Influence of temperature and soil type on inhibition of urea hydrolysis by N-(n-butyl) thiophosphoric triamide in wheat and pasture soils in south-eastern Australia.
Nitrogen (N) fertiliser use in Australia has increased from 35 Gg N in 1961 (FAO 2006) to 848 Gg N in 2007-08 (IFA 2009). Urea is currently the main form of N fertiliser used because of its relatively low manufacturing cost and low transportation cost per unit of N (Roy and Hammond 2004). However, urea hydrolyses rapidly to ammonia (N[H.sub.3]) in soil due to high localised pH, which can lead to large losses through N[H.sub.3] volatilisation. The amount of N[H.sub.3] lost depends on environmental conditions and soil pH, buffering capacity, moisture, and the urease activity of the system. In extreme cases, up to 70% of the applied N can be lost (Kiss and Simihaian 2002).
One method for reducing N[H.sub.3] volatilisation following application of urea is to use compounds that inhibit urease activity at the soil surface and allow the urea to move into the soil before hydrolysis. Much of the N[H.sub.3] released from hydrolysis would then be retained by the soil.
A range of urease inhibitor compounds is available, and one of the more successful is N-(n-butyl) thiophosphoric triamide (NBPT) (Medina and Radel 1988; Kiss and Simibaian 2002; Watson 2005; Chen et al. 2008). Studies on the benefits of NBPT to wheat cropping have shown that it can improve yield under some conditions (Karamanos et al. 2004). NBPT has also been found to inhibit nitrification in some soils (Bremner et al. 1986) and reduce nitrous oxide ([N.sub.2]O) emission (Singh et al. 2004), but it does not directly inhibit denitrification (Wang et al. 1991).
Wheat cropping and pasture systems are the two largest N-fertiliser users in Australia, accounting for 74% of total N use (Chen et al. 2008). Surface-broadcast applications of urea to wheat cropping and pasture systems is carried out in many parts of Australia, so the potential exists for N[H.sub.3] volatilisation as a result of urea hydrolysis. Losses of N[H.sub.3] of 28% of applied N from temperate dairy pastures (Eckard et al. 2003) and up to 24% from wheat-cropping systems (Chen et al. 2008; Turner et al. 2010) have been reported for Australian conditions. NBPT could play a role in reducing these losses, but only a few field trials have been carried out on fanning systems in Australia (Turner et al. 2010), and the effectiveness of NBPT in these two contrasting agricultural systems is not well known. Temperature and soil type are two of the key factors controlling the effectiveness of NBPT (Chai and Bremner 1987; Carmona et al. 1990; Antisari et al. 1996; Watson and Miller 1996), but little information is available for Australian soils.
This paper reports the results of laboratory experiments to assess the usefulness of NBPT for reducing urea hydrolysis from typical wheat and pasture soils in Victoria, Australia, under temperature conditions likely to be experienced during the time these soils are fertilised. Laboratory trials, by controlling environmental conditions (for example temperature and moisture) and using homogenised soil samples, enable detailed investigation of the influence of climatic variables on urea hydrolysis. Such information can be incorporated into a process model, and therefore be used to develop decision-support tools for N-fertiliser management incorporating urease inhibitors.
Materials and methods
Soil samples were collected from two representative wheat-growing areas (Kalkee 36.54[degrees]S, 142.2[degrees]E, Longerenong 36.67[degrees]S, 142.30[degrees]E) and one dairy pasture at Dookie (36.37[degrees]S, 145.70[degrees]E) in Victoria, Australia. The sampling depth was 0-100 mm. All samples were air-dried and ground to pass a 2-mm sieve before analysis. Selected soil properties at the three sites are given in Table 1. The soil texture classes for the soil under cropping ranged from clay loam at Kalkee to medium clay at Longerenong, with alkaline pH. Samples from Longerenong with and without stubble were included to evaluate the effect of elevating the urease activity of the same soil. In contrast, the soil from the pasture site was a strongly acidic sandy loam. Urease activity was measured using the non-buffered method of Dalal (1985).
The range of temperatures chosen (5, 15, 25[degrees]C) encompasses temperatures likely to be experienced at the soil surface across the three locations during the time of fertiliser application.
