Paddy Water Environ (2004) 2:115–124DOI 10.1007/s10333-004-0050-7

A R T I C L E

Hideo Nakasone · Muhammad Akhtar Abbas ·Hisao Kuroda

Nitrogen transport and transformation in packed soil columnsfrom paddy fields

Received: 18 July 2003 / Accepted: 11 May 2004 / Published online: 19 August 2004� Springer-Verlag 2004

Abstract Our understanding of nitrogen transformationin paddy fields or wetlands is limited due to the complexinteractions between soil, water, and biomass. Therefore,we studied transformation patterns resulting from the oxiclevel in the soil, and studied saturated (anoxic) and un-saturated (oxic) flow conditions. We present a modeldesigned to predict concentrations of nitrate and ammo-nium at several soil depths resulting from the processes ofnitrification, denitrification, and ammonification. Modelequations were obtained that describe NO3-N and NH4-Nconcentrations in terms of position, rate constant, andaverage flow velocity. Although many parameters wereincluded in the model equations, some were determinedfrom the literature and others were derived from ex-periments. A sensitivity analysis of the rate coefficientsfor NO3-N and NH4-N revealed that they are extremelysensitive to denitrification and ammonification respec-tively. Experimental results show that there were largedifferences in the transformations of NO3-N and NH4-N,the water pressure distributions, and the oxygen reductionpotentials (ORP) between saturated and unsaturated porewater flow conditions. The performance of the model forsequential transformations during the transport of NO3-Nis well documented under both saturated and unsaturatedflow conditions.

Keywords Diffused pollution · Non-point source ·Denitrification · Aquatic ecosystem · Acidification oflake · Low pH · Runoff model

Introduction

The behavior of nitrogen in relation to plant growth inpaddy soils has been extensively studied (De Datta et al1986). Recently, nitrogen runoff from farmlands has beenshown to be one of the main substances that causes eu-trophication in lakes and reservoirs (Burt et al 1993).High nitrate concentrations are commonly observed inwater due to eluviation from upland fields (Burt et al1993). This water is used for irrigation by collecting itdirectly or into a reservoir. On the other hand, it is wellknown that nitrogen content decreases in paddy fields orwetlands by means of denitrification (Moshiri 1993).Therefore, it is important to know which processes occurin the soil layers of paddy fields, especially in the rootzone of rice plants. Nitrogen transformation throughoutthe paddy soil layers is a complex process, which explainswhy there are only a few studies on this subject in theliterature. Changes in topography sometimes create dif-ferent percolation patterns, even in paddy fields. Thesechanges cause different flow patterns, which can bemainly divided into “saturated” or “unsaturated” flows.Each paddy field has a plow sole of thickness ~10 cm (DeDatta et al 1986). The plow sole, which controls thepercolation, is just below the plow layer (root zone).Therefore, it is believed that nitrogen transformation dif-fers depending on the flow conditions (De Datta et al1986), although this hypothesis has not been verified. Tothis end, we have evaluated nitrogen dynamics in packedcores in the laboratory, using anoxic and oxic soil col-umns that have been modified to mimic the actual soillayer of paddy fields. Mathematical modeling has beenused to investigate water quality issues in a variety ofways, and has proved to be useful tool for predicting theeffects of different management policies, determining theextents of pollution hazards, or testing the sensitivities ofthe processes involved. Therefore, we have also devel-oped a model that can be used for water quality man-agement in paddy fields or wetlands (although wetland isslightly different from paddy fields).

H. Nakasone ()) · H. KurodaSchool of Agriculture,Ibaraki University,Ami-chuoh 3–21–1, Inashiki, 300–0393 Ibaraki, Japane-mail: nakasone@mx.ibaraki.ac.jp

