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a Dep. of Plants, Soils, and Biometeorology
b Dep. of Agricultural Systems Technology and Education
c Dep. of Biology, Utah State Univ., Logan, UT 84322
* Corresponding author (jennyn{at}cc.usu.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: AS, ammonium sulfate • DC, dairy-waste compost • LW, liquid dairy-waste
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Appropriate use of the plant nutrients contained in the animal waste requires the ability to predict the release of inorganic nutrients from organic forms. Mineralization and nitrification are the key processes that determine the release and availability of animal waste N in soils. Nitrification produces soil nitrate, which is highly mobile and when produced in excess of plant demand can end up in surface and ground waters causing pollution (Kirchmann et al., 2002; Pierzynski and Gehl, 2005). The maximum contaminant level for nitrate allowed in portable water according to United States Environmental Protection Agency (USEPA) is 10 mg NO3–-N L–1 (USEPA, 1992) and nitrate levels in ground water above this limit have been measured in various part of the USA with agricultural sources of N being significant contributors (Sogbedji et al., 2000; Jabro et al., 2001). Better understanding of coupled mineralization-nitrification is therefore central to the ability to predict and manage soil N.
Repeated applications of dairy manure (Peacock et al., 2001), pig slurry (Hastings et al., 1997), wastewater effluent (Oved et al., 2001), and other organic amendments (manure, sewage sludge, and straw) (Marschner et al., 2003) affect plant nutrient availability through microbial mediated transformations and their products. Nitrogen-mineralization and nitrification rates have been significantly increased through addition of sheep manure (Sorensen, 2001), dairy shed effluent (Zaman et al., 1999), and cattle slurry (Muller et al., 2003). Zaman et al. (1999), for example, reported gross N mineralization rates of 6.1 and 3.4 mg N kg–1 soil d–1 16 d after field application of dairy shed effluent and NH4Cl, respectively, as compared with 1.5 mg N kg–1 soil d–1 for no treatment. Muller et al. (2003) reported a 20-fold increase in gross nitrification rate shortly after cattle slurry application (10.92 mg N kg–1 d–1) in comparison with the control (0.56 mg N kg–1 d–1). Addition of pig slurry (Chantigny et al., 2001) and cocomposted sewage sludge (Sims, 1990) has also resulted in immediate but short-term increases in inorganic N contents, particularly NH4+. The magnitude of the effect of animal wastes on N transformation rates is dependent on the time between application and measurement, the type of the waste, the number of applications, and local environmental and edaphic factors. These factors make the extrapolation of results from one area to another difficult.
The objective of this study was to investigate how repeated application of treated dairy-waste affects soil nutrient pools, N transformations, N availability to plants, and silage corn yields over a 5-yr period in an agricultural soil. Short-term dynamics of N transformations were also investigated using N15 pool dilution techniques and these results are described in a companion paper (Habteselassie et al., 2006).
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The soil treatments are control (with no N fertilization), AS at 100 kg N ha–1 (AS100), ammonium sulfate at 200 kg N ha–1 (AS200), DC at low level (DC100), DC at high level (DC200), LW at low level (LW100), and LW at high level (LW200). The low and high level treatments were applied to provide approximately 100 and 200 kg ha–1 of available N, taking into account actual N contents determined before application, contributions from mineralization, and credits from previous year applications. Initially, we assumed 10% of the applied compost N would become available during the growing season, and 5% in the following season while 100% of the N contained in the LW was considered as available in the current year. These assumptions are based on previous laboratory incubation experiments (Shi et al., 1999; Shi, 1998). During 2000 through 2002, the 10% mineralization estimate was considered to be an underestimate of actual N release and application rates were lowered to decrease excess nitrate accumulations. Actual application rates and characteristics of the materials applied are summarized in Table 2. The fertilizer treatments are applied every year in early May and incorporated into the top 15 cm of the soil with a small tractor disc rotor tiller. Incorporation of amendments followed application as soon as possible depending on soil moisture conditions. The DC and AS treatments were usually delivered in one application whereas the LW treatment was applied in two to three applications over several days to allow for infiltration.