Soil (40 g, oven-dry equivalent) was placed in 250-mL vials and pre-wetted to 60% water-filled pore space (WFPS) two days before commencement of the experiment. The 60% WFPS was chosen as it would provide the greatest opportunity to demonstrate the capabilities of the inhibitor. First, this was a moisture content that provided an evenness of wetting of the soil samples and so ensured that the conditions of the incubation were homogeneous. At lower moisture contents we found that areas of wet and dry existed, and so it was difficult to ensure a homogeneous treatment effect. Under very dry conditions, rates of urea hydrolysis will be slow as well, and so the effect of rapid urea hydrolysis and pH elevation would not occur. Second, at wetter moisture contents the potential benefit from the inhibitor is lessened, as in the field under such conditions, the urea granules would be quickly dissolved and diffused into the soil matrix. Therefore, the intensity of localised high pH due to hydrolysis of granules would be less, leading to less N[H.sub.3] volatilisation.
On Day 0, granular urea [+ or -] NBPT was surface-applied to the Kalkee soil at the rate of 160 kg N/ha (1400 mg/kg soil) (the rate that can be used for a single application per season at sowing). For the Longerenong and Dookie soils, a rate of 100 kg N/ha (840 mg/kg soil) was used (a rate indicative of a high split application). Urea granules averaged 30 mm diameter. NBPT was applied at a rate of 0.1% w/w of urea, which led to active ingredient application rates equivalent to 3 [micro]g/g soil (Kalkee) and 1.8 [micro]g/g soil (Longerenong, Dookie).
Vials were closed and incubated in the dark at 5[degrees]C, 15[degrees]C, and 25[degrees]C. The vials were aerated every three days, and lost moisture was replenished every seven days. Triplicate samples were collected from each of the treatments at five sampling times over 21 days and extracted with 2M KCl with 10% phenylmercuric acetate (PMA) (soil : solution ratio 1 : 2.5) for analysis of urea, using a Skalar SAN(++) segmented flow autoanlayser (Skalar Analytical BV 2005).
[Q.sub.10] values for urea hydrolysis were determined using an exponential function fitted using the statistics program SPSS version 13. Statistical analyses were performed using the R program of Hornik (2009).
The pasture soil from Dookie had the highest urease activity (186 [micro]g N/g soil.h) and the wheat soil from Kalkee had the lowest activity (54 [micro]g N/g soil.h) (Table 1). The two soils from the stubble management plots at Longerenong had intermediate urease activities at 78 (no stubble) and 90 (stubble) [micro]g N/g soil.h. Urea hydrolysis rates followed the order Dookie > Longerenong + stubble> Longerenong - stubble>Kalkee (Fig. 1), directly related to the urease activity (Table 1).
Rates of urea hydrolysis significantly (P < 0.001) increased with increasing temperature in all soils. After two weeks (Days 14 and 15), essentially all urea was hydrolysed at all temperatures for all soils (Fig. 1 and Table 2), and urea was hydrolysed more rapidly in the Dookie soils than in the wheat soils. The urea hydrolysis rates were highest at 25[degrees]C and lowest at 5[degrees]C. In the Dookie soil, urea from the no-inhibitor treatments was essentially completely hydrolysed (<4% remaining) by Day 3 at all temperatures. In the Longerenong soils, urea was completely hydrolysed by Day 5 at 25[degrees]C, by Day 7 10 at 15[degrees]C, and by Day 14 at 5[degrees]C. In the Kalkee soil, urea was completely hydrolysed by Day 7 at 25[degrees]C, by Day 10 at 15[degrees]C, and by Day 15 at 5[degrees]C.
[FIGURE 1 OMITTED]
The NBPT effectively retarded urea hydrolysis, providing increased time for the urea to move into the soil, for 14-15 days at 5[degrees]C and 15[degrees]C in all of the wheat soils (Kalkee and Longerenong) but for less time at 25[degrees]C (Fig. 1). The ability of the NBPT to reduce urea hydrolysis rates over 14 days (Table 2) provides a good assessment of its benefit in the field, as within 14 days we expect the urea under most situations to have moved into the soil. Urea hydrolysis rates in the Longerenong soils at 5[degrees]C and 15[degrees]C were essentially the same in the presence of NBPT. In the Kalkee soil, slightly more urea was hydrolysed at 5[degrees]C than at 15[degrees]C. In the Dookie soil, urea was completely hydrolysed in the urea + NBPT treatments by Day 7 at 15[degrees]C and 25[degrees]C and by Day 14 at 5[degrees]C.
The amount of urea hydrolysed was significantly (P < 0.001) affected by temperature, soil, and treatment ([+ or -]NBPT) across all samples, and the interaction between these three factors.
