Nitrogen Dynamics and Biomass Production in a Vertical Flow Constructed Wetland Cultivated with Forage Rice and their Mathematical Modeling

ABSTRACT Forage rice has high potential to produce biomass, and the vertical flow (VF) constructed wetland in which forage rice is cultivated is one of the effective ways to achieve the purification of eutrophicated water and biomass production simultaneously. To design and manage the VF constructed wetlands cultivated with forage rice adequately, nutrient dynamics and the growth of the rice should be understood quantitatively. In this study, we performed a series of experiments replicating VF constructed wetlands involving the cultivation of a variety of forage rice ("Kusahonami") using river water (supply rate : 0.1, 0.2, and 0.6 m3/(m2·day)) for 169 days. The results showed that the rice biomass increased with the river water supply rate. A mathematical model was developed based on these experimental observations in order to quantitatively explain the nitrogen dynamics in VF constructed wetlands cultivated with forage rice. The changes in both the rate of nitrogen assimilation by rice and the denitrification rate with the change in the rate of water supply were simulated with the proposed model

Nitrogen Dynamics and Biomass Production in a Vertical Flow Constructed Wetland Cultivated with Forage Rice and their Mathematical Modeling

Masaki SAGEHASHI*, Sheng ZHOU*, Tatsuro NARUSE**, Mari OSADA**,

Masaaki HOSOMI*

* Graduate School of Engineering, Tokyo University of Agriculture and Technology, Tokyo

184-8588, Japan

** (Former Affiliation) Graduate School of Engineering, Tokyo University of Agriculture and

Technology, Tokyo 184-8588, Japan

ABSTRACT

Forage rice has high potential to produce biomass, and the vertical flow (VF) constructed wetland in which forage rice is cultivated is one of the effective ways to achieve the purification

of eutrophicated water and biomass production simultaneously To design and manage the VF constructed wetlands cultivated with forage rice adequately, nutrient dynamics and the growth

of the rice should be understood quantitatively In this study, we performed a series of experiments replicating VF constructed wetlands involving the cultivation of a variety of forage

rice ("Kusahonami") using river water (supply rate : 0.1, 0.2, and 0.6 m3 /(m 2 ·day)) for 169 days The results showed that the rice biomass increased with the river water supply rate A mathematical model was developed based on these experimental observations in order to quantitatively explain the nitrogen dynamics in VF constructed wetlands cultivated with forage rice The changes in both the rate of nitrogen assimilation by rice and the denitrification rate with the change in the rate of water supply were simulated with the proposed model

Keywords: forage rice, nitrogen dynamics, vertical flow wetland

INTRODUCTION

Excess nutrient loading in a water environment causes various problems such as deterioration of water quality, landscape damage, etc Nutrient loading sources can be divided into two types, namely, point and non-point sources, with the latter sources usually more difficult to mitigate Constructed wetlands represent one of the promising techniques for removing nutrients from relatively large-scale water bodies This technique can be utilized in response to non-point nutrient sources Many studies have been performed to clarify the performance of various constructed wetlands (e.g.,

Nungesser and Chimney, 2006; Gu et al., 2006; Behrends et al., 2007; Zhou and

Hosomi, 2008a)

The constructed wetlands can be roughly divided into two types, i.e., the free water surface flow (FWSF) and the sub-surface flow (SSF) types, with the latter including the horizontal sub-surface flow (HSSF) and vertical flow (VF) types (Zhou and Hosomi, 2008b) In the VF constructed wetland, the wastewater is poured into the soil layer with

a distribution pipe, and the treated water flows out from the bottom through a drainage pipe (Brix and Arias, 2005)

Forage rice has received attention as biomass cultivated in constructed wetlands The

water penetration rate is an important factor for the growth of rice in paddy field Meanwhile, the water penetration rate in VF constructed wetland can be controlled artificially Moreover, the VF constructed wetland has high potential for nitrogen removal (Zhou and Hosomi, 2008b) Therefore, the VF constructed wetland cultivated with forage rice is a fascinating subject To design an adequate VF constructed wetland using forage rice, it is required to understand in detail the dynamics of nutrients in the soil layer and resultant rice growth