M. A. AbbasDept. of Structure & Env. Engineering,Faisalabad Univ. of Agriculture,38040 Faisalabad, Pakistan

The fundamental principle of nitrogen transport andtransformation in paddy soil is closely related to solutetransport phenomena. Nitrogen models with varying de-grees of complexity have been reported (Misra et al 1974;Iskandar and Selim 1981; Kalaurachchi and Parker 1988;Rolston et al 1990; Singh and Kirk 1993). Most of themodels include the transformation of nitrogen in unsat-urated field conditions (Starr et al 1974; Selim andIskandar 1981; Tindall et al 1995; Yamaguchi et al 1996).A few models simulate wetlands (Reddy 1986; Rao et al1984), but some of them are truncated, ignoring processesof vital importance such as nitrification and denitrifica-tion (Singh and Kirk 1993), while others are only one-process models (Yamaguchi et al 1992a). Modeling ofeffluent loads from farmlands has been attempted in Eu-rope and the USA, but little or no attention has been givento modeling paddy fields (Misawa 1988). There are bigdifferences in the transfer and transformation of nitrogenbetween upland fields and paddy fields, since soil con-ditions are completely different due to the ponding irri-gation used in paddy fields. The models presented byMisawa (1988) are conceptually astute, but the modelscannot calculate nitrogen concentrations at any desiredsoil depth. Simple applications of nitrogen models topaddy soils, particularly soils with different oxic levels,are non-existent. Therefore, it is important to performsimulations of the soil profiles of paddy fields. In thispaper, we report on experimental data regarding thechanges in nitrogen transformation that occur with dif-ferent flow conditions in paddy soils, and on the devel-opment of a model to simulate the nitrogen conditions inpaddy fields. We also compare the observed data with themodel developed.

Theoretical considerations

The model deals primarily with characteristic changes innitrogen levels through paddy soil layers. A schematicdiagram of the model is shown in Fig. 1. The nitrogenspecies include NH4-N and NO3-N in the soil solution,organic N (Org-N), and total nitrogen (T-N), where T-N isNH4-N, NO3-N, and Org-N. The NO2-N concentrationwas so small compared to NO3-N that we ignored it in thisstudy. Nitrogen movement and transformation are con-sidered through: (1) denitrification of NO3-N; (2) nitrifi-cation of NH4-N to NO3-N; (3) ammonification of Org-Nto NH4-N; (4) adsorption of NH4-N, and; (5) nitrateleaching. Nitrate leaching will occur when there is per-colation.

It should be emphasized that several simplifying as-sumptions were made during model development. Theyare: (i) the nitrification processes are simplified to a one-step process; (ii) only one pool of ammonificable Org-Nis assumed; (iii) adsorption-desorption of NH4-N is aninstantaneous reaction between that in soil solution andthe exchange sites of the soil matrix; (iv) biological up-take is not considered, although we transplanted rice inthe soil columns.

Nitrogen transport and transformation are traditionallydescribed by the convection-dispersion equation (CDE),including a reaction term:

Ri@Ci

@t¼ D

@2Ci

@x2� V

@Ci

@xþ Fi i ¼ 1; 2; . . . ð1Þ

where Ri is a retardation factor (Jury et al 1991), Ci is theconcentration of the nitrogen species in soil solution, D isthe hydrodynamic dispersion coefficient, V is the porewater velocity, Fi is the rate of transformation of thenitrogen species, x is the distance within the soil, and t istime. V is highly dependent on soil moisture and is vari-able for saturated and unsaturated conditions. The MIM(mobile-immobile) water model (Jury et al 1991) is ap-plicable to uplands; its validity to paddy fields is notknown. Applying CDE to soil columns of present ex-periments, it is assumed that the flow is controlled by thelayer of least permeability, and that V can be taken asconstant for both soil columns because there is a plowsole in the paddy field (Tabuchi and Yamafuji 1992).Assuming a first-order reaction rate for nitrification(Watts and Hanks 1978) and denitrification (Yamaguchiet al 1996), the rate of N transformation is expressed as:

Fi ¼ �kiCi ð2Þwhere Fi, i=1,2, ... indicates a transformation process, ki

is the rate constant, and Ci is the concentration of species iper unit volume of soil solution. It is assumed that NO2-Nand NO3-N do not participate in ion exchange reactions.The rate of increase in the concentration of NH4-N andNO3-N in soil solution at any column depth is made up ofnet dispersive-convective influx and production due tobiochemical reactions. The boundary conditions for thesimultaneous solution of Eqs. 1 and 2 are:

for NO3 � N Ci ¼ C0 x ¼ 0t ¼ 0Ci ¼ 0 x > 0t ¼ 0

for NH4�N and Org� N Ci ¼ 0 x ¼ 0t ¼ 0Ci ¼ Cint x > 0t ¼ 0

The boundary conditions cover the situation whereNO3-N is continually supplied onto a soil surface that

