Rice production in Arkansas is one of the top three crop commodities in terms of cash receipts. Researchers and farmers report that nitrogen (N) needs to be managed according to a variety of factors with two important ones being soil and fertilizer type. The objectives of this experiment were to determine: 1) the degree to which floodwater-incorporated N applied as urea or as ammonium sulfate infiltrates intact cores (7.2-cm dia., 10-cm depth) containing DeWitt siltloam soil, and 2) the distribution of N during 12 h of ponding. Inorganic-N concentrations were analyzed at 2-cm depth intervals in cores following removal of the flood. Nitrogen from applied fertilizer was recovered as ammonium. Ammonium sulfate-N remained in the top 4 cm of soil with concentrations of 375 µg N g-1 in the surface 2 cm and 300 µg N g-1 at the 2 - 4 cm depth after 12 hr of ponding. At all depth intervals below 4 cm, ammonium sulfate-N remained below 30 µg N g-1. In contrast, after 12 h of ponding, N in soil receiving urea was 105 µg N g-1 in the top 2 cm and 173 µg N g-1 at 2-4 cm. At 4-6, 6-8, and 8-10 cm, N was 109, 108, and 35 µg N g-1, respectively, after 12 h of ponding. These results demonstrate immediate and deeper movement of ammonium into silt loam soil receiving urea as compared to ammonium sulfate, demonstrating how the form of N in fertilizer affects its movement into the soil profile

Brye, Kristofor R.

Copenhaver, Lindsay M.

Miller, David M.

Norman, Richard J.

Savin, Mary C.

Tomlinson, Peter J.

English

UARK (University of Arkansas )

Discovery, The Student Journal of Dale Bumpers College of Agricultural, Food and Life Sciences Volume 7 Article 6 Fall 2006 Infiltration and short-term movement of nitrogen in a silt-loam soil typical of rice cultivation in Arkansas Lindsay M. Copenhaver University of Arkansas, Fayetteville Mary C. Savin University of Arkansas, Fayetteville David M. Miller University of Arkansas, Fayetteville Peter J. Tomlinson University of Arkansas, Fayetteville Kristofor R. Brye University of Arkansas, Fayetteville See next page for additional authors Follow this and additional works at: https://scholarworks.uark.edu/discoverymag  Part of the Agronomy and Crop Sciences Commons, Botany Commons, and the Horticulture Commons Recommended Citation Copenhaver, L. M., Savin, M. C., Miller, D. M., Tomlinson, P. J., Brye, K. R., & Norman, R. J. (2006). Infiltration and short-term movement of nitrogen in a silt-loam soil typical of rice cultivation in Arkansas. Discovery, The Student Journal of Dale Bumpers College of Agricultural, Food and Life Sciences, 7(1), 14-18. Retrieved from https://scholarworks.uark.edu/discoverymag/vol7/iss1/6 This Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Discovery, The Student Journal of Dale Bumpers College of Agricultural, Food and Life Sciences by an authorized editor of ScholarWorks@UARK. For more information, please contact scholar@uark.edu. Infiltration and short-term movement of nitrogen in a silt-loam soil typical of rice cultivation in Arkansas Authors Lindsay M. Copenhaver, Mary C. Savin, David M. Miller, Peter J. Tomlinson, Kristofor R. Brye, and Richard J. Norman This article is available in Discovery, The Student Journal of Dale Bumpers College of Agricultural, Food and Life Sciences: https://scholarworks.uark.edu/discoverymag/vol7/iss1/6 Infiltration and short-termmovement of nitrogen in a silt-loam soil typical of rice cultivation in ArkansasLindsay M. Copenhaver*, Mary C. Savin†, David M. Miller§, Peter J.Tomlinson‡, Kristofor R. Brye‡‡, and Richard J. Norman§§ABSTRACTRice production in Arkansas is one of the top three crop commodities in terms of cash receipts.Researchers and farmers report that nitrogen (N) needs to be managed according to a variety offactors with two important ones being soil and fertilizer type. The objectives of this experimentwere to determine: 1) the degree to which floodwater-incorporated N applied as urea or asammonium sulfate infiltrates intact cores (7.2-cm dia., 10-cm depth) containing DeWitt silt-loam soil, and 2) the distribution of N during 12 h of ponding. Inorganic-N concentrations wereanalyzed at 2-cm depth intervals in cores following removal of the flood. Nitrogen from appliedfertilizer was recovered as ammonium. Ammonium sulfate-N remained in the top 4 cm of soilwith concentrations of 375 µg  N g-1 in the surface 2 cm and 300 µg N g-1 at the 2 - 4 cm depthafter 12 hr of ponding. At all depth intervals below 4 cm, ammonium sulfate-N remained below30 µg N g-1. In contrast, after 12 h of ponding, N in soil receiving urea was 105 µg N g-1 in thetop 2 cm and 173 µg N g-1 at 2-4 cm. At 4-6, 6-8, and 8-10 cm, N was 109, 108, and 35 µg N g-1,respectively, after 12 h of ponding. These results demonstrate immediate and deeper movementof ammonium into silt loam soil receiving urea as compared to ammonium sulfate, demonstrat-ing how the form of N in fertilizer affects its movement into the soil profile.DISCOVERY VOL. 7, FALL 200614* Lindsay M. Copenhaver is a junior in the Department of Crop, Soil, and Environmental Sciences.