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a Dpto. de Producción Vegetal, Universidad de Almería, 04120 La Cañada, Almería, Spain
b USDA-ARS, Animal and Natural Resources Institute, Bldg. 163F, BARC-East, Beltsville, MD 20705
* Corresponding author (jmeising{at}anri.barc.usda.gov)
Received for publication July 3, 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DM, dry matter • GC, ground cover • II, immediate incorporation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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; the remainder is in much less labile, organic compounds (Beauchamp and Paul, 1988). Slurry NH+4–N represents a large pool of plant-available N that, if used effectively, could appreciably reduce mineral N fertilizer requirements for crop production. Large and variable N loss by ammonia (NH3) volatilization occurs following land application of cattle slurry (Pain and Thompson, 1988; Jarvis and Pain, 1990). Commonly, 30 to 70% of slurry NH+4–N is lost within a week of application (Pain and Thompson, 1988). These large N losses are an agronomic problem because of uncertainty as to the amount of slurry NH+4–N retained in soil for crop–forage production. They can also create environmental problems by contributing to N enrichment of nearby surface waters and natural terrestrial ecosystems (van Breemen and van Dijk, 1988; Schulze et al., 1989; Fisher and Oppenheimer, 1991; Valigura et al., 1994).
Farmers, advisors, and environmental managers require information that (i) quantifies NH3 volatilization from slurry applications, (ii) identifies major factors influencing these losses, and (iii) suggests practical methods of control. For the Mid-Atlantic region of the USA, unlike northwestern Europe (e.g., Pain and Thompson, 1988; Jarvis and Pain, 1990, 1997; Nielsen et al., 1991), there is currently very little quantitative data on NH3 loss following slurry application under field conditions (Thompson et al., 1997).
Published research data have been obtained from either grassland or arable soils. One English study (Thompson et al., 1990a) directly compared these two surfaces and reported that losses were 50% greater from grassland. To assist with "customizing" NH3 volatilization loss factors for extension advice, and for more accurate extrapolation of research data, data from direct comparisons of NH3 volatilization from arable and grassland soils are needed. The dry matter (DM) content of cattle slurry positively affects NH3 volatilization (Sommer and Olesen, 1991; Moal et al., 1995) under cool northwestern European conditions. However, it is uncertain whether DM will similarly affect NH3 volatilization under summer Mid-Atlantic conditions, where temperatures can be much higher.
Various methods have been shown to reduce NH3 loss following slurry application to soils (e.g., injection, banded application, subsequent irrigation, acidification; see Jarvis and Pain, 1990). However, one of the most practical methods for U.S. livestock farms, where corn (Zea mays L.) is commonly grown, may be incorporation into arable soil following conventional surface application. As NH3 volatilization is most rapid immediately after slurry application (Thompson et al., 1987), incorporation must be carried out as soon as possible to minimize NH3 loss. Information is required on the relative effectiveness of common tillage implements when used to incorporate cattle slurry immediately after surface application to arable land.
A series of field studies were conducted to assess the effect of the following factors on NH3 volatilization following cattle slurry application: (i) grass cover compared with bare soil, (ii) slurry dry matter content, and (iii) immediate incorporation with three different cultivation implements (tandem disk-harrow, chisel plow, and moldboard plow). As surface crusting was a characteristic of the field soil used, the effect of surface crusting was also examined.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Ammonia Volatilization Measurement
The system of small wind tunnels used for the collection of volatilized NH3 was very similar in design and size to that described by Lockyer (1984). The system consisted of six individual wind tunnel units. Each wind tunnel unit has two parts: (i) a transparent section formed from polycarbonate sheet, flexed and pinned to the ground along each edge to form an inverted "U"-shaped canopy covering an area of 1 m2 (2.0 x 0.5 m), and (ii) a steel duct of circular section of 40 cm i.d., containing an electrically powered fan blade and motor, to draw air through the canopy section. The canopy section was placed over a 1-m2 (2.0 x 0.5 m) treated plot, directing all NH3 volatilized from the plot surface to the steel duct section where a measured proportion of air flow was sampled.