Fig. 1 Schematic diagram of nitrogen cycle (note that fertilizerinput and ammonium exchange are not fully documented in thenitrogen model)

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initially does not contain NO3-N. The soil is supposed toundergo transformation processes such that Org-N givesrise to NH4-N, and added NO3-N undergoes denitrifica-tion readily after its entry into the soil. The solution ofEq. 1 yields:

Ci ¼ C0Pi ð3ÞIn the above equation, if t!1 and x is finite and for

steady-state conditions, Pi is given by:

Pi ¼ expx

2DiVi �

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

V2i þ 4Diki

q

� �� �

ð4Þ

Full details of the derivation of this can be found inCho (1971) and Misra et al (1974). Combining Eqs. 3 and4, we obtain the steady-state concentration profile of ni-trogen. Applying the Binomial Theorem to Eq. 4 revealsan approximate concentration profile. The solution is:

Pi¼expxVi

2Di1�

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1þ 4Diki

V2i

s !( )

¼expxVi

2Di1� 1þ 2Diki

V2i

� 2D2i k2

i

V4i

þ 4D3i k3

i

V6i

� . . .

� �� �� �

ð5ÞIf the value of Diki is very small compared to Vi

2,higher values can be neglected. Di is the largest for nitratesolution and is equal to 2.4 cm2/d (Yamaguchi et al1992b). The value of ki for the nitrification rate is 0.05/day in this study (see Table 1). Therefore, the maximumvalue of Diki/Vi

2 is 1/3. Any higher value can be neglect-ed. Pi is approximated to:

Pi ffi exp � kix

Vi

� �

ð6Þ

Therefore, the approximate steady-state concentrationprofile of nitrogen is:

Ci ffi C0 exp � kix

Vi

� �

ð7Þ

Nitrification and denitrification

Equation 7 provides the foundation for modeling nitrifi-cation and denitrification (Cho 1971; Starr et al 1974).Both can then be expressed by Eqs. 8 and 9 respectively.

NH4nþ1 ¼ NH4n exp �KNx

V

� �

ð8Þ

NO3nþ1 ¼ NO3n exp �KDx

V

� �

ð9Þ

where NH4n+1 is the concentration of NH4-N at the n+1th

soil depth (mg/L), NO3n+1 is the concentration of NO3-Nat the n+1th soil depth (mg/L), NH4n is the initial con-centration of ammonium at the nth soil depth (mg/L),NO3n is the initial concentration of nitrate at the nth

soil depth (mg/L), KN is the coefficient of nitrification(day�1), KD is the coefficient of denitrification (day�1), xis the soil depth between two soil layers (cm), and V isthe pore water velocity (cm/day). So, when n=0, it meansthat the NH4-N and NO3-N concentrations derived are forsurface water, and when n=1, it means that the NH4-Nand NO3-N concentrations derived are for 2.5 cm soildepth.

Ammonification

The ammonification of organic nitrogen to ammonium isdescribed by a first-order rate equation (Rao et al 1984)derived from the equations presented by Stanford andSmith (1972). If the organic nitrogen is of reasonablyuniform composition, the net release of inorganic nitrogenshould reflect the decline in organic nitrogen and followfirst-order kinetics (Burns 1980).

d Org� Nð Þdt

¼ �k Org� Nð Þ ð10Þ

where Org-N is the organic nitrogen content of the soil, tis the time, and k is the rate constant for the net release ofinorganic nitrogen. Integrating Eq. 10 gives:

Nt ¼ N0e�kt ð11ÞNt and N0 are the amount of Org-N present in the soil

at time t and time 0 respectively. The expression in Eq. 11yields:

Nm ¼ N0 1� e�kt�

ð12Þwhere Nm is the net amount of inorganic nitrogen(NH4-N) released. This equation has been successfullyapplied (Stanford and Epstein 1974; Watts and Hanks1978; Hadas et al 1989; Shiratani et al 1996). Accordingto Eq. 12, the following expression is employed for theprocess of ammonification:

NH4nþ1 ¼ Org� Nn 1� exp�KMx

V

� �

ð13Þ

Table 1 Rate coefficients for paddy soil in unsaturated flow con-ditions

Depth(cm)

Column

KM (day–1) KN (day–1) KD (day–1) KR (day–1)

2.5 0.15 0.001 0.7 –7.5 0.15 0.001 0.7 –

12.5 0.15 0.001 0.7 –20.0 0.03 0.01 0.5 –30.0 0.03 0.01 0.5 –40.0 0.05 0.05 – �0.01850.0 0.05 0.05 – �0.00660.0 0.05 0.05 – �0.00670.0 0.05 0.05 – �0.00680.0 0.05 0.05 – �0.006

105.0 0.03 0.03 – �0.006135.0 0.03 0.03 – 0.018

KM: coefficient of mineralization; KN: coefficient of nitrification;KD: coefficient of denitrification; KR: coefficient of aeration

117

where Org-Nn is the initial organic nitrogen (mg/L) at thenth depth, and KM is the coefficient of ammonification(day�1).