† Mary C. Savin is the faculty sponsor and an associate professor in the Department of Crop, Soil, and Environmental Sciences.§ David M. Miller is a professor in the Department of Crop, Soil, and Environmental Sciences.‡ Peter J. Tomlinson is a graduate student in the Department of Crop, Soil, and Environmental Sciences.‡‡ Kristopher R. Brye is an associate professor in the Department of Crop, Soil, and Environmental Sciences.§§ Richard J. Norman is a professor in the Department of Crop, Soil, and Environmental Sciences.THE STUDENT JOURNAL OF THE DALE BUMPERS COLLEGE OF AGRICULTURAL, FOOD AND LIFE SCIENCES 15INTRODUCTIONArkansas has been the nation’s leading rice-produc-ing state since 1973, ranking first in acres planted andproducing about 40 to 45% of the U.S. rice crop annual-ly (Slaton, 2001). Approximately 55, 35, and 9% of therice grown in Arkansas is produced on silt-loam, clay,and sandy-loam soils, respectively. Field preparation hasthe primary objective of removing winter vegetation andreducing the chance of seedling drift, so most Arkansasrice production is a delayed-flood, direct dry-seeded cul-ture, with flooding of fields beginning at the end of Mayand early June.Nitrogen (N) fertilizer is one of the most importantinvestments, monetarily and environmentally, in a suc-cessful rice crop (Wilson et al., 2005). Nitrogen accountsfor approximately 67% of the fertilizer (N + P + K)applied to rice (Vlek and Byrnes, 1986). While theamount of N required depends on rice culture, soil con-ditions, cultural practices, crop rotations, and other fac-tors (Wilson et al., 2005), the goal of any fertilizationprogram is to apply the optimal rate that will result inmaximal yields.Due to the complicated transformations N canundergo and the potential for inefficient N use, N is dif-ficult to manage in flooded soil systems (Reddy andPatrick, 1984). Recovery by rice can be as low as 20 to40%, if managed poorly, leading to extensive N losses(DeDatta et al., 1988). Because nitrate serves as an elec-tron acceptor during denitrification, the use of nitratesfor fertilization is avoided. Urea ((NH2)2CO) is anammonium-forming N source that is widely available,relatively inexpensive, and has a large percentage (46%)of N. More than 90% of the total fertilizer-N is appliedas urea instead of other forms (Vlek and Byrnes, 1986).Ammonium sulfate is another reduced-N source but isgenerally more expensive and has lower N content(21%), which also increases application costs (Wilson etal., 2005).The movement of N from urea and ammonium sul-fate on a silt-loam soil was analyzed in a laboratory studywith a simulated field “flood.” The objective was toMEET THE STUDENT-AUTHORI graduated from Conway High School in 2004and enrolled in the University of Arkansas in fall 2004as an environmental, soil, and water sciences majorwith minors in chemistry and global agriculture, food,and life sciences. I was awarded the Honors collegeAcademy Scholarship as well as the AgricultureBeginning Scholarship, Charles A. Stutte MemorialScholarship, R.P. and Mildred BartholomewScholarship, Johnnie N. Jenkins Scholarship, andDelta Classic Scholarship from the Dale BumpersCollege of Agricultural, Food and Life Sciences andfrom the Department of Crop, Soil andEnvironmental Sciences. I am a member of theEnvironmental, Soil and Water Sciences Club.I began working in the Soil Biology andMicrobial Ecology lab with Dr. Savin during the fall ofmy freshman year, where I was introduced to andbegan working on y honors research thesis concerningnitrogen movement in Arkansas rice soils. I appliedand received the Student Undergraduate ResearchFellowship (SURF) and the Carroll Walls ResearchStipend for the 2006 academic year. I attended theAmerican Society of Agronomy meeting in Salt LakeCity in fall of 2005 where I placed second in the Undergraduate Oral Research Symposium. I competed in the2006 Gamma Sigma Delta undergraduate research competition and placed second. I plan on continuing mygraduate studies after I receive my environmental, soil, and water sciences degree.Lindsay M. Copenhavermeasure the immediate movement of N into the surfacesoil and compare how the different fertilizer N formsaffected the depth to which N moved within 12 h ofapplication and water ponding. It was hypothesized thatN from urea would move farther down into the soil thanN from ammonium sulfate.MATERIALS AND METHODSSoil cores.Fifty-four intact soil cores (7.2 cm-dia., 10 cm-length)were collected from a 1.1 x 2.3 m plot at the Universityof Arkansas Rice Research and Extension Center,Stuttgart. Cores were kept intact inside plastic sleeves,placed on ice for transportation, and stored at 4°C. Atthe same time, five samples were taken for bulk densityand 10 samples were taken for chemical composition(Mehlich III, total C & N, pH, N, OM, EC, P & K). Soilchemical composition was