A 93.0 A 97.5 A 190.4 A 107.8 A 121.5 A 229.3 A 2512 ZHU Bo et al. Journal of Integrative Agriculture 2016, 15(11): 2507–2514

4. Discussion m–2 was the only factor affected by winter cover ryegrass (Table 4). It can be explained that the decreased yield of The effects of cover crops on the subsequent main crop early rice by ryegrass was caused by the reduced effective yields have been studied in various cropping systems tillers. Ryegrass reduced early and double rice yield in N around the world (Blanco-Canqui et al. 2012). Our previous fertilized plots in 2010 and 2011 (Table 3). These results study had reported double rice grain yield decline by using indicate that N fertilizer application in the succeeding rice ryegrass as a winter cover crop when no N fertilizer was seasons couldn’t change the negative effect of ryegrass on applied during double rice seasons (Zhu et al. 2012c). In rice growth. In double rice cropping systems in the studied the current study, yield and N uptake reduction of early rice region, only a few days gap can be expected between the and double rice attributed to ryegrass were observed both harvest of early rice and the transplanting of late rice. Early under no N application and under N applied at 100 and rice straw is usually incorporated into the soil after harvest. 200 kg ha–1 (Tables 3 and 5). Yield reduction of the main The decomposition of early rice straw may provide late rice crops by non-leguminous cover crops perennial ryegrass with slow released organic N as reported by Xu et al. (2010). (Lolium perenne L.), Brachiaria brizantha and Brachiaria This effect diluted the negative effect of ryegrass on late rice, ruziziensis had been reported by Bergkvist et al. (2011) and resulting in statistically non-significant difference in late rice Nascente et al. (2013). Increased competition for resources grain yield; N uptake and fertilizer N use efficiency in most including soil water and nutrients between a cover crop and cases in this study. the main crop have been deemed as the main reasons for Numerous studies have reported on cover crop effects yield decline of the main crop (Garibay et al. 1997; Hooks on retaining soil native N after the main crop harvest which and Johnson 2001). Since ryegrass has a vigorous growth may otherwise be lost into the environment (Hooker et al. character, it can effectively remove residual N from the soil 2008; Möller and Reents 2009; Gabriel and Quemada after the main crop harvest (Quemada et al. 2013; Valkana 2011; Ramírez-García et al. 2015). In the present study, et al. 2015). In a double rice cropping system in southern winter cover crop ryegrass increased fertilizer N agronomic China, as observed by Zhu et al. (2012c), ryegrass retained use efficiency for early rice and late rice in 2009 and 2011 171 kg ha–1 soil residual N in its above-ground biomass, (Table 6), and increased N uptake efficiency in the three resulting in a 29% reduction of soil mineral N contents in experimental years (Table 7). These results indicated that 0–30 cm soil depth. Thus, we attribute the decline of rice less N may be lost into the environment during double rice grain yield and N uptake to the decreased soil native N seasons after winter covering with ryegrass. Thus, further by the harvest of ryegrass. The results of rice yield com- studies were recommended to measure year round N losses ponents indicated that the number of early rice panicles such as nitrate leaching and N2O emission from double rice

Table 6 Effect of N management and winter cover treatment on AEN of early rice, late rice and double rice in 2009, 2010 and 2011 in Huarong County N rate Winter 2009 2010 2011 (kg ha–1) cover Early rice Late rice Double rice Early rice Late rice Double rice Early rice Late rice Double rice 100 WF 11.3 b 5.1 a 8.2 b 10.1 a 4.6 a 7.4 a 12.6 b 11.8 a 12.2 b RG 20.8 a 4.7 a 12.8 a 10.8 a 4.3 a 7.6 a 14.7 a 15.6 a 15.2 a Mean 16.1 A 4.9 A 10.5 A 10.5 A 4.5 A 7.5 A 13.7 A 13.7 A 13.7 A 200 WF 7.1 b 6.1 a 6.6 b 8.7 b 4.4 a 6.6 a 8.5 b 10.7 a 9.6 b RG 13.9 a 6.9 a 10.4 a 9.7 a 4.1 a 6.9 a 9.9 a 10.8 a 10.3 a Mean 10.5 B 6.5 A 8.5 A 9.2 B 4.3 A 6.7 A 9.2 B 10.7 B 9.9 B

Table 7 Effect of N management and winter cover treatment on fertilizer REN of early rice, late rice and double rice in 2009, 2010 and 2011 in Huarong County N rate Winter 2009 2010 2011 (kg ha–1) cover Early rice Late rice Double rice Early rice Late rice Double rice Early rice Late rice Double rice 100 WF 16.7 b 13.0 a 14.8 b 20.2 b 13.6 a 16.9 a 14.5 b 19.6 a 17.0 b RG 27.9 a 9.9 a 18.9 a 26.3 a 13.0 a 19.6 a 24.8 a 23.2 a 24.0 a Mean 22.3 A 11.4 A 16.9 A 23.2 A 13.3 A 18.3 A 19.6 A 21.4 A 20.5 A 200 WF 16.3 b 12.1 a 14.2 b 16.0 b 12.5 a 14.3 b 13.7 b 16.8 a 15.3 b RG 19.8 a 17.0 a 18.4 a 20.0 a 12.3 a 16.2 a 19.1 a 18.2 a 18.7 a Mean 18.0 B 14.6 A 16.3 A 18.0 B 12.4 A 15.2 B 16.4 B 17.5 A 17.0 B ZHU Bo et al. Journal of Integrative Agriculture 2016, 15(11): 2507–2514 2513

paddy fields when ryegrass was used as a winter cover in annuum L.). Nutrient Cycling in Agroecosysterms, 89, southern China. 399–412. Cassman K G, Pingali P L. 1995. Extrapolating trends from 5. Conclusion long-term experiments to farmers’ fields: The cas of irrigated rice system in Asia. In: Barnett V, Payne R, Steiner R, eds., Agricultural Sustainability: Economic, Environmental and The growth of early rice could be suppressed by winter cover Statistical Considerations. John Wiley & Sons, New York. crop ryegrass, leading to a reduction of grain yield and N pp. 63–84. uptake of early rice and double rice. However, less reduc- Gabriel J L, Quemada M. 2011. Replacing bare fallow with tions in rice yield and N uptake were observed at the high cover crops in a maize cropping system: Yield, N uptake and –1 N application of 200 kg N ha , indicating additional N was fertilizer fate. 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