Adsorption

The relation between the amount of substance adsorbedand the concentration of the substance in solution isknown as the adsorption isotherm (Jury et al 1991). Thelinear adsorption isotherm, a special case of the Freund-lich isotherm, has been used to attempt to determine theammonium adsorption in paddy soils (Selim and Iskandar1981; Rao et al 1984; Misawa 1988), and is given as:

NH4nþ1 ¼ KA � NH4n � KDE � NH4n ð14Þwhere KA is the coefficient of adsorption and KDE is thecoefficient of desorption (dimensionless).

Leaching

The mechanism that controls nitrogen leaching is thetransport of percolating water through the soil column. Itis therefore calculated as the product of water flow andnitrate concentration (Johnsson et al 1987). The solutionphase of nitrogen in the form of NH4-N is less prone toleaching due to retardation by solid phase partitioningassociated with cation exchange on negatively chargedsoil surfaces (Pierzynski et al 1994). Hence, ammonium isnot assumed to be easily transportable by water flow.Shiratani et al (1996) has applied the following equationfor the leaching of nitrate.

NO3nþ1 ¼ KL � V � NO3n ð15Þwhere KL is the leaching coefficient (day/cm).

Effect of air intrusion

Obviously, NO3-N concentrations are not subjected tovigorous denitrification under unsaturated conditions, es-pecially in oxic zones. The ammonificated NH4-N is ni-trified because of air intrusion, and the soil layer behaveslike upland field soil. Therefore, it is assumed that NO3-Ndoes not undergo complex transformation processes. Forsimplicity, the transport and transformation of NO3-N canbe expressed by the following equation:

NO3nþ1 ¼ NO3n exp�KRx

V

� �

ð16Þ

where NO3n+1 is the concentration of nitrate at the n+1th

depth (mg/L), NO3n is the concentration of nitrate at thenth depth (mg/L), and KR is the coefficient of aeration(day�1).

Governing equations

The following governing equations are used for simula-tion, using estimates for the concentrations of nitrogen(gain or loss) in aggregate processes as well as theleaching fraction of nitrate-nitrogen (Eqs. 8–16):

NTnþ1 ¼ DNnþ1 þ NHn � NFnþ1ð Þ þ LCnþ1 ð17Þ

NHn ¼ MNn þ NFn � ADSn ð18Þ(all values are in mg/L), where NTn+1 is the concentrationof NO3-N at the n+1th soil depth, DNn+1 is the concen-tration of NO3-N remaining after denitrification at then+1th soil depth (from Eq. 9), NHn is the concentration ofNH4-N at the nth soil depth (from Eq. 8), NFn+1 is theconcentration of NH4-N remaining after nitrification atthe n+1th soil depth (from Eq. 8), LCn+1 is the NO3-Nleaching at the n+1th soil depth, NHn is the concentra-tion of NH4-N at the nth soil depth, MNn is the concen-tration of NH4-N via ammonification of Org-N at the nth

soil depth (from Eq. 13), and ADSn is the adsorption ofNH4-N at the nth soil depth.

Evaluation of model

Measured and predicted concentrations of NO3-N andNH4-N were compared to evaluate the reliability of themodel. The average deviation (AD) between the measuredand simulated data is calculated using the followingequation:

AD ¼

P

j¼n

j¼1ABS xjp � xjm

�

nð19Þ

where j (=1,2,3,.........,n) is the number of observations,xjm is the measured data, and xjp is the predicted data.Sensitivity analyses were conducted for the rate coeffi-cients of ammonification, nitrification, and denitrificationin order to identify the model’s sensitivity. The sensitivitycoefficient y is defined as:

y ¼ @R

@P� PR

� �

ð20Þ

where R is the result from the model (nitrate or ammo-nium concentration), and P is the value of the rate coef-ficient being investigated.