determined at the Soil TestLaboratory at the University of Arkansas, Fayetteville.Infiltration and movement of N in top 10 cm of soil.Nitrogen (90 mg N for each fertilizer) was applied tothe center of cores (202.3 mg urea or 444.5 mg ammoni-um sulfate). Just enough water was added to dissolve thefertilizer, and then a flood was applied and maintainedusing Mariotte bottles (Fig. 1). The principle of theMariotte bottle is that the pressure inside the bottle atthe bottom of the bubble tube is at atmospheric pres-sure, which then maintains the water surface in the soilcore at the same height as the end of the bubble tube(Bouwer, 1986) (Fig. 1).Cores (four replications for a total of 48 cores) wereleached at time intervals of 0.5, 1, 2, 4, 8, or 12 h for eachfertilizer. When the allotted time elapsed, the floodwaterand leachate were collected; volumes were measured,and frozen. Each core itself was capped and immediate-ly placed in a -80°C freezer. Background N concentra-tions before (three replications) and after 12 h of flood-ing (three replications) were determined in cores notreceiving N fertilizer.Analysis.The frozen cores were cut at 2-cm depth intervalswith a band saw. Each thawed section was homogenizedwith a glass stirring rod. Moisture content was deter-mined gravimetrically after drying 5 g of wet soil at105°C for 24 h using the following equation:θg = (W – D)/D   x  100 (1)where θg = gravimetric moisture, W = wet soil (g), andD = dry soil (g). Inorganic N was measured in 2M KClsolutions (1:10 soil:extract) after shaking for 1 h and fil-tering through a Whatman #42 filter. Filtrate was storedat 4°C, or frozen if analysis could not be conducted with-in a month of extraction, before colorimetric analysis ofnitrate and ammonium (Mulvaney, 1996) on a nutrientautoanalyzer (Skalar Instruments, Norcross, Ga.). In theanalysis procedure, NO3- extracted from soil with 2MKCl is reduced to NO2- by passage through a column ofcopperized cadmium, and the NO2- formed is deter-mined by a modified Griess-Ilosvay method (Mulvaney,1996). NH4+ extracted from soil with 2M KCl is deter-mined by measuring the intensity of the emerald greencolor that forms upon treatment of an aliquot of theextract with salicylate and hypochlorite at high pH. Acatalyst (sodium nitroprusside) increases the rate andintensity of color development, and a chelating agent(EDTA) prevents the precipitation of divalent and triva-lent cations as hydroxides (Mulvaney, 1996).Mean N concentrations and standard deviations werecalculated for each depth and time interval.Concentrations were analyzed and compared across fer-tilizer type and over time using analysis of variance.Background soil-N concentrations were subtracted fromthe measured 12-h concentrations to obtain fertilizer-Nconcentrations.RESULTS AND DISCUSSIONThe soil chemical properties of the DeWitt silt loam(fine, smectitic, thermic, Typic Albaqualf) are summa-rized in Table 1. According to Brady and Weil (2002), therange for an average silt-loam soil bulk density is 0.9 –1.5 g/cm3, with a typical medium-textured soil having abulk density of 1.25 g/cm3. The DeWitt silt-loam bulkdensity is well within the reported range with an averagebulk density of 1.38 g/cm3. Carbon and pH values alsofell within normal ranges of 0.9-3.3% for carbon and 5-7 for pH (Brady and Weil, 2002).Neither fertilizer contained N in nitrate form andbecause of the short time intervals used in this study,nitrification was not expected to occur. Fertilizer wasexpected to be recovered as NH4+-N. In fact, after sub-tracting out background nitrate levels after 12 h ofponding, almost all inorganic N recovered was ammoni-um (data not shown).Floodwater incorporated NH4+-N into soil to varyingdegrees within the 12-h ponding time utilized in thisstudy. The concentrations of NH4+-N at the 0-2 cmdepth ranged from 523 µg N g-1 soil measured after 0.5 hto 375 µg N g-1 soil after 12 hours. Nitrogen at the 2-4 cmdepth after 0.5 h was 154 µg N g-1 soil and after 12 h was300 µg N g-1 soil. These results represent an accumula-tion in the upper 4 cm. There was a sharp, visible down-ward trend over time in 0-2 cm depth, accompanied bya similarly apparent increase in the 2-4 cm depth (Fig.DISCOVERY VOL. 7, FALL 20061617THE STUDENT JOURNAL OF THE DALE BUMPERS COLLEGE OF AGRICULTURAL, FOOD AND LIFE SCIENCES2). Below 2-4 cm, accumulation was slow, and inconsis-tent with concentrations ranging from a background 9µg N g-1 to 29 µg N g-1 soil after 12 h. There was no sig-nificant downward movement below 4 cm when com-pared to the upper 4 cm (Fig. 2). These results wereexpected because the positive charge of ammonium(NH4+) associates with negative charges on soil-particlesurfaces.In contrast to ammonium sulfate applied-N, therewas deeper movement of NH4+-N from surface-appliedurea (Fig. 3). In order to measure increases in NH4+ withurea applications, NH4+ must be released during break-down of composition of urea, leading to the expectationthat