Rice productivity and profitability with slow-release urea containing organic-inorganic matrix materials

Yang YANGBinmei LIUXiaoyu NILiangzhi TAOLixiang YUYe YANGMengxi FENGWenjin ZHONG and Yuejin WU*

1Engineering Laboratory of Environment-friendly High-efficiency Fertilizers and Pesticides of Anhui Province,Hefei Institutes of Physical Science,Chinese Academy of Sciences,Hefei 230031(China)

2The Innovative Academy of Seed Design,Chinese Academy of Sciences,Beijing 100101(China)

3Henan Xinlianxin Fertilizer Co.,Ltd.,Xinxiang 453700(China)

ABSTRACT Applying slow-release fertilizers is possible means for reducing nitrogen(N)loss in rice production.Matrix-based fertilizers represent novel slow-release fertilizers.To date,there is little consensus about the effect of combined addition of organic and inorganic matrix materials on rice production.We developed a slow-release urea fertilizer with selected organic and inorganic matrix materials.The study aimed to:i)determine the effect of the slow-release urea on rice yield,profit,and agronomic efficiency and ii)elucidate its possible mechanisms.A two-year field experiment was conducted during 2015–2016.Besides,laboratory experiments were conducted to determine the potential N loss risk.Three treatments were set up:control without N application(CK),regular urea treatment(RU,150 kg N ha-1),and slow-release urea treatment(SU,150 kg N ha-1).The results showed that rice biomass and grain yield were significantly higher in SU than in RU(P＜0.05).The higher panicle density in SU was largely responsible for the greater grain yield.Net profit in SU was≥US$450 ha-1,higher than in RU.Agronomic efficiency was significantly greater in SU than in RU(P＜0.05).Rice height,root area,leaf chlorophyll,leaf nitrate reductase activity,and leaf glutamine synthetase activity were larger in SU than in RU.Less N loss and greater soil N availability were partly responsible for the improvements in rice growth traits and physiological parameters in SU.Overall,the slow-release urea is a promising fertilizer for rice production.

Key Words:ammonia emission,ferric sulfate,matrix-based fertilizer,montmorillonite,nitrogen leaching,path analysis

INTRODUCTION

Rice is an important crop for global food security.Rice production is largely dependent on fertilizer inputs,such as nitrogen(N)fertilizer(Yanget al.,2018a).With sufficient N supply,rice yields can increase by more than 100%(Adviento-Borbe and Linquist,2016).As N application greatly contributes to rice production,rice growers usually apply N fertilizers at high rates(Huanget al.,2015).Large amounts of N application result in high risk of N loss and environmental pollution(Jianget al.,2017;Yaoet al.,2018).For example,about 1.7 Tg N were lostviaammonia emissions in paddy fields in China in 2013,which accounted for 17.7%of applied N(Wanget al.,2018).Rice production requires large amounts of water supply,and thus N leaching and runoffrisks are also high(Li P Fet al.,2018).In total,more than half of the applied N was lost in rice fields(Genget al.,2015).Considering the economic and environmental benefits,it is important to reduce N loss and increase grain yield and N use efficiency in rice production.

There are several ways to reduce N loss and increase crop productivity,such as modifying application methods and applying highly efficient fertilizers.Miet al.(2016)reported that split application of N fertilizers increased rice grain yield compared with single basal applications.Huanget al.(2016)found that split application and deep placement of N fertilizers reduced ammonia emissions in paddy fields.However,split application and deep placement are difficult to be accepted by rice growers(especially smallholder farmers),due to greater requirements for labor and agricultural machinery(Zhanget al.,2016).Development and application of controlled-or slow-release fertilizers are beneficial for environment-friendly and sustainable production of rice(Li P Fet al.,2018;Yanget al.,2018b).As these fertilizers do not require additional labor and agricultural machinery compared with common fertilizers,they are more likely to be accepted by rice growers.