Two separate computer programs were developed todeal with saturated and unsaturated conditions in thepaddy field soil. The model is versatile and can accom-modate as many data sets as necessary, but the number ofdepth increments within the soil column is restricted to12. The model predicts the nitrate-nitrogen and ammo-nium-nitrogen at the required depths.

118

Experimental apparatus and method

Two soil columns (135 cm long with inner dimensions of45�45 cm) were used in the laboratory experiments. The soil layerswere packed in boxes constructed from plywood with an acrylicsheet on the front. The columns were furnished with sample filters(5 cm, porous ceramic tubes), ORP sensors (platinum whirled), andmanometer tubes (25 cm tubes with perforation) at 11 points alongthe soil depth. In addition, one column (20–80 cm column depth)was provided with air entrance tubes to create unsaturated (oxic)flow conditions, as shown in Fig. 2. Water was sampled with asample syringe connected to the sample filter. The temperature inthe experimental room was maintained at around 25 �C, althoughhumidity was not controlled.

The soil layers: plow layer (root zone; RTN is clay), plow sole(hard pan; HPN is clay), and subsoil (SBL is Kanto loam) werecarefully packed to bulk densities of 0.69, 0.81, and 0.79 g/cm3 forsoil depths of 0–15, 15–35, and 35–85 cm (disturbed soil). Thebottom end of each column was filled with a 50 cm layer of sand(SND). In general, the saturated hydraulic conductivity (Ks) of eachpaddy soil layer was 10�3 to 10�5 cm/s for the root zone, 10�6 to10�7 cm/s for the plow sole, and 10�3 to 10�5 cm/s for the subsoil.Therefore, the percolation rate was 6 mm/day and fixed throughexperiments. Iwata et al (1995) reported similar values for paddy

soils from different regions in Japan. The percolation rate wasdetermined from the hydraulic conductivity for the plow sole. Thesoils used were taken from actual paddy fields. The experimentalapparatus is shown in Fig. 3.

The soil columns received continuous application of KNO3(20 mg/L as NO3-N) as the source of nitrogenous fertilizer over aperiod of two years, maintaining 5 cm ponded water depth. Soilanalyses were conducted on the chemical and physical properties ofthe soil layers. The results of these are summarized in Table 2. Toestablish a soil system similar to that in actual paddy fields, wepreconditioned the soil columns by saturating them with tap waterfor 45 days prior to NO3-N application and data collection. Sampleswere collected and analyzed for NO2-N, NO3-N, NH4-N and T-N.The important factors to consider are the saturated and unsaturatedflow conditions in paddy soil. Such flow phenomena cause the soillayers to be anoxic or oxic for saturated or unsaturated flows re-spectively, depending upon the aeration level. To check bothconditions, we measured the water pressure distributions and theoxygen reduction potentials (ORP) at different column depths, asshown in Figs. 4 and 5. The ORP was measured whether the soilcondition was oxic or anoxic. Dissolved oxygen (DO) is also im-portant, but we did not measure it because the analysis of DO in soil

Fig. 2 Positions of sample filter, ORP sensor, manometer tube, andair entrance tube

Fig. 3 Experimental apparatus used in this study (changes in thewater level are seen by the manometer)

Table 2 Chemical and physical properties of the packed soil col-umns

Soil layers (cm) I II III IV

Soil properties Rootzone Hardpan Subsoil Sand(0–15) (15–25) (25–85) (85–105)Clay Clay Loam Sand

N (g/kg) 0.305 0.424 0.130 0.002C (g/kg) 3.328 5.569 1.492 0.020C/N 10.91 13.13 11.47 10.00Bulk density(mg/m3)

0.69 0.81 0.79 1.51

Moisturecontent (kg/kg)

1.03 0.818 0.625 0.073

Fig. 4 Water pressure distribution in soil columns on 18th March1996

119

solution is difficult and inaccurate, especially for unsaturated flowconditions. Figure 4 shows a positive hydraulic head up to 12.5 cmdepth, which becomes negative due to suction beyond the under-lying plow sole. As the suction continues in the unsaturated area, itremains negative except for a few variations. On the other hand, itattains a positive head according to a linear function in saturatedconditions. ORP observations reflect the redox potentials of the soillayers, as shown in Fig. 5. The anaerobic state is defined by anegative ORP throughout the column depth, while the aerobic statehas positive ORP. At a depth of 30 cm it is around +200 mV, whichindicates that air has intruded at this depth. This clearly shows thatthe nitrogen transformation differs depending on the flow condi-tions.

