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On-line version ISSN 0718-5839
Chilean J. Agric. Res. vol.72 no.1 Chillán Mar. 2012
Chilean Journal of Agricultural Research 72(1) January - March
Effect of Nitrogen Inhibitors on Nitrous Oxide Emissions and Pasture Growth After an Autumn Application in Volcanic Soil
Efecto de Inhibidores de Nitrógeno en las Emisiones de Óxido Nitroso y Crecimiento de la Pradera después de la Aplicación Otoñal en un Suelo Volcánico
Erika Vistoso1*, Marta Alfaro1, Surinder Saggar2, and francisco Salazar1
1Instituto de Investigaciones Agropecuarias INIA, Casilla 24-O, Osorno, Chile. "Corresponding author (firstname.lastname@example.org).
2Landcare Research, Private Bag 11052, Manawatu Mail Centre, Palmerston North 4442, New Zealand.
A range of technologies is available to reduce greenhouse gas emissions from agricultural systems. Among these, applying N transformation inhibitors is promising in reducing nitrous oxide (N2O) emissions in both arable and pastoral soils. A randomized complete block design with six replicates was used to quantify the effect of urease (UI) and nitrification (NI) inhibitors on N2O emissions and pasture production from permanent pasture in an Andisol after autumn-applied urea. The N2O emissions were measured periodically for 8 wk with PVC stationary chambers. Mineral N, DM yield, N concentration, and N uptake were analyzed. Our results show that adding fertilizer (40 kg N ha-1) significantly increased N2O emissions by 281% over the control treatment (P < 0.05) and adding N inhibitors to fertilizer N resulted in a 10%-29% reduction in emissions. Both inhibitors initially reduced nitrate-N production in the Andisol (Osorno series) either by slowing urea hydrolysis or the nitrification process. Increases in pasture yield (8%-13%), N concentration (7%-9%), and N uptake (46%-56%) were measured by adding N inhibitors with urea. Emission factors, measured under field conditions in Chile after autumn- applied urea and N inhibitors (applied alone or combined), were very low (0.052%-0.114%) compared with values reported in the literature and the Intergovernmental Panel on Climate Change (IPCC) default value (1%). Using these technologies could contribute to Chile's nationally established goal of a 20% reduction in greenhouse gas emissions by 2020.
Key words: Andisol, emission factor, urease inhibitor, nitrification inhibitor, urea.
Una gama de tecnologías están disponibles para reducir las emisiones de gases de efecto invernadero desde sistemas agrícolas. Entre éstas, la aplicacion de inhibidores de transformacion del N es considerada prometedora para reducir emisiones de oxido nitroso (N2O) en suelos de cultivo y praderas. Un diseno de bloques completos al azar con seis repeticiones se utilizó para cuantificar el efecto de inhibidores de ureasa (UI) y nitrificación (NI) sobre emisiones de N2O y produccion de pradera permanente sobre un Andisol después de la aplicacion otonal de urea. Las emisiones de N2O se midieron periodicamente durante 8 semanas en cámara estacionaria de PVC. Se analizaron N mineral, rendimiento de MS, concentracion de N, y absorcion de N. Los resultados muestran que la aplicacion de fertilizante (40 kg N ha-1) aumentó significativamente las emisiones de N2O en 281% sobre el tratamiento control (P < 0,05) y generó 10-29% de reduction de emisiones. Ambos inhibidores redujeron la production de N-nitrato en el Andisol serie Osorno por detener la hidrólisis de urea o el proceso de nitrificación. Se midieron aumentos en production (8-13%), concentration N (7-9%) y absorción N (46-56%) con la adición de inhibidores del N con urea sobre el tratamiento control. Los factores de emision medidos bajo condiciones de campo en Chile por la aplicacion otonal de urea e inhibidores del N (aplicados solos o combinados) fueron muy bajos (0,052-0,114%) en comparacion con los valores reportados en literatura y valores por defecto del Panel Intergubernamental para el Cambio Climático (IPCC) (1%). El uso de estas tecnologías puede contribuir al objetivo establecido a nivel nacional de reducir las emisiones de gases de efecto invernadero en un 20% para el ano 2020.