To date,there are several types of controlled-or slowrelease fertilizers,such as coated fertilizer(Li P Fet al.,2018),stabilized fertilizer(Afsharet al.,2018),and matrixbased fertilizer(Yanget al.,2017).Coated fertilizers control nutrient releaseviacoating materials on the surface of fertilizer granules.However,the production cost of coated fertilizers is high(Niet al.,2013).Stabilized fertilizers reduced N lossviaregulating the activities of soil enzymes(Huanget al.,2016).However,the performance of stabilized fertilizers is sensitive to field conditions(such as soil pH)(Franciscoet al.,2011).Matrix-based fertilizers decrease N releaseviaflocculation and adsorption effects of the matrix materials added to the fertilizer granules(Yanget al.,2017).The production cost of matrix-based fertilizers is lower than those of other slow-release fertilizers,due to the much cheaper matrix materials(Niet al.,2013).Hence,matrix-based fertilizers are likely to be accepted by crop growers and have been produced in thirteen provinces in China.However,the performance of matrix-based fertilizers may be interfered by flooding conditions in paddy fields.Therefore,it is important to develop matrix-based fertilizers with better flocculation and adsorption performances.

Matrix materials can be divided into two groups,i.e.,inorganic matrix materials(such as clay minerals)and organic matrix materials(such as polyacrylamide)(Yanget al.,2017).The flocculation and adsorption performances are different between inorganic and organic matrix materials(Entry and Sojka,2008;Aliet al.,2018).Combined addition of inorganic and organic matrix materials improved flocculation and adsorption performances and promoted the growth of wheat in a laboratory experiment(Entry and Sojka,2008).Nevertheless,there is little consensus about the effects of matrix-based fertilizers(with inorganic and organic matrix materials)on field crop production.

We developed a novel slow-release urea with selected organic and inorganic matrix materials.The current study aimed to:i)determine the effect of slow-release urea on rice yield,profit,and agronomic efficiency(AE)and ii)elucidate the working mechanisms of the slow-release urea.Findings of this work will support further development of fertilizers based on organic-inorganic matrix materials for environment-friendly and cleaner production of crops.

MATERIALS AND METHODS

Site and materials

Field experiments were conducted during two rice growing seasons,i.e.,June 23–October 7,2015 and June 26–October 7,2016.The plots were located at the experimental base of Hefei Institutes of Physical Science(31°54′N,117°11′E,26 m above sea level),Chinese Academy of Sciences,Hefei,China.Selected properties of the paddy soil(0–20 cm layer)were:clay(＜2 μm),497.3 g kg-1;silt(2–20 μm),409.2 g kg-1;sand(20–2 000 μm),93.5 g kg-1;field capacity,29.1%;pHwater,6.4;total N,1.01 g kg-1;and organic carbon,10.57 g kg-1.Rainfall totaled 450.2 mm during the 2015 growing season and 455.4 mm during the 2016 growing season(Fig.1).Daily air temperatures averaged 27.17°C during the 2015 growing season and 26.89°C during the 2016 growing season(Fig.1).The current study used a japonica rice cultivar,i.e.,Oryza sativaL.Dangyujing 8.

Fig.1 Daily rainfall and air temperature during the rice growing season in 2015(a)and 2016(b)at the experimental site of the experimental base of Hefei Institutes of Physical Science,Chinese Academy of Sciences,Hefei,China.

Nitrogen fertilizers used in the current study were regular urea(46.4%N)and slow-release urea(43.2%N).The slowrelease urea containedca.5% organic-inorganic matrix material,which mainly consisted of polyacrylamide(0.04%),starch(0.06%),montmorillonite(3.0%),and ferric sulfate(1.9%).The organic-inorganic matrix material was added to molten urea to produce slow-release urea granules using the tower granulation method as described by Yanget al.(2017).Single super phosphate(16%P2O5)was used as phosphorus(P)fertilizer.Potassium chloride(60%K2O)was used as potassium(K)fertilizer.All fertilizers used in the current study were provided by Henan Xinlianxin Fertilizer Co.,Ltd.,Xinxiang,China.