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Soil Analysis
The preseason soil samples were sampled from each plot from the 0- to 30- and 30- to 60-cm depth, four cores (2.5 cm in diam.) were composited for each plot and depth in the field. For indices of available P and K, soil samples were air-dried and sieved before extraction with 0.5 M NaHCO3 (pH 8.5) solution. The extractable P was then determined using the ascorbic acid method (Kuo, 1996). The K was determined by flame atomic emission spectrometry (Varian, SpectraAA-200). Soil pH was measured on 1:1 ratio of soil/water with pH meter (ORION, model 290A). Organic C was determined using the Walkley-Black method (Nelson and Sommers, 1996). Total N was determined by dry combustion (Leco CHN 2000 Autoanalyzer, Leco Corp., St. Joseph, MI). Determinations for extractable Ca, Mg, Na, and K (1M ammonium acetate, pH 7.0) and for extractable metal elements (Zn, Fe, Cu, and Mn) (0.005 M DTPA in 0.1 M triethanolamine, pH 7.3) were done at Utah State University analytical lab with standard fertility methods.
Inorganic Nitrogen Pool Size and Nitrification Potential
Approximately 15 g of the preseason soil samples at field moisture was put into plastic specimen cups containing 75 mL of 2 M KCl immediately after collection. The cups were shaken for 1 h and filtered in a prerinsed Whatman No. 1 filter paper. The filtrates were frozen until analysis for NH4+ and NO3– using Lachat N Autoanalyzer (QuickChem Systems, 1992, 1993). To determine the extent of nitrate movement in the soil profile, soil was sampled after corn harvest from the center of each plot with a Giddings soil corer (5 cm diam.) to 180-cm depth (depth intervals 0–15, 15–30, 30–60, 60–90, 90–120, 120–150, 150–180 cm). The soils samples were extracted with 2 M KCl and analyzed for NO3–-N as described above.
Nitrification potential was determined by the shaken soil slurry method (Hart et al., 1994) from the surface soil (0–15 cm) approximately 90 d after planting time. After moist sieving through a 2-mm screen, about 15 g of moist soil from each plot was placed into 250-mL Erlenmeyer flasks and 100-mL phosphate buffer with 1 mM NH4+-N was added to these flasks. The flasks were continuously shaken for 24 h at 200 rpm (Stark, 1996). At 2, 4, 22, and 24 h after the beginning of shaking, 10-mL aliquots were removed. The aliquots were centrifuged at 8000 x g for 10 min. The (NO2– + NO3–)-N was analyzed by the Lachat N Autoanalyzer (QuickChem Systems, 1992; 1993). Nitrification potential was the slope of linear regression of concentrations of (NO2– + NO3–)-N versus time.
Corn Plant Nitrogen Content and Yield
Yield and plant N contents were determined as described previously (Shi et al., 2004). Nitrogen content was determined using Kjeldahl digestion and distillation method (Bremmer and Mulvaney, 1982). Cornstalk testing for nitrate was conducted at the end of the growing season to assess N availability and overall N status of the crop (Binford et al., 1992).
Statistical Analysis
Statistical analysis of multiple year data for total soil N, soil organic C, available P, available K, chopped corn and leaf N contents, nitrification potential, pH, and silage corn yield was performed with repeated measures analysis of variance (Proc Mixed) with year as repeated measures factor. The effect of the different treatments on soil nitrate and ammonium concentrations at various depths over multiple years was analyzed with a split-split plot design analysis of variance with treatment as the main plot, year as a subplot, and depth as a split-subplot. Significantly different means were separated by Tukey's Studentized Range (HSD) test. All statistical analyses were performed at 95% confidence level (P 
0.05) with the SAS software (SAS Institute Inc., Cary, NC, 2001).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Soil Organic Carbon and Total Nitrogen
Two years of repeated application of DC200 significantly increased SOC versus the other treatments (data not shown). From 1997 through 2002 approximately 63 and 126 Mg C ha–1 were applied to the DC100 and DC200 treated soils. In contrast, <2 Mg C ha–1 was applied in the LW200 treated soils. In 2002, the SOC was more than doubled in the DC200 treated soils with the average difference of 7.3 g C kg–1 soil between the DC200 and the rest of the treatments (Table 3). This C stored in the soil organic matter accounts for approximately 11.1 and 6.6% of the total amount of C applied in DC200 and DC100, respectively. These percentages are reasonable considering microbial efficiencies and SOM recycling over a 5-yr period (Hyvonen et al., 1998). The SOC of the plots receiving the other treatments and the control were similar with no significant difference among each other (Table 3). DC200 resulted in the highest total soil N content in year 2002 whereas the other treatments did not result in significantly different total N content from one another (Table 3). The increase in total soil N content associated with DC200 in 2002 was 0.76 g N kg–1 soil (89% increase) as compared with 1998 (Table 3).
