Modelling DCD effect on nitrate leaching under controlled conditions.
The high input of nitrogen (N) fertilisers to intensive pasture production systems poses a risk to the environment. In New Zealand, up to 200kgN/ha is applied annually to grass/clover pastures (Thomas et al. 2005). Nitrogen fertilisers are generally applied when pasture growth is slow, in spring and autumn. During these periods the soil remains generally wet, and the leaching of nitrate by subsequent rainfall is high.
Inhibitors can be used to increase the efficiency of N fertilisers by slowing down N transformations. Two categories of inhibitors can be used to control N dynamics in soils: urease and nitrification inhibitors. In this study, only a nitrification inhibitor was used and thus is discussed here. A nitrification inhibitor acts on the oxidation of ammonium (N[H.sub.4.sup.+]) into nitrite (N[O.sub.2]) (mostly N[H.sub.4.sup.+] to N[O.sub.2]) or of N[O.sub.2] into nitrate (N[O.sub.3.sup.-]). If it slows down the conversion of N[O.sub.2] to N[O.sub.3.sup.-], toxic N[O.sub.2] accumulation occurs. This results in an accumulation of N[H.sub.4] in the soil and decreased N[O.sub.3.sup.-], which is highly susceptible to leaching. It also decreases [N.sub.2]O emission through denitrification (Di and Cameron 2002a).
The effect of dicyandiamide (DCD) on nitrification of urine N has been examined by several researchers (McCarty and Bremner 1989; Cookson and Cornforth 2002). In a lysimeter study with urine application in autumn, Di and Cameron (2002b) found that DCD decreased nitrate leaching by 76% and nitrous oxide emissions by 82%.
DCD is non-toxic, contains 66.67% N, and degrades to C[O.sub.2], N[H.sub.3], and [H.sub.2]O. As DCD is a specific inhibitor, it is not detrimental to the general microbial community and only interrupts the reproductive cycle of nitrosomonas, thereby retarding the nitrification process (Bolan et al. 2004). But it is readily soluble in water, which could result in its leaching, thus lowering of its efficiency (Williamson et al. 1996).
Studies have shown that several factors influence the degradation of DCD by microorganisms, including temperature, soil pH, organic matter content, and soil moisture content (Hallinger et al. 1990). Di and Cameron (2004) observed that the higher the temperature, the faster the degradation, with a half-life for DCD of 110-115 days at 8[degrees]C, and 18-25 days at 20[degrees]C. DCD shows a higher efficiency at lower soil moisture, at an optimum level for nitrification (Puttanna et al. 1999).
Rodgers et al. (1985) found that DCD degrades more slowly in acidic soils. At pH 4-4.3 the degradation rate of DCD over 60 days was 4% compared with 48% in a soil with a pH of 6.8. As DCD affects only autotrophic organisms, a high content of organic matter tends to decrease DCD efficiency in the soil (Kutzova et al. 1993).
The source of N[H.sub.4] ions also influences the efficacy of DCD. McCarty and Bremner (1989) found that DCD was less efficient when N[H.sub.4] ions were derived from urea fertilisers rather than from ammonium sulfate. This could be due to increasing pH during urea hydrolyses, resulting in faster nitrification than in acid-forming fertilisers, such as ammonium sulfate (Keeney 1980). Thus, the relative inhibition of urea-derived N[H.sub.4] ions by nitrification inhibitors is lower.
Repeated application of DCD and its concentration also influence its degradation. Rajbanshi et al. (1992) found that the first application resulted in a delayed DCD degradation, whereas for repeated applications this lag phase disappeared. Threshold DCD concentrations for effective inhibition ranging from 5 to 25 mg DCD/kg soil have been found in incubation studies (Puttanna et al. 1999; Di and Cameron 2004).
The objectives of this study were to determine the extent to which DCD decreased nitrate leaching following urea application to laboratory soil columns and to ascertain whether the convection dispersion equation (CDE), which included terms for hydrolysis of urea, nitrification, and its inhibition by DCD, could be used to describe this decreased nitrate leaching. Both an incubation and a column leaching experiment were set up with urea and DCD applied to Tokomaru soil.