Experimental design

Both field experiments consisted of three treatments,i.e.,control without N application(CK),regular urea treatment(RU),and slow-release urea treatment(SU).The experiments followed a randomized block design with three replicates.The size of each plot was 100 m2(10 m×10 m).A 1-m gap was left between the plots.In RU and SU,N fertilizer(regular or slow-release urea)was applied at a rate of 150 kg N ha-1.Before planting rice,70%N fertilizer was applied into the 0–20 cm layer,while at the tillering stage,the remaining 30%N fertilizer was applied as top dressing.The P(60 kg P2O5ha-1)and K fertilizers(60 kg K2O ha-1)were applied into the 0–20 cm layer in all plots before planting rice.Rice was planted by hand with a 20-cm spacing between plants.During rice growing seasons,the surface water in paddy fields was maintained at a depth ofca.5 cm.Weeds in the fields were manually removed.

A N leaching experiment was conducted to investigate N lossvialeaching,which included three treatments,i.e.,CK,RU,and SU.The experiment was conducted with five replicates.Nitrogen leaching was studied following the method reported by Yi(2009)and Yanget al.(2018a).Briefly,8.5 kg air-dried paddy soil was mixed thoroughly with 0.471 g N(as regular or slow-release urea).Then,leaching tubes(20 cm in diameter,40 cm in height)were packed with the mixture.Thereafter,2.5 L distilled water was added to the leaching tube to saturate the mixture,and after 12 h of equilibrium,more distilled water was added to the leaching tube to maintain a 5-cm water layer during leaching.Leachates were collected,940 mL each time(at 14–17 min intervals).Twelve leachates were collected in total.

An experiment was carried out to investigate N lossviaammonia emission,which included three treatments,i.e.,CK,RU,and SU.Each treatment was conducted with five replicates.Ammonia emission was studied following the method described by Yanget al.(2018a).Briefly,a mixture of 8.5 kg air-dried paddy soil and 0.471 g N(as regular or slow-release urea)was put in an emission chamber(20 cm in diameter,40 cm in height).The moisture of the soil column in each emission chamber was then adjusted to field capacity.Soil columns were incubated at 25°C.The volatilized ammonia was sampled using the acid trapping method described by Yanget al.(2018a).Ammonia was sampled at 3-d intervals until the 30th day of the experiment.

Sampling and laboratory analyses

Rice biomass,grain yield,and yield components were determined at maturity stage.Each entire plot,except for border rows,was harvested.Moisture of rice straw and grains was determined by oven-drying subsamples using the method described by Yanget al.(2018a).Panicle density was determined based on data from a 1-m2(1 m×1 m)area from each plot.Twenty panicles were collected randomly from each plot for measuring grain number per panicle.Thousand grain weight was determined using an automatic kernel-counting system.

At the anthesis stage of rice plants,paddy soil samples were collected from the 0–20 cm layer.In each plot,five soil cores were taken in a W shape.Thereafter,soil cores from a same plot were mixed thoroughly and treated as a single sample.Then,soil samples were extracted with 1 mol L-1potassium chloride solution and the concentration of inorganic N(ammonium N plus nitrate N)in the extracts was determined using a continuous flow analyzer(AA3,Bran+Luebbe,Germany),as described by Yanget al.(2018b).

Also at the anthesis stage,ten flag leaves were collected from each plot.Leaf samples from a same plot were mixed and cut into pieces to provide a single sample.Then,the samples were used for determination of nitrate reductase(NR)activity,glutamine synthetase(GS)activity,and chlorophyll concentration.The NR and GS activities were measured using the method reported by Sinhaet al.(2015).Chlorophyll concentration was determined using the method described by Fenget al.(2009).

The leachates were analyzed for urea-N using a spectrophotometer(Lambda 35,PerkinElmer,Co.,Ltd.,USA)as described by Niet al.(2013).The ammonia emission samples were analyzed using the titration method to determine the amount of volatilized ammonia-N with 0.005 mol L-1sulfuric acid solution,as described by Yanget al.(2017).