Palabras clave: Andisol, factor de emision, inhibidor de la ureasa, inhibidor de la nitrificación, urea.
Livestock production (dairy and beef) in southern Chile is based on direct grazing of permanent pastures that are mainly on volcanic soils (Goic and Rojas, 2004); it has been intensified in the last decade with an increasing stocking rate. This has resulted in an increased use of N fertilizer and a greater accumulation of organic residues, e.g., dairy slurry (Alfaro and Salazar, 2008). Urea is the main N inorganic fertilizer in these livestock production systems. Thus, Chilean agricultural soils receive an annual application of 0.42 million Mg urea, which is estimated will reach 0.48 million Mg by 2020 (ODEPA, 2010).
Nitrogen is a nutrient that strongly regulates pasture production, but it can also contribute to environmental degradation through nitrate (NO3-) leaching, ammonia (NH3) volatilization, and nitrous oxide (N2O) emission (Bolan et al., 2004; Saggar et al., 2009; Martinez-Lagos et al., 2010; Nunez et al., 2010). Nitrous oxide is a potent greenhouse gas (GHG) with a global warming potential 310 times greater than CO2 and contributes to ozone layer depletion (Crutzen, 1981). In many agriculturally-based countries, including Chile, New Zealand, Ireland, and Uruguay, N2O emissions and other GHGs are increasing (UNFCCC, 2011). Greenhouse gas emissions from the agricultural sector in Chile have increased by 33% between 1984 and 2003 (González et al., 2009).
Several approaches have been recommended to mitigate the economic and environmental impacts of N losses (Saggar et al., 2009; Luo et al., 2010). One of these is the use of urease and nitrification inhibitors (Di and Cameron, 2002; 2005). Numerous studies in the last decade have evaluated the use of these inhibitors to reduce N gaseous and leaching losses and increase N-use efficiency in cropping and pastoral soils of the USA, Europe, and New Zealand (Di and Cameron, 2005; 2006; Subbarao et al., 2006; Saggar et al., 2009; Luo et al., 2010). However, no published information is available about the effect of N inhibitors on N loss (N2O-N emission) mitigation in Chilean pastoral soils and little information has been published about the seasonal variability of CO2 and N2O emission fluxes from volcanic ash soil under different agricultural uses and management (Munoz et al., 2010; 2011) in southern-central Chile.
We hypothesize that using N cycle inhibitors will be effective in reducing N2O emissions in a grassland Andisol in southern Chile. Thus, the aim of this research study was to quantify the effect of urease and nitrification inhibitors on N2O emissions and pasture yield in an Andisol after autumn-applied urea.
MATERIALS AND METHODS
The experiment was carried out at the Instituto de Investigaciones Agropecuarias (INIA), Centro Remehue (40°35' S, 73°12' W). The soil at the experimental site is an Andisol (Osorno series) managed under permanent pasture (Lolium perenne L., Holcus lanatus L., and Dactylis glomerata L.) with good drainage and a 60-120 cm depth (CIREN, 2003).
Prior to starting the experiment, legumes and soil worms were eliminated to control the N input as N biological fixation (Picloram, a.i. 0.25 L ha-1) and as part of pest management, respectively. Black caterpillars (Dalaca palens B.) and white worms (Phytoloema herrmanni Germ.) were eliminated early in autumn with the entomopathogenic fungus (Beauveria bassiana Vuill. Qu-B931 strain and Metarhizium anisopliae Metschn. Qu M270 strain, respectively, with doses of 1012 spores ha-1.
Climatic data. Daily rainfall and temperature during the experimental period, were recorded with an automatic weather station (CR10X-1M Model, Campbell Scientific Inc., Logan, Utah, USA) placed within 3 km of the experimental site.