Increasing temperature from 5[degrees]C to 15[degrees]C and 25[degrees]C decreased the inhibitory effectiveness of NBPT significantly (P<0.001) in the Dookie soil (Fig. 1d), with inhibition for <7 days at 15[degrees]C and 25[degrees]C and for >10 days at 5[degrees]C. However, for the Kalkee soil, inhibition by NBPT only declined when the temperature increased to 25[degrees]C but not from 5[degrees]C to 15[degrees]C. (P<0.001) (Fig. la), with >60% of the urea remaining in the NBPT treatments at 5[degrees]C and 15[degrees]C at Day 14, and <10% remaining at 25[degrees]C. In the Longerenong soils (Fig. 1b, c), there was complete hydrolysis of NBPT-amended urea at 25[degrees]C by Day 14, and >50% remained in the 5[degrees]C and 15[degrees]C treatments. Based upon the temperature response we observed (Fig. 1), in situations where urea application occurs and the soil surface temperature is >25[degrees]C, we would not expect the inhibitor to reduce urea hydrolysis rates on the Dookie soil, as it was not performing well at 25[degrees]C, and in the wheat-cropping soils we would expect some delay in urea hydrolysis but complete hydrolysis of NBPT amended urea within 14 days.
The increased urea hydrolysis with increasing temperature is not surprising as it has been found in other studies in the range 5-45[degrees]C (Kumar and Wagenet 1984; Moyo et al. 1989; Wang et al. 1996; Rawluk et al. 2001) but is much more profound (high [Q.sub.10] value) in these temperate soils than reported elsewhere (Xu et al. 1993; Trasar-Cepeda et al. 2007). The [Q.sub.10] values for urea hydrolysis over the temperature range 5-15[degrees]C were 2.9, 3.7, and 2.6 and over 15-25[degrees]C were 1.9, 4.6, and 2.8, for Kalkee, Longerenong--stubble, and Longerenong+stubble, respectively ([R.sup.2]>0.8 for all three wheat soils). Urea hydrolysis in the Dookie soil was not well described by the exponential function, and the results (Fig. 1) show that there is extremely rapid urea hydrolysis in this soil at all temperatures. The effectiveness of NBPT in these Australian soils was strongly influenced by temperature and soil urease activity, as has been found in different soils from around the world (Chat and Bremner 1987; Carmona et al. 1990; Watson et al. 2008). NBPT was more effective in the soil with low urease activity (Kalkee, Longerenong), remaining effective for at least l0 days at temperatures up to 25[degrees]C. NBPT applied at a commercially recommended concentration (0.1% w/w urea) was less effective at reducing the rate of urea hydrolysis for the soil with high urease activity (Dookie), particularly at high temperature, with >70% of the applied NBPT-amended urea hydrolysed by Day 3 at 25[degrees]C (Fig. ld). Two other properties of the Dookie soil stand out as being different from the wheat-cropping soils: organic carbon (OC) and pH. The Dookie soil had a much higher OC level than the wheat soils (11% v. <1.5%) and a lower pH than the alkaline wheat-cropping soils (pH 5.4 v. >7.5). These factors may have influenced the performance of the inhibitor, by affecting either the degradation of the inhibitor or the ability of the inhibitor to perform. Higher concentrations of NPBT may be required if we apply urea in a warm season, such as summer for irrigated pasture.
The NBPT reduced urea hydrolysis more effectively and lasted longer under cooler conditions in the wheat soils (>60% reduction of the rate of hydrolysis for more than two weeks at 5[degrees]C and 15[degrees]C). However, at 25[degrees]C it still remained effective for more than one week, which is often adequate in the field as most N[H.sub.3] volatilisation will occur in the first week after urea application in the warm season (Cat et al. 2002). In all soils except Kalkee we observed that at the cooler temperatures (5[degrees]C and 15[degrees]C) the rate of urea hydrolysis in NBPT-amended urea treatments flattened after three days, possibly due to the time required for conversion of NBPT to the inhibitory oxygen analogue NBPTO (Christianson and Howard 1994). The NBPT was effective much more rapidly at 25[degrees]C, possibly due to the rapid conversion of NBPT to the oxygen analogue. In addition, the NBPT will be decomposing, which is affected by environmental conditions including pH, and the ratio of decomposition to conversion to the oxygen anologue is dependent upon many factors including environmental conditions and application rate of NBPT (Hendrickson and Douglass 1993). In our study, NBPT was more effective in the alkaline soils (Kalkee, Longerenong) than in the acidic soil (Dookie), supporting the results of Hendrickson and Douglass (1993); however, the soils in our study also differed in other aspects, most noticeably urease activity and OC.