Rice growth traits(i.e.,plant height,root area,and leaf area index)were measured at the anthesis stage as well.In each plot,ten plants were sampled for measurement.Plant height was measured using a ruler.Root samples were collected according to the method described by Chenet al.(2014).Briefly,soil cores(30 cm in diameter,30 cm in height),which consisted of root system and bulk soil,were excavated manually from the field.Root samples with bulk soil were soaked in water and washed thoroughly using tap water.Then,the separated root samples were measured for root area with a root analysis system(WinRHIZO,Regent Co.,Ltd.,Canada).Leaf area was measured using a leaf analysis system(WinFOLIA,Regent Co.,Ltd.,Canada).Leaf area index was calculated as the ratio of total leaf area to planting area(Yanget al.,2018b).

Calculations and statistical analyses

Path analysis was conducted to determine the direct and indirect contributions of yield components to grain yield according to Leng(1992)and Qiaoet al.(2013)using data from all three treatments(i.e.,CK,RU,and SU)with three replicates(n=9).The total contribution of a yield component to grain yield was calculated as direct contribution plus indirect contribution.Yield components which had greater total contribution were more likely to decide the final value of grain yield.

Costs and profits in rice production were assessed.Nitrogen fertilizer cost was calculated as N application rate multiplied by N fertilizer price.Other costs included the costs of P and K fertilizers,seed,irrigation,field management,and land rent.Rice income was calculated as rice grain yield multiplied by rice grain price.Net profit was calculated as rice income minus total costs(N fertilizer cost+other costs).Fertilizer AE was defined as the increase in grain yield due to N application(Yanget al.,2018b).

Nitrogen leaching ratio was defined as the ratio of N leached from N fertilizer to total N in N fertilizer,and ammonia emission ratio was defined as the ratio of N in the volatilized ammonia to total N in N fertilizer(Yanget al.,2018a).The dynamics of cumulative N leaching ratio and cumulative ammonia emission ratio were fitted using the following logistic equation described by Zhanget al.(2009)and Nakahata and Osawa(2017):

whereyis cumulative N leaching ratio or cumulative ammonia emission ratio,xis leaching sequence number or time after urea application,anda,b,andkare constants.The constantarepresents the maximum value ofy;kreflects the changing rate ofy,with a largerkindicating thatyreaches its peak value faster.Whenxis equal to ln(b/k),the slope of the fitting curve or theychanging rate has the maximum value ofak/4.

Means were compared by the least significant difference(LSD)test(α=0.05)using Proc ANOVA in SAS 9.1(SAS Institute Inc.,USA).Path analysis was performed using Proc REG and Proc CORR in SAS 9.1.Logistic equations were fitted using OriginPro 2015(OriginLab Corporation,Northampton,USA).

RESULTS

Rice biomass,grain yield,and yield components

Compared with CK,the N-applied treatments(RU and,especially,SU)significantly increased rice biomass(P＜0.05)(Table I).During 2015,the treatment SU significantly increased biomass by 33.9%and 87.5%compared with the treatment RU and CK,respectively(P＜0.05);during 2016,biomass in SU was 15.0%and 69.5%higher than that in RU and CK,respectively(P＜0.05).

Compared with CK,the N-applied treatments(RU and,especially,SU)significantly increased grain yield(P＜0.05)(Table I).During 2015,grain yield in SU was 18.2%and 72.1%greater than that in RU and CK,respectively(P＜0.05);during 2016,SU significantly increased grain yield by 20.9%and 80.4%compared with RU and CK,respectively(P＜0.05).

Compared with CK,the N-applied treatments(RU and,especially,SU)significantly increased panicle density(P＜0.05),while they had little effect on grain number per panicle and thousand-grain weight(P＞0.05)(Table I).During 2015,panicle density in SU was 8.5%(P＞0.05)and 58.0%(P＜0.05)greater than that in RU and CK,respectively;during 2016,SU increased panicle density by 10.6%(P＞0.05)and 59.7%(P＜0.05)compared with RU and CK,respectively.Path analysis(Fig.2)showed that the total contribution of panicle density was 0.941 1 during 2015 and 0.914 7 during 2016,which was greater than those of grain number per panicle and thousand-grain weight.