Movement of nitrate in soil solution is controlled by convection and hydrodynamic dispersion:
[q.sub.N] = [q.sub.W][C.sub.N] - [lambda][q.sub.W] [partial derivative][C.sub.N]/[partial derivative]z (1)
where [q.sub.N] is the solute flux of nitrate, [C.sub.N] is the soil solution concentration of nitrate (mol/[m.sup.3]), [q.sub.w] is the Darcy flux density (m/h), [lambda] is the dispersivity (m), and z is depth (m). For non-adsorbed solutes, such as nitrate in most soils, [C.sub.N] = [M.sub.N]/[theta], where My is total amount of nitrate in the soil (mol/[m.sup.3]) and [theta] is the volumetric water content ([m.sup.3]/[m.sup.3]).
To account for nitrogen transformations, both the hydrolysis of urea to N[H.sub.4.sup.+] ions and the oxidation of N[H.sub.4.sup.+] ions to N[O.sub.3.sup.-] ions need to be considered.
Following Wagenet et al. (1977) these transformations are both treated as first-order reactions. Thus, for urea, denoted by the subscript u:
[partial derivative][M.sub.U]/[partial derivative]t = -[partial derivative][q.sub.U]/[partial derivative]z - [K.sub.U][M.sub.U] (2)
where [q.sub.U] is the flux of urea, and [K.sub.U] is the rate constant for urea hydrolysis [[h.sup.-1].
For N[H.sub.4.sup.+], denoted by the subscript a:
[partial derivative][M.sub.A]/[partial derivative]t = -[partial derivative][q.sub.A]/[partial derivative]z + [K.sub.U][M.sub.U] - [K.sub.A][M.sub.A]e - [K.sub.A][M.sub.A]r(1 - e) (3)
where [K.sub.A] is the rate constant for nitrification [[h.sup.-1]], and r and e factors for the nitrification inhibition by DCD accounting for degradation and efficiency. The value of r is given by:
r(t) = (1 - [M.sub.DCD](t)/[M.sub.DCDo]) (4)
where [M.sub.DOCDo] is the initial mass of DCD. Assuming that the degradation of DCD can be described by a first-order decay process, the mass of DCD ([M.sub.DCD]) is given by (Jury et al. 1991):
[M.sub.DCD](t) = [M.sub.DCDo] exp(-[micro]t) (5)
where [micro] the first-order decay rate constant [[h.sup.-1].
For nitrate, denoted by the subscript n:
[partial derivative][M.sub.N]/[delta]t = -[partial derivative][q.sub.N]/[partial derivative]z + [K.sub.A][M.sub.A]e - [K.sub.A][M.sub.A]r(1 - e) (6)
For the initial concentrations relevant to the study described here, we assume that [M.sub.A], [M.sub.N], and [M.sub.U] after packing were the same throughout the entire column, and [M.sub.U] was equivalent to the mass of urea applied to each treatment. By saturating the columns from the bottom, the ascending water flow pushed both nitrate and urea to the top of the columns. If we assume uniform displacement of the water originally in the soil ([[theta].sub.i]) with the invading solution from the bottom, and assume that solute dispersion and diffusion are negligible in the repacked soil column, the depth of soil containing urea and nitrate can be calculated using:
z = [[theta].sub.i]/[[theta].sub.s] L (7)
where [[theta].sub.i] is the initial volumetric water content [[m.sup.3]/[m.sup.3]] before saturation, [[theta].sub.s] is the saturated volumetric water content [[m.sup.3]/[m.sup.3]], and L is the length [m] of the column. With [[theta].sub.i] of ~0.13 [m.sup.3]/[m.sup.3] and [[theta].sub.s] of ~0.47, it was assumed that the entire nitrate was concentrated in the upper 56mm. Mineralisation and denitrification were assumed to be negligible during the experiment.
For the inlet boundary, a third-type boundary condition was assumed (van Genuehten and Wierenga 1986):
-[lambda][q.sub.W] [partial derivative][C.sub.N]/[partial derivative]z + [q.sub.W][C.sub.N] = [q.sub.W][C.sub.O]
where [C.sub.O] is the concentration of the applied solution.
For the lower boundary condition, it was assumed that the soil column was part of an effectively semi-infinite system, as suggested by van Genuchten and Wierenga (1986). Thus:
[partial derivative][C.sub.N]/[partial derivative]z ([infinity], t) = 0 (9)
The above equations were solved numerically using an explicit finite difference scheme, written in Visual Basic[TM] within Microsoft[R] Excel. For no flow periods [q.sub.w] was set to zero.