urea will be able to move farther down into the soil.Concentrations of NH4+-N were 130 µg N g-1 soil after0.5 h and 105 µg N g-1 soil after 12 hours at 0-2 cm (Fig.4). Nitrogen was approximately 175 µg N g-1 soil at the 2-4 cm depth after a 0.5 h and remained at 175 µg N g-1after12 h of ponding. In contrast, NH4+ concentrationsat the 4-8 cm depths were higher than those measured insoil receiving ammonium sulfate (Figs. 2 and 3).Ammonium concentrations at the 4-8 cm depth alsoincreased during 12 h of flooding. At the 4-6 and 6-8 cmdepths, 75 and 39 µg N g-1 soil, respectively, were meas-ured after 0.5 h of ponding and concentrations reachedapproximately 109 µg N g-1 soil at both depths after 12 h(Fig. 3). Although concentrations of urea were not ashigh in the top 4 cm as ammonium sulfate, concentra-tions below 4 cm increased over time (Fig 3).While N from ammonium sulfate stayed in the sur-face 4 cm of soil, N from urea infiltrated farther and wasaccumulating at depths below 4 cm during 12 h of pond-ing. These results have significance because if ureabreaks down to release ammonium before a flood isestablished, fertilizer N will behave more like ammoni-um sulfate and N will not infiltrate as far. Any ammoni-um that remains at the surface and under aerobic condi-tions can undergo nitrification. Nitrate can leach withdownward water movement and move into the anaero-bic zone. In the anaerobic zone, nitrate can undergo den-itrification. Some cultural practices take 5 d to establisha flood. These results suggest that management practicesthat prevent the breakdown of urea and the subsequentaccumulation of NH4+-N near the soil surface need fur-ther investigation.ACKNOWLEDGMENTSThis project was funded by the Rice Research andPromotion Board, and Division of Agriculture,University of Arkansas.LITERATURE CITEDBrady, N. C. and R. R. Weil. 2002. The Nature andProperties of Soils. 13th ed. Prentice Hall, UpperSaddle, N.J.Bouwer, H. 1986. Intake rate: cylinder infiltrometer. p.837-838. In: A. Klute (ed.)  Methods of Soil AnalysisPart 1. Physical and Mineralogical Methods.Second Edition. SSSA Book Ser. 9. SSSA. Madison, Wisc.DeDatta, S.K., R.J. Buresh, M.I. Samson, and W. Kai-Rong. 1988. Nitrogen use efficiency and nitrogen-15 balances in broadcast-seeded, flooded, and trans-planted rice. Soil Sci. Soc. Am. J. 52:849-855.Mulvaney, R.L. 1996. Nitrogen-Inorganic Forms.p.1152-1159. In: D.L. Sparks et al. (eds.) Methods ofSoil Analysis. Part 3. Chemical Methods-SSSA BookSer. 5. SSSA. Madison, Wisc.Reddy, K.R. and W.H. Patrick, Jr. 1984. Nitrogen trans-formations and loss in flooded soils and sediments.Crit. Rev. Environ. Control 13:273-309.Slaton, N. A., (ed.) 2001. Rice Production Handbook.MP192-10-M-1-01RV. Cooperative ExtensionService. Little Rock, Ark.Wilson, C.E., N. A. Slaton, and R. J. Norman. 2005.Nitrogen Fertilization of Rice. CooperativeExtension Service. [online]<http://www.aragriculture.org/cropsoilwtr/rice/Publications/HTML/nitro-fert.asp>. 26 Sept. 2005.Vlek, P.L.G. and B.H. Byrnes. 1986. The efficacy and lossof fertilizer N in lowland rice. p. 131-147. In: S.K.DeDatta and W.H. Patrick, Jr. (eds.) Nitrogen econ-omy of flooded rice soils. Martinus Nijhoff, TheHague.DISCOVERY VOL. 7, FALL 200618 Fig. 1. Mariotte Bottle. Two tubes are inserted through the stopper. One tube (A) is for siphoning the water to thesoil core and the other tube (B) allows air into the bottle. The bottom of this “bubble” tube is set at the level at whichthe water surface in the soil core is to be maintained (Bouwer, 1986). Table 1. Mean soil properties (± standard deviation) of a DeWitt silt-loam soil (fine, smectitic, thermic, TypicAlbaqualf) collected from the University of Arkansas Rice Research and Extension Center, Stuttgart (n=5 for bulkdensity, n=10 for all other properties).pH ECz(umhos/cm)P(mg/kg)K(mg/kg)C(%)N(%)OM(%)Bulk Density(g/cm3)6.37(0.03)164.80(7.16)10.43(0.39)108.34(3.25)0.95(0.02)0.09(0.00)2.37(0.14)1.38(0.02)zEC is electrical conductivity and OM is organic matter.01002003004005006000-2 2-4 4-6 6-8 8-10 10-12Soil Depth (cm)AS-derived-N(µgN/gsoil)0.5hr 4hr 12hrFig. 2. Mean NH4+-N concentrations (± standard deviation) from ammonium sulfate(AS) recovered after time intervals of 0.5, 4 or 12 h at 2-cm depth intervals in soilcores containing DeWitt silt loam.  01002003004005006000-2 2-4 4-6 6-8 8-10 10-12Soil Depth (cm)UreaDerived-N(µgN/gsoil)0.5hr 4hr 12hrFig. 3. Mean NH4+-N concentrations (± standard deviation) from urea recovered after time intervals of 0.5, 4 or 12 h at 2-cm depth intervals in soil cores containing DeWitt silt loam.  Fig. 2. Mean NH4+-N concentrations (± standard devi-ation) from am o ium sulfate (AS) recovered aftertime intervals of 0.5, 4 or 12 h at 2-cm depth intervalsin soil cores containing DeWitt silt loam. Fig. 3. Mean NH4+-N concentrations (± standard devi-ation) from urea recovered after time intervals of 0.5, 4or 12 h at 2-cm depth intervals in soil cores containingDeWitt silt loam.

