Fertilizer nitrogen loss via N2 emission from calcareous soil following basal urea application of winter wheat

ZHANG Yukun,WANG Rui,PAN Zhnlei,LIU Yn,ZHENG Xunhu,JU Xiotng,ZHANG Chong,BUTTERBACH-BAHL Klus,d nd HUANGBinxing

aState Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry,Institute of Atmospheric Physics,Chinese Academy of Sciences,Beijing,China;

bCollege of Earth and Planetary Sciences,University of Chinese Academy of Sciences,Beijing,China;

cCollege of Resources and Environmental Sciences,China Agricultural University,Beijing,China;

d Institute for Meteorology Climate and Atmospheric Environmental Research,Karlsruhe Institute of Technology,Garmisch-Partenkirchen,Germany

ABSTRACTThe ratio of nitrous oxide(N2O)to N2Oplus nitrogen gas(N2)emitted from soils(N2O/(N2O+N2))is regarded as a key parameter for estimating fertilizer nitrogen(N)loss via N2 emission at local,regional or global scales.However,reliable measurement of soil N2 emissions is still difficult in fertilized soil-crop systems.In this study,the Nloss via N2 emission following basal urea application(with a dose of 150 kg N ha-1)to a calcareo us soil cultivated with winter wheat was quanti f i ed using the helium-based gas-f l ow-soil-core technique.Emissions of N2 and N2O from sampled fresh soils were measured under simulated f i eld soil temperature and oxygen conditions.Our observation performed on the f i rst day after irrigation and rainfall events showed the highest N2 and N2Oemissions,which amounted to approximately 11.8 and 3.8μg Nh-1 kg-1 dry soil,corresponding to 3304 and 1064μg Nm-2 h-1,respectively.The N2O/(N2O+N2)molar ratios within about 10 days following fertilization ranged from 0.07 to 0.25,which were much larger than those at the other time.During the one-month experimental period,the urea-N loss via emissions of N2,N2O,and N2+N2O was 1.6%,0.6%,and 2.2%,respectively.Our study con f i rms that the widely applied acetylene-inhibition method substantially underestimates fertilizer N losses via N2 emissions from calcareous soils cultivated with winter wheat.

KEYWORDS Denitri f i cation;fertilizer nitrogen loss;N2 emission;N2O/(N2O+N2);calcareous soil

1.Introduction

Anthropogenic activities have dramatically increased the amount of reactive nitrogen(Nr)circulating in the biosphere and atmosphere,creating a series of environmental consequences,such as eutrophication,acidi f i cation,and air pollution(Fowler et al.2013).Terrestrial denitri f i cation converts 30%-60%of the Nrback into nitrogen gas(N2)(Ciais et al.2013),and thus contributes to close the global nitrogen(N)cycle(Gruber and Galloway 2008).Moreover,denitri f i cation is an important process of fertilizer Nloss from agricultural systems(Bouwman et al.2013),as it converts nitrate and nitrite into nitric oxide(NO),nitrous oxide(N2O),and N2.Among these gases,the N loss through N2emission is especially most uncertain in terrestrial N budgets because the high background atmospheric N2concentration makes soil N2emissions difficult to measure(Davidson and Seitzinger 2006).This situation is greatly hampering accurate quanti f i cation of terrestrial N budgets using experimental or modeling approaches.

Previous studies have mainly quanti f i ed the N loss via denitri f i cation in rice paddy f i elds or wetlands(Burgin and Groffman 2012;Wang et al.2017).By contrast,research on upland soils has been much less common.Most previous studies on N loss through denitri f i cation in upland soils have been based on the acetylene inhibition (AI)method(e.g.Cheng et al.2004;Ju et al.2009).Meanwhile,only a few researchers have adopted other methods that can speci f i cally determine N2emission,such as the gas-f l ow-soil-core(GFSC)technique(e.g.Scheer et al.2009)or the15N gas f l ux method(e.g.Buchen et al.2016).However,many studies have reported that the AI method,which usually determines the N loss in the form of N2+N2O,can severely bias the results owing to its shortcomings(Groffman et al.2006).Because of its sensitivity and high costs,the15N gas f l ux method is rarely applied in croplands,even though it directly measures N2emissions in-situ(e.g.Buchen et al.2016).The helium-based GFSC technique (Butterbach-Bahl,Willibald,and Papen 2002;Molstad,Dorsch,and Bakken 2007;Wang et al.2011)allows for direct measurements of the emissions of N2,N2O,and NO from soils under given conditions.Using the GFSC technique,in this study we attempted to quantify the fertilizer N loss via N2emission in an upland soil.In the North China Plain(NCP),typically with calcareous soils and widely cultivated with irrigated winter wheat and rainfed summer maize,excessive fertilizer-N is usually used as part of an intensive cultivation of both crops.For instance,Zhao et al.(2006)reported that N fertilizers were conventionally applied in the NCP at a rate of 550-600 kg N ha-1y-1,which more than doubled the optimum demand,286 kg N ha-1y-1,for both crops(Ju et al.2009).However,fertilizer-N losses via N2emission due to denitri f i cation in intensive agricultural soils have been poorly quanti f i ed.In particular,existing studies use different methods that feature very large uncertainties.For example,the reported fertilizer-N loss in the form of N2+N2O via soil denitri f i cation in wheat-maize rotation f i elds of the NCP ranges from 0.1%-3.3%to 13%-29%.The former rates of loss,which were measured using the AI method(Ju et al.2009),are obviously much lower than the later ones determined via the mass balance approach (Cai and Yang 1998).In fact,neither method separately quanti f i es the losses of individual denitri f i cation gases,i.e.NO,N2O,and N2.