Methods and materials
The soil used for the experiments was the topsoil of Tokomaru silt loam, a Typic Fragiaqualf (Cowie 1978) collected from a dairy farm mixed pasture consisting of ~70% ryegrass (Lolium perenne) and ~30% white clover (Trifolium repens). The soil has an organic carbon content of 3.43%, pH 5.8, and a clay content of 22%, with mica/illite, chlorite, kandite, kaolinite, smectite, and vermiculite as the dominant clay minerals (Table 1). The soil was air-dried and passed through a 2-mm sieve.
For the incubation experiment, 355 g dry soil was placed in plastic bags and mixed with urea and/or DCD diluted in distilled water. Four different treatments, with 2 replicates each, were used with (a) control, (b) DCD at a rate of 15 kg DCD-N/ha (8.5 mg DCD-N/kg soil), (e) urea at a rate of 600 kg N/ha (340mg N/kg soil), and (d) DCD at a rate of 15 kg DCD-N/ha plus urea at a rate of 600 kg N/ha. The concentration of urea and DCD solutions was adjusted so that 20 mL of solution was added to each bag. Then another 50mL of distilled water was added to each bag resulting in a gravimetric water content of ~0.15[m.sup.3]/[m.sup.3]. The soil was incubated at 20[degrees]C for 8 days. During this time pH, KCl-extractable N[H.sub.4.sup.+], and N[O.sub.3.sup.-], and DCD concentrations were monitored.
Column leaching experiments
For the column leaching experiment, the same 4 treatments as described for the incubation methods were used. The 355-g oven-dry soil samples were put into plastic bags and mixed with urea and/or DCD diluted in water, and adjusted to a gravimetric water content of ~0.15 [m.sup.3]/[m.sup.3]. Columns of 200 mm height and 50 mm diameter were packed with the soil mixed with various treatments at a bulk density of 0.885 Mg/[m.sup.3]. After packing, the columns were saturated from the bottom with a solution of 5.5 x [10.sup.-6] M Ca[Cl.sub.2]. Again 2 replicates were made for each treatment, resulting in 16 columns because 2 different leaching rates were used. Half the columns were leached 1 day after the packing and the other half 1 week later. The top and bottom were covered with aluminium foil to avoid water losses.
The columns were leached with the same Ca[Cl.sub.2] solution of 5.5 x [10.sup.-6] M at a flow rate of 18mm/h using a peristaltic pump. The outflow was collected at the bottom of the columns in aliquots of 30 mL. After about 17 h, which corresponds to leaching of ~3 pore volumes, the experiments were terminated. A subsample of the soil was used for the determination of the resident concentrations of N[H.sub.4.sup.+] and N[O.sub.3.sup.-], and another for the gravimetric water content.
Ammonium and nitrate-N concentrations in the soil were measured on 2 M KCl extracts of moist soil immediately after the leaching events. Both the soil extracts and the leachates were analysed for N[O.sub.3] and N[H.sub.4] using an auto analyzer (Blackmore et al. 1987).
DCD was measured by a spectrophotometer using the method described by Vilsmeier (1979). For this method the DCD-treated soils were extracted with distilled water at a rate of 1:2. After centrifugation and filtration, 2 mL of the extract was mixed with 0.5 mL of diacetyl and 1 mL of the naphtol reagent. Thirty min after adding the reagents, the colour was measured with the spectrophotometer at 570 nm.
Results and discussion
The changes in pH values measured in the incubation experiments over the 8-day period are shown in Fig. 1a. There was no significant difference between the DCD treated and untreated control (-urea) soils, with a slight decrease over the 8-day period from 5.8 to 5.5. Thus, DCD alone had no effect on the pH of the soil. The pH of the urea and urea + DCD treated soils increased from 5.8 to 6.6 within 1 day after incubation because of ammonification of the added urea. While the pH of the urea-treated soil decreased gradually to 5.3, the pH of the urea + DCD treated soil remained high over the 8-day incubation experiment. This suggests that DCD successfully inhibited the nitrification of ammonium to nitrate, which is indicated by a decrease in pH. Similarly, Cookson and Cornforth (2002) observed that the pH of a soil treated with urine and DCD stayed significantly higher over a period of 35 days.