Treatments and experimental design
A randomized complete block design with six replicates tested the effectiveness of the urease inhibitor (UI) and nitrification inhibitor (NI) treatments: Control, Urea, Urea+UI, Urea+NI, and Urea+UI+NI. Nitrogen as urea (46% N) was applied at the rate of 40 kg N ha-1. Each treatment was applied to a 2 m2 (1 m x 2 m) area plot with a 0.5 m buffer zone around each plot. Urease inhibitor and NI were added manually at the rate of 0.25 g N-(n-butyl) thiophosphoric triamide kg-1 N (NBPT, Agrotain International, USA) and 10 kg dicyandiamide ha-1 (DCD, Alfa Aesar, USA), respectively.
All plots received 65 kg P ha-1 (triple superphosphate, 20% P), 125 kg K ha-1 (potassium chloride, 52% K), 24 kg Mg ha-1 (magnesium oxide, 51% Mg), and 40 kg S ha-1 (gypsum, 18% S) as basal nutrients.
Gas sample collection and analysis
Gaseous emissions of N2O were measured periodically for 8 wk by the closed static chamber techniques described by Saggar et al. (2004). Briefly the N2O gas was accumulated in an air-tight static chamber (25 cm diameter and 20 cm depth) placed in each plot, and gas samples were collected daily in vials (22 mL capacity). The static chambers were closed once a day with a hermetic PVC lid; samples were taken at 0, 30, and 60 min for three alternate days during the first week, twice per week between the 2nd and 5th weeks, and then once a week for 3 more weeks.
The N2O concentration in the gas samples was determined with a Gas Chromatograph (Perkin Elmer® Precisely, Clarus 600 Model, Shelton, USA) fitted with an 63Ni electron capture detector (ECD) and a CarboxenTM 1010 PLOT column (30 m x 0.32 mm ID, Sigma-Aldrich Co. LLC., St. Louis, USA). The oven, injector, and detector temperatures were operated at 80, 260, and 360 °C, respectively. The carrier gas was helium with a flow of 4 mL min-1. The limit detection for the methodology is 0.1478 and 0.1401 for 1 and 5 mg L-1, respectively. The N2O fluxes were calculated with the equations described in Saggar et al. (2004).
Estimation of emission factor
The proportion of N emitted as N2O-N (EF) was estimated for each treatment with Equation :
Emission factor (EF) = [(A - B)/C]/100 
where A is N2O-N emitted in the treated plot, B is N2O-N emitted by the control treatment, and C is the amount of applied N, considering that it constitutes the N inhibitor.
Soil and plant analysis
At the start of the experiment, six representative soil samples (0-10 cm depth) were collected, air-dried, and sieved through a 2 mm sieve and stored in plastic containers for analysis. These samples were analyzed for soil texture and chemical characteristics by the methods outlined in Sadzawka et al. (2006). Briefly soil pH was measured potentiometrically in water and CaCl2 by soil suspensions at a 1:2.5 soil:solution ratio. Organic matter was estimated by wet digestion through a modified Walkley-Black method. Exchangeable base cations (Ca, Mg, K, and Na) and exchangeable Al were extracted with 1 M NH4Ac at pH 7.0 and 1 M KCl, respectively, and analyzed by atomic absorption spectrophotometry (AAS). Phosphate and sulfate were extracted with 0.5 M NaHCO3 at pH 8.5 and 1 M Ca(H2PO4)2 and analyzed by the Murphy and Riley method and turbidimetry (Tabatabai, 1982), respectively.
In addition, during the experimental period (28 April to 22 June 2011), a composite soil sample (0-10 cm depth) was collected every week from each plot and immediately analyzed for NH4+-N and NO3-- N in accordance with the method described by Sadzawka et al. (2006). Soil moisture in a subsample of this soil (n = 2) was measured for weight loss after drying the sample at 105 °C for 24 h (Rowell, 1997).