In this study,soil samples were collected after basal urea application to a winter wheat f i eld in the NCP.Using the GFSCtechnique,we directly measured the emissions of N2,N2O,and NO from the fresh samples under the given conditions of soil temperature,moisture,and oxygen,which were the same as those in-situ at soil sampling.The objectives were to(1)identify the temporal variations in N2emissions and N2O/(N2O+N2) ratios following the basal N fertilization,and(2)quantify the rate of urea-N loss via N2emission.

2.Materials and methods

2.1.Description of experimental f i eld site

The investigated experimental f i elds,which werelocated in the northern suburbs of Beijing and belonged to the Shangzhuang Agricultural Experimental Station(39°48′N,116°28′E)of China Agricultural University,were long-term rotationally cultivated with winter wheat and summer maize.There is a typical calcareous f l uvo-aquic soil,which on average contains 28%clay(<0.002 mm),32%silt(0.002-0.05 mm),40%sand(0.05-2 mm),0.71%organic carbon,and 0.08%total Nin the0-20cm layer,showing an average pH(H2O)value of 8.1 and a bulk density(BD)of 1.4±0.1 g cm-3in the cultivation horizon.

The conventional fertilization treatment(Ncon)of a long-term experiment initiated 12 years ago for the typical winter wheat-summer maize rotation system was selected for thisstudy.It hasthree replicated square f i eld plots(64 m2for each),which were designed by completely randomized blocks with those of other f i eld treatments.The location of each f i eld plot was f i xed since establishment of the experiment.The winter wheat was sown in early October and harvested in late May or early June.The preceding crop straw was fully removed from the f i eld.Urea wasapplied asthe basal Nfertilizer for the winter wheat,at a rate of 150 kg N ha-1,prior to rotary plough tillage(to 10 cm in depth)followed by sowing.Both phosphorus and potassium fertilizers were basally applied together with the urea at a rate of 100 kg ha-1in phosphorus pentoxide and potassium oxide.During the observation period(1-26 October 2017)of this study,28 mm of sprinkling irrigation was supplied on 7 October to promote seed germination(Figure 1(a)).For more detailed information about the experimental site and f i eld management practices,refer to Huang et al.(2013)and Huang,Ju,and Yang(2017).

2.2.Field sampling and soil-core preparation

To determine the soil emissionsof N2,aswell as N2Oand NO,topsoil(0-20 cm)samples were collected from the winter wheat f i eld of Nconafter basal fertilization.Using the GFSCtechnique,we directly measured the emissions of these three gases from the soil samples incubated under soil conditionsof temperature,moisture,and oxygen that were the same as those in-situ when the soil samples were collected.

In thisstudy,thesoilswere sampled seven timesin the one-month period following basal fertilization.The soil sampling wascarried out once every two daysduring the f i rst two weeks following fertilization and/or following an irrigation/rainfall event(but with exceptions on 8 and 10 October due to excessively wet soil for f i eld sampling operation).During the remaining period,the soil sampling was conducted weekly.About 0.8 kg of soil was randomly collected each time for each replicated plot.When soil samples were collected,soil environmental variables(temperature,moisture,and oxygen concentration of the soil air)were recorded(see subsection 2.4 for details).The collected soil samples were stored under cool conditions and transported to the laboratory within 2 h after sampling.Prior to measurement,the soil sample from each plot was f i lled into four stainless-steel rings(5.6 cm in diameter,4 cm in height)and repacked at a BD of 1.4 g cm-3,which wasthe same asthe value measured in-situ.The four repacked soil cores were enclosed into one of the three incubation vessels of the applied GFSC system for simultaneous dynamical measurements of N gas emissions from the three f i eld replicates.

