Application of controlled-release urea increases maize N uptake,environmental benefits and economic returns via optimizing temporal and spatial distributions of soil mineral N

Mingxue SUN ,Juan LI ,Lili ZHANG ,Xiaomeng SU ,Ning LIU ,Xiaori HAN ,Songjiang WU,Zhentao SUN,* and Xiangdong YANG,*

1Institute of Agricultural Resources and Regional Planning,Chinese Academy of Agricultural Sciences/State Key Laboratory of Efficient Utilization of Arid and Semi-arid Arable Land in Northern China/Key Laboratory of Plant Nutrition and Fertilizer,Ministry of Agriculture and Rural Affairs,Beijing(100081)

2Agricultural Development Service Center of Zhuanghe,Zhuanghe 116499(China)

3Agricultural Technology Extension Center,Wafangdian 116300(China)

4College of Land and Environment,Shenyang Agricultural University,Shenyang100866(China)

ABSTRACT The creation of controlled-release urea(CRU)is a potent substitute for conventional fertilizers in order to preserve the availability of nitrogen(N)in soil,prevent environmental pollution,and move toward green agriculture.The main objectives of this study were to assess the impacts of CRU’s full application on maize production and to clarify the connection between the nutrient release pattern of CRU and maize nutrient uptake.In order to learn more about the effects of CRU application on maize yields,N uptake,mineral N(Nmin)dynamics,N balance in soil-crop systems,and economic returns,a series of field experiments were carried out in 2018-2020 in Dalian City,Liaoning Province,China.There were 4 different treatments in the experiments:no N fertilizer input(control,CK);application of common urea at 210 kg ha-1 (U),the ideal fertilization management level for the study site;application of polyurethane-coated urea at the same N input rate as U(PCU);and application of PCU at a 20%reduction in N input rate(0.8PCU).Our findings showed that using CRU(i.e.,PCU and 0.8PCU)may considerably increase maize N absorption,maintain maize yields,and increase N use efficiency(NUE)compared to U.The grain yield showed considerable positive correlations with total N uptake in leaf in U and 0.8PCU,but negative correlations with that in PCU,indicating that PCU caused excessive maize absorption while 0.8PCU could achieve a better yield response to N supply.Besides,PCU was able to maintain N fertilizer in the soil profile 0-20 cm away from the fertilization point,and higher Nmin content was observed in the 0-20 cm soil layer at various growth stages,particularly at the middle and late growing stages,optimizing the temporal and spatial distributions of Nmin.Additionally,compared to that in U,the apparent N loss rate in PCU was reduced by 36.2%,and applying CRU(PCU and 0.8PCU)increased net profit by 8.5%to 15.2%with less labor and fertilization frequency.It was concluded that using CRU could be an effective N fertilizer management strategy to sustain maize production,improve NUE,and increase economic returns while minimizing environmental risks.

Key Words:apparent N loss,fertilization management,green agriculture,maize yield,N balance,N use efficiency

INTRODUCTION

With a planting area of 41.3 million hectares and a total yield of 2.61×102million tons in 2020,maize(Zea maysL.)has emerged as China’s first major food crop(National Bureau of Statistics,2020).The Northeast region of China,one of the main production areas of maize,produced nearly 30% of the country’s total (National Bureau of Statistics,2018).The use of nitrogen(N)fertilizer is crucial for maize production,but the average N use efficiency(NUE)of maize in China is low with only 21.0%-29.1%(Chenet al.,2011;Yu and Shi,2015),because reactive N in the soil-crop system is easily lost caused by the high temperatures and frequent rainfall occurring during the maize growing season (Luet al.,2019).

It is often necessary to use top-dressing fertilizers to meet the N demands for maize production throughout its entire growing season.However,it will be a labor-intensive and challenging job to implement this practice in maize fields.Currently,in the Northeast region of China,many farmers apply excessive amounts of N fertilizers with one-time application to meet high yield requirements.Unfortunately,this approach leads to significant losses of active N,mainly through leaching losses(Fanget al.,2006;Hanet al.,2016;Yanet al.,2016).To address the challenge of increasing maize productivity while minimizing environmental risk,it is crucial to enhance the NUE of maize throughout its entire growing season.By optimizing N application rates,notable improvements in grain yield and reduced environment pollution can be achieved,contributing to the long-term sustainability of soil health.For instance,Renet al.(2022)have demonstrated that a 40%reduction in N surplus can be attained by simply optimizing the N application rate without requiring changes in farmers’ operational practices.This reduction can be achieved with a 15%-19% decrease in N application.Furthermore,research has shown that the combined application of 60%controlled-release fertilizers(CRFs)and 40%urea can enhance environmental sustainability by 2.2%-4.6%in Southwest China(Lyuet al.,2021).Controlled-release urea(CRU)application offers an effective solution to improve NUE and reduce environmental risks by synchronizing N release with the maize’s demand throughout the growing season(Azeemet al.,2014;Qiaoet al.,2016;Yanget al.,2021).China,being the largest consumer of CRFs,has significant potential for large-scale adoption of CRU.The increasing global usage of CRU in agricultural production indicates that CRU is likely to be a prominent direction for future developments in novel fertilizers(Trenkel,2010).