The speed of the fan in the steel duct section was manually adjusted with a variable voltage controller to maintain the air velocity at a target value of 1 m s-1. Air velocity within each tunnel was measured every 10 s using a TSI (St. Paul, MN) hot-wire anemometer positioned centrally in the forward part of the steel duct with output recorded on a Campbell Scientific (Logan, UT) 21X data logger. The relationship between air velocity and total air flow within the tunnels had been previously established using a balometer.
The NH3 concentration of air entering and leaving the transparent canopy section, of each tunnel, was sampled with gas washing bottles containing 0.002 mol L-1 H3PO4. The air entering the canopy was sampled with 125-mL bottles containing 80 mL of the acid solution; the air leaving was sampled with 250-mL bottles containing 120 mL of acid solution. Air entering the steel duct section from the canopy was sampled with a star-shaped sampler system, with six arms of 19 cm length, located 20 cm in front of the fan blade in the steel duct. The six arms were joined at a central hub from where sampled air was drawn into the outlet acid trap. Along each arm of the sampler system were three holes, at distances of 7, 14, and 18 cm with corresponding diameters of 3.0, 4.0, and 4.4 mm. The six-arm sampler was designed to ensure an even sampling of the total air stream in the steel duct (D. Lockyer, personal communication, 1996).
Each acid trap was connected to a small pump and a flow meter. The flow meters for the inlet and outlet air were set at 4 and 5 L m-1, respectively. At the end of each sampling interval, both acid traps on each tunnel were replaced with fresh bottles of acid. The exposed H3PO4 solutions were then made up to known volumes and stored at 3°C prior to analysis for NH+4–N with a Lachat (Milwaukee, WI) flow injection analyzer system, using the salicylate method (Kempers and Kok, 1989).
A commercial power hookup provided electrical energy for the tunnel motors, hot-wire anemometer systems, and small pumps.
The rate of NH3 volatilization, for each time period, for each tunnel was calculated as follows:
NH3 concentration) = NH3 concentrationOUT - NH3 concentrationIN, NH3 concentration x VolumeTOTAL)/(treated area x time) Rate of NH3 volatilization data are generally expressed in units of kg N ha-1 h-1 or g N ha-1 h-1. In Study DM1, because of slight differences in the slurry NH+4–N application rate between different DM content slurries, NH3 volatilization rate data are expressed as percent applied slurry NH+4–N lost ha-1 h-1. Total and cumulative loss data are expressed as percent applied slurry NH+4–N lost, for all studies. When presented graphically, NH3 volatilization data are presented in the mid-point of the sampling interval.
Following slurry application, the tunnels were immediately placed on the treated plots, and measurements commenced within 5 min. The cover sections remained in place for the duration of each study. Wind barriers were placed both in front of and behind each tunnel to reduce the effect of ambient wind on air velocity within the tunnel. The barriers consisted of either sheets of plastic stretched between vertical wooden posts, or sections of plywood held upright by metal pegs.
Slurry Treatments
The cattle slurries used in the studies were obtained from housed dairy cattle at BARC. The cattle were fed a diet based on corn silage. A small amount of sawdust, which was used as bedding, would have been contained in the slurry. The slurries used in the two studies examining the effect of dry matter content (Studies DM1 and DM2) were collected 24 to 48 h prior to application. The different slurry dry matter content treatments were prepared the day before commencing the studies. For the other studies, fresh slurry was collected during the 24-h period prior to application.
Descriptions of the treatments examined in each study, the dates they began, and the study duration are given in Table 1. Relevant details of the composition of the slurries are given in Table 2 . Details of the slurry, NH+4–N, and total N application rates are provided in Table 3 .
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In Studies GC1, DM1, and DM2, 8 L of slurry was applied to plots measuring 2.0 by 0.5 m, giving an application rate equivalent to 80 m3 ha-1 (Table 3). In Study GC2, 8 L of slurry was applied to a plot measuring 1.75 by 0.5 m, giving an application rate equivalent to 91 m3 ha-1 (Table 3). The shorter plot was used in Study GC2 to ensure that all of the applied slurry was well within the tunnel cover when rain threatened.