Fig.2 Path analysis of the contributions of panicle density(x1),grain number per panicle(x2),and thousand-grain weight(x3)to rice grain yield(y)during the 2015(a)and 2016(b)growing seasons in the field experiment conducted at the experimental base of Hefei Institutes of Physical Science,Chinese Academy of Sciences,Hefei,China.Values 0.886 6,0.218 3,and 0.173 4 during 2015 and 0.791 4,0.283 6,and 0.036 3 during 2016 represent direct contributions,while the other values represent indirect contributions.Total contribution(TC)was calculated as the sum of direct contributions and indirect contributions.

TABLE IRice biomass,grain yield,and yield components during the 2015 and 2016 growing seasons in the field experiment conducted at the experimental base of Hefei Institutes of Physical Science,Chinese Academy of Sciences,Hefei,China

Net profits and AE

Compared with CK,the N-applied treatments(RU and,especially,SU)significantly improved the profitability of rice production(P＜0.05)(Table II).Nitrogen fertilizer cost in SU was US$23.5 and US$23.3 ha-1greater than that in RU during 2015 and 2016,respectively.Rice income in SU was US$473 and US$500 ha-1greater than that in RU during 2015 and 2016,respectively(P＜0.05).Therefore,the net profit in SU was US$450 and US$476 ha-1greater than that in RU during 2015 and 2016,respectively(P＜0.05).Nitrogen application is crucial for maintaining rice profitability.Net profit in CK(without N application)was low and even negative.

Fertilizer AE was improved in SU(Table II).During 2015,AE in SU was 58.1%greater than that in RU(P＜0.05).During 2016,SU significantly increased AE by 63.7%compared with RU(P＜0.05).

Soil inorganic N content and leaf N assimilation enzyme activities

Compared with CK,the N-applied treatments(RU and,especially,SU)significantly increased soil inorganic N concentration(P＜0.05)(Table III).In 2015,soil inorganic N in SU was 6.0%and 46.3%greater than that in RU and CK,respectively(P＜0.05).In 2016,SU increased soil inorganic N by 17.6%and 32.8%compared with RU and CK,respectively(P＜0.05).

The activity of NR in rice leaves was significantly improved in the N-applied treatments(RU and,especially,SU)compared with that in CK(P＜0.05)(Table III).During 2015,SU increased NR activity by 35.9%and 163.1%compared with RU and CK,respectively(P＜0.05).During 2016,NR activity in SU was 25.9% and 124.2% greater than that in RU and CK,respectively(P＜0.05).Compared with CK,the N-applied treatments(RU and,especially,SU)significantly improved GS activity in rice leaves(P＜0.05)(Table III).In 2015,GS activity in SU was 12.8%and 24.4%greater than that in RU and CK,respectively(P＜0.05).In 2016,SU increased GS activity by 27.9% and 65.6%compared with RU and CK,respectively(P＜0.05).

TABLE IINet profits of rice production and agronomic efficiencies(AEs)of the N fertilizers during the 2015 and 2016 growing seasons in the field experiment conducted at the experimental base of Hefei Institutes of Physical Science,Chinese Academy of Sciences,Hefei,China

Potential N loss risk

Cumulative N leaching ratio in RU was consistently higher than that in SU(Fig.3a).The N leaching dynamics were well fitted using logistic equations(P＜0.01)(Table IV).The maximum cumulative N leaching ratio in RU(a=46.33%)was larger than that in SU(a=37.25%).Cumulative N leaching ratio in RU(k=2.20)reached its peak value much earlier than that in SU(k=0.49).The largest N leaching rate was much higher in RU(ak/4=25.48)than in SU(ak/4=4.56).Furthermore,the largest N leaching rate appeared earlier in RU(ln(b/k)=0.56)than in SU(ln(b/k)=1.57).

The treatment RU had a consistently higher cumulative ammonia emission ratio than the treatment SU(Fig.3b).Dynamics of cumulative ammonia emission ratio in RU and SU were well described using logistic equations(P＜0.01)(Table IV).The greatest cumulative ammonia emission ratio in RU(a=9.03%)was higher than that in SU(a=6.05%).Cumulative ammonia emission ratio in SU(k=0.41)reached its peak value earlier than that in RU(k=0.38).The greatest ammonia emission rate was larger in RU(ak/4=0.86)than in SU(ak/4=0.62).The greatest ammonia emission rate in RU(ln(b/k)=3.58)appeared earlier than that in SU(ln(b/k)=3.88).