The pasture was mown with a lawn mower when it was 20 cm high and clippings were collected and weighed fresh. A subsample was then taken to determine DM content at 60 °C for 48 h (Sadzawka et al., 2007), and then DM production was calculated as kg DM ha-1. Herbage samples were analyzed for total N by the Kjeldahl digestion method in accordance with the method described by Sadzawka et al. (2007).
All data were statistically analyzed (ANOVA) with the Genstat version 12.0 (Genstat, 2003) to determine statistical significance. Comparison of means was done by a Least Significant Difference Test (LSD) at a 95% confidence level.
RESULTS AND DISCUSSION
The experimental site had good soil fertility for pasture and animal production. It had high OM (230 g kg-1) and a moderately acid reaction (Table 1). Soil P status was high but S concentration was low, which is common in most Andisols of this area (Alfaro et al., 2006).
Table 1. Initial soil characteristic (0-10 cm, n = 3, ± standard error of the mean). Date: 29 April 2011.
1According to CIREN (2003).
2Al SAturation: proportion of available Al in relation to total cation content (Ca + Mg + K + Na + Al).
Rainfall, temperature, and soil moisture
During the measurement period, rainfall was lower (167 and 144 mm for the mean and experimental period, respectively) than the 34-yr mean for this site (Figure 1); mean air temperature (8.4 °C) was similar to the 34-yr average (8.8 °C), and soil moisture varied between 68% and 93%.
Yield. Applying N increased DM yield, which indicates that this soil responds to N input (P < 0.05) according to Teuber et al. (1988), and resulted in 45%-50% more DM yield than the control treatment (Table 2). Dry matter yield with added urea was higher (1252 ± 246.17 kg DM ha-1) than the control treatment (910 ± 151.22 kg DM ha-1) by 37%. Our lower response to urea is consistent with autumn-applied N when the DM response to added N is lower than spring applications (Teuber et al., 1988). Higher DM yield responses have also been reported (128% increase) in pastoral systems by adding increasing N rates (Mora et al., 2007).
Table 2. Yield, N concentration, N plant uptake, N use efficiency, and N emission factor per treatment after autumn-applied N to a permanent pasture (n = 6, ± standard error of the mean).
UI: urease inhibitor; NI: nitrification inhibitor; ns: non significant.
Adding N inhibitors to urea individually (urea+UI and urea+NI) or combined (urea+UI+NI) produced a marginally higher DM yield (1325 ± 142.48 to 1363 ± 132.81 kg ha-1) than urea alone (P > 0.05, Table 2) in accordance with the results of Di and Cameron (2006) and Luo et al. (2007).
The observed DM yield trend between these treatments suggested that N conserved with inhibitors was used by the pasture plants to enhance their productivity as shown by the N uptake and N use efficiency data (Table 2). The lack of difference between treatments could be related to the high spatial variability in pasture growth commonly found in permanent pastures. We only had six replicated plots (1 m x 1 m) to measure pasture yield. Considering pasture DM variability measured during this autumn growth (compared with other seasons of the year) a larger number of replicates would be required to test the statistical significance of differences in DM yield, N content, and N uptake obtained in this study. The highest autumn DM yield during the 16-wk study period was obtained in the urea+UI+NI treatment.
Nitrogen concentration. Pasture N concentration was lower in the control treatment (3.5 ± 0.18%, Table 2) than in the treatments receiving only urea and urea with N inhibitors (applied alone or combined with urea). There was no difference in N concentration in plots from urea and urea with N inhibitor treatments (P > 0.05, Table 2). Thus, applying N alone or with N inhibitors increased pasture N concentration by 7%-9% and improved their protein level. The pasture N/protein contents obtained during autumn-applied N are within the adequate range recommended for animal nutrition and also within the range generally found in this region's N-fertilized pastures (Whitehead, 2000; Anrique et al., 2008).