Numerous studies have been conducted to assess the effects of CRU on maize production,with a particular focus on its impact on maize growth,N supply,and the environment.The majority of these studies have shown that a one-time application of CRU can improve maize growth and yield,enhance NUE,and minimize negative environmental effects(Zhaoet al.,2013;Genget al.,2016;Zhenget al.,2016).However,it is worth noting that some studies have reported contrasting results.For instance,no significant differences were also observed in maize yields,NH3volatilization,and N2O emissions between CRU and urea applications (Li,2018;Xieet al.,2020).The inconsistency in these findings could be attributed to various factors such as crop traits,soil properties,fertilizer types used,field management practices,and climatic conditions.It is important to consider these factors when evaluating the effectiveness of CRFs like CRU,as they can significantly influence the study results.Further research is needed to better understand the specific conditions under which CRU can consistently contribute to improved maize production and reduced environmental risks.

In order to assess the overall impact of CRU application in maize production,various analysis methods such as metaanalysis,life cycle assessment,ecosystem economic benefits,and adjusted emergy accounting have been employed(Zhanget al.,2019;Yaoet al.,2021).Zhuet al.(2020) reported that the use of CRU can significantly increase maize yield by 7.4%and total N uptake(TNK)by 9.2%.Additionally,CRU application has been found to increase maize yield by 5.3%,improve NUE by 24.1%,and reduce active N losses(Zhanget al.,2019).Taking environmental benefits into account,CRU application can also lead to a 6.4%increase in revenue(Yanget al.,2021).Overall,CRU offers a winwin strategy for increasing staple grain production while mitigating climate change globally.However,the specific mechanisms through which CRU affects crop growth,soil,and the environment are not yet fully understood.Some studies have suggested that CRU application can match N supply with plant demand,maintain N availability in the soil,improve the content of mineral N(Nmin),and enhance N accumulation and assimilation during specific growth stages of maize(Jiaet al.,2014;Genget al.,2016;Zhenget al.,2016;Penget al.,2017;Fenget al.,2021;Rahmanet al.,2021).Nevertheless,a detailed and comprehensive explanation of the role of CRU in the soil-plant-environment system is still lacking.Therefore,there is an urgent need for integrated analysis to understand how CRUs function within this system.Furthermore,previous reports have not compared the effects of CRFs with optimal conventional fertilizer management practices (Lawrenciaet al.,2021).To address these gaps,field experiments were conducted in Dalian City,Liaoning Province,China to investigate the effectiveness of CRU(a polyurethane urea with a 3-month release period)on maize yield,N uptake,dynamics of Nminin the soil during the growing season,N balance in the soilcrop system,and economic benefits.The study aimed to test the hypothesis that CRU application can maintain or increase maize yield and N uptake while providing environmental benefits and economic returns by synchronizing with crop demand and optimizing the spatial distribution of soil Nmin.The assessment also focused on how CRU application can maintain Nmincontent in the 0-20 cm soil layer and reduce Nminleaching,thereby improving ecological sustainability.By comprehensively analyzing the effects of CRU on the soil-maize system,this study aims to provide a theoretical guide for optimizing future CRU application in Liaoning Province in Northeast China.

MATERIALS AND METHODS

Site description

The experiments were conducted over the course of three maize growing seasons,from 2018 to 2020.Two different sites were selected for the experiments.The first site was located in Zhuanghe,Dalian City,Liaoning Province,China(122°55′N,39°41′E),and the experiments were carried out in 2018.The second site was in Wafangdian,Dalian City,Liaoning Province,China(121°37′N,39°35′E),and the experiments were conducted in 2019 and 2020.The weather data used in the study,as shown in Fig.1,was obtained from the Shenyang City Public Weather Service Platform.Both experimental sites have a typical temperate continental monsoon climate.The average air temperature during the maize growing season was 21.8,21.6,and 21.0°C in 2018,2019,and 2020,respectively.The cumulative precipitation during the maize growing season was 537.5,594.4,and 718.2 mm in 2018,2019,and 2020,respectively(Fig.1).The soil at the experimental sites was classified as clay brown loam.The basic physical and chemical properties of the 0-20 cm soil are listed in Table I.

Fig.1 Dynamics of daily mean temperature and precipitation at experimental sites for three maize growing seasons in 2018-2020.