Fig.3 Dynamics of cumulative N leaching ratio(a)and cumulative ammonia emission ratio(b)in the laboratory N leaching and ammonia emission experiments with treatments of regular urea(RU)and slow-release urea(SU)applied to the paddy soil at 0.055 g N kg-1 soil.Error bars are standard deviations(n=5).

Rice growth traits and leaf chlorophyll

Compared with CK,the N-applied treatments(RU and,especially,SU)had significantly positive effects on rice plant height(P＜0.05)(Table V).During 2015,plant height in SU was 8.4% and 18.5% larger than that in RU and CK,respectively(P＜0.05).During 2016,SU increased plant height by 3.3% and 16.1% compared with RU and CK,respectively(P＜0.05).

Root area of rice was significantly greater in the Napplied treatments(RU and,especially,SU)than in CK(P＜0.05)(Table V).In 2015,the root area in SU was 22.6%and 34.9%larger than that in RU and CK,respectively(P＜0.05).During 2016,SU increased root area by 23.6%and 51.3%compared with RU and CK,respectively(P＜0.05).

Compared with CK,the N-applied treatments(RU and,especially,SU)significantly increased the leaf area indexof rice(P＜0.05)(Table V).During 2015,SU increased leaf area index by 5.8%(P＞0.05)and 121.5%(P＜0.05)compared with RU and CK,respectively.During 2016,leaf area index in SU was 8.7%(P＞0.05)and 136.6%(P＜0.05)greater than that in RU and CK,respectively.

TABLE IIISoil inorganic nitrogen(N)content and nitrate reductase(NR)and glutamine synthetase(GS)activities in rice leaves at the anthesis stage during the 2015 and 2016 growing seasons in the field experiment conducted at the experimental base of Hefei Institutes of Physical Science,Chinese Academy of Sciences,Hefei,China

TABLE IVParameter values of the logistic equation,y=a/(1+b×exp(-k×x))a),used to fit the dynamics of cumulative N leaching ratio and cumulative ammonia emission ratio in the laboratory N leaching and ammonia emission experiments with treatments of regular urea(RU)and slow-release urea(SU)applied to the paddy soil at 0.055 g N kg-1 soil

TABLE VRice plant height,root area,leaf area index,and leaf chlorophyll at the anthesis stage during the 2015 and 2016 growing seasons in the field experiment conducted at the experimental base of Hefei Institutes of Physical Science,Chinese Academy of Sciences,Hefei,China

Leaf chlorophyll concentration was significantly higher in the N-applied treatments(RU and,especially,SU)than in CK(P＜0.05)(Table V).During 2015,leaf chlorophyll in SU was 35.6% and 121.3% larger than that in RU and CK,respectively(P＜0.05).During 2016,SU significantly increased leaf chlorophyll by 41.4%and 96.5%compared with RU and CK,respectively(P＜0.05).

DISCUSSION

One of the crop growers’requirements for a novel fertilizer is the improvement of crop productivity(Yanget al.,2018a).Previous studies have reported that the application of controlled-or slow-release fertilizers was beneficial for improving rice productivity(Li P Fet al.,2018;Yanget al.,2018a).The novel slow-release urea used in the current study was different from any previously reported slow-release fertilizers.It is necessary to assess the effects of the slow-release urea on rice productivity.Our findings(Table I)showed that the slow-release urea had significantly positive effects on rice biomass and grain production.Therefore,the slowrelease urea meets the rice growers’requirements for rice productivity.Additionally,Jiaoet al.(2018)reported that both irrigation practices and N application in paddy fields contributed greatly to rice grain yields,and thus rice productivity would be improvedviaoptimizing irrigation and N fertilization practices.Nortonet al.(2017)also highlighted the importance of regulating soil water conditions to increase rice grain yields.Irrigation is an essential field management practice for rice production,and thus it is worthy of investigating whether rice productivity can be further improved in the SU treatmentviaadjusting irrigation practices.

