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Department of Agronomy and Horticulture and USDA-ARS, University of Nebraska-Lincoln, Lincoln, NE 68583
* Corresponding author (beghball1{at}unl.edu)
Received for publication March 13, 2002.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CP1, compost applied every year • CP2, compost applied every other year • CTL, control • EC, electrical conductivity • FRT, synthetic N fertilizer • MBC, microbial biomass carbon • MN1, manure applied every year • MN2, manure applied every other year • PMN, potentially mineralizable nitrogen • WFP, water-filled porosity
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil management and organic amendments, such as animal manure and compost, can affect soil organic C pools, soil nutrients, and microbial environments and activities, which are some of the controlling factors in the emission of CO2, N2O, and CH4 to the atmosphere (Intergovernmental Panel on Climate Change, 1996; Conrad, 1996). At the end of a 10-yr experiment, application of cattle manure and red clover (Trifolium pratense L.) hay resulted in accumulation of the most labile and biologically active soil organic C pools (Wander et al., 1994; Wander and Traina, 1996). Biologically active C promotes microbial activities and CO2 emissions from soils. Emission of N2O depends on the form of N found in manure (Intergovernmental Panel on Climate Change, 1996) and rate of manure application. Lessard et al. (1996) reported that total N2O emission from soil over a 185-d period increased from 0.7 to 1.0 kg N2O-N ha-1 when dairy cow manure application was increased from 170 to 339 kg total N ha-1. Agronomic practices in general reduce CH4 oxidation capacity of soils (Mosier et al., 1991; Kessavalou et al., 1998). In northeastern Colorado, CH4 oxidation capacities of tilled and irrigated soils cropped to winter wheat (Triticum aestivum L.) and corn were 90% lower than grassland soils (Bronson et al., 1992; Bronson and Mosier, 1993).
Continued elevated emissions of greenhouse gases after termination of manure application to soil not only poses a threat of increased atmospheric concentration of greenhouse gases but also increased risk of soil C and N depletion. In the long run, this could lead to the deterioration of soil health previously derived from manure application. For most reported research, measurements of greenhouse gas emissions were made within a year of organic material amendment. Little is known about biological and chemical processes occurring in soils several years after the termination of organic amendment. No literature (using the CAB abstracts database) was found to evaluate both greenhouse gas emission and soil attributes four years after manure or compost application terminated. One relevant study was reported by Wagner-Riddle et al. (1997), who evaluated only N2O emission continuously within two years after manure application without evaluation of key soil attributes. A thorough knowledge of the residual effects of manure application on fluxes of greenhouse gases and key soil attributes may provide valuable information for developing efficient manure management practices for crop production.
The objective of this study was to evaluate the residual effects of manure, compost, and N fertilizer on fluxes of soil CO2, N2O, and CH4, and on soil C and N indicators four years after manure and compost application had stopped.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Total Carbon and Total Nitrogen Inputs
Treatments were applied in autumn from 1992 to 1995 for the cropping seasons of 1993 to 1996. The rates of beef cattle manure, composted feedlot manure, and N fertilizer were based on a need by corn for 151 kg N ha-1 yr-1 to achieve an expected yield level of 9.4 Mg ha-1. The application rates were based on assumption that 40, 20, 10, and 5% of total N in manure and 20, 20, 10, and 5% of total N in compost will be plant available in the first, second, third, and fourth year after application. Detailed characteristics of manure and compost and application rates were presented by Eghball and Power (1999). Total C and total N (corrected for ammonium loss during drying) of manure and compost were determined on air-dried samples based on the method described by Schepers et al. (1989). More than 11.5 Mg C ha-1 (Table 1) and 900 kg N ha-1 (Table 2) were added to the soil by 4-yr (1992–1995) manure or compost application. Manure or compost was applied by hand in late autumn after corn harvest, followed by disking within two days after application. Every year all plots were disked in the spring before corn planting. Plant residue incorporated into the soil was estimated to be 90 to 95% after spring disking.