Experimental design

The cropping system in the experiments is single maize every year and the experiments were performed in a randomized plot design with three replicates during three maize growing seasons.The plot size was 3 m× 7 m (width×length).Four treatments were established including:1)no N fertilizer input(control,CK);2)application of conventional urea at a rate of 210 kg N ha-1(U),representing the optimum fertilization management at the local site;3)application of CRU (a polyurethane-coated urea with a 3-month N release period) at the same N input rate as U (PCU);and 4) application of PCU with a 20% reduction in N input(0.8PCU),at a rate of 168 kg N ha-1.The fertilizers used in the experiments were urea(46%N),CRU(44%N),calcium superphosphate(12%P2O5),and potassium chloride(60%K2O).For the U treatment,the fertilizer N was applied with 60%as a basal fertilizer and 40%as a topdressing fertilizer at the twelve-leaf collar(V12)stage.The P and K fertilizers were applied as basal fertilizer at one time before maize sowing,at a rate of 90 kg P2O5ha-1and 90 kg K2O ha-1,respectively.In the PCU and 0.8PCU treatments,CRU was applied once before maize sowing.The maize variety used in the experiments was Danyu 2199.Maize was planted in early May and harvested in early October.The row spacing for maize planting was 60 cm,with a plant spacing of 40 cm.Ridge farming was employed,and fertilizer was incorporated into soil at a depth of 13 cm.Field management practices for all treatments followed local practices as needed.Due to the annual rainfall of approximately 600 mm,irrigation was not necessary for the maize fields as the rainfall provided sufficient moisture for maize growth.

TABLE ISoil basic physical and chemical properties in the 0-20 cm layer at the two experimental sites,Zhuanghe and Wafangdian,Dalian City,Liaoning Province,China

Samplingand measurement

Soil sampling was conducted randomly before sowing in the 0-20 cm soil layer at both experimental sites.Five soil samples were collected from each plot,and these samples were mixed to create one composite sample.The soil samples were divided into 2 parts,one part immediately taken to the laboratory to measure soil Nminand the other part air-dried at room temperature.The air-dried samples were used to determine soil organic matter,pH,total N,total P,total K,alkali-hydrolysable N,Olsen-P,and NH4OAc-K,according to the methods described by Lu (2000).In 2018,three composite soil samples were collected from each plot in the 0-20,20-40,and 40-60 cm soil layers before maize sowing and after harvest to determine the Nmincontent.In 2019,nine soil samples (from U and PCU treatments)and three soil samples(from CK)were collected from each plot in the 0-20,20-40,and 40-60 cm soil layers on the 1st,7th,14th,21st,28th,60th,90th,and 143rd days after fertilization to determine the Nmincontent.Before Nmindetermination,the fresh soil samples were passed through a 2-mm sieve and soaked in a 0.01 mol L-1CaCl2solution for mineral N extraction.The resulting filtrate was used to measure the concentrations of ammonium-Nand nitrate-Nusing a three-channel flow analyzer(TRAACS2000,Bran and Luebbe,Germany).The Nmincontent was calculated as the sum ofandcontents.

At the maturity stage of maize in each year(2018-2020),three representative plant samples were collected from each plot.The maize plants were divided into stems,leaves,beans,and cobs,and each part was weighed separately.The plant samples were then oven-dried at 105°C for 30 min and further dried to a constant weight at 75°C.After drying,the samples were ground and passed through a 2-mm sieve,and their weights were recorded.The N concentrations in plant parts were determined using the Kjeldahl method(Lu,2000).The TNK was calculated as the sum of the N contents in all above-ground plant parts.Total N uptake is an important parameter used to calculate NUE.The maize yield from each plot was recorded at harvest.

Temporal and spatial distributions of Nmin in micro-zone experimental trial

In 2019,a micro-zone experimental trial was conducted at Shenyang Agricultural University in Shenyang,Liaoning Province.The purpose of the trial was to determine the temporal and spatial distributions of Nminduring the maize growing period.The trial consisted of 3 fertilization treatments,i.e.,control with no N input (CK),urea (U),and polyurethane-coated urea with a 3-month N release period(PCU),with three replicates.The sampling schematic diagram of the micro-zone trial is shown in Fig.2.Each small plot had an area of 0.36 m2(0.6 m×0.6 m).Fertilization was carried out by applying 16 g of N fertilizer into the 10-20 cm soil layer in the center of each small square under PCU and U treatments on May 1,2019.There were 12 sampling points in each small plot.Soil samples were collected at each sampling point from the 0-20,20-40,and 40-60 cm soil layers on the 21st and 143rd days after fertilization.Destructive sampling was performed,meaning that the entire plant was removed from each micro-zone for analysis.The fresh soil samples collected were used to determine the concentrations ofand.Except for the specific fertilization practice mentioned above,all other agricultural management practices for this micro-zone trial were performed similarly to those described in the previous experiments.

Fig.2 Sampling schematic diagram of temporal and spatial distributions of mineral N in micro-zones.

Calculations

The NUE(%),N agronomic efficiency(NAE)(kg kg-1),partial productivity of N fertilizer(PPNF)(kg kg-1)were calculated as follows(López-Bellidoet al.,2005):

whereUNandU0are the TNK (kg N ha-1) at maturity with and without N fertilizer input,respectively,YieldNand Yield0are the maize yields(kg ha-1)in the treatments with and without N fertilizer input,respectively,andFNis the rate of N fertilizer input(kg N ha-1).