The interaction between soil and temperature had a significant (P < 0.001) influence on the rate of urea hydrolysis in this study, with increasing temperature above 15[degrees]C increasing the amount of urea hydrolysed in the Longerenong and Kalkee soils more than in the Dookie soil. This shows that the benefits of NBPT for slowing urea hydrolysis will depend upon the soils to which the urea is applied. The outcomes suggest that NBPT could be used effectively to reduce N losses from wheat-cropping soils in southern Australia with temperatures ranging from 5[degrees]C to 25[degrees]C. For systems with high urease activity such as the pasture soil, or where temperatures are higher (25[degrees]C), a greater rate of NBPT may be required to effectively reduce the rate of urea hydrolysis. The low level of inhibition seen in the pasture soils (3-10 days) may still provide sufficient time for water, as rainfall or irrigation, to wash the urea into the soil and prevent N[H.sub.3] loss.
The NBPT was able to reduce urea hydrolysis rates in wheat-cropping soils from southern Victoria at 5[degrees]C, 15[degrees]C, and 25[degrees]C. Temperature affected the ability of the NBPT to reduce urea hydrolysis rates in these soils, and it was effective for shorter periods at higher temperature. The influence of temperature on the ability of NBPT to reduce urea hydrolysis was dependent upon soil type. In an acidic, organic soil with higher urease activity (Dookie), NBPT was less effective and temperature had less effect on the inhibitory ability of NBPT than in alkaline, less organic, lower urease activity soils (Kalkee, Longerenong). NBPT therefore has the potential to reduce N losses from Australian agricultural systems by slowing the rate of urea hydrolysis, and even when this is for a short time, it may still provide flexibility in farm management.
The authors acknowledge Ms Sirikanda Watcharathai and Dr Xing Chen for assistance in the running of the experiments, and acknowledge financial support from ARC, GRDC, DAFF, and Incitec Pivot Limited.
Manuscript received 8 November 2010, accepted 23 February 2011
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H. C. Suter (A,D), P. Pengthamkeerati (A,B), C Walker (C), and (D). Chen (A)
(A) Melbourne School of Land and Environment, The University of Melbourne, Vic. 3010, Australia.
(B) Environmental Technology Research Unit (EnviTech), Department of Environmental Science, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand.
(C) Incitec Pivot Limited, PO Box 54, North Geelong, Vic. 3215, Australia.
(D) Corresponding author. Email: firstname.lastname@example.org
Table 1. Description of soils used Wheat cropping soils Kalkee Longerenong Longerenong - stubble + stubble Texture Clay loam Medium clay Medium clay Total C (%) 1.42 1.18 1.32 Total N (%) 0.14 0.09 0.09 C : N ratio 10 13 15 [pH.sub.w] 7.8 8.1 8.1 Urease activity ([micro]g 54 78 90 N/g soil.h) (A) N[H.sub.4.sup.+] ([micro]g/g) 2.1 2.8 2.8 N[O.sub.3] ([micro]g/g) 2.5 15.3 11.3 Pasture soil Dookie (A) Texture Fine sandy loam Total C (%) 9.9 Total N (%) 0.99 C : N ratio 10 [pH.sub.w] 5.4 Urease activity ([micro]g 186 N/g soil.h) (A) N[H.sub.4.sup.+] ([micro]g/g) 35 N[O.sub.3] ([micro]g/g) 53 (A) Includes soil and thatch. Table 2. Urea remaining (% of applied) at Day 14 (Day 15 at Kalkee) after fertiliser application Data are triplicate means Treatment Temp Kalkee Longerenong Longerenong ([degrees]C) - stubble + stubble Urea 5 2.2 2.0 1.6 15 0.0 0.0 0.1 25 0.0 0.0 0.0 l.s.d. (P=0.05) 3.1 0.1 3.8 NBPT 5 70 66 77 15 66 65 55 25 0.3 1.1 0.1 l.s.d. (P=0.05) 4.2 3.5 11 l.s.d. (P=0.05) 3.2 2.1 6.6 (urea + NBPT) Treatment Temp Dookie l.s.d. ([degrees]C) (P=0.05) Urea 5 0.0 2.5 15 0.0 0.2 25 0.0 0.0 l.s.d. (P=0.05) 0.0 NBPT 5 0.7 2.1 15 0.0 9.3 25 0.7 1.7 l.s.d. (P=0.05) 0.6 l.s.d. (P=0.05) 0.3 (urea + NBPT)