Nitrogen uptake. Pasture N uptake increased significantly when N was applied compared to the control treatment, but no differences were observed among N treatments (P > 0.05, Table 2). The highest N uptake was obtained in the pasture receiving urea+UI+NI. Overall, the N-fertilized pasture treatment removed an extra 18 kg N ha-1, which resulted in an increase of 38%-46% in N use efficiency (Table 2). All treatments receiving N met the potential dairy cow protein requirements for a 30 L milk d-1 production (NRC, 2001).
Soil N transformations
The NH4+-N and NO3-- N concentrations on day 1 fluctuated between 39.7 ± 2.28 to 73.2 ± 0.81 mg kg-1 dry soil and 6.5 ± 0.65 to 11.1 ± 0.33 mg kg-1 dry soil, respectively (Figure 3).
Figure 3. Distribution of NH4+-N (a) and NO3"-N (b) concentration after applying only urea and with urease (UI) and nitrification (NI) inhibitors after autumn-applied N to permanent pasture (n = 3, ± standard error of the mean).
Applying urea initially increased NH4+-N concentrations (58.9 ± 3.74 mg kg-1 dry soil), urea+UI (67.7 ± 1.95 mg kg-1 dry soil), and urea+NI (73.2 ± 0.81 mg kg-1 dry soil) with a subsequent decline (Figure 3a). The NH4+-N concentrations at the end of the experimental period were 24.3 ± 0.00, 23.3 ± 1.01, 12.5 ± 1.01, 27.7 ± 2.36, and 21.6 ± 0.34 mg kg-1 dry soil in the control, only urea, urea+UI, urea+NI, and urea+UI+NI treatments, respectively.
The NO3-- N concentration was low in all the treatments before starting the experiment and increased 27 d after the treatment was applied (Figure 3b), thus corroborating previous studies by Cartes et al. (2009) and Vistoso and Alfaro (2010) regarding the effect of applying N on N mineralization. This has implications for the N dynamics in this soil.
Adding urea+UI+NI was more effective in inhibiting the transformation of the nitrification process in the Osorno soil (Figure 3).
Nitrous oxide emissions
The N2O-N fluxes were low and, on average, varied between 0.001 ± 0.0001 and 8.567 ± 1.8795 g N2O-N ha-1 d-1 with three emission peaks in all treatments during the measurement period (Figure 2a). The first high peak was observed 4 d after urea was applied (8.6 ± 1.88 g N2O-N ha-1 d-1) and emissions were seven- and ten fold greater than in the control treatment. The second peak occurred in all treatments, except the control, 18 d after treatments were applied and was the highest in the urea+UI treatment (2.2 ± 0.35 g N2O-N ha-1 d-1) followed by the urea and urea+NI treatments (1.7 ± 0.16 and 1.6 ± 0.05 g N2O-N ha-1 d-1, respectively). The third peak occurred in the urea+UI (1.9 ± 0.21 g N2O-N ha-1 d-1), urea (1.8 ± 0.15 g N2O-N ha-1 d-1), urea+NI (1.5 ± 0.09 g N2O-N ha-1 d-1), and urea+UI+NI (0.1 ± 0.18 g N2O-N ha-1 d-1) treatments 25 d after they were applied. These N2O emissions continued to decrease in all the treatments up to base levels 35 d after treatments were applied. Higher N2O-N emission peaks were associated with rainfall events and increasing temperatures during the measurement period (Figure 1) because they were related to restricted aeration and high water content in the soil, according to Saggar et al. (2007) and Luo et al. (2008).
Figure 2. fluxes (a) and total emissions (b) of nitrous oxide after applying only urea and with urease (UI) and nitrification (NI) inhibitors after autumn-applied N to permanent pasture (n = 6, ± standard error of the mean).
Different letters indicate a statistical difference between treatments (LSD test, P < 0.05).