Soil Nminaccumulation(Naccum)was calculated as follows(Sainjuet al.,2007):

whereCNminis the soil Nminconcentration(mg kg-1),Dis the soil depth(cm),andρis the soil bulk density(g cm-3).

The N balances (0-60 cm soil layer) in the soil-crop system were calculated according to the following equations:

where NMN is the net mineralized N,Nris the residual soil mineral N,Niis the initial soil mineral N,Nsis the apparent N loss,Nfis the N fertilizer application rate,ANRR is the apparent N recovery rate,TNKFis the TNK in fertilization treatments,TNKCKis the TNK in CK,ARNR is the apparent residual Nminrate,Nr,Fis the Nrin fertilization treatments,Nr,CKis the Nrin CK,and ANLR is the apparent N loss rate.

Measurement of N release characteristics of CRU in water

To measure the N release rate of CRU,10 g CRU particles were weighed and packed into a nylon net bag.The net bag containing the CRU particles was then soaked in 200 mL distilled water in a plastic bottle.The bottle was covered and incubated at 25°C.Each sample was replicated twice to ensure accuracy.The N release rate of the CRU samples was measured on the 1st,4th,7th,10th,14th,21st,and 28th days after incubation.On each measurement day,all the extract from the bottle was poured out and the bottle was then replenished with 200 mL deionized water to continue the incubation.The release amount of N in the extract was determined using thep-dimethylaminobenzaldehydespectrophotometry method (ISO,2016).Based on the N release curve shown in Fig.3,it can be observed that the N release period of CRU was approximately 3 months.The N release curve exhibited an S-shaped pattern during the incubation in 25°C water (Fig.3).This release pattern closely matched the maize N uptake mode,indicating that the N release from CRU is well-suited for meeting maize’s N requirements.

Fig.3 N release curve of controlled-release urea,a polyurethane-coated urea with a 3-month N release period,over time in 25 °C water.

Statistical analysis

The data processing and analysis of variance(ANOVA)were conducted using Microsoft Excel 2019(Microsoft Corporation,USA)and SAS(SAS Systems,USA),respectively.The effects of fertilization treatments (T) on maize yield,TNK,NUE,NAE,PPNF,and economic benefits and year(Y)during 2018-2020,as well as the interaction effects between T and Y,were analyzed using two-way ANOVA.The means were separated using the least significant difference(LSD)test at a significance level of 5%.Additionally,correction analyses were performed for maize yield,TNK,N uptake in above-ground plant parts,and NUE in the three fertilization treatments.Linear regression analysis was conducted,and figures illustrating the changes in Nmincontent with sampling time were created using Origin 2018.The N balance in the soil-maize system was calculated.The majority of Nminconsumption by the maize plant occurred in the 0-60 cm soil layers due to its root distribution.N inputs included fertilizer N input,initial soil Nmin,and net mineralized N during the maize growing period,while N outputs consisted of TNK,Nrin the 0-60 cm soil layer after harvest,and Ns.Origin 2018 (Origin Lab,USA) was also utilized for generating other figures.Furthermore,Matlab(MathWorks,Inc.,USA)was employed to create the temporal and spatial distributions of Nminin the U and PCU treatments.

RESULTS

Effects of fertilization treatments on maize yield

The effects of T and Y on maize yields were found to be significant(P ＜0.001)individually.However,their interaction(Y×T)effect showed no significant differences(Fig.4).Over the course of 3 years(2018-2020),the maize yield in CK ranged from 8.0 to 9.1 t ha-1.The yields in the fertilization treatments varied between 8.3 and 12.1 t ha-1.The average maize yields for CK,U,PCU,and 0.8PCU treatments were 8.5,10.1,10.5,and 10.9 t ha-1,respectively.It should be noted that the maize yields in the treatments were relatively low in 2018 due to unfavorable weather conditions,particularly a heavy rainfall of 213 mm on August 20,2018(Fig.1a),which led to maize lodging.Furthermore,a significant difference(P ＜0.05)in maize yield was observed between 0.8PCU and U treatments.This suggests that PCU could result in higher yields compared to U under extreme climate conditions,such as heavy rainfall.

Fig.4 Effects of different fertilization treatments, i.e.,control with no fertilizer N input (CK),urea (U),a polyurethane-coated urea with a 3-month N release period (PCU),and 20% reduction in PCU (0.8PCU),on maize yields in 2018-2020.Vertical bars indicate standard errors of the means (n=3).Bars with the same letter(s) within a year are not significantly different at P ＜0.05 using the least significant difference test.The significance of year(Y),treatment(T),and their interaction(Y×T)on maize yield are shown.The asterisks***indicate significant difference at P ＜0.001.NS=not significant.