The purpose of this study is thus to clarify the effects of the water supply rate on the dynamics of nitrogen in VF constructed wetlands cultivated with forage rice This was done both by experimental and model analysis approaches

MATERIALS AND METHODS

Pot Experiment

A series of experiments was performed at an open-air experimental station located in Ibaraki Prefecture, Japan Figure 1 shows the outline of the experimental apparatus used

in this study The apparent density and porosity of the ando soil were 0.44×106 g/m3 and 0.676 m3/m3, respectively Gravels (diameter = ca 1 to 2 cm) were put into the gravel zone, and unwoven cloths were installed at the upper part of the gravel zone and the water effluent port River water obtained from the Sanno-gawa River was supplied at the top surface of the pot, and it penetrated into the soil zone and flowed out from the bottom of the pot Soil water samplers (DIK-8391, Daiki Rika Kogyo, Japan) were installed at depths of 10 cm and 20 cm in the soil zone to sample the soil interstitial water The water supply rate was controlled to achieve preset supply rates, and the water level was adjusted by back pressure at the water effluent port

In this study, a kind of forage rice known as “Kusahonami” was employed

"Kusahonami" is a new variety developed for whole-crop silage It can produce a larger

quantity of biomass, and has a higher tolerance for nitrogen loading than the commonly

used rice variety (Zhou and Hosomi, 2008a; Sakai et al., 2003) Two rice seedlings were

transplanted in a pot on May 8, 2005 and harvested on October 24, 2005

In the experiments, the river water supply rates were set at 0.1, 0.2, and 0.6 m3/(m2·day), and the time courses of the concentrations of inorganic nitrogen compounds (i.e., ammonium, nitrate, and nitrite nitrogen) in the supplied water, soil interstitial water, and effluent water were monitored as well as the leaf number and plant height of the rice The concentrations of inorganic nitrogen ions were measured with an ion chromatograph, and the total nitrogen (T-N) concentration in water was analyzed by absorption spectrophotometry after decomposition with potassium peroxodisulfate (K2S2O8) (Hosomi and Sudo, 1986) Furthermore, the weight of rice in each pot was measured at the end of the experiment after being oven-dried at 80 °C for 48 h

water supply

surface water depth = 0.05 m

ando soil zone depth = 0.35 m gravel zone depth = 0.05 m

water effluent

penetration

forage rice

Fig 1- Experimental Pot Used in This Study

Mathematical Modeling

Structure

A mathematical model which describes the fate of inorganic nitrogen compounds in soil, water and rice was developed The model structure is shown in Fig 2 The gravel zone was not considered in the model

0.20 m

0.05 m 0.05 m

0.10 m

0.35 m

Nitrification

Nitrification

Advection

& diffusion

Advection

& diffusion

Advection

& diffusion

diffusion

Advection

& diffusion

Advection

& diffusion

Advection

& diffusion

diffusion

gas

Denitrifi -cation

Denitrifi -cation

Denitrifi -cation

Plant uptake

Plant uptake

Plant uptake

Plant uptake

Plant uptake

Plant uptake

NH ms, s

NH ls, s

Adsorption/

desorption

Adsorption/

desorption

Adsorption/

desorption

Nitrification

Water supply

Water supply

Fig 2- The Structure of the Model Developed in This Study

The model is composed of four compartments, namely the surface water compartment

(surf), upper soil compartment (us), middle soil compartment (ms), and lower soil

compartment (ls) In each compartment, complete mixing was assumed Basically, the

nitrogen (NO2+3-N) The adsorption of NO2+3-N on soil was ignored

The organic nitrogen dynamics was not considered because the differences between

organic nitrogen (T-N minus inorganic nitrogen) in supplied water and in effluent was

not so significant (supplied water = 0.80±0.50 mg-N/L; effluent = 0.39±0.22 mg-N/L