[FIGURE 1 OMITTED]
The amounts of N[H.sub.4.sup.+] monitored over the 8-day period in the incubation bags are shown in Fig. 1b. They were very low for the control and DCD treatments. In both the urea and urea + DCD treated soils, the amount of N[H.sub.4.sup.+] reached its maximum 1 day after the beginning of the incubation, with 300 and 330 [micro]gN[H.sub.4]-N/g soil for the urea and urea + DCD treatment, respectively. There was no significant difference between these 2 treatments before the second day. Thus, after 1 day of incubation with urea the ammonification process had occurred. After I day, the N[H.sub.4.sup.+] concentration in the urea-treated soil decreased gradually, but remained significantly higher for the urea + DCD treatment. In the urea + DCD treatment 88% of the N[H.sub.4.sup.+] remained, while in the urea treatment, only 57% of the N[H.sub.4.sup.+] remained after 8 days. Linear regression of these measurements gave a half-life for N[H.sub.4.sup.+] of 9 days for the urea treatment and 31 days for urea + DCD treatment. Irigoyen et al. (2003) found, in an incubation study with 60 kg N/ha, half-lives of N[H.sub.4] of 6 days without DCD and 18 days with DCD at a rate of 1.8 mg DCD-N/kg soil. Di and Cameron (2004) found half-lives of 22 days without DCD (25 kg-N/ha as urea + 1000 kg-N/ha as urine) and 55 days with DCD (7.7 lag DCD-N/g soil). These differences could be due to the amount of nitrogen added as well as to the rate of DCD applied and the soil temperature.
DCD effectively decreased the nitrification of N[H.sub.4] in the incubation bags. The decrease of N[H.sub.4.sup.+] over the 7-day period (from Day 1 to 8) in the urea treatment occurred at a rate of [k.sub.1] = 0.003/h, and this was ~4 times slower than for the urea + DCD treatment. Assuming that the amount of DCD added to the soil was effective at inhibiting all the nitrification process (e = 0 in Eqn 3), and thus that the reduction of N[H.sub.4] in the urea + DCD treatment would only be due to the decay of DCD, this would give a half life for DCD of 5.2 days. This is a decay rate constant [micro] of -0.0055. Assuming a half-life of 20 days with [micro] = -0.0015, as found by Di and Cameron (2004) at 20[degrees]C, this gave an optimised efficiency of 82%, and thus e = 0.18 (Fig. 1d). These values will be used later for modelling the column leaching experiments.
For nitrate, there were no differences between the control and DCD treatment, with an initial concentration of about 20 [micro]g N[O.sub.3]-N/g soil and a slow increase to about 40 [micro]g N[O.sub.3]-N/g (Fig. 1c). As expected from the N[H.sub.4.sup.+] results, the N[O.sub.3.sup.-] concentration in the urea-treated soil increased faster and reached a much higher value compared with the concentration in the urea + DCD treatment. The increase in the urea treatment from about 20 to 200 [micro]g N[O.sub.3]-N/g is consistent with the nitrification rate obtained from the N[H.sub.4] measurements discussed above. This shows that in the urea treatment the nitrification process had taken place after 1 week. Linear regression analysis of the increase in N[O.sub.3] concentrations revealed that the nitrification in the urea-treated soil was 5 times faster than the increase in the urea + DCD treatment.
The breakthrough curves (BTC) of DCD are shown in Fig. 2. The peak concentration was around 1 pore volume, which is equivalent to about 94 mm. Recoveries between 106 and 130% were obtained from these BTCs, which might be because of the dark colour of the leachates, which interfered with the DCD measurement. Degradation of DCD, assuming a half-life of 20 days, would mean that only 80% of the applied DCD should remain after a period of 8 days. Despite the high recoveries, the results indicate that DCD can easily be leached to depth, thereby decreasing its efficiency to decrease nitrification of N[H.sub.4.sup.+], which is readily adsorbed by soils. Also shown in Fig. 2 is the modelled BTC for one of the DCD treatments. The measured BTCs are more dispersed, which might partly be because of measurement errors.
[FIGURE 2 OMITTED]
Leachate pH measurements
The pH values measured in the leachates are presented in Fig. 3a, b for the columns leached after 1 and 7 days. For the columns leached 1 day after packing, the pH changes in the leachates were quite similar for the control and DCD treatment, with a gradual decrease due to N[O.sub.3.sup.-] leaching with H as counter ions (as well as basic cations, especially calcium, and ammonium in the urea treatment as counter ions) up to around 1 pore volume (PV). This was followed by an increase to about the initial pH, which was probably due to leaching of mainly basic cations as the counter ions. In the urea treatment, the decrease in pH was much faster, with a minimum around 0.5 PV. In contrast to these treatments, the pH in the urea + DCD treatment increased, with a maximum value around 1 PV. This was probably due to N[H.sub.4.sup.+] leaching with OH as counter ions.