Effects of fertilization treatments on TNK and NUE

According to Table II,the T,Y,and their interaction significantly influenced(P ＜0.05)TNK and NUE in maize.Application of CRU(PCU and 0.8PCU)resulted in a significant improvement in TNK compared to application of U.Specifically,in the PCU treatment,TNK in maize increased by 19.1%(Zhuanghe in 2018),15.5%(Wafangdian in 2019),and 15.8%(Wafangdian in 2020),respectively.On average,the PCU and 0.8PCU treatments increased TNK by 16.7%and 19.5%,respectively,compared to the U treatment.Significant differences in NUE were also observed between the CRU (PCU and 0.8PCU) and U treatments over the 3 years.In PCU and 0.8PCU treatments,maize NUE was 17.1%-18.1%and 14.6%-24.9%,respectively,higher than that in the U treatment.On average,the NUE in the PCU and 0.8PCU treatments increased by 21.2% and 24.2%,respectively,compared to the U treatment.These results indicate that applying CRU can significantly enhance maize NUE.Additionally,the interaction of Y and T had a significant effect on PPNF.In 2019 and 2020,the PPNF in the 0.8PCU treatment,which had a lower N fertilizer input,was significantly higher than those in the PCU and U treatments.Overall,the findings suggest that applying CRU,specifically PCU and 0.8PCU,can lead to increased TNK,NUE,and PPNF in maize cultivation compared to applying U alone.

Correlation analyses of various indicators under different fertilization treatments

Correlation analyses were conducted to explore the relationship among various indicators,including above-ground biomass,maize yield,TNKs of maize in different plant parts,and NUE under different fertilization treatments(Table III).Maize yield and biomass in the U,PCU,and 0.8PCU treatments had positive correlations with TNK and TNK in bean.Additionally,a significant positive correlation(P ＜0.05)was observed between maize yield and TNK in leaf in the U treatment.However,maize yield had a negative correlation with TNK in leaf in the PCU treatment.This suggests that a higher N amount in leaf did not necessarily result in a higher maize yield in the PCU treatment.On the other hand,a significantly positive correlation(P ＜0.01)was found between maize yield and TNK in leaf in the 0.8PCU treatment.These findings align well with the above results,which highlight that applying CRU(PCU and 0.8PCU)led to improved N uptake,maintained or increased maize yield compared to applying U,and subsequently increased NUE.The potential for increasing maize yield was particularly notable when applying PCU at a 20%lower N application rate.Overall,these correlation analyses support the notion that applying CRU can enhance N uptake and improve maize yields compared to conventional urea application,with corresponding increases in NUE.

TABLE IIEffects of different fertilization treatments,i.e.,control with no fertilizer N input(CK),urea(U),a polyurethane-coated urea with a 3-month N release period(PCU),and 20%reduction in PCU(0.8PCU),on N use efficienciesa) in maize in 2018-2020 and effects of year(Y),treatment(T),and their interaction(Y×T)using two-way analysis of variance(ANOVA)

Effects of different fertilization treatments on temporal and spatial distributions of mineral N

The temporal and spatial distributions of Nminin the soil reflects the soil’s ability to supply N in different treatments.Mineral N consists ofand,both of which showed a decreasing trend with increasing soil depth during the maize growing period(Fig.5).Thecontent in the 0-20 cm soil layer increased shortly after fertilization and gradually decreased over time,reaching lower levels ranging from an average of 0.5 to 0.8 mg kg-1at the maturity stage of maize.Thecontent in the 20-60 cm soil layer fluctuated within a narrow range.A regression analysis revealed a gradually decreasing trend ofin the 0-20 cm soil layer over time.Thecontent in the 20-60 cm soil layer was smaller than that in the 0-20 cm layer.At harvest,thecontents in the PCU and U treatments reached lower levels,with an average of 9.8 and 1.1 mg kg-1,respectively.Furthermore,thecontent in the PCU treatment was higher than that in the U treatment in different growing periods and soil layers.The dynamics of Nminin different treatments followed a similar pattern to that of.On the 60th day after fertilization(V12 stage),the average Nmincontent in the 0-60 cm soil layer for the U and PCU treatment was 46.8 and 56.6 kg ha-1,respectively.On the 90th day after fertilization(tasseling stage),the average Nmincontent in the 0-60 cm soil layer in the U treatment was 62.9 kg ha-1,significantly lower than that (118.1 kg ha-1)observed in the PCU treatment.Adequate Nminlevels still remained in the 0-60 cm soil layer at the maturity stage,being approximately 4 times higher in the PCU treatment than that in the U treatment.These findings suggest that PCU can maintain N availability throughout the entire maize growing period.

Fig.5 Regression analyses on(a,d,and g),(b,e,and h),and mineral N(c,f,and i)contents in 0-20(a-c),20-40(d-f),and 40-60(g-i)cm soil layers as time extended under different fertilization treatments,i.e.,control with no fertilizer N input(CK),urea(U),and polyurethane-coated urea with a 3-month N release period(PCU).n=10 for CK and n=72 for U and PCU.