(Q=0.6); 0.36±0.26 mg-N/L (Q=0.2); 0.53±0.57 mg-N/L (Q=0.1); average±SD of

observations Q is the water supply rate [m3/(m2･day)].) compared to that of inorganic

nitrogen in the experimental results

Furthermore, total nitrogen in rice was employed as a state variable Above-ground rice

biomass, and underground rice biomass were calculated from the total nitrogen in rice

with a certain proportional constant obtained from our experiments

Basic Equations

The nitrogen flow in the system can be described by the following mass conservation

equations

i NH up i nit i NH dif i NH adv

dt

dNH

, , , , , ,

i NO up i den i nit i NO dif i NO adv

dt

dNO

, , , , , , ,

∑

=

i

i NO up i

i NH up

dt

dN

, , ,

where NHi and NOi represent NH4-N and NO2+3-N in the ith layer [g-N/m2], respectively,

and Nrice is the rice nitrogen [g-N/m2] The terms radv,NH,i and radv,NO,i are the NH4-N and

NO2+3-N inflow into the ith layer due to advection, respectively The terms rdif,NH,i and

respectively The advection was calculated using conventional equations The diffusion

in soil was calculated from the soil water content, soil porosity, and diffusion coefficient

in water based on the literature (Millington and Quirk, 1961; Shearer et al., 1973)

Adsorption

Ammonium nitrogen is adsorbed on soil After Jury and Horton (2004), linear and

instantaneous adsorption were assumed, and the concentration of nitrogen in soil

interstitial water, CNH,i, is calculated as

( ads NH s i s) i

i

NH

z K

NH C

Δ

⋅ +

⋅

=

θ ρ

,

where Kads,NH is the linear adsorption coefficient (3.48×10-6) [m3/g-soil], ρs is the soil

density (440,000) [g/m3], θs is the water content (0.676) [m3/m3], and Δzi is the depth of

the ith compartment The adsorption coefficient was obtained from an adsorption

experiment (data not shown), and the soil density and water content were determined

based on the physical properties of the soil mentioned above

Nitrification and Denitrification

The nitrification at the surface water is described as

surf )

(T nit nit,surf

where k nit,surf is the nitrification rate constant at the surface water [/day], χ nit is the

temperature constant for nitrification (1.05) [-] (based on Mayo and Bigambo, 2005),

and T is the temperature [°C] The parameter k nit,surf was calibrated in this study The

temperature variation is described below On the other hand, the denitrification at the

surface was ignored

Mayo and Bigambo (2005) employed a model which considered the nitrification and

denitrification both by biofilm on soil aggregates and plant roots Based on this report,

we assumed two mechanisms for r nit,i and r den,i as follows

i ) (T nit i

r,i nit,r,i nit,s,i

i ) (T nit i nit

z

w k k

NH χ

k

⎠

⎞

⎜⎜

⎝

⎛

Δ

⋅ +

=

⋅

⋅

i

T den i

i r i r den i s den i

T den i den i

z

w k

k NO k

⎠

⎞

⎜⎜

⎝

⎛

Δ

⋅ +

=

⋅

⋅

, ,

) 20 ( ,

where k nit,i is the overall nitrification rate constant at the ith compartment at 20 °C [/day],

[/day], k nit,r,i is the nitrification rate constant by the bacteria attached to the root at 20 °C

[/day], w r,i is the root weight at the ith compartment [g/m2], k den,i is the overall

denitrification rate constant at the ith compartment at 20 °C [/day], k den,s,i is the

denitrification rate constant by the bacteria not attached to the root at 20 °C [day], k den,r,i

is the denitrification rate constant by the bacteria attached to the root at 20 °C [/day],

