A Comparative Study of Nitrogen Loss after Application of Biochar Coated Urea and Common Urea in Vegetable Soil at Chaihe Catchment of Dianchi Lake

Huihui WANG,Zhengyi HU,Xiaoqi ZHU,Guohui ZHOU

College of Resources and Environment,University of Chinese Academy of Sciences,Beijing 100049,China

With the increase of multiple crop indexes,land use degree becomes increasingly higher[1].In order to ensure crop yield,China possesses the highest application amount of nitrogen fertilizer in the world[2].Excessive application of nitrogen fertilizerand unreasonable fertilization modes not only cause the waste of fertilizer and reduce the utilization efficiency of nitrogen fertilizer[3],but also bring pollution risks to water resource and environment,such as the eutrophication of water bodies,nitrate pollution of groundwater[3-4]and air pollution[5].At present,exploring an effective approach to control and reduce agricultural pollution to the environment,especially from the source,has become a hot issue in agricultural and the environmental research in recent years.

A large number of studies have shown that slow/controlled-release fertilizer can improve the utilization efficiency of nitrogen fertilizer and reduce nitrogen loss[6],especially ammonia volatilization and leaching losses[7].Due to complex production technology and high cost of slow/controlled-release materials,prices of slow/controlled-release fertilizer remain high.However,most commercially available polymer coating materials are non-biodegradable high molecular weight polymers synthesized with chemical methods[6],which easily cause damage to soil structure,and continuous application of these materials for several years will cause pollution to soil.Biochar can absorb and retain nutrients in the fertilizer,achieve the slow/controlled-release effect,significantly reduce soil nitrogen loss,improve soil fertility,increase crop yield and maintain soil ecosystem balance due to its porosity,large specific surface area,surface negative charge,charge density and other properties[8].Low-cost available crop straw biomass feedstock can be processed by carbonization to prepare biochar coating materials as slow/controlledrelease fertilizer.Preparing environmentally friendly biochar coated slow/controlled-release fertilizer can not only effectively solve the problem of slow/controlled-release material pollution,but also reduce nitrogen loss in farmland soil.

As one of the main flood control and irrigation channels in Jinning County,Chaihe catchment is located in the east of Jinning County,Kunming City,through which water eventually flows to Dianchi Lake.Chaihe catchment is also the major vegetable production base in Kunming City,with high agricultural cropping index and application amount of chemical fertilizers and pesticides.Therefore,controlling and reducing nonpoint source pollution caused by nitrogen fertilizer in farmland from the source has important significance for alleviating water eutrophication in Chaihe catchment and Dianchi Lake.In this study,a pot experiment was carried out to compare effects of different nitrogen levels and application of newly developed biochar coated urea and common urea on the yield ofBrassica campestrisL.and nitrogen loss caused by ammonia volatilization and nitrogen leaching invegetable soil at Chaihe catchment,which provided the basis for scientific and reasonable application of nitrogen fertilizer and control ofagricultural nonpoint source pollution in vegetablegrowing area at Chaihe catchment of Dianchi Lake.

Materials and Methods

Experimental materials

SoilTypical vegetable soil was collected as experimental soil from Chaihe catchment in Jinning County,Kunming City,Yunnan Province (in the south of Dianchi Lake),which mainly consisted of red soil that was developed from sandstone and conglomerate,pH 6.34,containing 2.1% organic matter,530 mg/kg total nitrogen,3.54 mg/kg available phosphorous.

FertilizerCommon urea(Urea)and biochar coated urea(BCU)were applied as nitrogen fertilizer;calcium superphosphate(CaP2H4O8)and potassium sulfate (K2SO4)were used as phosphate fertilizer and potassium fertilizer,respectively.Specifically,BCU(nitrogen content:29% )was produced by Professor Hu Zheng-yi from College of Resources and Environment,University of Chinese Academy of Sciences.Based on water dissolution test at 23℃,the dissolution time of nitrogen from BCU was about 28 d.

Plant materialThe potted plant wasB.campestrisL.,which was purchased from Beijing JingYan YiNong Sci-Tech Development Center.