Infiltration and short-term movement of nitrogen in a silt-loam soil typical of rice cultivation in Arkansas

ScholarWorks@UARK

Discovery, The Student Journal of Dale Bumpers College ofAgricultural, Food and Life SciencesVolume 7 Article 6Fall 2006Infiltration and short-term movement of nitrogenin a silt-loam soil typical of rice cultivation inArkansasLindsay M. CopenhaverUniversity of Arkansas, FayettevilleMary C. SavinUniversity of Arkansas, FayettevilleDavid M. MillerUniversity of Arkansas, FayettevillePeter J. TomlinsonUniversity of Arkansas, FayettevilleKristofor R. BryeUniversity of Arkansas, FayettevilleSee next page for additional authorsFollow this and additional works at: https://scholarworks.uark.edu/discoverymagPart of the Agronomy and Crop Sciences Commons, Botany Commons, and the HorticultureCommonsThis Article is brought to you for free and open access by ScholarWorks@UARK. It has been accepted for inclusion in Discovery, The Student Journalof Dale Bumpers College of Agricultural, Food and Life Sciences by an authorized editor of ScholarWorks@UARK. For more information, pleasecontact scholar@uark.edu, ccmiddle@uark.edu.Recommended CitationCopenhaver, Lindsay M.; Savin, Mary C.; Miller, David M.; Tomlinson, Peter J.; Brye, Kristofor R.; and Norman, Richard J. (2006)"Infiltration and short-term movement of nitrogen in a silt-loam soil typical of rice cultivation in Arkansas," Discovery, The StudentJournal of Dale Bumpers College of Agricultural, Food and Life Sciences. University of Arkansas System Division of Agriculture. 7:14-18.Available at: https://scholarworks.uark.edu/discoverymag/vol7/iss1/6Infiltration and short-term movement of nitrogen in a silt-loam soil typicalof rice cultivation in ArkansasAuthorsLindsay M. Copenhaver, Mary C. Savin, David M. Miller, Peter J. Tomlinson, Kristofor R. Brye, and Richard J.NormanThis article is available in Discovery, The Student Journal of Dale Bumpers College of Agricultural, Food and Life Sciences:https://scholarworks.uark.edu/discoverymag/vol7/iss1/6Infiltration and short-termmovement of nitrogen in a silt-loam soil typical of rice cultivation in ArkansasLindsay M. Copenhaver*, Mary C. Savin†, David M. Miller§, Peter J.Tomlinson‡, Kristofor R. Brye‡‡, and Richard J. Norman§§ABSTRACTRice production in Arkansas is one of the top three crop commodities in terms of cash receipts.Researchers and farmers report that nitrogen (N) needs to be managed according to a variety offactors with two important ones being soil and fertilizer type. The objectives of this experimentwere to determine: 1) the degree to which floodwater-incorporated N applied as urea or asammonium sulfate infiltrates intact cores (7.2-cm dia., 10-cm depth) containing DeWitt silt-loam soil, and 2) the distribution of N during 12 h of ponding. Inorganic-N concentrations wereanalyzed at 2-cm depth intervals in cores following removal of the flood. Nitrogen from appliedfertilizer was recovered as ammonium. Ammonium sulfate-N remained in the top 4 cm of soilwith concentrations of 375 µg  N g-1 in the surface 2 cm and 300 µg N g-1 at the 2 - 4 cm depthafter 12 hr of ponding. At all depth intervals below 4 cm, ammonium sulfate-N remained below30 µg N g-1. In contrast, after 12 h of ponding, N in soil receiving urea was 105 µg N g-1 in thetop 2 cm and 173 µg N g-1 at 2-4 cm. At 4-6, 6-8, and 8-10 cm, N was 109, 108, and 35 µg N g-1,respectively, after 12 h of ponding. These results demonstrate immediate and deeper movementof ammonium into silt loam soil receiving urea as compared to ammonium sulfate, demonstrat-ing how the form of N in fertilizer affects its movement into the soil profile.DISCOVERY VOL. 7, FALL 200614* Lindsay M. Copenhaver is a junior in the Department of Crop, Soil, and Environmental Sciences.† Mary C. Savin is the faculty sponsor and an associate professor in the Department of Crop, Soil, and Environmental Sciences.§ David M. Miller is a professor in the Department of Crop, Soil, and Environmental Sciences.‡ Peter J. Tomlinson is a graduate student in the Department of Crop, Soil, and Environmental Sciences.‡‡ Kristopher R. Brye is an associate professor in the Department of Crop, Soil, and Environmental Sciences.