Rate coefficients

To validate the model, two essential requirements must be met.First, an extensive database from well-designed experiments isneeded. Second, we need to select model parameters that providethe “best fit” to the experimental data. To accomplish this, certainadjustments are needed to simulate the experimental data. The ratecoefficients are summarized as follows: KM=coefficient of ammo-nification, KN=coefficient of nitrification, KD=coefficient of deni-trification, KA=coefficient of adsorption, KL=coefficient of leach-ing, and KR=coefficient of aeration. These parameter values areestimated from the literature (Misawa 1988; Shiratani et al 1996) oradjusted to improve the agreement between measured and simu-lated nitrogen profiles. The estimated parameters were the ad-sorption and leaching coefficients, while the adjusted parameterswere the coefficients for ammonification, nitrification, and deni-trification (Misra et al 1974; Shiratani et al 1996; Yamaguchi et al1996). Since the experiments were conducted for two different soilconditions—unsaturated and saturated—the rate coefficients arepresented for both of these conditions in Tables 1 and 3.

Results and discussion

The nitrogen concentrations for NH4-N and NO3-N weresimulated over 105 weeks following the application ofKNO3 as the source of nitrate-nitrogen in the paddy soilcolumns. The experimental data shown in Fig. 6 indicatethe measured concentration of nitrogen with the applica-tion of NO3-N. Typical patterns for NO2-N, NO3-N, NH4-N and T-N concentrations, for both conditions, are shownin Figs. 6a, b. The NO2-N concentrations do not appear inFigs. 6, 7, 8, 9, 10, 11, and 12, since the concentrationswere less than 0.05 mg/L.

Figures 6a, b show that the nitrate nitrogen was nearlydenitrified up to the 2.5 cm layer. However, nitrogenconcentrations begin to reappear from the 7.5 cm layer.This phenomenon may be explained by assuming thatdenitrifying bacteria grown at the soil surface died and themicrobes decomposed there. These accumulated microbeswere ammonificated and transferred slowly downwardwith soil solution, so that high NH4-N concentrationsappear in the soil layer just below. There would be NH4-Nreleaseed from the soil, although in small amounts. Theammonificated nitrogen was not oxidized quickly in thecase of the saturated flow condition. High NH4-N con-centrations were transferred to the 60 cm layer. Instead,nitrogen was removed almost completely at 30–80 cmdepth with unsaturated flow conditions, since ammonifi-cation, nitrification, and denitrification are enhanced un-der such conditions. We estimated that high denitrifica-tion was taking place at these depths, since some parts ofthe soil layer were anoxic even though the flow was un-saturated (oxic conditions). Anyhow, our experiment re-vealed that there are relevant differences in nitrogentransformation patterns in different flow conditions.

Case I: Unsaturated flow conditions

The transport and transformation processes of the modelhave six “rate coefficients”. KM, KN, KD, and KR arepresented in Table 1. KA and KL are constant for bothflow conditions, and they are KA=0.03, KL=0.012 (day/

Fig. 5 ORP, as measured in soil columns on 18th March 1996

Table 3 Rate coefficients for paddy soil in saturated flow condi-tions

Depth(cm)

Column

KM (day–1) KN (day–1) KD (day–1)

2.5 0.15 0.001 0.77.5 0.15 0.001 0.7

12.5 0.15 0.001 0.720.0 0.03 0.005 0.530.0 0.03 0.005 0.540.0 0.05 0.008 0.350.0 0.05 0.008 0.360.0 0.05 0.008 0.370.0 0.05 0.008 0.380.0 0.05 0.008 0.3

105.0 0.03 0.001 0.3135.0 0.03 0.001 0.3

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Fig. 6 a Nitrogen concentrationunder unsaturated flow condi-tions on 10th June 1996. b Ni-trogen concentration undersaturated flow conditions on10th June 1996

Fig. 7 Simulated and measured concentrations of nitrate and am-monium at 2.5 cm soil column depth under unsaturated conditions

Fig. 8 Simulated and measured concentrations of nitrate and am-monium at 20 cm soil column depth under unsaturated conditions