Bigambo, 2005) The parameters k nit,s,i, k nit,r,i, k den,s,i and k den,r,i were calibrated in this

study

The root weights at the ith layer are calculated as

tot r i

tot

rice Nw

rice a

tot r tot

f

N w w

w

,

where ζi is the root abundance at the ith layer [g/g], wr,tot is the total weight of the rice

roots [g/m2], fabove is the above-ground fraction of rice (0.7) [g/g] (based on the

measurement in this study), wtot is the total weight of the rice [g/m2], wa is the weight of

the above-ground part of the rice [g/m2], and fNw,rice is the rice nitrogen / weight ratio

[g-N/g] The variations of ζi and fNw,rice are discussed below

Nitrogen Uptake by Rice

The rice growth rate is affected by radiation, temperature, and growth stage (Horie

1987; Horie et al., 1991), and the growth can be described by the logistic model (Sheehy,

J.E et al., 2006) The nitrogen uptake by rice is closely related with growth Assuming

Monod-type nitrogen uptake kinetics (Selim and Iskandar, 1981) and ignoring the

temperature effects, the NH4-N and NO2+3-N uptake by rice is described as

i NO i NH N

i NH i

r gs rad p

a

a p a up N i

NH

C w

f f w

w w k r

, ,

, ,

,

, , ,

−

⋅

i NO i NH N

i NO i

r gs rad a

a a

up N i

NO

up

C C

K

C w

f f w

w w

k r

, ,

, ,

max ,

max , , ,

−

⋅

max rad

IR

IR

where kN,up is the maximum nitrogen uptake by rice per unit root weight [g-N/g-root],

based on the observations in this study), frad is the radiation factor, fgs is a factor

dependent on growth stage, wr,i is the root weight per unit area [g-root/m2], KN is the

half-saturation constant for nitrogen uptake (1.0) [g/m3] (based on Selim and Iskandar,

1981), IR is the daily radiation [MJ/m2], and IRmax is the maximum daily radiation

during the experimental period [MJ/m2] The variations of fgs are discussed below

Forcing Functions

Some forcing functions were included in the model The daily average temperature and

daily irradiation were obtained from a weather station near the experimental site (Japan

Meteorological Agency) The photosynthetic activity is controlled by the LAI (leaf area

index), and it varies according to the growth stage Nitrogen uptake depends on the

photosynthetic activity, and fgs was assumed to be as shown in Fig 3 based on the

observed variation of leaf number (see "Observation of Rice Growth" in "RESULTS

AND DISCUSSION") The factor fNW,rice was determined as shown in Fig 4 based on

another experimental observations (data not shown) The vertical distribution of the root

abundance, ζi, was assumed to be as shown in Fig 5

Calculation

The mass balance equations were solved numerically by the 4th Runge-Kutta methods

using STELLA ver 9.0.3J (isee systems, inc., USA) The calculation step (Δt) was set at

1/180 day The initial condition of the nitrogen allocation was calculated by a

sufficiently long (100 days) simulation (Δt =1/100 day) without rice under the

corresponding water supply rate and the temperature on the transplantation day

0.0 0.2 0.4 0.6 0.8 1.0

f gs

[-Time after transplantation [day]

0 0.01 0.02 0.03 0.04 0.05

Time after transplantation [day]

f Nw

0.0 0.2 0.4 0.6 0.8 1.0

upper soil middle soil lower soil

Time after transplantation [day]

ζ i

RESULTS AND DISCUSSION

Observation of Rice Growth

The time courses of leaf number and plant height are shown in Fig 6 In the 0.6

m3/(m2∙day) case, the water level was increased from early October, and clogging was presumed However, the discharge rate was maintained, and we concluded that the effect of this clogging on the rice growth was not critical Obviously, the leaf number and plant height were increased with the water supply rate As described earlier, the photosynthesis activity is controlled by the leaf area Roughly, the plant height increased until 120 days after transplantation, and thereafter a constant height was maintained in every case The leaf area index (LAI), however, increased until heading and then

decreased (Hasegawa et al., 1991) To determine whether this observed stoppage in the growth in plant height is related to the heading, the factor dependent on growth stage, fgs,

is assumed as before (Fig 3)

