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
Journal of Water and Environment Technology, Vol 7, No 4, 2009 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/(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 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 Address correspondence to Masaki SAGEHASHI, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Email:sagemasa@cc.tuat.ac.jp Received February 16, 2009, Accepted August 6, 2009 - 251 - Journal of Water and Environment Technology, Vol 7, No 4, 2009 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 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 to 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 - 252 - Journal of Water and Environment Technology, Vol 7, No 4, 2009 forage rice water supply (river water) surface area = 0.08 m2 surface water depth = 0.05 m penetration ando soil zone depth = 0.35 m water effluent gravel zone depth = 0.05 m Fig 1- Experimental Pot Used in This Study Nrice Water supply Middle soil Upper soil NHsurf Lower soil Surface water 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 The gravel zone was not considered in the model Advection & diffusion NHus,s NHus,w Adsorption/ desorption Advection & diffusion NHms, s NHms,w Adsorption/ desorption Water supply Nitrification Plant uptake NHls, s NHls,w diffusion Advection & diffusion NOus Plant uptake Denitrifi -cation 0.05 m Advection & diffusion NOms Nitrification Advection & diffusion Adsorption/ desorption 0.05 m NOsurf Plant uptake Nitrification Plant uptake gas Denitrifi -cation 0.20 m Advection & diffusion Plant uptake Plant uptake NOls Nitrification Denitrifi -cation 0.10 m diffusion Effluent Effluent Fig 2- The Structure of the Model Developed in This Study - 253 - 0.35 m Journal of Water and Environment Technology, Vol 7, No 4, 2009 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 model describes the dynamics of ammonium nitrogen (NH4-N) and nitrate + nitrite 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 dNH i = radv , NH , i + rdif , NH ,i − rnit , i − rup , NH ,i dt (1) dNOi = radv , NO ,i + rdif , NO , i + rnit , i − rden, i − rup , NO ,i dt (2) dN rice = ∑ rup , NH , i + ∑ rup , NO , i dt i i (3) 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 rdif,NO,i are the NH4-N and NO2+3-N inflow into the ith layer due to diffusion, 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 - 254 - Journal of Water and Environment Technology, Vol 7, No 4, 2009 C NH ,i = (K ads , NH NH i ⋅ ρ s + θ s ) ⋅ Δzi (4) 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 (T − 20 ) rnit,surf = knit,surf ⋅ χ nit ⋅ NH surf (5) where knit,surf is the nitrification rate constant at the surface water [/day], χnit is the temperature constant for nitrification (1.05) [-] (based on Mayo and Bigambo, 2005), and T is the temperature [°C] The parameter knit,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 rnit,i and rden,i as follows ⎛ w ⎞ (T − 20 ) (T − 20 ) rnit,i = knit , i ⋅ χ nit ⋅ NH i = ⎜⎜ k nit,s,i + knit,r,i ⋅ r,i ⎟⎟ ⋅ χ nit ⋅ NH i Δzi ⎠ ⎝ (6) w ⎞ (T − 20) ⎛ (T − 20 ) rden, i = kden ,i ⋅ χ den ⋅ NOi = ⎜⎜ k den, s ,i + kden, r ,i ⋅ r ,i ⎟⎟ ⋅ χ den ⋅ NOi Δzi ⎠ ⎝ (7) where knit,i is the overall nitrification rate constant at the ith compartment at 20 °C [/day], knit,s,i is the nitrification rate constant by the bacteria not attached to the root at 20 °C [/day], knit,r,i is the nitrification rate constant by the bacteria attached to the root at 20 °C [/day], wr,i is the root weight at the ith compartment [g/m2], kden,i is the overall denitrification rate constant at the ith compartment at 20 °C [/day], kden,s,i is the denitrification rate constant by the bacteria not attached to the root at 20 °C [day], kden,r,i is the denitrification rate constant by the bacteria attached to the root at 20 °C [/day], and χden is the temperature constant for denitrification (1.05) [-] (based on Mayo and Bigambo, 2005) The parameters knit,s,i, knit,r,i, kden,s,i and kden,r,i were calibrated in this study The root weights at the ith layer are calculated as - 255 - Journal of Water and Environment Technology, Vol 7, No 4, 2009 wr,i = ζ i ⋅ wr , tot (8) wr , tot = (1 − f above ) ⋅ wtot (9) wtot = wr ,tot + wa = N rice f Nw,rice (10) 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 rup , NH ,i = k N ,up ⋅ rup , NO ,i = k N , up ⋅ f rad = wa , p − wa wa , p ⋅ f rad ⋅ f gs ⋅ wr ,i ⋅ C NH ,i K N + C NH ,i + C NO ,i wa , max − wa C NO ,i ⋅ f rad ⋅ f gs ⋅ wr ,i ⋅ wa , max K N + C NH ,i + C NO , i IR IRmax (11) (12) (13) where kN,up is the maximum nitrogen uptake by rice per unit root