Fig. 9 Simulated and measured concentrations of nitrate and am-monium at 60 cm soil column depth under unsaturated conditions

Fig. 10 Simulated and measured concentrations of nitrate andammonium at 2.5 cm soil column depth under saturated conditions

121

cm). Figures 7, 8, and 9 show the distribution of NH4-Nand NO3-N in soil solution under unsaturated (oxic)conditions. At a soil column depth of 2.5 cm, which is thefirst sampling point in the root zone, the model gives agood approximation of the NO3-N until the 55th week,and has a fair accuracy from the 85th week onwards too,as depicted in Fig. 7. The observed concentration between54th and 88th is fluctuating. The 2.5 cm soil column depthis very near to the irrigation water on the soil surface.Hence there was direct entry of surface water in this case,because we took out sample water by suction. Referringto Fig. 8, it is obvious that both measured and simulatedvalues show promising similarities. At the zone of aera-tion, the 30–80 cm soil depth, it is apparent that NH4-N isnitrified, resulting in a fractional value, and a similaroutcome is noticed for the predicted results too. Measuredand predicted results show that the fluctuation in N de-creases with soil depth. NO3-N was fairly accurate exceptthat there was peak concentration of NO3-N at 60–80 cmdepth (Fig. 9 for 60 cm. This implies either the pre-ex-istence of NO3-N or a relatively high rate of nitrification.The peak becomes wider and drifts downward with re-spect to time. It seems likely that this drifting correlateswith Darcy’s law. It does not disappear in unsaturatedconditions. Nevertheless, it leached down and ultimatelydisappeared from the columns. The model is thereforeadequate for use with the soil column under unsaturatedconditions.

Case II: Saturated flow conditions

As the name implies, the structure of this model is in-herently the same as that used above except there is noconsideration of an air entrance zone in order to maintainthe saturated environment that creates the anoxic state.Evidently, five rate coefficients except KR are involved inthe calculations. The rate coefficient values are presentedin Table 3. Figures 10, 11, and 12 show measured andmodel predictions of NO3-N and NH4-N in saturatedconditions.

Figures 10 and 11 show a comparison of measured andsimulated results at 2.5 cm and 20 cm soil column depths.Concentration distributions show that the fluctuations inNO3-N at 2.5 cm column depth (Fig. 10) may be relatedto the applied concentration of NO3-N. The sample filterfixed at 2.5 cm depth was very close to the surface andthere is the possibility that surface water with a highNO3-N concentration entered the filter and got collectedin the syringe through suction, rather than soil solution.The simulated results failed to predict the peak NO3-Nconcentrations starting from the 31st week up to the 85th

week. In the plow layer, all nitrogen transformation pro-cesses were essentially similar in both soil columns;therefore, the applied rate coefficients must have samevalues. This principle is followed in the model simulation.NO3-N peaks might be due to the direct entry of influentinto the sample collector, or the pre-existence of NO3-Nderived from the parent soil. This pattern also contra-

dicted the high nitrogen removal capacity (nearly 90%) ofthe paddy soil.

At 20 cm in the plow sole in Fig. 11 no concentrationof NO3-N was found, and the model predicts a similarpattern. Simulated NH4-N also has a similar pattern tothat observed. In contrast, the model overestimates theconcentration of NH4-N, particularly at 60 cm. FromFig. 12, it is clear that the ammonium nitrogen concen-tration is predicted accurately up to the 31st week, whenan abrupt change in the NO3-N caused a different patternin the NH4-N. High concentrations of T-N (for example;12.0 and 11.9 mg/L at 40 and 60 cm soil column depthsrespectively, as observed on 10th June 1996) were foundin the experimental data that may include reasonableamounts of organic nitrogen. Many models address theheterogeneity of organic nitrogen in soil and soil waterby incorporating multiple organic nitrogen fractions(Johnsson et al 1987). As we saw earlier, only one pool oforganic nitrogen is encountered in the present modelcalculations. Hence, overestimation of NH4-N cannot beignored. The adsorption also cannot be ignored.