For the studies examining immediate incorporation (Studies II1, II2, and II3), slurry was applied with a tractor-pulled Houle (Drummondville, QC, Canada) EL 54-3600 tanker with a low trajectory spreading bar (40 cm above ground). One nozzle was used to apply slurry in a 2.0-m-wide band. The actual application rate for each immediate incorporation study (Table 3) was determined by weighing the slurry collected in trays placed on the ground, in a position adjacent to and downwind from the experimental plots.
Ground Cover Studies
Studies GC1 and GC2 examined the effect of grass cover compared with bare soil on NH3 loss following slurry application. Slurry was applied to both bare soil and to a well-established tall fescue (Festuca arundinacea L. cv. Kentucky 31) grass sward, which was cut to a height of approximately 3 cm. In both studies, the bare soil was free of plant material, two tunnels (duplicates) were used for each treatment, and measurements were made for 4 d.
In Study GC1, the surface of the bare soil was cultivated, using a hand rake, prior to slurry application to break up a surface crust and provide a tilled soil surface. Slurry was applied at 1430 h on 9 June 1997. The acid traps were changed after 3, 6, 19, and 24 h; thereafter, they were changed two or three times per 24-h period.
In Study GC2, the crusted bare soil was not raked prior to slurry application. Slurry was applied at 0830 h on 26 June 1997. The acid traps were changed after 3, 6, 12, and 24 h; thereafter, they were changed two or three times per 24-h period.
Dry Matter Studies
Study DM1 examined the effect of slurry dry matter content on NH3 volatilization following slurry application to bare soil that had been raked prior to slurry application. The different DM contents (Table 2) were obtained by sequentially sieving slurry through 12.7-, 1.0-, and 0.5-mm sieves, and then adding the collected solid material, in different ratios, to the sieved or raw slurry. This procedure produced slurries with a range of DM contents, but with similar NH+4–N and total N contents (Table 2). Dry matter contents of the raw and sieved slurry were 117 and 54 g kg-1. Slurry was applied at 1400 h on 8 July 1997. One tunnel was used for each of the six treatments. The acid traps were changed after 3, 6, 18, and 24 h; thereafter, they were changed two or three times per 24-h period. Measurements were made for 3 d.
Study DM2 examined the effects of slurry DM content and surface soil crusting on NH3 volatilization following slurry application. Three slurries with different DM contents were applied to soil that had either a surface crust or a loose soil surface. The slurry DM contents were 56, 96, and 113 g kg-1 (Table 2); these corresponded to, respectively, sieved slurry (0.5 mm), sieved slurry with some solids addition, and raw slurry. The crusted soil had a surface crust that had formed naturally, and the loose surface soil was hand-raked immediately prior to applying the slurry treatments. Slurry was applied at 0900 h on 16 July 1997. One tunnel was used for each of the six treatments. The acid traps were changed after 2, 4, 6, 8, 12, and 24 h; thereafter, they were changed two or three times per day. Measurements were made for 4 d.
Immediate Incorporation Studies
Studies II1, II2, and II3 examined the effect of immediate incorporation on NH3 volatilization. In each study, there were four treatments: surface-applied slurry that was not incorporated, and surface-applied slurry that was incorporated immediately after application with a tandem disk-harrow, a chisel plow, or a moldboard plow. The three studies commenced between 0900 and 1100 h on the dates indicated in Table 1. In each study, one incorporation treatment was randomly assigned to a new area of soil and a tunnel was randomly assigned to measure NH3 loss. This protocol produced an immediate incorporation investigation with a completely randomized design with three replicates over time.
Slurry was applied in 5- by 2-m bands, using the tractor-pulled Houle tanker. Bands of slurry were parallel and separated laterally by 4 m, and longitudinally by 6 m. The 2- by 0.5-m canopy of each tunnel was placed across the band of slurry, 3 m from the beginning of the band. The prevailing wind direction was perpendicular to the bands and directly into the canopy openings. This arrangement of the slurry bands and tunnels ensured that the exhaust from one tunnel had minimal effect on the inlet NH3 concentration of the other tunnels.