Experimental design

The pot experiment was carried outin the greenhouse ofBeijing Academy of Agriculture and Forestry Sciences.One blank treatment(CK)and two nitrogen fertilizer treatments(Urea and BCU)were designed at four nitrogen application levels.Thus,a total of six treatments were set,including CK,U1,U2,U3,B2 and B3 that contained 0,400,320,280,320 and 280 mg/kg nitrogen,respectively.Specifically,Urea was applied in treatments U1,U2 and U3;BCU was applied in treatments B2 and B3.Each treatment was repeated three times.All these treatmentswere applied with 275 mg/kg P2O5and 62 mg/kg K2O.

The plastic pots were 20 cm in diameter and 15 cm in height,which were loaded successively (from bottom to top)with 2 cm of gravel(to cover the drain hole),600 g of experimental soil,and 400 g of soil that was mixed with nitrogen,phosphate and potassium fertilizers(approximately 5 cm in thickness),respectively.After seed germination,B.campestrisL.seedlings were transplanted to the plastic pots,five seedlings per pot,with the planting depth of 0.5-1.0 cm.During the experimental process,B.campestrisL.seedlings were irrigated quantitatively with 500 ml of pure water each time,to reduce the interference of other factors.The positions of plastic pots were changed successively every 2 d,to reduce the error brought by position differences.The pot experiment lasted 45 d from May 30,2013 to July 14,2013.

Sample collection and analysis

Closed chamber absorption method is a common method for ammonia collection[9].As an absorbent,dilute sulphuric acid exhibits higher recovery,precision and sensitivity than boric acid[9].In this study,using 0.01 mol/L dilute sulphuric acid as absorbent,volatilized ammonia was collected regularly in a closed chamber every day after fertilization.After collection,ammonium nitrogen content was determined with indophenol blue colorimetric method[10].Ammonia volatilization amounts in various urea fertilizer treatments and CK were compared until those in urea fertilizer treatments and CK remained consistent for several continuous days.

Atthe growth stage ofB.campestrisL.,leaching water was collected five times to determine ammonium nitrogen,nitrate nitrogen and total nitrogen.Ammonium nitrogen was determined with indophenol blue colorimetric method;nitrate nitrogen was determined with dual-wavelength ultraviolet spectrophotometric method;total nitrogen amount was determined with alkaline potassium persulfate digestion-ultraviolet spectrophotometric method[10].At the end of the experiment,B.campestrisL.roots were washed to determine the fresh weight as plant biomass.

Data processing

The determined results were averaged as experimental data.In this study,the amount of ammonia volatilization and nitrogen leaching in Urea and BCU treatments have subtracted that in CK treatment.Experimental data were analyzed using Excel and SPSS 17.0 software.

Results and Analysis

Ammonia volatilization loss

As shown in Fig.1A and Fig.1B,the maximum ammonia volatilization amount was proportional to nitrogen application amount in three Urea treatments(U1＞U2＞U3).The amount of ammonia volatilization reached the maximum at 3-5 d after Urea application.Specifically,the amount of ammonia volatilization in U1 reached the maximum on the 3rdd;the amount of ammonia volatilization in U2 and U3 reached the maximum on the 5thd.Since the 8thd after Urea application,ammonia volatilization amount in various treatments declined rapidly and tended to be consistent,indicating the end of ammonia volatilization in Urea treatments.BCU treatments possessed similar dynamic curves of ammonia volatilization to Urea treatments,and the maximum ammonia volatilization amount was proportional to nitrogen application amount(B2＞B3).Compared with Urea treatments at the same nitrogen application level,the maximum ammonia volatilization amount in BCU treatments was reduced by 18.4% -21.7%.Within 8 days after fertilization,ammonia volatilization amount in BCU treatments was lower than that in Urea treatments.However,since the 8thd after fertilization,ammonia in Urea treatments was almostcompletelyvolatilized,while ammonia in BCU treatments was still volatilizing.In the subsequent monitoring period,the amount of ammonia volatilization in BCU treatments was higher than that in Urea treatments.Accordingly,BCU fertilizer could release nitrogen slowly over time.