Figure 6 illustrates the temporal and spatial distributions of Nminin the U and PCU treatments on the 21st(seedling stage)and 143rd(at harvest)days after fertilization.In each sampling time,the Nmincontent showed a decreasing trend with increasing distance from the fertilization point in both the U and PCU treatments within each soil layer.On the 21st day after fertilization,the Nmincontent in the 0-20 cm soil layer at a radius of 10,20,and 30 cm in the U treatment was 18.6,17.3,and 10.7 mg kg-1,respectively(Table IV).In the PCU treatment,the Nmincontent in the 0-20 cm soil layer at the corresponding radius was 31.5,27.9,and 14.4 mg kg-1,respectively.Compared to U,the PCU treatment significantly increased the Nmincontent by 69.0% (radius 10 cm) and 61.0%(radius 20 cm)in the 0-20 cm soil layer;however,there was no significant increase at a radius of 30 cm.The Nmincontent in the U treatment decreased noticeably with increasing soil depth.In the PCU treatment,there were no significant differences in the Nmincontent between 20-40 and 40-60 cm soil layers,although the Nmincontent in the 0-20 cm soil layer was significantly higher than that in the 20-60 cm soil layer.The Nmincontent in the 0-60 cm soil layer exhibited a decreasing trend over time,particularly in the 0-20 cm soil layer.

On the 143rd day after fertilization,the Nmincontent showed no large temporal and spatial variations in the 0-60 cm soil layer in the U treatment,ranging from 1.1 to 2.4 mg kg-1.However,in the PCU treatment,there was still more Nmindistributed around the fertilization point,with 18.4 mg kg-1at a radius of 10 cm and 9 mg kg-1at a radius of 20 cm in the 0-20 cm soil layer,which was much higher than that in the U treatment.The distribution of Nminappeared to be more centralized in the PCU treatment,while it was relatively scattered in the U treatment(Fig.6).This indicates that CRU releases nutrients gradually and in a controlled manner,limiting the wide migration of fertilizer-N and optimizing the temporal and spatial distributions of mineral N.It can be inferred that CRU has a good N supply ability,thereby improving maize yield.

TABLE IVTemporal and spatial distributions of mineral N content at radii of 10,20,and 30 cm from the fertilization point in 0-20,20-40,and 40-60 cm soil layers on the 21st and 143rd days after fertilization under two fertilization treatments,i.e.,urea(U)and polyurethane-coated urea with a 3-month N release period(PCU)

Fig.6 Temporal and spatial distributions of mineral N in PCU(a and b)and U(c and d)treatments on the 21st(a and c)and 143rd(b and d)days after fertilization.Take the fertilization point on soil surface as origin(0,0,0)and X,Y,and Z axes represent east-west direction,south-north direction,and up-down direction,respectively.PCU=polyurethane-coated urea with a 3-month N release period;U=urea.

Effects of different fertilization treatments on N balance in soil-crop system

The N balance in the soil-crop system in 2018 is shown in Table V.The N fertilizer inputs in the U and PCU treatments accounted for the majority of the total N input,with a proportion of 54.8%.The TNK was the main N output in all treatments,representing 88.9%in CK,50.1%in the U treatment,and 53.8%in the PCU treatment.The results from Table V indicate that TNK and Nrin the PCU treatment were significantly higher than those in the U treatment.Additionally,the Nsin the PCU treatment was significantly lower compared to the U treatment.The ANRR and ANLR in the PCU treatment were 18.1%higher and 36.2%lower,respectively,than those in the U treatment.These findings suggest that applying CRU(at the same N input rate as U)can provide a greater amount of fertilizer N for crops to absorb,while minimizing fertilizer N losses through various pathways compared to U application.This indicates that CRU helps retain more fertilizer N within the soil-crop system,thereby contributing to the improved NUE observed.

Economic benefits

The economic return and benefit analysis,following the methodology described by Thompsonet al.(2000)and Liet al.(2020),are presented in Table VI.The results of benefit analysis showed that both Y and T had significant effects(P ＜0.001 orP ＜0.01)on yield benefit,net benefit,net benefit change relative to U,and return on investment for maize production.However,there was no significant(P ＞0.05)interaction effect between Y and T on these economic parameters.Due to unfavorable weather conditions,the net benefits in different treatments were relatively low in 2018 than in 2019 and 2020.However,in 2018,the net benefit in the 0.8PCU treatment was 48.2% higher (P ＜0.05)than that in the U treatment,primarily due to lower labor cost and higher yield benefit.On average,application of 0.8PCU increased the farmer’s income by 1 495.9 and 839.2 CNY ha-1,respectively,compared to U and PCU.Furthermore,in 2018,application of 0.8PCU significantly improved(P ＜0.05)the return on investment,with a 22.3%increase compared to U.Although the cost of CRU was higher than that of U,the total cost of applying CRU was lower than that of U,indicating that selecting CRU as a N fertilizer input is a profitable approach.Overall,the economic analysis suggests that applying CRU,particularly at a rate of 0.8PCU,can enhance farmers’income and provide a better return on investment compared to using conventional urea.