Immediate incorporation was achieved by having another tractor with the tillage implement follow several meters behind the slurry tanker. Immediately after each implement had incorporated the slurry, the tunnel was positioned across the band of treated soil, and measurements were begun. The sequence of slurry application, incorporation, tunnel placement, and start of measurement was completed for each treatment prior to applying slurry for another treatment. This ensured that measurements commenced within 5 min of treatment application. Once the tunnels were positioned, soil was placed along the edges of each tunnel cover and firmed to ensure a good seal between the tunnel cover and the underlying soil. Measurements were made for 5 d for each of the three studies. For the unincorporated surface-applied treatment, the slurry-treated soil to both sides of the tunnel was incorporated with a rotovator within 5 min of positioning the tunnel in order to minimize background concentrations of atmospheric NH3. The unincorporated treatment was always placed in the most downwind position at the beginning of the trials to minimize background NH3 levels.
In Study II1, the acid traps were changed after 2, 4, 6, 10, 20, and 24 h; and thereafter, two or three times per day. In Study II2, they were changed after 2, 4, 6, 8, 10, 22, and 24 h; and thereafter, two or three times per day. In Study II3, they were changed after 2, 6, 10, and 24 h; and thereafter, one to three times per day.
Analysis of Slurries
The NH+4–N content of the slurries was determined by distillation with MgO using 10 g of slurry, diluted to 250 mL with deionized water and distilled in a macro-Kjeldahl distillation unit. About 100 mL of distillate were collected in 4% boric acid and titrated with standardized sulfuric acid to determine NH+4–N. Total N was determined by Dumas combustion using a LECO (St. Joseph, MI) CHN analyzer. Slurry DM content was determined by drying 50 mL in a forced-air oven at 105°C for 48 h.
Measurement of Environmental Parameters
During each study, data on rainfall, soil and air temperature, atmospheric relative humidity (RH), and solar radiation were collected. Rainfall was sampled with a tipping bucket rain gauge, soil and air temperature with thermocouples, RH with a pyschrometer, and solar radiation with a pyranometer; data was recorded on a Campbell 21X data logger. Where environmental data are presented in figures, the data are averaged for the corresponding time interval for which the rate of NH3 volatilization was measured.
Statistical Analyses
The limited number of wind tunnels available for each study dictated the need for compromise between number of replicates and number of treatments. We chose to emphasize treatment comparisons with limited replication, that is, two replicate tunnels in Studies GC1 and GC2, and replicates in time for Studies II1–3. This approach has been commonly employed in studies using similar wind tunnel systems to examine NH3 volatilization, using either nonreplicated comparisons (e.g., Lockyer et al., 1989; Sommer and Olesen, 1991; Sommer et al., 1991; Thompson et al., 1990b) or limited replication with duplicates or triplicates (e.g., Moal et al., 1995; Pain et al., 1989b; Sommer and Ersbøll, 1994; Thompson et al., 1990a). Statistical estimates of uncertainty, in the current studies, were calculated by pooling variances among replicated studies, or by pooling variances over time, as described below.
Methods Used to Analyze Rates of NH3 Volatilization and Total NH3 Loss
Treatment comparisons for the rate of NH3 volatilization, in the GC and DM studies, used a pooled variance term obtained from the duplicated rate data for each sampling interval in Studies GC1 and GC2. Linear regression analysis of the standard deviation (SD) of NH3 volatilization rates versus the mean rate, for each sampling interval, indicated a significant relationship (r2 = 0.39) with larger SD values associated with larger means. Rates of NH3 volatilization in Studies GC1 and GC2 were consequently separated into high-loss and low-loss groups to allow pooling of similar variance estimates (M. Kramer, personal communication, 2001). The criterion for separation was the relative concentration of NH3 in outlet to inlet air. Data were placed in the high-loss group if the NH3 concentration of the outlet air was at least ten times that of the inlet air. The high-loss group contained NH3 volatilization rates of at least 100 g NH3–N ha-1 h-1. The pooled variance for the high-loss group (23 df) yielded a SD of ±105 g N ha-1 h-1, and that of the low-loss group (21 df), a SD of ±15 g N ha-1 h-1. The pooled variance used to examine the rate variable percent of applied NH+4–N lost ha-1 h-1 in Studies DM1 and DM2 was calculated using the same approach, using data from Studies GC1 and GC2 after expressing the results on a percentage loss basis. Least significant differences (LSD) for comparing rates of NH3 volatilization between treatments for individual sampling intervals in the GC and DM studies used the appropriate pooled variance values referred to above.
