Min Huang*,Xuefeng Zhou,Yingbin Zou

Southern Regional Collaborative Innovation Center for Grain and Oil Crops(CICGO),Hunan Agricultural University,Changsha 410128,Hunan,China

Keywords:

A B S T R A C T Zero-tillage has become increasingly attractive in rice production in China.This study was conducted to determine the feasibility of two possible improved N management practices with fewer N applications in zero-tillage rice:(1)two split applications of urea at 75 kg N ha?1at mid-tillering and 45 kg N ha?1at panicle initiation(U120—2),and(2)a single application of cross-linked polyacrylamide-coated urea(a slow-release fertilizer)at midtillering at a rate of 150 kg N ha?1(PCU150—1).Three field experiments were conducted to compare grain yield and N-use efficiency among several N treatments:a zero-N control(CK),U120—2,PCU150—1,a single application of urea at mid-tillering at a rate of 150 kg N ha?1(U150—1),and a commonly recommended N management practice for conventional tillage rice(three split applications of urea with 75 kg N ha?1as basal,30 kg N ha?1at mid-tillering,and 45 kg N ha?1at panicle initiation)(U150—3).Treatments with N application(U120—2,PCU150—1,U150—1,and U150—3)produced 1.08—3.16 t ha?1higher grain yields than CK.Grain yields under both U120—2and PCU150—1were comparable to that in U150—3.Recovery efficiency of N(REN),agronomic N-use efficiency(AEN)and partial factor productivity of applied N(PFPN)were increased under U120—2and were similar under PCU150—1to those under U150—3.U150—1showed lower grain yield,REN,AEN,and PFPNthan U150—3.These results suggest that U150—3can be replaced with U120—2to achieve both an increase in N-use efficiency and a reduction in number of N applications and or by PCU150—1to achieve a maximum reduction in number of N applications in zero-tillage rice production in China.

1.Introduction

Rice is the staple food crop for about 65%of the population in China[1].Although rice production in China has shown remarkable growth in the past five decades,several key problems in the Chinese rice production system prevent a sustainable increase in rice production [2].Overuse of fertilizers,especially N fertilizer,has been one of the major problems confronting rice production in China[2,3].The average rate of N application for rice production in China is 180 kg ha?1,about 75%higher than the world average[4].Because of the high rate of N application,only 20%—30%of N is taken up by the rice plant and a large proportion of N is lost to the environment[5].This results not only in waste of the applied N but also in many environmental problems,such as soil acidification,increased greenhouse gas emissions,and surface water eutrophication[3,6].

A sustainable increase in rice production in China has been constrained by changes in socioeconomic and physical environments, such as decreased labor availability and degraded soil[2,3].Rice production technologies must be developed that will be labor saving and environmentally friendly and will maintain rice yield potential[7].Conventional tillage(by moldboard plowing and rotavating)is the most widely used method for land preparation in Chinese rice production[8].However,this practice not only requires a large amount of energy and labor[9]but also accelerates mineralization of organic matter,reduces soil fertility,increases water consumption,and damages the chemical and physical properties of the soil[10].In recent years,zero-tillage has been become increasingly attractive in Chinese rice production[7],owing to its benefits including saving fuel,equipment,and labor as well as conserving soil[11].

As compared with conventional tillage,zero-tillage can cause accumulation of rice roots and soil N in the surface soil layer[12,13].Thus,zero-tillage rice has more roots distributed in the soil layer with higher N content,indicating that less N fertilizer may be required in zero-tillage rice.However,Norman et al.[14]reported that zero-tillage rice took a long time to take up basal N,resulting in an increase in N loss and a consequent increase in N fertilizer requirement.Huang et al.[15]also observed that N uptake was delayed in zero-tillage rice.They further found that inhibition of root growth caused by accumulation of inhibitory pseudomonads in the rhizosphere was responsible for the poor N uptake in zero-tillage rice at early growth stages[16].These reports suggest that the basal N rate may need to be decreased to reduce N loss in zero-tillage rice.But in fact,rice farmers in China generally follow the same N management for zero-tillage rice as for conventional tillage rice[7].Even worse,owing to shortage of labor and increased labor costs,many rice farmers apply fertilizers only once,before crop establishment,to avoid in season fertilizer application[2].For these reasons,reducing the number of N applications should be a primary goal in improving rice N management to meet the changes in the socioeconomic environment.In this regard,it has been suggested[17]that the supply of N by a single application of slow-release fertilizer is likely to satisfy plant requirements.