TABLE VN balancea) (0-60 cm soil layer)in soil-crop system under different fertilization treatments,i.e.,control with no fertilizer N input(CK),urea(U),and polyurethane-coated urea with a 3-month N release period(PCU),in 2018

DISCUSSION

Effects of different fertilization treatments on maize yield and N uptake

In this study,application of PCU significantly increased the average N uptake and NUE of maize by 16.7% and 21.1%,respectively,compared to U.These findings are consistent with previous results of Huet al.(2013).The use of CRU has been shown to increase N uptake by 13%(Li,2018;Xieet al.,2020)and improve NUE by 21.9%-24.1%(Zhanget al.,2019;Zhuet al.,2020).This improvement in N uptake and NUE is mainly attributed to the integrated patterns of synchronizing N release with crop needs and reducing various N losses from the soil-crop system,which leads to increased N uptake by crop (Zhanget al.,2019;Yaoet al.,2021).Additionally,the application of CRU delays N release,providing higher total N supply and a greater proportion of N at later stages.However,it was observed that the increase in NUE in the PCU treatment did not result in a significant increase in maize yield (by 4.3% on average) compared to the U treatment (Fig.3),which aligns with the findings reported by Li(2018).One possible reason for this is that the N application rate used in this study was already optimized for this region,resulting in very little yield gap between the PCU and U treatments.Moreover,it was found that leaf N content showed a negative correlation with maize yield in the PCU treatment(Table III),suggesting that higher leaf N content did not translate into higher yields.This indicates that the oversupply of N in the PCU treatment led to luxury absorption of N by the crop,which is supported by the results showing relatively high leaf N uptake in relation to total N uptake at harvest(Table III).Previous reports have also indicated that excessive use of N fertilizer does not provide yield benefits(Ahmedet al.,2017)and that an appropriate application rate of CRU can promote the translocation of N from stem and leaf into grains(Liet al.,2020).In this study,it was observed that the 0.8PCU treatment had similar maize yields to the PCU and U treatments,but had significantly higher NUE than the U treatment and higher PPNF than the PCU treatment.This supports the idea that an appropriate reduction in N application rate with CRU,at the optimized urea N level for this region,can maintain maize yield while increasing NUE.Furthermore,the yield in the 0.8PCU treatment was significantly higher than that in the U treatment in 2018,indicating that maize in the 0.8PCU treatment exhibited better stress resistance under unfavorable weather conditions.The results from the 3-year study confirmed that PCU could achieve stable or increased crop yields with a 20%reduction in N input rate,demonstrating that CRU application can ensure maize yield compared to U.

Effects of different fertilization treatments on soil mineral N distribution,N balance and N losses in soil-crop system

Our study demonstrated that PCU was able to maintain higher content of Nmin,particularly,in the 0-60 cm soil layer compared to U at different maize growth stages.This is consistent with previous reports that have shown that,compared to U,CRU application leads to higher soil Nmincontent,especially in shallow layers,particularly at later growth stages (Huet al.,2013;Zhenget al.,2016;Liuet al.,2019).It has been observed thatin the U treatment tends to migrate to deeper soil layers.In our study,the total Nmincontent in the 0-60 cm soil layer in the PCU treatment was 198.3 kg ha-1,which was 14 times that in the U treatment after harvest,consistent with previous findings (Liuet al.,2019).At the maize tasseling stage,maize plants still require a great amount of N,and PCU can provide suitable N supply instead of U.

The N absorption patterns of maize can be summarized as follows:the N content in maize is the lowest at the V4 stage(seedling stage,about 30 d after sowing),lower at the V12 stage (large trumpet stage,about 45 d after sowing),the highest at the R3 stage (milk stage,about 75 d after sowing),and higher at the R6 stage(physiological maturity stage,about 110 d after sowing)(Garciaet al.,2018).Zhaoet al.(2015) reported that soilcontent showed a decreasing trend as maize grew after CRU application,which is consistent with our findings.Specifically,the highest soil Nmincontent was observed in the PCU treatment at the jointing stage,approximately 30 d after sowing.At this stage,bothand Nmincontents exhibited a release peak,followed by a subsequent decline.After 60 d of sowing,theand Nmincontents remained relatively stable(Fig.S1,see Supplementary Material for Fig.S1).