Figure 1.Temporal dynamicsof(a)precipitation,irrigation,and soil temperature,and(b)moisture and oxygen concentration in soil air during the observation period.Soil moisture is expressed as water-f i lled pore space(WFPS).The soil data were measured at the 10 cm depth.

2.3.Laboratory measurement of N gas emissions from soil cores

The GFSCtechnique adopted in this study for detection of N2,N2O,and NOemissions from incubated soil cores has been described in detail in previous publications(Liao et al.2013;Wang et al.2011).Immediately after gas-tight sample installation,the air in soil pores and the enclosure headspace was replaced with an N2-free arti f i cial atmosphere (20% oxygen in helium)at a temperature above 0°Cbut below 4°Cby thoroughly f l ushing for 20-25 h.Then,the enclosure headspaces were continuously f l ushed under the‘f i eld soil condition'for 4 h to re-establish a‘natural'gradient of gases within each soil core.Thereafter,four emission rates(6 h for each)were measured for each of the three nitrogenousgases during a 24-h period with headspace f l ushing at the‘f i eld soil condition'.The temperature and oxygen concentrationsmeasured in-situ at the time of f i eld sampling were set for the incubated soil cores to establish the‘f i eld soil conditions'.The moisture of soil cores was not adjusted since it could represent the f i eld water condition at sampling.This was because(1)the fresh soils were immediately incubated after sampling and(2)the soil air replacement and the following headspace f l ushing did not signi f i cantly reduce the soil moisture(Wang et al.2013).

The emission rate of each gaswasdetermined with f i ve headspace gas concentrations measured over a 4-h period at 1-h intervals.The detection limits of the applied GFSCsystem for the N2,N2O,and NO emission rateswere0.65,0.002,and 0.01μg Nh-1kg-1of dry soil(d.s.),respectively.The measured emission rate of each gas for a replicated f i eld plot on the sampling day was reported as the average of the four measurements of the corresponding vessel.The gas emission rates inμg N h-1kg-1d.s.were also converted to units ofμg Nm-2h-1for the0-20 cm f i eld soil layer(with an average BDof 1.4 g cm-3).

2.4.Auxiliary f i eld measurements

Daily precipitation was automatically recorded using a meteorological station located adjacent to the experimental f i eld.The moisture,temperate,and oxygen concentration of soil air at the 10-cm depth in each f i eld plot were automatically recorded at 30-min intervals using a combined moisture and temperature sensor(5TM,Decagon Devices Inc.,USA)and an oxygen sensor(SO-100,Apogee Instruments,Logan,UT,USA).The data acquisition was interrupted during the soil plough tillage stage(3-6 October)before the wheat was sown.According to the soil BD(g cm-3)and a theoretical soil particle density(ρ,g cm-3),the recorded soil water content(θv,cm3cm-3)was converted to water-f i lled pore space(WFPS,%)following WFPS=θv/(1-BD/ρ).

3.Results

3.1.Environmental variables

The soil temperature ranged from 9.1 to 20.9°C(mean:13.9°C)during the one-month experimental period(Figure 1(a)).The irrigation and subsequent precipitation supplied 80 mm of water to the f i eld within four days(Figure 1(a)),and thus increased the soil moisture from 29%to 70%WFPS,while simultaneously decreasing the oxygen concentrations in soil air([O2])from 21%to 18%(Figure 1(b)).These two variables showed a signi f i cant negative correlation,which could be described by[O2]=-0.077WFPS+24.36(R2=0.60,p<0.05).

3.2.Temporal variations of N gas emissions and their molar ratios

Before basal fertilization,as Figure 2(a)shows,the emissions rates of the three nitrogenous gases were all low enough to be close to their detection limits.Fertilization greatly promoted Ngasemissionsfrom soil.For instance,the magnitudes of N2,N2O,and NO emission rates on average reached 1.8,0.3,and 0.5μg N h-1kg-1d.s.corresponding to 504,84,and 140μg Nm-2h-1,respectively,on the f i rst day after fertilization,which were approximately 2,96,and 108 times higher than those prior to urea application.Subsequent irrigation,occurring on the fourth day after urea incorporation into the soil,and heavy rain falling immediately after,substantially stimulated the emissions of N2and N2O.On the f i rst day after three rainy days,both gases showed maximum emissions,of 11.8±6.6 and 3.8±2.1μg N h-1kg-1d.s.(Figures1(a)and 2(a)),corresponding to 3304±1848 and 1064±588μg Nm-2h-1for N2and N2O,respectively.On this day,however,the NO emission level dropped to a value(around 0.02μg N h-1kg-1d.s.)very close to its detection limit.Afterwards,the emission rates of both N2and N2O also quickly decreased,to levels of approximately 1.1 and 0.005μg N h-1kg-1d.s.,respectively(Figure 2(a)).