Based on the above considerations and a commonly recommended N management practice for conventional tillage rice(three split applications of urea at 75 kg N ha?1as basal,30 kg N ha?1at mid-tillering,and 45 kg N ha?1at panicle initiation)(U150—3),we designed two possible improved N management practices for zero-tillage rice in China:(1)two split applications of urea at 75 kg N ha?1at mid-tillering and 45 kg N ha?1at panicle initiation(U120—2),and(2)a single application of cross-linked polyacrylamide-coated urea(a slow-release fertilizer)at mid-tillering at150 kg N ha?1(PCU150—1).The objective of this study was to test the feasibility of these two N management practices.

2.Materials and methods

Three field experiments were conducted.The details of these experiments were as follows:

2.1.Field experiment I

This experiment was conducted in Nanxian(29°21′N,112°25′E,30 m a.s.l.),Hunan province in a single rice-growing season in 2011.A rice—oilseed rape rotation with conventional tillage was followed in the field before the experiment was conducted.The soil of the experimental field was a purple calcareous clayey soil with the following properties in the upper 20 cm layer:pH 7.73,organic matter 28.8 g kg?1,total N 1.92 g kg?1,available P 29.1 mg kg?1,and available K 81.2 mg kg?1.The hybrid rice cultivar Liangyoupeijiu was grown under three N treatments(Table 1).The N treatments were arranged in a completely randomized block design with three replications and plot size of 20 m2.Rice plants were established by zero-tillage transplanting.Pre-germinated seeds were sown in a seedbed.Twenty-five-day-old seedlings were transplanted at a hill spacing of 20 cm×20 cm with two seedlings per hill.Phosphorus(60 kg P2O5ha?1)was applied as basal fertilizer.Potassium (105 kg K2O ha?1)was split equally between basal and panicle-initiation applications.Water management employed a strategy of shallow flooding(to a depth of 1—2 cm)—midseason drainage—reflooding(to a depth of 5—8 cm)—moist intermittent irrigation.Weeds were managed by a combination of herbicide spraying(20%paraquat,diluted 5 mL L?1and applied at 750 L ha?1at 7 days before transplanting)and hand weeding.Insects and diseases were intensively controlled by chemicals to avoid yield loss.At maturity,grain yield was determined from a 5-m2area in each plot and adjusted to a standard moisture content of 0.135 g H2O g?1.

2.2.Field experiment II

This experiment was conducted in Haikou(19°45′N,110°11′E,26 m a.s.l.),Hainan province in the early rice-growing season in 2012.A double rice cropping system with conventional tillage was followed in the field before the experiment was conducted.The soil of the experimental field was a sandy loam with the following properties in the upper 20 cm layer:pH 5.92,organic matter21.4 g kg?1,total N 1.08 g kg?1,available P 34.8 mg kg?1,and available K 115 mg kg?1.The hybrid rice cultivar Liangyoupeijiu was grown under four N treatments(Table 1).The N treatments were arranged in a completely randomized block design with three replications and plot size of 20 m2.Rice plants were established by zero tillage seedling throwing.Seedling throwing is a simplified cultivation technology in which rice seedlings with soil on their roots are thrown by hand into fields.Pre-germinated seeds were sown in seedling trays.Thirty-day old seedlings were thrown at a hill spacing of 20 cm×27 cm with two seedlings per hill.Crop management followed the practices used in field experiment I.At maturity,grain yield was determined from a 5-m2area in each plot and adjusted to the standard moisture content of 0.135 g H2O g?1.

Table 1–Description of N treatments.