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Soil and Temperature Measurements
Soils were analyzed for bulk density, volumetric water content, electrical conductivity (EC1:1), and pH1:1 (where the subscript 1:1 indicates soil weight to water ratio). Soil EC and pH were measured according to the procedure of Smith and Doran (1996). Soil water-filled porosity (WFP) was calculated as the ratio of volumetric water content to soil total porosity. Assumptions for particle density and density of water are 2.65 and 1.0 Mg m-3, respectively. Potentially mineralizable nitrogen (PMN), microbial biomass carbon (MBC), and NO3–N were determined after corn harvest on 8 Sept. 1999. The PMN and MBC were measured with the anaerobic incubation method as described by Gajda et al. (2001) and microwave irradiation method as described by Islam and Weil (1998), respectively.
The residual effects of manure and compost on soil greenhouse gas emissions and soil C and N indicators were evaluated from 14 June to 24 Nov. 1999 (four years after the last application of manure and compost). At each time of gas sampling, soil temperature at the 7.5-cm depth was measured adjacent to the gas chamber. Air temperature data and winter soil temperatures were gathered from a weather station located 3 km from the experiment. Four 1.8-cm-diameter soil cores were collected at depth intervals of 0 to 7.5, 7.5 to 15, and 15 to 30 cm within a 1-m distance from the gas-sampling chamber.
Gas Measurements
Measurements of greenhouse gases were made with the vented surface gas chamber technique (Hutchinson and Mosier, 1981). Gas samplings were conducted on 14 June, 27 July, 31 Aug., 1 Oct., and 24 Nov. 1999. At each sampling date, a flux measurement was taken in each of the 18 plots (three replications in each of the six treatments). The chambers were removed between sampling dates. The chamber (a cylindrical surface chamber, 15 cm in diameter and 15 cm high) was placed on a weed-free representative interrow area within each plot. Interrow was chosen because it occupied 74% of the area (row area was defined as a 20-cm-wide strip centered over the row). The chamber was inserted 7.5 cm deep into the soil and capped creating an effective headspace of 1334 cm3. Gas samples were collected from the chambers at 0, 15, and 30 min with a 20-mL polypropylene syringe. Gas was sampled after purging the gas repeatedly 10 to 20 times with the syringe. The 20-mL samples were transferred to 12-mL evacuated serum vials and transported to the laboratory.
Gas samples collected were analyzed for CO2, N2O, and CH4 within 72 h after sampling. The CO2 content was determined with a gas-chromatographic system with a thermal conductivity detector maintained at 110°C as described by Weier et al. (1993). The N2O and CH4 contents were determined with an automated gas chromatographic system with a Poropak Q (Palo Alto, CA) column and electron capture detector, and a Poropak N column and flame ionization detector, respectively, as described by Arnold et al. (2001).
Gas Flux Calculation
For simplicity, gas concentrations measured at 0, 15, and 30 min are represented as C0, C15, and C30, respectively. Our data indicated that gas concentration could steadily increase, steadily decrease, or fluctuate with time. Existence of flux (positive or negative) is determined by the difference of gas concentration (increase or decrease) with time. The absolute value of the difference between two gas concentrations (C15 - C0, C30 - C15, C30 - C0) was considered greater than zero only if it was greater than two times the standard error of gas measurements in our laboratory as described by Arnold et al. (2001). Otherwise the concentration difference and thus gas flux was equal to zero. This approach was taken to ensure that the gas concentration difference exceeded laboratory measurement error.
If the gas concentration inside the chamber steadily increased with time (C0 < C15 < C30) in a nonlinear trend, that is, when the ratio of (C15 - C0)/(C30 - C15) > 1, the flux calculation was made as described by Hutchinson and Mosier (1981):
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If the gas concentration inside the chamber steadily increased (C0 < C15 < C30) or decreased with time (C0 > C15 > C30) and the ratio of (C15 - C0)/(C30 - C15) 
1, then the flux was calculated from the equation:
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C/t is the average rate of change of concentration between C15 - C0 and C30 - C15.
This approach avoids underestimation of flux, especially when the rate of change of concentration between 0 and 15 min is much lower than that between 15 and 30 min. When gas concentration fluctuates with time (C0 < C15 > C30 or C0 > C15 < C30), it indicates that gas flux trend is inconsistent within a 30-min period (emission followed by uptake or vice versa). In this case, C/t is represented by the average rate of change of concentrations of C15 - C0 and C30 - C0.