Statistical analysis of total NH3 loss (as percent of applied slurry NH+4–N lost) was conducted by calculating a pooled variance term from those studies that had duplicates or triplicates. Specifically, the residual mean square from Studies GC1 and GC2 (each with 2 df) was pooled with the residual mean square from Studies II1, II2, and II3 (with 7 df) after confirming variance homogeneity using F tests (Snedecor and Cochran, 1980). The II1, II2, and II3 studies contributed 7 df to the pooled variance because the surface-applied treatment of II2 was not included, due to a loose vacuum line on the outlet air sampling pump, which produced an unreliable data point. The resulting pooled variance (11 df) of 15.6 (SD of ±4%) was used to calculate the LSD values for the treatment comparisons of total NH3 loss (as percent of applied slurry NH+4–N) for the GC and DM studies.
Analysis of Individual Studies
Treatment comparisons in the two ground cover studies used the pooled variance terms for rate of NH3 volatilization (g N lost ha-1 h-1) and total NH3 loss (percent of applied slurry NH+4–N lost) described above. The corresponding LSD values for the rate of NH3 volatilization in the high-loss group at P 
0.05 and P 0.01 were 218 and 295 g N ha-1 h-1, respectively. The LSD values for the rate of NH3 volatilization in the low-loss group at P 0.05 and P 0.01 were 32 and 43 g N ha-1 h-1, respectively. The LSD values for total NH3 loss, over the duration of the study, at P 0.01 and P 0.05 were, respectively, 12.0 and 8.7% of applied slurry NH+4–N.
In the unreplicated DM1 and DM2 studies, treatment comparisons for rate of NH3 volatilization (percent of applied NH+4–N lost ha-1 h-1) and total NH3 loss (percent of applied slurry NH+4–N lost) used the appropriate pooled variance terms described previously. The LSD values for the rate of NH3 volatilization for high-loss periods were, respectively, 0.6 and 0.8% of applied NH+4–N lost ha-1 h-1 at P 0.05 and P 0.01. In Study DM2, which had a complete factorial treatment design, the previously mentioned, 11 df pooled variance term for total NH3–N lost was used in an analysis of variance (ANOVA) to test for treatment effects. This allowed testing of the main effects of slurry DM content (three levels), soil surface condition (two levels), and the interaction of these two factors. Linear regression analyses were also used to examine the effects of slurry DM content on the rate of NH3 volatilization and on the total loss in specific time periods. Because of the limited replication, and the field nature of these studies, a statistically significant level of probability of P 0.10 was used for interpreting these regression analyses.
The three immediate incorporation studies were combined and analyzed as a single investigation using total loss expressed as percent of applied slurry NH+4–N, with three replicates in time. A one-way ANOVA with an arc sine transformation was used to compare the tillage treatments using the 7 df residual mean square term from the combined three immediate incorporation studies. The arc sine transformation is recommended for comparing percentage data containing values close to zero (Snedecor and Cochran, 1980). The unincorporated surface-applied treatment had only two replicates due to a loose vacuum hose as noted above.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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0.05) up to 24 h, but not significant thereafter. In Study GC2 (Fig. 1b), differences were highly significant (P 0.01) up to 6 h, but not significant thereafter.
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0.01). Total NH3 loss in Study GC2 was 40% of applied slurry NH+4–N from grass and 28% from bare soil (Fig. 2b); this difference was also highly significant. The total 4-d loss from grass was 1.7 times that from loose bare soil in Study GC1, and 1.4 times that from crusted bare soil in Study GC2. In Study GC1, air temperature during the first 6 h averaged 25°C, and thereafter was 12 to 30°C. In Study GC2, air temperature averaged 31°C during the first 6 h, and then was 15 to 30°C. These temperatures are generally typical of this time of the year.
Dry Matter Study 1
The hourly rate of NH3 volatilization is plotted against time in Fig. 3a
, for four of the six different DM content slurries. The rate is expressed as percent applied slurry NH+4–N lost ha-1 h-1 to adjust for slight differences in the rates of NH+4–N applied with the different DM content slurries (Table 3).