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Available Phosphorus and Potassium
DC100 and DC200 resulted in significantly higher level of available P and available K in 0- to 30-cm depth than AS and LW100, which did not show statistically significant difference among each other (Table 3). LW200 resulted in comparable P and K levels as DC100. Annual tests on preseason soil samples indicated that plots receiving compost had sufficient available P and K for plant growth and no supplements were needed for these plots after 1999. Phosphorus and K measurement at a lower depth (30–60 cm) in 2001 and 2002 (data not shown) indicated that DC200 resulted in significant P and K increases lower in the profile. Management for N availability resulted in excess levels of P and K in the compost treatments. Randall et al. (2000) has previously noted that annual applications of dairy-waste manure at rates to meet N demands of corn could lead to excess build up of P that might be of an environmental concern. While extractable Cu and Fe were not statistically different for the soils with different history, Mn increased from 8 to 14 mg kg–1 soil and Zn increased from 2 to 8 mg kg–1 soil in a comparison of the control versus DC200 soils in 2002.
Preseason Inorganic Nitrogen Pool Size
The overall effects of treatment, year, or depth and the interactions between treatment and year were significant for nitrate concentration. The effects of block and the interaction between treatment and depth were not significant. Year was the only factor that had a significant effect on ammonium concentration. DC100 and DC200 significantly increased the nitrate concentration over the 5-yr period as compared with the other treatments, which did not show any significance difference among each other. In all the years, the nitrate concentration in plots receiving DC200 was significantly higher than those receiving LW, AS, or control (Fig. 1). DC100, however, showed a significantly higher nitrate concentration than the other treatments only in years 1998, 2000, and 2001 and AS and control in years 1999 and 2002. After steadily increasing from 1998 to 2000, the nitrate concentration in plots receiving DC decreased in 2001 in comparison with the previous year mainly because of the reduction in the amount of compost applied due to higher mineralization credit (Table 2). The nitrate concentrations for DC100 and DC200 were 3.05 and 2.36 mg kg –1 soil in 1998 and increased to 6 and 9.43 mg kg –1 soil in 2002 in the top 0- to 30-cm layer, respectively. This is a two to four fold increase in NO3–-N concentration.
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Nitrate was the major inorganic N form at both soil depths. The preseason ammonium concentration was not significantly affected by the various treatments and was always a small proportion of the inorganic N pool size (Fig. 1). Since ammonium is generally rapidly nitrified to nitrate, the low ammonium concentrations were expected. The preseason soil samples were collected in early May right before application of the various treatments, almost a year after treatment application for the previous year. Ammonium levels during the early growing season (approximately 50 d after amendment) were higher for the AS100 and AS200 treated soils but this difference was not found at mid-season (approximately 90 d [Shi et al., 2004] and data not shown). Nitrate was also the dominant inorganic N form in a 112-d laboratory incubation study (Shi and Norton, 2000). In Shi's study, ammonium concentration in the ammonium sulfate amended soil decreased rapidly reaching similar levels to that of the control and compost amended soil after 40 d of incubation. Hadas et al. (1996) noted that in laboratory study of compost mineralization, ammonium was almost completely nitrified by the end of the first week. Similarly, Van Kessel et al. (2000) reported that during an incubation experiment of various components of dairy compost the bulk of mineralized ammonium was observed to have been nitrified to nitrate with in 14 to 28 d. In this study, the dominant fate of added ammonium was nitrification although some ammonia may have volatilized during application as the pH of the soil is high (>8).