Estimation of Daily and Annual Fluxes
Since gas measurements were not made every day, daily gas fluxes were estimated with the relationships between measured fluxes and soil properties. Total CO2–C flux is the sum of flux contributions of soil microorganisms and invertebrates (especially during nongrowing season), and root plus root-induced microbial respiration (during growing season). For lack of better terms and for simplicity, CO2 flux from root and root-induced microbial respiration were grouped as root respiration. Among measured soil properties, soil temperature best described CO2–C flux from microorganisms and invertebrates as a simple linear function (0.05 probability level and r2 = 0.30). Soil temperature data (from our soil measurements) was used to estimate daily gas-flux contribution of soil microorganisms and invertebrates. Root respiration during corn growth was determined as the difference between measured gas flux and the estimated gas flux of soil microorganisms and invertebrates. Daily root respiration during the growing season was estimated by linear interpolation of root respiration between consecutive gas measurements. Annual (1 Jan.–31 Dec. 1999) CO2 emission was estimated by totaling the estimated (based on soil temperature) and measured daily gas fluxes. Assumption of temporal linearity between sampling periods may not be always valid, therefore the annual CO2 emissions are estimates that may not be the accurate annual losses of CO2.
Interpolations for daily fluxes of N2O or CH4 were not done because there was no clear relationship between N2O or CH4 flux with other measured soil variables such as soil temperature or WFP.
To evaluate the effects of WFP, EC, and pH on CO2–C fluxes across different sampling times and soil temperatures, fluxes were adjusted to a uniform soil temperature of 25°C by using the following equation as described by Parkin et al. (1996):
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Statistical Analysis
Analysis of variance was conducted with the Mixed Model of SAS (SAS Institute, 1992; Littell et al., 1996). In this analysis, treatment was the component of fixed effect and replication was the random effect. To evaluate how treatment means and differences change with time, analyses of repeated measures were performed by using autoregressive order 1 covariance (Littell et al., 1996). A probability level < 0.05 was considered significant.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soil Carbon Gas Fluxes
Carbon Dioxide
Soil receiving manure or compost had similar CO2 emission as the control or fertilized soil (Fig. 1)
. The residual effect of manure or compost on CO2 emission was not significant four years after manure and compost applications had stopped. It appears that the large native pool of organic matter and the annual addition of C from plant materials (more than 4.1 Mg C ha-1 yr-1) after termination of manure application masked the residual effects (if any) of manure and compost amendments on CO2–C emission.
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In a laboratory experiment, Linn and Doran (1984) found that microbial respiration increased with WFP (from 10 to 60%) reaching a maximum at WFP of 60% and then microbial respiration decreased for WFP > 60%. In our study the effects of WFP on CO2–C flux were inconsistent. A possible reason, besides variability of soil environment in the field, is that the moisture range did not vary widely among sampling times. The WFP ranged from 34 to 50% at all sampling times in the 0- to 7.5-cm depth except on 24 November, where WFP ranged from 50 to 80%. Within each sampling time, the WFP was similar among treatments in all soil depths. These similarities indicate that when the organic matter pool was large and plots were under similar cultural practices, the residual effects of applied manure and compost on WFP were not detectable.
Using the Q10 value of 1.8, the projected root respiration at 25°C during the growing season showed no difference among treatments within each sampling time (Fig. 1). However, there were significant differences in root respiration among sampling times in the order: 27 July > 14 June > 31 August (Fig. 1). The average root respiration rates on 27 July, 14 June, and 31 August were 23.0, 10.0, and 3.1 kg CO2–C ha-1 d-1, respectively. These root-respiration rates were equivalent to 51, 32, and 10% of total CO2–C fluxes for those dates, respectively. As with Eghball and Maranville (1993), corn root volume on 14 June (when plants have six to eight leaves) was expected to be smaller than that of 27 July (when corn was silking and pollinating), causing less root respiration. The root activity on 31 August was lower than that on 27 July or 14 June because corn reached the beginning of dent stage and approached physiological maturity on 31 August. Soil temperature and WFP were not expected to be the cause of these differences because they were similar among the sampling dates. The average soil temperatures on 14 June, 27 July, and 31 August were 26, 27, and 27°C, and WFP values were 51, 52, and 54%, respectively.