Figure 2.Temporal dynamics of(a)N2,N2O,and NOemissions from the soil of the winter wheat f i eld following basal fertilization,and molar ratiosof(b)N2O/(N2O+N2)and(c)NO/N2O.The downward black and gray arrowsindicate the datesof basal fertilization(3 October)and sprinkling irrigation(7 October),respectively.The vertical bars represent the standard errors of three spatial replicates.

The N fertilization,as well as the subsequent irrigation/rainfall,resulted in signi f i cant variations in molar ratios of the N gas product.As Figure 2(b)illustrates,N2O/(N2O+N2)ratios increased from a level close to zero before fertilization,to 0.07-0.25.Afterwards,the N2O/(N2O+N2)ratios gradually dropped back to the near-zero level.The ratios of NO/N2O were approximately 3.5,but exhibited no signi f i cant difference between before and after fertilization under conditions of low soil moisture(<30%WFPS).However,they decreased signi f i cantly,to 0.03-0.21,when the soil moisture increased to over 60%WFPS.As the soil moisture dropped below 55%,however,the NO/N2O ratios rose again to the initial level or even higher(Figure 2(c)).

3.3.Fertilizer-N loss via emissions of gases

The soil N2,N2O,and NO emissions cumulated to 1456.9±628.1,309.5±161.3,and 40.1±7.2μg N kg-1d.s.,corresponding to 4.1±1.8,0.9±0.5,and 0.11±0.01 kg N ha-1,respectively,during the onemonth period following fertilization.Assuming that the emission rate prior to N addition could represent the background gas emissions under unfertilized conditions,the rates of fertilizer-Nloss via N2,N2O,and NO emissions during the observation period following basal fertilization were estimated at 1.6%±1.2%,0.6%±0.3%,and 0.07%±0.01%,respectively.

4.Discussion

4.1.Fertilizer-N loss via N2 emission

In this study,the soil emissions of N2and N2Ofollowing basal fertilization of winter wheat on average accounted for 1.6%and 0.6%of theapplied urea-N,respectively.This fertilizer-N loss rate via emission of N2+N2O(2.2%on average)washigher than the range(0%-0.4%)measured by the AImethod for wheat f i eldsin China(adapted from Wang and Yan(2016)).In particular,our loss rate was 21 times higher compared to the average value(0.1%)measured using the AImethod in a winter wheat f i eld with similar soil,climate,and management practices(Ju et al.2009).On thecontrary,thisstudyresulted in amuch lower fertilizer-N loss via N2emission as compared to some other previous results,e.g.10.5%(corn f i eld)measured by the15N gas f l ux method(Li et al.2002),13%-29%(wheat f i eld)determined by the15Nmassbalancemethod(Caiand Yang 1998),and 10%-70%(irrigated cotton f i eld)detected by the GFSCtechnique(Scheer et al.2009).The higher soilmoisture and temperature than thisstudy may account for the much higher losses observed in corn and cotton f i elds by Liet al.(2002)and Scheer et al.(2009).

It should benoted that thisstudymight to someextent have underestimated the fertilizer-N losses via emissions of N2,N2O,and/or NO.Two reasonslikely account for the possible underestimation.Oneisthat abrupt changesand especially thetrue emission peaksmight havebeen unfortunately missed dueto thelimitationsof instrument capacity aswell as inconvenience of f i eld sampling operation(Figure2(a)).Becauseof theselimitations,low observation frequencies of at most once every two days following fertilization,irrigation and heavy rainfall eventshad to be adopted for soil sampling and subsequent gas measurements.The other is the lack of a true control treatment without Napplication.Deductionof thebackground emissionsestimated aspre-fertilization levelslikely biased the resultsdue to the unavoidable‘memory'effect of preceding fertilizer amendments.Therefore,further studies are still needed to assess the effects of improving measurement scheduleson the accuracy of fertilizer-Nlossvia N2emission measured using the method adopted in this study.The assessment also needs to involve the use of other techniquessuitable for undisturbed measurements in-situ,such as the15N gas f l ux method(Stevens et al.1993).