The statistical analysis between simulated and ob-served data is summarized in Table 4. For unsaturatedexperiments, the average deviation (AD) of NO3-N wasgood, while the value for NH4-N reflects disagreements ashigh as 4.5 (at 30 cm soil column depth). For saturatedconditions, the simulated NO3-N was similar to that ob-

Fig. 11 Simulated and measured concentrations of nitrate andammonium at 20 cm soil column depth under saturated conditions

Fig. 12 Simulated and measured concentrations of nitrate andammonium at 60 cm soil column depth under saturated conditions

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served, except at 2.5 cm depth, which is attributed to vari-ations in observed data due to the direct entry of surfacewater. Modeling NH4-N in saturated conditions producedreasonable results, with some overestimation, as reflectedin the values of the average deviation. Further, experi-ment errors should also be included in the AD.

Sensitivity analysis was conducted, based upon 10%variations in the values of the rate coefficients, givingyKM, yKN, and yKD for KM, KN, and KD respectively. Ifwe consider the absolute value of y for variations inNO3-N, the model is extremely sensitive to KD at 2.5 cm,moderately sensitive to KM at 80 cm, and slightly sen-sitive to KN at 20 cm. In the case of NH4-N, the effects ofvarying KM and KN are nearly identical, as ammonifi-cation and nitrification are mainly responsible for theNH4-N balance. The model is insensitive to KD from thisperspective. The results are given in Tables 5 and 6.

Conclusions

This study has clarified that nitrogen transport andtransformations in paddy soil differ depending on the flowpatterns, which in turn depend upon whether there is airintrusion or not. The rate coefficients tested should bebeneficial to further modeling approaches. The simulationresults suggest that this model can be used. Ignoring thesharp peaks encountered early on in the experimentsperformed under unsaturated conditions, the model’s pre-dictions for NO3-N agreed reasonably well with observeddata. The simulated NH4-N concentration occasionallyshowed fluctuations from the measured values. This wasascribed to uncertainty about the rate of ammonification,which is one of the main sources of NH4-N, and thesimplifying assumptions inherent in the model. A sensi-tivity study of the transformation processes shows deni-trification to have a greater effect on the results predictedfor NO3-N than ammonification and nitrification. In caseof NH4-N, the model is extremely sensitive to ammoni-

Table 4 Statistical analysis showing average deviations between simulated and measured data

Soil depth (cm) 2.5 7.5 12.5 20 30 40 50 60 70 80 105 135

NO3–N (unsaturated flow; mg/L) �0.51 0.10 0.02 0.47 0.19 �0.01 �0.13 �0.16 �0.7 �0.4 0.47 0.09NH4–N (unsaturated flow; mg/L) �0.40 �0.13 1.09 1.03 4.49 �0.08 0.15 0.19 0.15 0.54 0.11 �1.02NO3–N (saturated flow; mg/L) �3.09 0.07 0.03 0.16 0.34 0.57 0.57 0.55 0.49 0.31 0.09 0.05NH4–N (saturated flow; mg/L) �0.08 1.74 1.76 0.43 0.06 1.24 1.85 2.65 0.59 2.41 1.29 �2.07

Table 5 Sensitivity analysis ofsimulated data for NO3–N

Columndepth (cm)

Unsaturated flow condition Saturated flow condition

yKM(%) yKN(%) yKD(%) yKM(%) yKN(%) yKD(%)

2.5 0 1 �261 0 1 �2447.5 5 12 �21 5 12 �13

12.5 25 69 �24 25 69 �4620 16 96 �4 16 96 1230 21 90 �2 21 90 740 21 82 �4 21 82 150 15 76 �5 15 76 �460 �12 60 �6 �12 60 �670 �63 31 �8 �63 31 �880 �161 34 �1 �161 34 �10

105 �53 20 4 �57 20 4135 �43 57 3 �43 57 3

Table 6 Sensitivity analysis ofsimulated data for NH4–N

Columndepth (cm)

Unsaturated flow condition Saturated flow condition

yKM(%) yKN(%) yKD(%) yKM(%) yKN(%) yKD(%)

2.5 58 0 0 58 0 07.5 38 0 23 38 0 23

12.5 16 �1 �3 16 �1 �320 21 �5 �2 21 �5 �230 21 �11 �1 21 �11 �140 15 �17 0 15 �17 050 �13 �31 0 �13 �31 060 �68 �61 1 �68 �61 170 �185 �131 6 �185 �131 680 281 162 �15 281 162 �15

105 �40 �35 3 �40 �35 3135 �198 66 11 �198 66 11

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fication, moderately sensitive to nitrification, and slightlysensitive to denitrification.

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