§§ Richard J. Norman is a professor in the Department of Crop, Soil, and Environmental Sciences.THE STUDENT JOURNAL OF THE DALE BUMPERS COLLEGE OF AGRICULTURAL, FOOD AND LIFE SCIENCES 15INTRODUCTIONArkansas has been the nation’s leading rice-produc-ing state since 1973, ranking first in acres planted andproducing about 40 to 45% of the U.S. rice crop annual-ly (Slaton, 2001). Approximately 55, 35, and 9% of therice grown in Arkansas is produced on silt-loam, clay,and sandy-loam soils, respectively. Field preparation hasthe primary objective of removing winter vegetation andreducing the chance of seedling drift, so most Arkansasrice production is a delayed-flood, direct dry-seeded cul-ture, with flooding of fields beginning at the end of Mayand early June.Nitrogen (N) fertilizer is one of the most importantinvestments, monetarily and environmentally, in a suc-cessful rice crop (Wilson et al., 2005). Nitrogen accountsfor approximately 67% of the fertilizer (N + P + K)applied to rice (Vlek and Byrnes, 1986). While theamount of N required depends on rice culture, soil con-ditions, cultural practices, crop rotations, and other fac-tors (Wilson et al., 2005), the goal of any fertilizationprogram is to apply the optimal rate that will result inmaximal yields.Due to the complicated transformations N canundergo and the potential for inefficient N use, N is dif-ficult to manage in flooded soil systems (Reddy andPatrick, 1984). Recovery by rice can be as low as 20 to40%, if managed poorly, leading to extensive N losses(DeDatta et al., 1988). Because nitrate serves as an elec-tron acceptor during denitrification, the use of nitratesfor fertilization is avoided. Urea ((NH2)2CO) is anammonium-forming N source that is widely available,relatively inexpensive, and has a large percentage (46%)of N. More than 90% of the total fertilizer-N is appliedas urea instead of other forms (Vlek and Byrnes, 1986).Ammonium sulfate is another reduced-N source but isgenerally more expensive and has lower N content(21%), which also increases application costs (Wilson etal., 2005).The movement of N from urea and ammonium sul-fate on a silt-loam soil was analyzed in a laboratory studywith a simulated field “flood.” The objective was toMEET THE STUDENT-AUTHORI graduated from Conway High School in 2004and enrolled in the University of Arkansas in fall 2004as an environmental, soil, and water sciences majorwith minors in chemistry and global agriculture, food,and life sciences. I was awarded the Honors collegeAcademy Scholarship as well as the AgricultureBeginning Scholarship, Charles A. Stutte MemorialScholarship, R.P. and Mildred BartholomewScholarship, Johnnie N. Jenkins Scholarship, andDelta Classic Scholarship from the Dale BumpersCollege of Agricultural, Food and Life Sciences andfrom the Department of Crop, Soil andEnvironmental Sciences. I am a member of theEnvironmental, Soil and Water Sciences Club.I began working in the Soil Biology andMicrobial Ecology lab with Dr. Savin during the fall ofmy freshman year, where I was introduced to andbegan working on y honors research thesis concerningnitrogen movement in Arkansas rice soils. I appliedand received the Student Undergraduate ResearchFellowship (SURF) and the Carroll Walls ResearchStipend for the 2006 academic year. I attended theAmerican Society of Agronomy meeting in Salt LakeCity in fall of 2005 where I placed second in the Undergraduate Oral Research Symposium. I competed in the2006 Gamma Sigma Delta undergraduate research competition and placed second. I plan on continuing mygraduate studies after I receive my environmental, soil, and water sciences degree.Lindsay M. Copenhavermeasure the immediate movement of N into the surfacesoil and compare how the different fertilizer N formsaffected the depth to which N moved within 12 h ofapplication and water ponding. It was hypothesized thatN from urea would move farther down into the soil thanN from ammonium sulfate.MATERIALS AND METHODSSoil cores.Fifty-four intact soil cores (7.2 cm-dia., 10 cm-length)were collected from a 1.1 x 2.3 m plot at the Universityof Arkansas Rice Research and Extension Center,Stuttgart. Cores were kept intact inside plastic sleeves,placed on ice for transportation, and stored at 4°C. Atthe same time, five samples were taken for bulk densityand 10 samples were taken for chemical composition(Mehlich III, total C & N, pH, N, OM, EC, P & K). Soilchemical composition was determined at the Soil TestLaboratory at the University of Arkansas, Fayetteville.Infiltration and movement of N in top 10 cm of soil.Nitrogen (90 mg N for each fertilizer) was applied tothe center of cores (202.3 mg urea or 444.5 mg ammoni-um sulfate). Just enough water was added to dissolve thefertilizer, and then a flood was applied and maintainedusing Mariotte bottles (Fig. 1). The principle of theMariotte bottle is that the pressure inside the bottle atthe bottom of the bubble tube is at atmospheric pres-sure, which then maintains the water surface in the soilcore at the same height as the end of the bubble tube(Bouwer, 1986) (Fig. 1).Cores (four replications for a total of 48 cores) wereleached at time intervals of 0.5, 1, 2, 4, 8, or 12 h for