2.3.Field experiment III

This experiment was conducted in Liuyang(28°09′N,113°37′E,43 m a.s.l.),Hunan province in the early rice-growing season in 2013.The double rice cropping system with conventional tillage was practiced in the field before the experiment took place.The soil of the experimental field was clayey with the following properties in the upper 20 cm layer:pH 5.89,organic matter 38.9 g kg?1,total N 2.37 g kg?1,available P 12.6 mg kg?1,and available K 107 mg kg?1.A hybrid rice cultivar,Lingliangyou 104,and an inbred rice cultivar,Zhongjiazao 17,were grown under five N treatments(Table 1).The experiment was laid out in a split-plot design with N treatments as main plots and cultivars as subplots,having three replications and a subplot size of 14 m2.Rice plants were established by zero-tillage seedling throwing.Pre-germinated seeds were sown in seedling trays.Twenty-three-day old seedlings were thrown at a hill spacing of 20 cm×15 cm with two seedlings per hill.Crop management followed the practices used in field experiment I.At maturity,rice plants were sampled from a 0.48-m2area in each subplot.Panicles were counted to calculate panicles m?2.Plants were separated into straw and panicles.Panicles were hand-threshed and filled spikelets were separated from unfilled spikelets by submersion in tap water.Three subsamples of 30-g filled spikelets and all unfilled spikelets were taken to count the spikelets.Dry weights of straw,rachis,and filled and unfilled spikelets were determined after oven-drying at 70°C to constant weight.Spikelets panicle?1,spikelets m?2,spikelet filling percentage,grain weight,total biomass production,and harvest index were calculated.Tissue N concentration was determined using a Skalar SAN Plus segmented flow analyzer(Skalar Inc.,Breda,The Netherlands).Total N uptake was the sum of N uptake in straw,rachis,and filled and unfilled spikelets(dry matter×corresponding N concentration).Grain yield was determined from a 5-m2area in each subplot and adjusted to the standard moisture content of 0.135 g H2O g?1.N-use efficiency parameters,including internal N-use efficiency(IEN),recovery efficiency of N(REN),agronomic N-use efficiency(AEN),and partial factor productivity of applied N(PFPN),were calculated following Dobermann[18].Briefly,IENwas calculated as the ratio of grain yield to total N uptake.RENwas calculated as the ratio of the increase in plant N accumulation at maturity that resulted from N fertilizer application to the fertilizer N rate.AENwas calculated as the increase in grain yield per unit of applied N.PFPNwas the grain yield per unit N applied.

Data were analyzed by analysis of variance and regression analysis(Statistix 8,Analytical Software,Tallahassee,Florida,USA).The statistical model used for analysis of variance included sources of variation due to replication and N treatment.Means of N treatments were compared according to the least significant difference test(LSD)at the 0.05 probability level.

3.Results

N application had significant effects on grain yield in all three experiments(Table 2).In Nanxian in 2011,Liangyoupeijiu produced 2.49—3.16 t ha?1higher grain yields in treatments with N application(U150—3and U120—2)than in CK.In Haikou in 2012,grain yields in Liangyoupeijiu were 1.08—1.81 t ha?1higher in treatments with N application(U150—3,U150—1,and PCU150—1)than in CK.In Liuyang in 2013,Lingliangyou 104 and Zhongjiazao 17 showed respectively 1.74—2.61 and 1.97—3.10 t ha?1higher grain yields in treatments with N application(U150—3,U120—2,U150—1,and PCU150—1)than in CK.There was no significant difference in grain yield between U120—2and U150—3in Liangyoupeijiu in Nanxian in 2011 and in Lingliangyou 104 and Zhongjiazao 17 in Liuyang in 2013.U150—1produced lower grain yield than U150—3by 8%in Liangyoupeijiu in Haikou in 2012 and by 10%in Lingliangyou 104 and Zhongjiazao 17 in Liuyang in 2013.The difference ingrain yield between PCU150—1and U150—3was not significant in Liangyoupeijiu in Haikou in 2012 or in Lingliangyou 104 and Zhongjiazao 17 in Liuyang in 2013.

Table 2–Grain yield in zero-tillage rice grown under different N treatments.