Therefore,compared to U,PCU can maintain higher soil Nmincontent at the later stages of maize growth(from 60 to 90 d after sowing),ensuring an adequate nutrient supply for the crops.In contrast,U application leads to a rapid increase in soil available N within two weeks,resulting in N deficiency during the late growing period(Zhuet al.,2020).Even when U is split-fertilized into 2 applications,it can be challenging to meet the N requirements of maize with long growing period in Northeast China.Thus,a one-offapplication of PCU is a promising alternative strategy.In addition to the above results,our study also focused on spatial variations in soil Nmincontent.Higher Nmincontent in the PCU treatment was observed at radii of 10 and 20 cm in the 0-20 cm soil layer,both on the 21st day and after harvest,compared to the U treatment.The controlled-release mechanism of PCU prevents fertilizer N from diffusing and migrating extensively.It has been demonstrated that PCU provides N in a sustainable manner,maintaining soil N availability at high levels and continuously supporting crop growth(Liuet al.,2021;Rahmanet al.,2021).

In terms of N balance in the soil-crop system,the application of CRU resulted in a 36.2% reduction in ANLR,an 18.1% increase in ANRR,and a 4-fold increase in Nrcontent compared to U.This finding supports the notion that CRU application promotes crop N uptake,decreases active N losses,and increases residual Nmincontent,which is consistent with previous research (Jiet al.,2017).In Northeast China,N leaching loss through conventional farm management practices is the primary pathway for N loss.In fact,it has been observed that N can leach to a depth of 3 m through a15N-labeled tracer field experiment (Yuanet al.,2021).In our study,no fertilizer N was found in the 0-60 cm soil layer after harvest in the U treatment,likely because fertilizer N had leached into soil layers deeper than 60 cm.However,the Nrcontent in the PCU treatment reached 57.2 kg ha-1and the ANRR was 18.1%,indicating that CRU could delay the leaching ofto deeper soil layers.Additionally,a portion of the residual Nminin the 0-60 cm soil layer remained available for the next crop.Therefore,replacing common urea with CRU is more beneficial for the sustainable development of agricultural systems.

Comprehensive effects of CRU in soil-crop system

Indeed,crop production systems are highly complex and can exhibit diverse responses to nutrient application.It is important to consider various factors,including agronomic,environmental,and economic aspects,when evaluating the effectiveness of fertilization management strategies in specific regions (Bruulsema,2009).To assess the suitability of a fertilization management strategy,it is necessary to conduct a comprehensive evaluation that combines scientific conclusions with the interests of farmers and the government.Economic evaluations play a crucial role in this process.In the case of CRU application,it has been found to provide economic advantages by saving labor and time while increasing yield benefits(Lawrenciaet al.,2021).This demonstrates the potential economic benefits associated with applying CRU as a fertilization management strategy.Proper management of N application through the use of CRU has the potential to achieve a better balance between yield,grain quality,and soil fertility,thereby promoting sustainable development in agriculture(Liuet al.,2021).Our study’s findings indicate that CRU outperformed conventional urea even under optimal application conditions.To maximize the benefits of CRU,it is important to combine its use with appropriate management practices.This includes adhering to the principles of 4R Nutrient Stewardship,i.e.,applying the right source of nutrients,at the right rate,at the right time,and in the right place.By following these principles,farmers can optimize the effectiveness of CRU and achieve sustainable agricultural practices.

CONCLUSIONS

The findings of this comprehensive 3-year study elucidated the effectiveness of CRU in various aspects of maize production in Northeast China.The PCU fertilizer,at an N input rate of 210 kg ha-1,was found to maintain maize yield and promote N uptake by an average of 16.7%compared to U.Additionally,it improved NUE by 21.1% on average.A reduced N input strategy of 0.8PCU,with a 20%reduction in N input,showed even better coordination in the N transformation of maize,resulting in higher increases in N uptake and NUE compared to PCU.Throughout the growing period,the Nmincontent in the soil was consistently higher in the PCU treatment compared to the U treatment,both in different growing periods and different soil layers.However,there was a decreasing trend in soil Nmincontent over time,particularly in the 0-20 cm soil layer.After harvest Nmin(ranging from 1.1 to 2.4 mg kg-1) showed no large spatial variation in the 0-60 cm soil layer for the U treatment.In contrast,the PCU treatment exhibited higher Nminconcentrations around the fertilization point,being 18.4 (radius 10 cm) and 9.0 (radius 20 cm) mg kg-1in the 0-20 cm soil layer.Furthermore,the PCU treatment effectively decreased apparent N losses and ANLR in the N balance of the soil-crop system,leading to higher economic benefits compared to the U treatment.This individual case study provides a theoretical basis for the application of CRU in maize production in Northeast China.Overall,the use of CRU can be considered as a farming practice to address various N management challenges faced in maize production.

ACKNOWLEDGEMENT

This work was supported by the National Key R&D Program of China (No.2022YFD1700605),the National Natural Science Foundation of China(Nos.31872177 and 31972511),and the Fundamental Research Funds for Central Non-profit Scientific Institution,China(No.1610132-023005).

SUPPLEMENTARY MATERIAL

Supplementary material can be found in the online version.

CONTRIBUTION OF AUTHORS

Mingxue SUN and Juan LI contributed equally to this work.