eachfertilizer. When the allotted time elapsed, the floodwaterand leachate were collected; volumes were measured,and frozen. Each core itself was capped and immediate-ly placed in a -80°C freezer. Background N concentra-tions before (three replications) and after 12 h of flood-ing (three replications) were determined in cores notreceiving N fertilizer.Analysis.The frozen cores were cut at 2-cm depth intervalswith a band saw. Each thawed section was homogenizedwith a glass stirring rod. Moisture content was deter-mined gravimetrically after drying 5 g of wet soil at105°C for 24 h using the following equation:θg = (W – D)/D   x  100 (1)where θg = gravimetric moisture, W = wet soil (g), andD = dry soil (g). Inorganic N was measured in 2M KClsolutions (1:10 soil:extract) after shaking for 1 h and fil-tering through a Whatman #42 filter. Filtrate was storedat 4°C, or frozen if analysis could not be conducted with-in a month of extraction, before colorimetric analysis ofnitrate and ammonium (Mulvaney, 1996) on a nutrientautoanalyzer (Skalar Instruments, Norcross, Ga.). In theanalysis procedure, NO3- extracted from soil with 2MKCl is reduced to NO2- by passage through a column ofcopperized cadmium, and the NO2- formed is deter-mined by a modified Griess-Ilosvay method (Mulvaney,1996). NH4+ extracted from soil with 2M KCl is deter-mined by measuring the intensity of the emerald greencolor that forms upon treatment of an aliquot of theextract with salicylate and hypochlorite at high pH. Acatalyst (sodium nitroprusside) increases the rate andintensity of color development, and a chelating agent(EDTA) prevents the precipitation of divalent and triva-lent cations as hydroxides (Mulvaney, 1996).Mean N concentrations and standard deviations werecalculated for each depth and time interval.Concentrations were analyzed and compared across fer-tilizer type and over time using analysis of variance.Background soil-N concentrations were subtracted fromthe measured 12-h concentrations to obtain fertilizer-Nconcentrations.RESULTS AND DISCUSSIONThe soil chemical properties of the DeWitt silt loam(fine, smectitic, thermic, Typic Albaqualf) are summa-rized in Table 1. According to Brady and Weil (2002), therange for an average silt-loam soil bulk density is 0.9 –1.5 g/cm3, with a typical medium-textured soil having abulk density of 1.25 g/cm3. The DeWitt silt-loam bulkdensity is well within the reported range with an averagebulk density of 1.38 g/cm3. Carbon and pH values alsofell within normal ranges of 0.9-3.3% for carbon and 5-7 for pH (Brady and Weil, 2002).Neither fertilizer contained N in nitrate form andbecause of the short time intervals used in this study,nitrification was not expected to occur. Fertilizer wasexpected to be recovered as NH4+-N. In fact, after sub-tracting out background nitrate levels after 12 h ofponding, almost all inorganic N recovered was ammoni-um (data not shown).Floodwater incorporated NH4+-N into soil to varyingdegrees within the 12-h ponding time utilized in thisstudy. The concentrations of NH4+-N at the 0-2 cmdepth ranged from 523 µg N g-1 soil measured after 0.5 hto 375 µg N g-1 soil after 12 hours. Nitrogen at the 2-4 cmdepth after 0.5 h was 154 µg N g-1 soil and after 12 h was300 µg N g-1 soil. These results represent an accumula-tion in the upper 4 cm. There was a sharp, visible down-ward trend over time in 0-2 cm depth, accompanied bya similarly apparent increase in the 2-4 cm depth (Fig.DISCOVERY VOL. 7, FALL 20061617THE STUDENT JOURNAL OF THE DALE BUMPERS COLLEGE OF AGRICULTURAL, FOOD AND LIFE SCIENCES2). Below 2-4 cm, accumulation was slow, and inconsis-tent with concentrations ranging from a background 9µg N g-1 to 29 µg N g-1 soil after 12 h. There was no sig-nificant downward movement below 4 cm when com-pared to the upper 4 cm (Fig. 2). These results wereexpected because the positive charge of ammonium(NH4+) associates with negative charges on soil-particlesurfaces.In contrast to ammonium sulfate applied-N, therewas deeper movement of NH4+-N from surface-appliedurea (Fig. 3). In order to measure increases in NH4+ withurea applications, NH4+ must be released during break-down of composition of urea, leading to the expectationthat urea will be able to move farther down into the soil.Concentrations of NH4+-N were 130 µg N g-1 soil after0.5 h and 105 µg N g-1 soil after 12 hours at 0-2 cm (Fig.4). Nitrogen was approximately 175 µg N g-1 soil at the 2-4 cm depth after a 0.5 h and remained at 175 µg N g-1after12 h of ponding. In contrast, NH4+ concentrationsat the 4-8 cm depths were higher than those measured insoil receiving ammonium sulfate (Figs. 2 and 3).Ammonium concentrations at the 4-8 cm depth alsoincreased