Panicles m?2and spikelets m?2were significantly higher,while spikelet filling percentage was generally lower,in treatments with N application(U150—3,U120—2,U150—1,and PCU150—1)than in CK(Table 3).The differences in spikelet panicle?1and grain weight between treatments with N application and CK were relatively small and inconsistent.There were no significant differences in all yield components between U120—2and U150—3.U150—1yielded significantly fewer spikelets panicle?1and spikelets m?2than U150—3,whereas the differences in panicles m?2,spikelet filling percentage and grain weight between them were not significant or consistent.Differences in yield components between PCU150—1and U150—3were insignificant or inconsistent.

Total biomass production was significantly greater in treatments with N application(U150—3,U120—2,U150—1,and PCU150—1)than in CK,whereas harvest index was significantly lower in treatments with N application than in CK(Table 3).The differences in total biomass production and harvest index between U120—2and U150—3were not significant.U150—1yielded significantly lower total biomass production than U150—3,whereas the difference in harvest index between them was not significant.There were no significant differences in total biomass production or harvest index between PCU150—1and

Total N uptake was significantly higher,while IENwas generally lower,in treatments with N application(U150—3,U120—2,U150—1,and PCU150—1)than in CK(Table 4).There were no significant differences in total N uptake and IENbetween U120—2and U150—3.Total N uptake was significantly lower under U150—1than under U150—3,whereas the difference in IENbetween them was insignificant.The differences in total N uptake andIENbetween PCU150—1andU150—3werenot significant.Grain yield was correlated positively with total N uptake but negatively with IEN(Fig.1).

U120—2generally showed higher REN,AENand PFPNthan U150—3,and the differences were significant except for RENand AENin Lingliangyou 104(Table 4).U150—1usually showed lower REN,AENand PFPNthan U150—3,and the differences were significant except for RENin Lingliangyou 104.There were no significant differences in REN,AENand PFPNbetween PCU150—1and U150—3.

4.Discussion

N is the most yield-limiting nutrient in rice cropping systems worldwide,and almost every farmer must apply N fertilizer to obtain a desirable grain yield[20].Many studies have been conducted to determine the response of rice to N application[5,21—23],but most of these studies have been performed under conventional tillage conditions.In this study,wedetermined the response of zero-tillage rice to N application.The yield increases of 1.08—3.16 t ha?1from applied N in zerotillage rice are close to those reported in conventional-tillage rice in China[5,23].The increased yield from applied N in zero-tillage rice was due to increases in both sink(panicles m?2and spikelets m?2)and source(biomass production).This finding is consistent with that reported for conventional tillage rice[22,23].

Table 3–Yield components,total biomass production,and harvest index in zero-tillage rice grown under different N treatments in Liuyang,Hunan province,China in 2013.

Table 4–Total N uptake,internal N-use efficiency(IEN),recovery efficiency of N(REN),agronomic N-use efficiency(AEN),and partial factor productivity of applied N(PFPN)in zero-tillage rice grown under different N treatments in Liuyang,Hunan province,China in 2013.

N is also the most difficult nutrient to manage because of many opportunities for loss[24].Matching N supply to crop N requirement is an essential component of improving crop N management,and one common way to achieve this match is split application of N[25].In China,three split applications of N at basal,mid-tillering,and panicle initiation stages is usually recommended in rice production.However,this N management has a major drawback,namely added labor input by farmers.In this study,we designed two possible improved N management practices(U120—2and PCU150—1)with fewer N applications for zero-tillage rice and compared them with a commonly recommended N management practice(U150—3).Our results showed that grain yields under both U120—2and PCU150—1were comparable with that under U150—3.REN,AEN,and PFPNwere increased under U120—2and maintained at the same level under PCU150—1as under U150—3.These results suggest that U150—3can be replaced with U120—2to achieve both a reduction in number of N applications and an increase in N-use efficiency,or by PCU150—1to achieve a maximum reduction in number of N applications in zero-tillage rice production in China.

Fig.1–Relationship of grain yield with total N uptake(a)and internal N-use efficiency(IEN)(b)in zero-tillage rice in Liuyang,Hunan province,China in 2013.