Based on linear interpolation of root-related respiration and the relationship between microbial respiration with soil temperature, the estimated annual losses of CO2–C in 1999 were 4.4, 5.1, 4.4, 4.5, 4.2, and 4.2 Mg ha-1 from the CTL, FRT, MN1, MN2, CP1, and CP2 plots, respectively. The estimated annual loss of CO2–C from the CTL and FRT plots was close (within 0.2 Mg C ha-1) to the corresponding average annual plant C input from plant materials of 4.2 and 4.9 Mg ha-1 in CTL and FRT plots, respectively. This finding suggests that for a spring disking tillage system, returning corn plant materials to the soil is necessary to maintain a balance between C input into the soil and CO2–C emissions from the soil.
Methane
Fluxes of CH4–C were not different among treatments at each sampling time and there was no consistent pattern of fluxes (Fig. 3)
. The soil could be a sink (negative fluxes) at a sampling time and could be a source (positive fluxes) at another time, despite the similarity of environmental factors among sampling times. High standard errors of means in each sampling time indicated that CH4–C fluxes were not significantly different from zero in most cases at the 0.05 probability level. The marginal CH4–C flux in our study was similar to the observations of Conrad (1996). Conrad (1996) pointed out that in most upland soils, CH4–C production is usually absent or marginal because the CH4–C flux is dominated by CH4–C oxidation, which is dependent on the dynamic composition of methanotrophs and other CH4–oxidizing microbes such as nitrifiers. The treatment means of CH4–C fluxes pooled from all samplings (±SE of 1.64 g ha-1 d-1) were 0.0, 0.7, 1.8, -0.18, and 0.79 g ha-1 d-1 on 14 June, 27 July, 31 August, 1 October, and 24 November, respectively. The pooled data from all samplings showed no definite relationship between CH4–C fluxes with soil temperature, WFP, EC, or pH.
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Differences in Total Nitrogen Input into Soil
Total N input into the soil, N removed from the soil by grain, and total N recycled as plant materials differ with treatments. In the CTL plot (no N input), 62 kg N ha-1 yr-1 soil N was removed by grain. In the FRT plots, average annual N input into soil (107 kg N ha-1 yr-1 ) was slightly higher than total N removed by grain (101 kg N ha-1 yr-1). In the manure- and compost-treated plots, total N input was much higher than the N uptake.
Soil Nitrous Oxide Emission
Fluxes of N2O-N were not different among treatments at each sampling time except on 14 June where CP1 and CP2 resulted in greater N2O-N fluxes than other treatments at the 0.10 probability level (Fig. 4)
. The means (±SE of 7.9 g ha-1 d-1) of N2O-N fluxes on 14 June from the CP1 and CP2 plots (30.5, and 24.5 g ha-1 d-1, respectively) were higher than the other treatments (less than 11 g ha-1 d-1). Kessavalou et al. (1998) associated high N2O-N flux in June with WFP being greater than 60%. Since WFP was similar among the treatments (less than 55% in the 0- to 15-cm depth), the difference could be due to nitrification as shown by soil EC. A significant linear relationship between EC1:1 level and soil nitrate concentration (EC = 0.008 x NO3–N, r2 = 0.47, P < 0.001) indicated that 47% of the EC variability was explained by soil nitrate. Weier et al. (1993) observed an increase in N2O emission with increasing EC1:1. Residual soil EC1:1 values in CP1 and CP2 on 14 June (ranging from 0.41 to 0.5 dS m-1) were higher than those of other treatments (ranging from 0.25 to 0.40 dS m-1). Rapid increase in spring temperature on recently tilled soil may have increased mineralization, ammonification, and nitrification processes resulting in higher nitrate and thus increased N2O flux in June. According to Bremner and Blackmer (1978) and Bremner et al. (1981), the increase in ammonification and nitrification processes can result in increased nitrous oxide emission, especially under aerobic conditions.
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Nitrogen loss by way of N2O-N emission four years after manure or compost application was negligible. No N2O measurement was taken one, two, or three years after manure or compost application. According to a study by Wagner-Riddle et al. (1997), N2O emission during June to November was nearly zero within one or two years after two-year manure application. By using the average flux of 14 June, 27 July, and 31 August to represent the daily N2O-N flux during the growing season (from corn emergence on 1 May to corn maturity on 30 September) and the average flux of 1 October and 24 November to represent daily N2O-N flux during the nongrowing season, estimated annual emission of N2O-N was 1.4 kg ha-1, which was slightly less than 1% of the 151 kg N ha-1 applied to the FRT plot in spring 1999. This estimate is similar to the N loss as N2O-N (ranging from 1 to 1.5% of applied N) under corn irrigated with high-nitrate ground water during a dry year (Qian et al., 1997). The estimate of 1% N2O-N loss per N added was also within the range (0.40–1.57%) of N2O-N losses in various studies summarized by Lessard et al. (1996).