[FIGURE 3 OMITTED]
For the columns leached 7 days after packing, the changes in pH all followed the same trend, with an initial decrease to ~1 PV due to N[O.sub.3.sup.-] leaching with H as counter ions followed by an increase, probably due to leaching of basic cations as the counter ions. The intensity of the pH drop was lowest for the urea + DCD and highest for the urea treatment.
The measured and simulated BTCs for nitrate are presented in Fig. 4 for Day 1 (D1) and Fig. 5 for Day 7 (D7). The replicates were always quite similar. For D1, nitrate leaching was very similar for all treatments. For the modelling purpose a dispersivity of 3 mm was used and a [k.sub.U] value of 0.2/h (Kumar and Wagenet 1984), which ensures that hydrolysis of urea is completed after 1 day. As a first attempt at modelling, the values for [k.sub.A], [mu], and e were those obtained from the incubation experiments with [k.sub.A] = 0.003/h, [mu] = 0.0015, and e = 0.18. The model with these values gave total amounts of N[O.sub.3.sup.-] leached well above those measured, especially for the urea treatments (see Table 2). Next a value for [k.sub.A] of 0.0015 was used, which falls within the range of values found by Tillman and Scotter (1991), and Iskandar and Selim (1981). A possible reason for the different [k.sub.A] for the incubation experiment and the leaching experiment is the water content during the studies, as will be discussed later. Best agreements between measurements and model were obtained with an inhibition efficiency of 100%, and thus e = 0. The CDE with these described the measured BTC curves reasonably well for the D1 experiment (see Fig. 5 and Table 2).
[FIGURES 4-5 OMITTED]
For D7, nitrate leaching in the urea + DCD treatment was also slightly higher, but compared with the urea treatment nitrate leaching was decreased by 71%. The model with the above parameters described the measured data for D7 reasonably well. Only for the urea treatment were the measured data more dispersed than the model prediction. The measured total amount of N[O.sub.3.sup.-] leached, however, was similar to that of the model prediction.
The mass balances of nitrogen for the various columns, including N[O.sub.3.sup.-] and N[H.sub.4.sup.+], are given in Table 2, with the average for the replicated columns. Recovery rates for nitrogen ranged between 70 and 109%. Recoveries > 100% might be due to mineralisation of organic matter, which would have a larger effect on the recoveries of the control and DCD treatments, with small amounts of nitrogen. The results also indicate that for the control and DCD treatment for both days, the major output was nitrate leaching. For the urea and urea + DCD treatments for D1, most of the nitrogen remained as N[H.sub.4.sup.+] in the soil, as nitrification would not yet have occurred. For the urea treatment for D7, most of the nitrogen was also leached as N[O.sub.3.sup.-]. In the urea + DCD treatment nitrification was decreased, resulting in a reduction of 71% compared with that in the urea treatment.
Di and Cameron (2002b) observed in their lysimeter study a decrease of 76% in nitrate leaching when urine was applied in autumn and a decrease of 42% when applied in spring. Williamson et al. (1996) found a reduction of only 18% in the cumulative amount of N leached. The inhibitory effect of DCD depends on both the soil temperature and the method of application. Di and Cameron applied DCD 5-9 times, thereby maintaining an inhibitory effect in the upper soil layer over a long period. The study described here would be similar to the experiment of Di and Cameron (2002b). Nonetheless, the slightly higher efficiency of DCD in decreasing nitrate leaching is probably due to the better mixing of soil and DCD before the leaching events. Inhibitors have been found to be more effective in incubation studies because of better mixing with the soil, compared with applications in the field (Slangen and Kerkhoff 1984).
Table 2 also shows the total amount of N[O.sub.3.sup.-] leached as obtained from the 2 modelling attempts with different model parameter values. The difference in the [k.sub.A] value for the incubation and leaching experiments is probably due to the different water content, of about 0.15 [m.sup.3]/[m.sup.3] for the incubation and 0.47 [m.sup.3]/[m.sup.3] for the leaching experiment. These are equivalent to about 0.23 and 0.71 relative water content, defined as volumetric water content/total porosity. The drier conditions in the incubation experiment seemed to have been favourable for nitrification. Similarly, Grundmann et al. (1995) found that the optimum relative water content for nitrification at 20[degrees]C was 0.36. However, in the leaching experiment the amount of DCD seemed to be sufficient to inhibit totally the slower nitrification process compared with that in the incubation experiment, where DCD only inhibited 82% of the nitrification process. Thus, in the leaching experiment, nitrification occurred at the same rate as DCD degradation. More experiments are needed to determine the relationship between water content and nitrification rate and the optimal rate of DCD application.