0 20 40 60 80 100 120

0 20 40 60 80 100 120 140 160

Time after transplantation [day]

0 20 40 60 80 100 120 140

Q = 0.6

Q = 0.2

Q = 0.1

Q = 0.6

Q = 0.2

Q = 0.1

Fig 6- Time Courses of Leaf Number and Plant Height (“Q” : river water supply rate into each pot [m 3 /(m 2∙day)])

Model Predictability

The final above-ground rice biomass of each experimental case as calculated with the finally calibrated parameters is shown in Fig 7 along with the measured values Underestimations and overestimation were occurred in the low-water-supply case and high-water-supply case, respectively However, considering the simple structure of the model, the predictability of the rice production seemed to be acceptable

0.0 0.5 1.0 1.5 2.0 2.5 3.0

measurement calculation

Fig 7- Comparison Between Calculated and Measured Above-Ground Rice Weight

Figure 8 shows the time courses of the measured and calculated NO2+3-N concentrations

in soil interstitial water and effluent water in each case Note that the calculated

NO2+3-N concentration in effluent water from about 2 months after transplantation was low in every case In the model calculation, overestimation was especially observed in the soil interstitial water in the 0.6 m3/(m2∙day) case However, as in the rice production prediction, the model predictability for the NO2+3-N concentrations in soil interstitial water and effluent water was permissible considering the simple structure of the model

transplantation was also low (Fig 9) In the 0.6 m3/(m2∙day) case, some measurements

in effluent water were relatively higher than others, and the model calculation was significantly different from these values (Fig 9) There was a possibility that these high

supplied water In this model, however, the concentration of NH4-N in the supplied

change in water quality cannot be predicted by the model However, the general trends

in the NH4-N concentration variations were predicted by the model calculation

Q = 0.1

0.0

0.5

1.0

1.5

2.0

2.5

0 20 40 60 80 100 120 140 160

Q = 0.2

0.0 0.5 1.0 1.5 2.0 2.5

0 20 40 60 80 100 120 140 160

Q = 0.6

0.0 0.5 1.0 1.5 2.0 2.5

0 20 40 60 80 100 120 140 160

Time after transplantation [day]

Time after transplantation [day] Time after transplantation [day]

g-Supplied water (assumed in calc.)

Supplied water (assumed in calc.)

(calc.)

(calc.)

Effluent water (calc.)

Effluent water (calc.)

1) Average of 10 cm and 20 cm depth 2) The middle compartment in the model

Supplied water

(meas.)

(meas.)

Effluent water (meas.)

Effluent water (meas.)

(“Q”: river water supply rate into each pot [m 3 /(m 2 ·day)])

Time after transplantation [day]

Time after transplantation [day] Time after transplantation [day]

Supplied water (assumed in calc.)

Supplied water (assumed in calc.)

(calc.)

(calc.)

Effluent water (calc.)

Effluent water (calc.)

1) Average of 10 cm and 20 cm depth 2) The middle compartment in the model

Supplied water

(meas.)

(meas.)

Effluent water (meas.)

Effluent water (meas.)

Q = 0.1

0.0

0.5

1.0

1.5

2.0

2.5

0 20 40 60 80 100 120 140 160

Q = 0.2

0.0 0.5 1.0 1.5 2.0 2.5

0 20 40 60 80 100 120 140 160

Q = 0.6

0.0 0.5 1.0 1.5 2.0 2.5

0 20 40 60 80 100 120 140 160

(“Q”: river water supply rate into each pot [m 3 /(m 2 ·day)])

Calibrated Parameters

Some parameters were calibrated based on experimental observations, and the finally calibrated values are shown in Table 1

The parameter knit,r,us was calibrated as 0.01 (m3/g-root)/day Considering the root weight during the experiment, the overall nitrification rate constant in the upper soil,