The residual effects of manure or compost pose no additional threat to the atmospheric quality. Also it poses no threat to the depletion of soil N. The 1.4 kg ha-1 yr-1 loss of soil N as N2O-N was negligible considering the large pool (more than 3.1 Mg ha-1) of total soil N. The large quantity of soil total N and the annual internal recycling of more than 34 kg plant N ha-1 yr-1 (Table 2) after termination of manure application may have masked the residual effects (if any) of manure and compost amendments on N2O-N fluxes.
Assessment of Soil Properties Relevant to Gas Emission
Soil pH
Soil pH at the 0- to 7.5-cm depth varied among treatments (Table 3). The pH in FRT plots (consistently <5.6) was lower than that of CTL plots (consistently >6.1). The pH in the FRT plots was lower than the accepted pH (between 6 to 7.5) for general plant growth and microbial activity (Smith and Doran, 1996). Acidity in FRT plots was produced during the formation of nitrate from ammonium fertilizer applied to the soil (Brady, 1984). In Canada, Bouman et al. (1995) showed a marked decrease of pH in prairie soil due to a long-term application of urea and anhydrous ammonia. The pH of CTL plots was consistently lower than that of CP1 or CP2 (consistently >6.5). Soil pH values at the 0- to 7.5-cm depth were in the order: CP1 = CP2 > MN1 = MN2 > CTL > FRT (Table 3). Residual effects of annual application of ammonium nitrate and diammonium phosphate resulted in a decrease of 0.5 pH unit compared with CTL plots and 0.7 to 1.0 units compared with manure or compost treatment. Lime contained in the manure and compost increased the soil pH (Eghball, 1999).
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Soil pH at the 15- to 30-cm depth varied among treatments at each sampling date except on 31 August (P = 0.08). Different from the pH at the 0- to 7.5- and 7.5- to 15-cm depths, the treatment pH averages were 6.0 or greater, indicating less acidification at deeper depths. The average pH in the FRT plots (ranging from 6 to 6.2) was similar to that in CTL plots.
Soil Electrical Conductivity
Soil EC ranged from 0.06 to 0.8 dS m-1 (Table 3), which was within the range acceptable for general plant growth and microbial activity (Smith and Doran, 1996). Soil EC at the 0- to 7.5-cm depth varied among treatments. The EC in CP1 and CP2 plots was always higher than EC of the CTL plots. Soil EC at the 0- to 7.5-cm depth decreased among treatments in the order: CP2 = CP1 > MN2 = MN1 = FRT > CTL. Similar to the 0- to 7.5-cm depth, soil EC at the 7.5- to 15-cm depth was higher in the compost treatments than in other treatments.
Soil EC at the 15- to 30-cm depth was in the order: CP2 = CP1 = FRT > CTL = MN1 = MN2. Despite small differences (<0.05 dS m-1) among treatments, salts from compost and nitrate from FRT plots reached deeper soil than the manure treatment. Higher EC levels in deeper soils of the FRT plots as compared with the CTL plots was partially due to nitrate leaching, as a significant linear relationship was observed between EC and soil nitrate concentration.
Soil Nitrate
There was no significant effect of treatments on soil nitrate concentration at the 0- to 7.5- or 7.5- to 15-cm depths (Table 3). Soil nitrate concentration decreased significantly with depth in all treatments except the FRT plots, which received NH4NO3 in 1999. Due to leaching of spring-applied ammonium nitrate, nitrate N concentration at the 15- to 30-cm depth (Table 3) and its total amount at the 0- to 30-cm depth (Table 4) was higher in the FRT than the other treatments. The decrease of nitrate with depth and the similarity between the CTL plots and the manure- and compost-treated plots indicates that the postharvest residual effects of manure and compost on nitrate leaching were negligible. High potentially mineralizable nitrogen (PMN) in manure and compost plots (Table 4) did not result in greater nitrate leaching than other treatments.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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