4.2.Molar ratios of N gas emissions

Thisstudy con f i rmed substantial N2emissions occurring simultaneously with N2Oat high soil moisture resulting from heavy rainfall/irrigation events following Nfertilization.Using the15Ngas f l ux method to measure N2lossesin corn f i eldsreceiving the same Ndose asthis study,Liet al.(2002)observed a peak emission of 2666 μg N m-2h-1following heavy rainfall after fertilizer addition,which is comparable with our maximum.Using a similar GFSC system,Scheer et al.(2009)observed a lower maximum N2emission(361μg Nm-2h-1on average)from soilsof irrigated cotton f i elds than thisstudy.Thelow N2emission wasprobablydueto the lower N application compared to this study(75 versus 150 kg N ha-1).However,much higher N2emissions from the denitri f i cation of calcareous soils in the NCP than those of this study(133-500 versus 11.8μg Nh-1kg-1d.s.)were observed under anaerobiosis,even though the soil was treated with lower(67%-81%)N doses(Wang et al.2013;Yuan et al.2017).These differences in N2emissions can be attributed to the oxygen availability determined by precipitation,irrigation,or experimental manipulation.This is because oxygen is the most important regulator for denitri f i cation(Bouwman et al.2013),in which the activity of N2O reductase encoded by the nos gene is generally thought to increase with decreasing oxygen availability(Chapuis-Lardy et al.2007).In this study,the observed lowest oxygen concentrationsin soilporeswereapproximately 18%,which is still high enough to induce a less efficient expression of the nos gene.Thereafter,the reduction of N2O to N2was limited and thus led to much lower N2emissions(Figure 2(a))than those under anaerobiosis(Wang et al.2013).The measurements under anaerobiosis resulted in N2O/(N2O+N2)ratios of approximately 0.37 for a calcareous soil(adapted from Wang et al.(2013)),which is higher than in our study(Figure 2(b)).Thissuggeststhat limitation by oxygen does not induce higher N2O/(N2O+N2)ratios than denitri f i cation under anaerobic conditions.

Our investigations showed that fertilization increased N2O/(N2O+N2)ratios,making them much higher than prior to N amendment(Figure 2(a,b)).However,our observed N2O/(N2O+N2)ratios(0.07-0.25)following urea application were lower than previously reported values(～0.51-0.69)for fertilized intact soil cores of a winter wheat f i eld in France(Mathieu et al.2006)and fertilized upland soils in China(Wang and Yan 2016).The aforementioned possible underestimation of N2and N2O emissions due to the adopted insufficient measurement frequencies might have contributed to the lower ratio valuesof thisstudy.However,thetruth remainsto be revealed by further studies.

The N2O/(N2O+N2)ratio(r)can be used as a key parameter,together with given N2O emissions(FN2O),to estimate soil N2emissions(FN2)at local,regional and/or global scales,following FN2=(1/r-1)FN2O(Butterbach-Bahl et al.2013;Schlesinger 2009).Accordingly,underestimated N2O/(N2O+N2)ratios would lead to overestimated N2emissions,and vice versa.This emphasizes the need for further research to elucidate the regulating factors that in f l uence the accuracy and precision of measured N2O/(N2O+N2)ratios for soils under different conditions.

5.Conclusion

Using the GFSC technique,emissions of N2,N2O,and NO from a calcareous soil following basal urea fertilization of winter wheat were measured under simulated f i eld conditions of temperature,and pore oxygen concentrations and fresh soil moisture.Our study reveals that irrigation/heavy rainfall following Nfertilization not only substantially promotes the emission of both N2and N2O,but also greatly enhances the N2O/(N2O+N2)ratio.Approximately 1.6%and 2.2%of the basally applied urea-N was estimated to have been lost via emissions of N2and N2+N2O,respectively.However,these percentages were possibly underestimations of the true losses owing to instrumental limitations and the inconvenience of f i eld sampling operations.These limitations might also have led to underestimated N2O/(N2O+N2)ratios,which is a potentially key parameter for estimating fertilizer-N losses via N2emissions under different conditions.Further studies are still needed to clarify this issue.

Acknowledgments

The authors thank Yongfeng FU,Xiaoming FENG,Lin WANG,Xiaoxia HU,Lei MA,and Hepu LIUfor their assistance with the f i eld and laboratory measurements.

Disclosure statement

No potential con f l ict of interest was reported by the authors.

Funding

This work was jointly supported by the National Key Research&Development Program[grant number 2017YFD0200100]and the National Natural Science Foundation of China[grant numbers 41877333,41303060,and 41830751].