Total N2O-N emissions were low in the control treatment and high in the urea treatment. Emissions decreased when inhibitors were applied (P < 0.05; Figure 2b) from 70.8 ± 12.82 g N2O-N ha-1 with only urea to 64.2 ± 12.82, 61.5 ± 11.71 and 50.0 ± 12.00 g N2O-N ha-1 with urea+UI, urea+NI, and urea+UI+NI treatments, resulting in 9%, 13%, and 29% N2O emission reduction, respectively.
Urease inhibitor was less effective in inhibiting urease hydrolysis since NH4+-N concentrations were higher (67.0 ± 1.95 mg kg-1 dry soil) in that treatment than in the treatment with only urea (58.9 ± 3.74 mg kg-1 dry soil) on the first day; however, this behavior was reversed over time in accordance with previous studies by Subbarao et al. (2006) and Zaman et al. (2007). This difference can be attributed to the experimental site spatial variability and temporary soil sampling variability.
The DCD nitrification inhibitor showed better efficacy at reducing N2O emissions because of its nitrification inhibition (Amberger, 1989) and prolonged residence time of ammoniacal-N in the soil, thus resulting in reduced N2O emissions (Di and Cameron, 2002; 2003). Therefore, UI plus NI combined with urea significantly reduced N2O emissions from added urea (P < 0.05, Figure 2a). Emissions decreased to base levels (0.31 ± 0.002 mg kg, n = 34) after day 35 despite an increase in soil NO3-N at that time; this could be related to the low potential denitrification in this soil type (Dixon et al., 2011), which contrasted with studies by Luo et al. (1999), Bolan et al. (2004), and Saggar et al. (2004; 2009) for N2O emissions in New Zealand's dairy-grazed permanent pastures.
The total amounts of N2O emitted from N fertilizer ranged from 0.052%-0.114% of total N added through urea and nitrification inhibitors. The emission factor (EF) from the autumn-applied N fertilizer was 0.1%, which is ten fold lower than the 1.0% default value recommended by the IPCC (2001) and currently used to estimate N2O emissions from applied N fertilizer in Chilean soils (González et al., 2009).
The results of this study indicate that N2O emissions and autumn-applied EF are low in Chilean grassland Andisols and emissions could be additionally reduced by applying N inhibitors. However, further research is required to fully understand the long-term variability and effects of using these products in Chilean grassland soils. These mitigation technologies can reduce N2O emissions and are an alternative in developing strategies for Chile to achieve the 20% GHG emission reduction target in 2020.
Applying urea fertilizer (40 kg N ha-1) in autumn to volcanic soil (Andisol, Osorno series) resulted in N loss as N2O-N emissions of only 0.114%. Emissions were significantly reduced (up to 29%) by applying N inhibitors (UI and/or NI) with the fertilizer. Applying N plus N inhibitors (alone or combined) decreased soil NH4+-N concentrations and increased soil NO3-- N concentrations. The urea+UI+NI treatment was more effective in inhibiting the nitrification process in the Osorno series soil. These inhibitors also increased DM yield (8%-13%), N concentration (7%-9%), and N uptake (46%-56%) (P > 0.05) resulting in an increased N use efficiency of 38%-46%. This is the first field study carried out in Chile that reports nitrous oxide emission factors (EF) from N fertilizer and emission reductions from N inhibitors (applied alone or combined) in a grassland soil. An EF value of 0.114 obtained for fertilizer is almost one-tenth of the current default IPCC value used in the Chilean GHG inventory. In addition, EF could be reduced to 0.052 by adding N inhibitors.
We thank the Comisión Nacional de Investigation Científica y Tecnológica (CONICYT), PDA-08 and MEC-CONICYT N° 80100011 projects, and the Instituto de Investigaciones Agropecuarias (INIA) for supporting this research. We also thank Hugo Flores, Biometric Specialist, INIA (Centro La Platina) for his statistical advice and to Luis Ramirez, Sara Hube, Josué Martinez-Lagos, and Ana Rosas for their help in the field and laboratory.
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Received: 4 October 2011.
Accepted: 12 December 2011.