Increases in grain yield are attributed to changes in yield components(Li Net al.,2018).We found that,when compared with the RU treatment,the SU treatment tended to increase panicle density and thousand grain weight,while it had inconsistent effect on grain number per panicle(Table I).Notably,the difference in yield components between RU and SU treatments was not significant.Therefore,it is hard to directly figure out which yield component contributed more to grain yield.Path analysis is a method suitable for quantification of the contributions of different yield components to grain yield(Qiaoet al.,2013).The results of path analysis showed that panicle density had the largest total contribution to grain yield,followed by grain number per panicle;while thousand grain weight had a small positive or even negative total contribution to grain yield(Fig.2).These findings indicate that panicle density was the major yield component influencing rice grain yield in the current study.Hence,increases in grain yield in the SU treatment were largely due to the increases in panicle density.Similarly,Chuet al.(2016)found that increases in productive tiller number(panicle density)were vital for rice grain yield,which increased grain yield by 18%–19%on average.Miniottiet al.(2016)reported that low rice grain yield was largely due to decreases in effective tiller number,indicating that panicle density is an important yield component influencing rice grain yield.Wanget al.(2016)found that sufficient supply of N led to greater panicle density and higher rice grain yield.Hence,the higher soil N availability in SU(Table III)could be partly responsible for the larger panicle density and grain yield(Table I).

Another requirement of crop growers for a novel fertilizer is that it can increase net profits in crop production(Sharma and Singh,2011;Yanget al.,2018a).Low-cost fertilizers are more likely to be accepted by crop growers(Sharma and Singh,2011).Low cost is one of the advantages of matrixbased fertilizers over other slow-release fertilizers(Niet al.,2013).However,the cost of matrix-based fertilizers is still higher than that of conventional fertilizers(Yanget al.,2017).It is important that the profit in matrix-based fertilizer treatments outweighs the cost.Otherwise,it is unlikely that crop growers would accept the fertilizer.Our findings showed that,although the N fertilizer cost in the SU treatment was higher than that in the RU treatment,rice income was much higher in SU than in RU treatment(Table II).Consequently,net profit in the SU treatment was≥US$450 ha-1higher than that in the RU treatment.These findings confirmed that,considering its profitability,the slow-release urea is acceptable for rice growers.Genget al.(2015)reported that rice grain yield in controlled-release urea treatments(applied at 70%of local rate)was similar to that in regular urea treatment(applied at 100%of local rate).Zhenget al.(2016)found that the combined application of regular urea and controlled-release urea increased crop yield and reduced fertilization costs.Hence,more work should be carried out to determine whether rice profitability can be improved by lowering slow-release urea application rate or combined application of slow-release urea with regular urea.

Agronomic efficiency is a parameter for evaluating the effectiveness of a fertilizer on improving crop productivity(Yanget al.,2017).Our results showed that agronomic effi-ciency was significantly greater in SU than in RU treatment(Table II).One explanation is that soil N availability was significantly higher in SU than in RU treatment(Table III),due to the lower N loss risk through N leaching and ammonia emission(Fig.3).Hence,more applied N was retained in the field to support rice growth in the SU treatment,leading to higher agronomic efficiency(Table II).Similarly,Liet al.(2017)reported that the application of controlled-release urea increased rice N uptake and reduced N loss(viaammonia emission and runoff),thus improving N use efficiency.Additionally,our results(Table III)also showed that the SU treatment significantly improved the activities of NR and GS in rice leaves.The improved leaf NR and GS activities(Table III)were beneficial for rice growth and N use,which were partly responsible for the increase in agronomic efficiency(Table II).Similarly,Yanget al.(2012)found that the application of controlled-release urea increased the activities of NR and GS in rice leaves,leading to more than 22%increase in apparent N use efficiency.Notably,de Borja Reiset al.(2018)reported that rice N use was closely related to the irrigation regimes,and rice N use was improved in the treatments where soil moisture was controlled at a moderate level.Nianget al.(2018)also found that rice N use was largely dependent on soil moisture conditions and water deficit greatly weakened the response of rice to N application.Li Zet al.(2018)reported that available N concentration in the soil was improved in treatments where soil moisture was controlled.Hence,more work should be carried out to determine whether rice N use efficiency in the SU treatment can be improved through modifying irrigation regimes.