As shown in Fig.1C,cumulative curves of ammonia volatilization in various urea fertilizer treatments exhibited a similar trend,but cumulative ammonia volatilization amount varied among different stages.Within the first 8 days of ammonia volatilization,cumulative ammonia volatilization amount in various urea fertilizer treatments increased rapidly.After 8 days of ammonia volatilization,various urea fertilizer treatments exhibited significant differences in the increase of cumulative ammonia volatilization amount.Specifically,cumulative ammonia volatilization amount in Urea treatments almostremained unchanged,while cumulative ammonia volatilization amount in BCU treatments increased constantly but slightly,which was consistent with the dynamic changes of ammonia volatilization from different fertilizers(Fig.1A,Fig.1B).At 1-20 days after fertilization,cumulative ammonia volatilization in three Urea treatments was higher than that in BCU treatments.Since the 20thd after fertilization,cumulative ammonia volatilization in two BCU treatments exhibited an overall trend of B2＞U3.In addition,under the same nitrogen sources and different nitrogen application levels,cumulative ammonia volatilization was proportional to the amount of nitrogen application.Moreover,at the same nitrogen application level,cumulative ammonia volatilization in Urea treatments was 3.7% -21.9% higher than that in BCU treatments.

Nitrogen leaching loss

As shown in Table 1,fertilizer type and amount of nitrogen application affected the leaching amount of different nitrogen forms.Specifically,cumulative leaching of nitrate nitrogen and total nitrogen in three Urea treatments was significantly higher than that in BCU treatments(P＜0.05);cumulative leaching amount of ammonium nitrogen in Urea treatments was remarkably lower than that in BCU treatments(P＜0.05).At different nitrogen application levels,leaching amount of ammonium nitrogen,nitrate nitrogen and total nitrogen was proportional to nitrogen application level.In three Urea treatments,leaching amount of ammonium nitrogen,nitrate nitrogen and total nitrogen demonstrated a descending order of U1＞U2＞U3;in two BCU treatments,leaching amount of ammonium nitrogen,nitrate nitrogen and total nitrogen was generally in a descending order of B2＞B3.Therefore,reducing the amount of nitrogen application mightdecline nitrogen leaching.In addition,ammonium nitrogen leaching exhibited no significant difference among three Urea treatments;nitrate nitrogen leaching in U1 was significantly higher than that in U2 and U3 (P＜0.05),but no significant difference was observed in nitrate nitrogen between U2 and U3;total nitrogen leaching in U1 was remarkably higher than that in U2 (P＜0.05);leaching amount of total nitrogen in U2 was remarkably higher than that in U3(P＜0.05).In two BCU treatments,different nitrogen leaching forms demonstrated a descending order of B2＞B3,with no significant difference.

In Urea treatments,ammonium nitrogen leaching accounted for 1.0% -2.0% of total nitrogen in leaching water;nitrate nitrogen in leaching water accounted for 76.0% -95.7% of total nitrogen.In BCU treatments,ammonium nitrogen and nitrate nitrogen in leaching water accounted for 29.7% -31.7% and 51.6% -53.5% of total nitrogen,respectively.Therefore,nitrate nitrogen was the main nitrogen leaching form from two fertilizer types,as well as a small amount of dissolved organic nitrogen,which was consistent with Xueet al[11-12].Moreover,the high leaching amount of ammonium nitrogen in BCU treatments also indicated that there was almostno ammonia volatilization in Urea treatments since the10thd,whileammoniainBCUtreatments was volatilized continuously.

Yield

As shown in Fig.2,plant biomass in various urea fertilizer treatments was higher than that in CK.In addition,plant biomass was proportional to nitrogen application amount in various urea fertilizer treatments with no significant difference.At the same nitrogen application level,plant biomass in BCU treatments was slightly higher than that in Urea treatments by 1.3% approximately. Reducing nitrogen applicationamountby20% -30% posed no significant influence on plant biomass in various urea fertilizer treatments.