In the urea treatment, for D7 a significant amount of N[H.sub.4.sup.+] was leached. In the urea + DCD treatment the decrease in N[O.sub.3.sup.-] leaching also resulted in a decrease in N[H.sub.4] leaching by 32% compared with that in the urea treatment, which is due to charge balance. The total nitrogen leaching (N[H.sub.4.sup.+] and N[O.sub.3.sup.-]) was decreased by 57% by DCD. Thus, DCD mitigates N leaching by decreasing nitrate leaching and nitrate-induced ammonium leaching. Nitrate leaching accounted for >80% of the cation leaching, including N[H.sub.4.sup.+] ions. The leaching of N[H.sub.4.sup.+] and the other cations accompanying nitrate leaching will be modelled in a subsequent paper.
The study demonstrated that DCD significantly decreased nitrogen losses by both nitrate and nitrate-induced ammonium leaching. The half-life of N[H.sub.4.sup.+] was increased from 9 to 31 days by DCD, which would make N[H.sub.4.sup.+] more available for plant uptake. Another positive aspect of DCD use was the effect of DCD on soil acidification, by slowing down the nitrification process and reducing the release of [H.sup.+] ions. The high solubility of DCD can result in substantial leaching of DCD, thereby reducing its effectiveness as a nitrification inhibitor.
The CDE, with source/sink terms for nitrogen transformations and the inhibition and efficiency of DCD on nitrification, could be used reasonably well to simulate the leaching of nitrate in the various treatments. However, values for the model parameters were different from those obtained from incubation experiments. To extend the use of the CDE for simulating nitrate leaching as affected by nitrification inhibitors in the field, relationships between mineralisation, nitrification, soil water content, temperature, and the inhibitory and efficacy effects of DCD would have to be established.
Manuscript received 22 December 2006, accepted 23 May 2007
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Iris Vogeler (A,C), Adeline Blard (B), and Nanthi Bolan (B)
(A) HortResearch, Palmerston North, New Zealand.
(B) Institute of Natural Resources, Massey University, New Zealand.
(C) Corresponding author. Email: email@example.com
Table 1. Characteristics of soil Soil Soil Sand Silt Clay Org.C locations classification (%) (g/kg) Tokomaru Typic Fragiaqualf 4.3 73.7 22 3.4 Soil Org.N pH CEC locations (%) (cmol/kg) Tokomaru 0.28 5.7 11.2 Table 2. Nitrogen mass balance (mg N) per column (average for 2 soil columns) on Day 1 and Day 7 RV, Recovery (%); Model 1: [k.sub.A] = 0.003/h, [micro] = 0.0015, e = 0.18; Model 2: [k.sub.A] = 0.0015/h, [micro] = 0.0015, e = 0 Initial soil N Amount leached conc. added (measured) Urea N[H.sub.4]+ N[O.sub.3]- N[O.sub.3]- N[H.sub.4]+ Day 1 Control 1.2 7.7 0 6 0.2 DCD 1.2 7.7 0 5.7 0.2 Urea 1.2 7.7 120 9.3 6.3 Urea + DCD 1.2 7.7 120 7.2 5.3 Day 7 Control 1.2 7.7 0 8.7 0.8 DCD 1.2 7.7 0 8.6 0.7 Urea 1.2 7.7 120 43.5 25.4 1.2 7.7 120 12.3 17.4 Final soil RV Amount leached conc. Model 1 Model 2 N[H.sub.4]+ N[O.sub.3]- N[O.sub.3]- N[O.sub.3]- Day 1 Control 1.0 2.5 109 7.8 7.7 DCD 1.4 4.2 129 7.7 7.7 Urea 91.9 7.2 89 19 13.5 Urea + DCD 90.1 3.5 82 13.7 10.1 Day 7 Control 2.8 2 100 8.1 7.9 DCD 1.7 1.9 91 7.9 7.8 Urea 33.8 2.5 78 57.0 35.6 Urea + DCD 83.7 2.1 86 28.9 14.7
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|Author:||Vogeler, Iris; Blard, Adeline; Bolan, Nanthi|
|Publication:||Australian Journal of Soil Research|
|Date:||Jun 1, 2007|
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