Fig.2–Relationship between grain yield and internal N-use efficiency(IEN)in zero-tillage rice.Data are a combination of those from the present study and the study of Xia et al.[31].

It is well known[5]that fertilizer N-use efficiency(REN,AEN,and PFPN)in rice can be increased by reducing N rate and by applying less N at the early vegetative stage.This strategy might be more efficient in zero-tillage rice,which has the characteristic of delayed N uptake[14—16].Consistently,in this study,a decrease in N rate(30 kg ha?1)at the early vegetative stage resulted in higher REN,AENand PFPNunder U120—2than under U150—3.However,because this study was conducted under moderate soil fertility conditions,the magnitude of decreased N rate in U120—2observe in the study might not be applicable under low soil fertility conditions.Yin et al.[21]observed that increased PFPNwas achieved without sacrificing yield in zero-tillage rice by reduction of the N rate from 150 kg ha?1to 120 and 105 kg ha?1in a low and a moderate soil fertility field,respectively.Thus,it might be necessary to adjust the magnitude of the N rate reduction in the improved N practice of U120—2according to the soil fertility status.This adjustment could be reliably achieved by application of the principles of site-specific N management[26].In addition,as a consequence of the reduced N rate at the early vegetative stage,the proportion of N applied during the vegetative period(PNVP)under U120—2(62.5%)was lower in the present study than those(70%—90%)in most previous studies[8,27,28].Huang et al.[29]performed a meta-analysis of zero-tillage effect on rice yield in China,finding that reducing PNVP from 90%to 70%—80%resulted in a negative effect of zero-tillage on rice yield.However,in the present study,the lower PNVP did not lead to a yield reduction in zerotillage rice.This finding suggests that zero-tillage effect on rice yield may not depend linearly on the PNVP.More research is needed to confirm and clarify this relationship.

Fertilizer N-use efficiency was also associated with fertilizer type.REN,AENand PFPNwere maintained under PCU150—1but decreased under U150—1as compared with U150—3.In this regard,a preliminary test showed that applying cross-linked polyacrylamide-coated urea resulted in about 20%lower cumulative NH3volatilization than applying urea at a rate of 150 kg N ha?1in a zero-tillage rice field(data not shown).Moreover,the magnitude of the reduction in NH3volatilization may be increased by improving the composition of slow release fertilizer[30].Further studies might yield new slow release fertilizers to increase N-use efficiency in zero-tillage rice.

IENreflects the ability of a rice plant to transform N acquired from all sources(soil,fertilizer)into grain yield[18].In this study,grain yield was correlated negatively with IENbut positively with total N uptake.This result is not in agreement with that of Xia et al.[31],who observed that grain yield was positively correlated with IENbut not with total N uptake in zero-tillage rice.This disagreement may be due to the large difference in the range of IENbetween these two studies.The IENwas 59.1—74.1 kg kg?1in the present study and 37.1—60.7 kg kg?1in the study of Xia et al.[31].In general,a very high IENimplies a deficiency of N,whereas a low one indicates poor internal N conversion[18].Combining the data from the present study and the study of Xia et al.[31]yields a quadratic correlation between grain yield and IENin zero-tillage rice(Fig.2).This relationship is not in agreement with that reported by Witt et al.[32].They calculated the IENfor achieving certain grain yield targets in rice using the QUEFTS model[33]and showed that IENgradually decreased from 68 to 41 kg kg?1with a gradual increase in grain yield from 1.00 to 9.99 t ha?1.Using the regression equation shown in Fig.2,we estimated that a high grain yield of more than 10 t ha?1could be achieved at an IENof 45—57 kg kg?1in zerotillage rice.This IENrange was lower than the optimal range(55—65 kg kg?1)reported by Dobermann[18].This difference might be partly explained by the following two points:(1)zero-tillage rice has the characteristic of superior postheading N uptake[12];and(2)a higher proportion of N distribution in straw at maturity generally lead to a lower IENin rice[34].These considerations again indicate that N management should be specific for zero-tillage rice.

Acknowledgments

This study was supported by the National Natural Science Foundation of China(31301267)and the China Agriculture Research System(CARS-01).

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