during 12 h of flooding. At the 4-6 and 6-8 cmdepths, 75 and 39 µg N g-1 soil, respectively, were meas-ured after 0.5 h of ponding and concentrations reachedapproximately 109 µg N g-1 soil at both depths after 12 h(Fig. 3). Although concentrations of urea were not ashigh in the top 4 cm as ammonium sulfate, concentra-tions below 4 cm increased over time (Fig 3).While N from ammonium sulfate stayed in the sur-face 4 cm of soil, N from urea infiltrated farther and wasaccumulating at depths below 4 cm during 12 h of pond-ing. These results have significance because if ureabreaks down to release ammonium before a flood isestablished, fertilizer N will behave more like ammoni-um sulfate and N will not infiltrate as far. Any ammoni-um that remains at the surface and under aerobic condi-tions can undergo nitrification. Nitrate can leach withdownward water movement and move into the anaero-bic zone. In the anaerobic zone, nitrate can undergo den-itrification. Some cultural practices take 5 d to establisha flood. These results suggest that management practicesthat prevent the breakdown of urea and the subsequentaccumulation of NH4+-N near the soil surface need fur-ther investigation.ACKNOWLEDGMENTSThis project was funded by the Rice Research andPromotion Board, and Division of Agriculture,University of Arkansas.LITERATURE CITEDBrady, N. C. and R. R. Weil. 2002. The Nature andProperties of Soils. 13th ed. Prentice Hall, UpperSaddle, N.J.Bouwer, H. 1986. Intake rate: cylinder infiltrometer. p.837-838. In: A. Klute (ed.)  Methods of Soil AnalysisPart 1. Physical and Mineralogical Methods.Second Edition. SSSA Book Ser. 9. SSSA. Madison, Wisc.DeDatta, S.K., R.J. Buresh, M.I. Samson, and W. Kai-Rong. 1988. Nitrogen use efficiency and nitrogen-15 balances in broadcast-seeded, flooded, and trans-planted rice. Soil Sci. Soc. Am. J. 52:849-855.Mulvaney, R.L. 1996. Nitrogen-Inorganic Forms.p.1152-1159. In: D.L. Sparks et al. (eds.) Methods ofSoil Analysis. Part 3. Chemical Methods-SSSA BookSer. 5. SSSA. Madison, Wisc.Reddy, K.R. and W.H. Patrick, Jr. 1984. Nitrogen trans-formations and loss in flooded soils and sediments.Crit. Rev. Environ. Control 13:273-309.Slaton, N. A., (ed.) 2001. Rice Production Handbook.MP192-10-M-1-01RV. Cooperative ExtensionService. Little Rock, Ark.Wilson, C.E., N. A. Slaton, and R. J. Norman. 2005.Nitrogen Fertilization of Rice. CooperativeExtension Service. [online]<http://www.aragriculture.org/cropsoilwtr/rice/Publications/HTML/nitro-fert.asp>. 26 Sept. 2005.Vlek, P.L.G. and B.H. Byrnes. 1986. The efficacy and lossof fertilizer N in lowland rice. p. 131-147. In: S.K.DeDatta and W.H. Patrick, Jr. (eds.) Nitrogen econ-omy of flooded rice soils. Martinus Nijhoff, TheHague.DISCOVERY VOL. 7, FALL 200618 Fig. 1. Mariotte Bottle. Two tubes are inserted through the stopper. One tube (A) is for siphoning the water to thesoil core and the other tube (B) allows air into the bottle. The bottom of this “bubble” tube is set at the level at whichthe water surface in the soil core is to be maintained (Bouwer, 1986). Table 1. Mean soil properties (± standard deviation) of a DeWitt silt-loam soil (fine, smectitic, thermic, TypicAlbaqualf) collected from the University of Arkansas Rice Research and Extension Center, Stuttgart (n=5 for bulkdensity, n=10 for all other properties).pH ECz(umhos/cm)P(mg/kg)K(mg/kg)C(%)N(%)OM(%)Bulk Density(g/cm3)6.37(0.03)164.80(7.16)10.43(0.39)108.34(3.25)0.95(0.02)0.09(0.00)2.37(0.14)1.38(0.02)zEC is electrical conductivity and OM is organic matter.01002003004005006000-2 2-4 4-6 6-8 8-10 10-12Soil Depth (cm)AS-derived-N(µgN/gsoil)0.5hr 4hr 12hrFig. 2. Mean NH4+-N concentrations (± standard deviation) from ammonium sulfate(AS) recovered after time intervals of 0.5, 4 or 12 h at 2-cm depth intervals in soilcores containing DeWitt silt loam.  01002003004005006000-2 2-4 4-6 6-8 8-10 10-12Soil Depth (cm)UreaDerived-N(µgN/gsoil)0.5hr 4hr 12hrFig. 3. Mean NH4+-N concentrations (± standard deviation) from urea recovered after time intervals of 0.5, 4 or 12 h at 2-cm depth intervals in soil cores containing DeWitt silt loam.  Fig. 2. Mean NH4+-N concentrations (± standard devi-ation) from am o ium sulfate (AS) recovered aftertime intervals of 0.5, 4 or 12 h at 2-cm depth intervalsin soil cores containing DeWitt silt loam. Fig. 3. Mean NH4+-N concentrations (± standard devi-ation) from urea recovered after time intervals of 0.5, 4or 12 h at 2-cm depth intervals in soil cores containingDeWitt silt loam.

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Infiltration and short-term movement of nitrogen in a silt-loam soil typical of rice cultivation in Arkansas

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