Nitrogen loss

Nitrogen loss refers to the sum of ammonia volatilization and nitrogen leaching amount.As shown in Table 2,nitrogen loss was proportional to nitrogen application amount,and nitrogen leaching amount was higher than ammonia volatilization amount.Therefore,in various urea fertilizer treatments,nitrogen loss was mainly derived from nitrogen leaching that accounted for 69.5% -81.2% of total nitrogen loss.After application of two nitrogen fertilizers at the same level,the amount of ammonia volatilization,nitrogen leaching and nitrogen loss varied significantly(P＜0.05),generally in a descending order of Urea treatments＞BCU treatments.The proportion of ammonia volatilization in Urea treatments and BCU treatments was 3.7% -4.1% and 3.2% -3.6%,respectively;the proportion of nitrogen leaching in Urea treatments and BCU treatments was 14.5% -16.2% and 6.9% -7.3%,respectively;the proportion of nitrogen loss was 18.1% -19.9% and 9.4% -9.7%,respectively.In various treatments,the amount of nitrogen leaching accounted for 6.9% -16.2%,which was similar to related studies[12-14].In three Urea treatments,ammonia volatilization,nitrogen leaching and nitrogenlossvariedsignificantlyamong differentnitrogen application levels(P＜0.05),generally in a descending order of U1＞U2＞U3.In two BCU treatments, ammonia volatilization varied significantly between different nitrogen application levels (B2＞B3),but no significant difference was observed in nitrogen in leaching water.After application of the same fertilizer,nitrogen loss was proportional to the rate of nitrogen application.Furthermore,after application of 320 and 280 mg/kg nitrogen,nitrogen loss in BCU treatments was reduced by 43.5% -45.5% compared with that in Urea treatments; specifically, ammonia volatilization was reduced by 3.7% -21.7%,and nitrogen leaching was reduced by 49.8% -52.1%.According to the results,reducing 20% -30% of ni-trogen application rate posed no significant influence on the yield.The application of BCU could reduce nitrogen loss more effectively.

Discussions

Volatilization,leaching and runoff are main pathways of nitrogen loss from fertilizer in farmland[15].In situ observation of nitrogen loss from fertilizer in farmland is restricted by various factors,such as cultivation conditions,environmental factors and human resources,resulting in great uncertainty in quantitative calculation of nitrogen loss.In this study,a pot experiment was carried out indoors under strictly controlled soil,fertilizer and water conditions to collect volatilized ammonia and leached nitrogen accurately.However,due to the limitations of pot experiment,there was no runoff water to be collected.In previous studies,nitrous oxide exhibited a low volatilization rate,which only accounted for 0.2% -0.4% and 0.03% -0.06% of volatilization loss and nitrogen loss,respectively[16-17].Therefore,the present study ignored the collection of nitrous oxide but simply focused on nitrogen monitoring in pathwaysassociated with remarkable nitrogen loss.

A large number of studies have shown that ammonia volatilization and nitrogen leaching are the main factors causing low utilization efficiency of nitrogen fertilizer.Especially,nitrogen leaching plays a dominant role.In this study,nitrogen leaching amount accounted for 69.5% -81.2% of nitrogen loss,suggesting that nitrogen loss is mainly caused by nitrogen leaching.Furthermore,nitrate nitrogen is the main form of nitrogen leaching,which accounted for 51.6% -95.7% of the total nitrogen leaching amount in this study.This might be due to the factor that soil nitrification promoted the conversion of ammonium nitrogen into nitrate nitrogen which was easier to leach.Existing research shows that urea applied into soil at Chaihe catchment can be completely converted into nitrate within 3 days after application[18].However,leaching amount of ammonium nitrogen in BCU treatments was significantly higher than that in Urea treatments(P＜0.05);leaching amount of nitrate nitrogen in BCU treatments was remarkably lower than that in Urea treatments (P＜0.05),which might be because ammonium nitrogen in BCU was not converted rapidly into nitrate nitrogen via soil nitrification,leading to large amounts of ammonium nitrogen in leaching water.Moreover,ammonium nitrogen can be easily absorbed by soil,and its loss is lower compared with nitrate nitrogen,which reduces the leaching of nitrate nitrogen.Therefore,BCU application can minimize nitrogen loss by reducing the nitrification of ammonium nitrogen.

In agricultural production,excessive application of nitrogen is the main cause of nitrate nitrogen leaching[19].The results indicated that reducing 20% -30% of nitrogen application amount could minimize nitrogen loss by 27.1% -34.9%,including 28.9% -37.5% of nitrogen leaching.Therefore,reducing nitrogen application amount is the key to minimizing nitrogen loss.Without affecting crop yield,applying BCU and reducing nitrogen amount by 20% -30% can effectively control nitrogen loss,especially leaching loss,thereby ensuring economic income of local vegetable growers,saving fertilizer inputs,declining the risk of agricultural nonpoint source pollution,and protecting local water resources and environment.

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