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The 54 g kg-1 DM content slurry had a similar general pattern of NH3 volatilization to the 74 g kg-1 DM content slurry but maintained lower rates throughout (data not presented). Ammonia volatilization from the 105 g kg-1 DM content slurry followed a similar pattern to the 117 g kg-1 DM content slurry but with consistently lower rates (data not presented).
Linear regression of rate of NH3 volatilization by slurry DM content (Fig. 4
, Table 4)
indicated a positive but nonsignificant relationship (r2 = 0.49, P > 0.10) for the 0- to 3-h interval, a highly significant positive relationship (P 0.01) for the 3- to 6- and 6- to 18-h intervals, and a significant, negative relationship (P 0.10) in four of the five subsequent time intervals. The effect of slurry DM content on the rate of NH3 volatilization changed from positive to negative approximately 18 h after application.
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0.01), and a significant, negative relationship for the 18- to 72-h period (r2 = 0.70, P 0.05). The total NH3 loss during the 72-h study was not significantly related to DM content (r2 = 0.09, P > 0.10). Thus, the more prolonged negative effect of DM content on the rate of NH3 volatilization during the 18- to 72-h period, when rates were relatively low, offset the positive effect during the 0- to 18-h period when rates were generally much higher.
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Dry Matter Study 2
On both the crusted and loose soil surfaces, higher total 4-d NH3 volatilization loss was determined from the most dilute slurry (56 g kg-1 DM) (Fig. 6)
. Linear relationships between the total loss and slurry DM content were not significant (P > 0.05) on either soil surface.
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As in Study DM1, more of the total NH3 loss occurred earlier from the higher DM content slurries. After 6 h on the loose soil surface, 44, 62, and 63% of the total 4-d loss had occurred from, respectively, the 56, 96, and 113 g kg-1 DM content slurries. Corresponding values on the crusted soil surface were 55, 63, and 70%.
For each slurry DM content, the total NH3 loss from the crusted soil surface was consistently higher than from the loose soil surface (Fig. 6). The average difference between total loss from the loose and crusted soil was 4.8% of applied slurry NH+4–N, which was not statistically significant (P > 0.05). The effect of the interaction between slurry DM content and soil surface condition on total NH3 loss was also not significant (P > 0.05).
The average loss from the three different DM content slurries on the crusted soil was significantly greater (P 0.05) than from the loose soil (6.3 vs. 5.3% of applied slurry NH+4–N, respectively) during the first 2 h. The smaller loss on the loose soil was most likely due to greater initial infiltration on the looser soil surface. There were no consistent effects of soil surface condition after the initial 2 h.
The average air temperatures for Study DM2 during the first and second 12-h periods were 29 and 27°C. For the remaining 72 h, the average air temperature was 31°C.
Immediate Incorporation Studies
In each of the three studies, the total NH3 loss from each of the three immediate incorporation treatments was much less than from the unincorporated slurry treatment (Table 5)
. The average total loss from unincorporated surface-applied slurry, in Studies II1 and II3, was 46% of applied slurry NH+4–N. The moldboard plow was the most effective implement, reducing the total NH3 loss to negligible amounts (0.3–3.0% of applied slurry NH+4–N). The total loss from the disk harrow treatment was of 2 to 8% of applied slurry NH+4–N. The chisel plow was the least effective implement, with total losses of 8 to 12% of applied slurry NH+4–N.
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0.01 and P 0.05, respectively. Total losses from the disk harrow and moldboard plow treatments were not significantly different (P > 0.05). In each of the three studies, the degree of reduction in NH3 loss was positively related to the degree of soil inversion–mixing provided by each implement; the order being: moldboard plow > disk harrow > chisel plow. Soil moisture data were not directly measured in these studies. However, rainfall records prior to the studies provided a qualitative indication of soil moisture status. Prior to Study II1, there was 18 mm of rain seven days before and 3 mm three days before the study. Study II2 commenced one week later, during which time there was no rain (i.e., quite dry soil conditions). Forty-six millimeters of rain fell six days before starting Study II3 (i.e., moist soil conditions). The largest losses from incorporated slurry were associated with the drier soil of Study II2, and the smallest losses with the wetter soil of Study II3. The larger NH3 loss from the shallow-incorporated slurry in drier soil is consistent with greater gas exchange potential, between the soil air and the atmosphere, in drier soil.