Nitrogen loss risk(viaN leaching and ammonia emission)in the current study was assessed with laboratory experiments(Fig.3),and thus the results reflected the potential N loss risk.Under field conditions,N loss risk is affected by some field factors such as temperature(Yanget al.,2015)and plant growth traits(Yanget al.,2019).Therefore,more studies are needed to investigate the actual N loss risk with the slow-release urea application under field conditions.Li P Fet al.(2018)found that the application of controlled-release fertilizers reduced the ammonium concentration in the surface water in paddy fields,and thus led to lower N runoffloss.Wanget al.(2018)found that ammonia emissions in fields generally increased as N application rate increased.Also,Yanget al.(2015)reported that greater soil ammonium concentration due to N application led to larger amounts of ammonia emissions in fields.Hence,it is important to reduce N application rate to alleviate ammonia emissions.Since the slow-release urea application reduced potential N loss risk(Fig.3)and significantly increased rice grain yield(Table I),it is possible to decrease its application rate to further reduce N loss without yield loss.Additionally,Yaoet al.(2018)reported that deep placement of urea reduced ammonia emission.Overall,it is worthy of investigating whether decreasing application rate and adjusting application depth(in soil profiles)can further reduce N loss associated with slow-release urea application.

Biomass and grain production are highly dependent on photosynthetic assimilation and nutrient uptake(Kanget al.,2012;Yanget al.,2012).Our findings showed that the treatment SU significantly increased rice height and leaf chlorophyll concentration(P＜0.05),while it also tended to increase leaf area index(P＞0.05)(Table V).Improvements in rice height,leaf chlorophyll concentration,and leaf area index were beneficial for photosynthetic assimilation,and thus promoted rice growth and development.Similarly,Chuet al.(2016)found that a greater leaf area index was beneficial for rice grain yield.Li Net al.(2018)reported that both a higher leaf area index and a higher leaf chlorophyll concentration were beneficial for crop growth and productivity.Sone and Sakagami(2017)found that maintaining a high concentration of leaf chlorophyll was important for rice photosynthesis and growth.Okamiet al.(2016)reported that a greater leaf chlorophyll concentration in the top canopy was beneficial for rice photosynthetic assimilation.The higher soil N availability in SU(Table III)was partly responsible for the improvements in rice height,leaf chlorophyll concentration,and leaf area index(Table V).Similarly,Liuet al.(2018)reported that the leaf area index of rice was closely related to N availability,and leaf area index was an indicator of N status in rice plants.Furthermore,our results(Table V)also showed that SU significantly increased root area,which was beneficial for rice uptake of nutrients(such as N)and reducing N loss risk.

CONCLUSIONS

The slow-release urea used in this study contained selected organic and inorganic matrix materials and was different from previously reported matrix-based fertilizers.The treatment SU significantly increased rice biomass and grain yield.The higher grain yield in SU was largely due to increases in panicle density.The net profit and agronomic efficiency in SU were significantly higher than that in RU.The treatment SU had lower potential N loss risk compared with the treatment RU,and thus led to greater soil N availability.Improvements in rice growth traits and physiological parameters in SU were partly due to the greater soil N availability.These improvements were beneficial for rice growth,which was partly responsible for the higher productivity and agronomic efficiency in SU.Considering the higher productivity,higher profitability,and lower N loss risk,we conclude that the slow-release urea is a promising fertilizer for rice production.

ACKNOWLEDGEMENTS

This study was supported by the National Key R&D Program of China(No.2017YFD0301302),the National Natural Science Foundation of China(Nos.31601828 and 31500300),Anhui Science and Technology Major Project(No.18030701205),and the Science and Technology Service Network Initiative of Chinese Academy of Sciences(No.KFJ-STS-QYZD-008).We appreciate the reviewers’comments.