weight [g-N/g-root], wa,p is the potential value of the rice above-ground biomass (3,000) [g/m2] (assumed 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 based on the observed variation of leaf number (see "Observation of Rice Growth" in "RESULTS - 256 - Journal of Water and Environment Technology, Vol 7, No 4, 2009 AND DISCUSSION") The factor fNW,rice was determined as shown in Fig based on another experimental observations (data not shown) The vertical distribution of the root abundance, ζi, was assumed to be as shown in Fig Factor depend on growth stage, fgs [-] 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 1.0 0.8 0.6 0.4 0.2 0.0 50 100 150 Time after transplantation [day] Factor depend on growth stage, fNw,rice [g-N/g] Fig 3- Assumed Time Course of the Factor Dependent on Growth Stage, fgs 0.05 0.04 0.03 0.02 0.01 0 50 100 150 Time after transplantation [day] Fig 4- Time Course of the Ratio Between Rice Nitrogen and Weight of Rice, fNw,rice - 257 - Root abundance at ith layer, ζi [g/g] Journal of Water and Environment Technology, Vol 7, No 4, 2009 1.0 0.8 upper soil middle soil lower soil 0.6 0.4 0.2 0.0 50 100 150 Time after transplantation [day] Fig 5- Assumed Time Course of Vertical Distribution of Root Abundance, ζi RESULTS AND DISCUSSION Observation of Rice Growth The time courses of leaf number and plant height are shown in Fig 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) 140 120 Q = 0.6 Q = 0.2 Q = 0.1 Q = 0.6 Q = 0.2 Q = 0.1 80 60 120 100 80 60 40 40 20 20 0 20 40 60 80 Plant height [cm] Leaf number [/hill] 100 100 120 140 160 Time after transplantation [day] Fig 6- Time Courses of Leaf Number and Plant Height (“Q” : river water supply rate into each pot [m3/(m2∙day)]) - 258 - Journal of Water and Environment Technology, Vol 7, No 4, 2009 Rice above-ground biomass [kg/m2] Model Predictability The final above-ground rice biomass of each experimental case as calculated with the finally calibrated parameters is shown in Fig 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 3.0 2.5 2.0 measurement calculation 1.5 1.0 0.5 0.0 0.1 0.2 0.3 0.4 0.5 0.6 Water supply rate [m /(m day)] Fig 7- Comparison Between Calculated and Measured Above-Ground Rice Weight Figure 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 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 The NH4-N concentration in effluent water in each case from about months after 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 values were caused by the temporal change in the concentration of NH4-N in the supplied water In this model, however, the concentration of NH4-N in the supplied water was assumed to be constant as was that of NO2+3-N Therefore, the temporal 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 - 259 - 2.5 NO2+3-N concentration [mg-N/L] NO2+3-N concentration [mg-N/L] Journal of Water and Environment Technology, Vol 7, No 4, 2009 Q = 0.1 2.0 1.5 1.0 0.5 0.0 20 40 60 2.5 Q = 0.2 2.0 1.5 Supplied water 1.0 Supplied water (assumed in calc.) 0.5 0.0 80 100 120 140 160 20 NO2+3-N concentration [mg-N/L] Time after transplantation [day] 2.5 40 60 80 100 120 140 160 Time after transplantation [day] Q = 0.6 Soil interstitial water1) (meas.) Soil interstitial water2) (calc.) Effluent water (meas.) 2.0 1.5 Effluent water (calc.) 1.0 1) Average of 10 cm and 20 cm depth 2) The middle compartment in the model 0.5 0.0 20 40 60 80 100 120 140 160 Time after transplantation [day] 2.5 NH4-N concentration [mg-N/L] NH4-N concentration [mg-N/L] Fig 8- Comparison Between Calculated and Measured NO2+3-N Concentration (“Q”: river water supply rate into each pot [m3/(m2·day)]) Q = 0.1 2.0 1.5 1.0 0.5 0.0 20 40 60 80 100 120 140 160 NH4-N concentration [mg-N/L] Q = 0.2 2.0 1.5 Supplied water 1.0 Supplied water (assumed in calc.) 0.5 0.0 Time after transplantation [day] 2.5 2.5 20 40 60 80 100 120 140 160 Time after transplantation [day] Q = 0.6 Soil interstitial water1) (meas.) Soil interstitial water2) (calc.) Effluent water (meas.) 2.0 Effluent water (calc.) 1.5 1.0 1) Average of 10 cm and 20 cm depth 2) The middle compartment in the model 0.5 0.0 20 40 60 80 100 120 140 160 Time after transplantation [day] Fig 9- Comparison Between Calculated and Measured NH4-N Concentration (“Q”: river water supply rate into each pot [m3/(m2·day)]) Calibrated Parameters Some parameters were calibrated based on experimental observations, and the finally calibrated values are shown in Table 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, - 260 - Journal of Water and Environment Technology, Vol 7, No 4, 2009 knit,us, was estimated to be