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The total NH3 loss from slurry applied to grass sward was 1.7 and 1.4 times that from bare soil in the two ground cover studies, giving an average value of 1.5. The similarity of air temperatures following application suggests that any effect due to the different application times (1430 and 0830 h) of the two studies would be small in relation to the ground cover effect.
The results from the two ground cover studies are consistent with a 1.5-fold difference in an English study (Thompson et al., 1990a). Grass sward presumably enhances NH3 loss by reducing the infiltration of slurry into soil, and the subsequent retention of NH+4 ions on soil exchange sites. Crop residues similarly increase NH3 loss following slurry applications (Döhler, 1991). These data demonstrate the need to consider the presence of grass sward or thick crop residues when considering NH3 loss factors for manure management purposes. As a general guideline, it is suggested that NH3 loss from cattle slurry surface-applied to grass sward be considered to be 1.5 times more than from bare soil.
Higher slurry dry matter content did not increase total NH3 loss from surface-applied cattle slurry in Studies DM1 and DM2. This contrasted the positive effect of slurry DM content reported by Sommer and Olesen (1991) and Moal et al. (1995) working under more moderate temperature conditions in, respectively, Denmark and Brittany, France. In Study DM1, there was a strong positive effect of DM content on NH3 volatilization for the first 18 h, followed by a clear negative effect during the remaining 54 h. The data from Study DM2 were generally supportive, with higher NH3 volatilization rates from the most dilute slurry after 12 to 24 h on both loose and crusted soil surfaces.
The positive effect of slurry DM content on NH3 volatilization during the first 12 to 24 h of the studies reported here is attributed to less infiltration into soil of the higher DM content slurries. The negative effect of slurry DM content on NH3 volatilization after 12 to 24 h is attributed to more rapid crust formation on the slurry surface, and more rapid drying of the applied slurry. A surface crust and drier applied slurry increases physical resistance to NH+4 and NH3 transport within the slurry matrix, and to NH3 release from the slurry surface (Thompson et al., 1990a, b). Visual observations were that the higher DM content slurries dried and produced surface crusts more quickly. The lack of diurnal increase in the rate of NH3 volatilization from the higher DM content slurries on the day after application, and the occurrence of diurnal affects with the more dilute slurries, is consistent with this hypothesis (see Fig. 3).
It is suggested that the overall effect of increasing slurry DM content on total NH3 volatilization loss depends on the relative influence of the contrasting effects of infiltration and surface crusting, and that prevailing weather conditions determine the balance between these two effects. Under mild climatic conditions, infiltration appears to be more influential with reduced infiltration promoting greater losses with high DM slurries (e.g., Sommer and Olesen, 1991; Moal et al., 1995). Under the hot summer conditions of the present studies, the two effects apparently balanced one another.
These data suggest that climatic conditions need to be considered when predicting total NH3 volatilization loss in relation to slurry DM content. This may be particularly relevant in regions of the USA where there are large seasonal changes in temperature.
Slurry DM content appreciably affected cumulative NH3 loss in the first several hours after slurry application. Consideration of this effect of slurry DM content may improve prediction of NH3 loss from slurry that is incorporated into soil several hours after surface application.
In the moist, loam-textured soil used, immediate incorporation with the moldboard plow, disk harrow, or chisel plow substantially reduced NH3 loss in each of three studies. The degree of reduction was positively related to the degree of inversion. Klarenbeek and Bruins (1991) observed that NH3 volatilization from pig slurry was directly related to the degree of soil inversion during immediate incorporation.
The results from the present studies demonstrate the potential for using immediate incorporation to substantially reduce NH3 volatilization loss from surface slurry applications. However, for the development of extension recommendations, similar studies should be performed on a range of soil texture classes; soil moisture effects should also be considered.
The data from Study DM2 and the ground cover studies suggest that soil surface crusts do not appreciably influence total NH3 loss. For management guidelines, it is suggested that surface soil crusts have only a small effect on NH3 loss following slurry applications under warm climatic conditions.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Soil Science Society of America Journal | Journal of Plant Registrations | The Plant Genome | |||