Postharvest Nitrate in the Profile
DC100 and DC200 resulted in considerably higher nitrate concentrations in the lower depth intervals as compared with the other treatments, suggesting occurrence of significant nitrate movement in these plots (Fig. 2). Although our irrigation management was devised to minimize excess water applications during the growing season to decrease leaching below the rootzone, some downward movement of excess nitrates may have occurred. Nitrate concentration tends to decrease from the surface layer down to the 30- to 60-cm depth and then increases to concentrations comparable with those of the surface layers, even higher in some cases (year 2000) followed by a decrease again in the lowest part of the profile (150–180 cm) (Fig. 2). The highest nitrate concentration (41.6 mg N kg–1 soil) was seen in the plots receiving DC200 at the 60- to 90-cm depth in 1999. By 1999, approximately 500 and 200 kg N ha–1 additional nitrate over the control had accumulated in the profile under the DC200 and DC100, respectively. These high levels of nitrate were not maintained in subsequent years, possibly due to nitrate movement down to the lower layers and/or plant uptake (Fig. 2). The nitrate concentration below 60 cm for Year 2001 was not as high as those of the previous years mainly because of the reduction in amount of compost applied.
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One possibility for synchronizing N release from compost with plant uptake is to sow cover crops (catch crops) with high N demand after the harvest of the major crop. Significant reduction in downward movement of nitrate has been observed after the use of such crops (Lewan, 1994; Brandi-Dohrn et al., 1997). Adoption of other management practices could also be employed to prevent downward movement of nitrate after harvest. The addition of amendments with high C/N ratio (such as straw residues) can be used to immobilize the nitrate to keep it in the root zone (Kirchmann et al., 2002). Nitrification inhibitors could potentially be used to control the conversion of ammonium, which is produced through compost mineralization, to nitrate thereby decreasing its mobility (Norton, 2000).
King (1984) and N'Dayegamiye et al. (1997) have pointed out the environmental risk of applying matured compost at high rate due to excess nitrate production and subsequent contamination of water bodies. According to Mamo et al. (1998), one time municipal solid waste compost application followed by N supply with conventional fertilization for two more years indicated the highest nitrate leaching potential as compared with plots receiving immature compost treatment or controls (N supply with conventional fertilization only). When C is not limiting, Dick et al. (2000) have shown that nitrate can be immobilized by microbes in subsoil horizons reducing its downward movement.
The nitrate concentration at the various depth intervals for the other treatments was not significantly different from the control and was below 2 mg N kg–1 soil, indicating absence of significant nitrate movement down through the profile in the plots receiving LW and AS. Comparing nitrate loss from various N sources, Jokela (1992) reported leaching potential of nitrate from plots receiving dairy manure to be equal to or slightly less than agronomically equivalent rates of ammonium nitrate fertilizers. While composting manure has been advocated as a method of stabilizing manure N and reducing leaching potential (Gagnon et al., 1998); the management of multiple year applications for the timing of N release remains problematic.
Nitrification Potential
The main effects on nitrification potential by treatment and year, and the interaction between treatment and year were significant. The block effect was not significant. All the treatments showed significantly higher mean nitrification potential than the control for the 5-yr period except for LW100. Plots receiving DC200 and LW200 showed the highest increase (Fig. 3). Averaged over the 5 yr, the control soil nitrification potential was 3.62 where as for the DC200 and LW200, it was 14.07 and 13.04 mg N kg–1 d–1, respectively. This is nearly a 300% increase in nitrifier population activity. For AS200, AS100, DC100, and LW100, the average rates were 11.4, 8.5, 8.0, and 6.9 mg N kg–1 d–1, respectively. The overall effect of repeated waste applications of all treatments was an increase in nitrification rates over time (Fig. 3). Nitrification potential a few weeks after application of the treatments in June 2000 was higher for DC, AS200, and LW200 (data not shown) than in August. This is expected, as ammonium availability is higher closer to the time of application.
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Similar to this study, previous authors have reported increased nitrification potentials as a result of organic amendments including sewage sludge (Hernandez-Apaolaza et al., 2000), DC (Shi et al., 2004), and spent mushroom compost and granular organic fertilizer (Laanbroek and Gerards, 1991). The change in nitrification potential by the treatments in this study is likely related to changes in the population size of ammonia and nitrite oxidizing bacteria but may also include kinetic effects caused by community composition changes (Norton, 2000).
Corn Plant Nitrogen Content, Silage Yield, and Nitrogen Recovery
The overall effects on corn plant N content measurements and silage yield of treatment and year, and the interaction between treatment and year were significant. DC consistently gave the highest yield. Averaged over 6 yr, all the treatments had a significantly higher yield than the control. AS200 and LW200 had no significant yield differences. AS100, however, gave better yield than LW100. The high level gave significantly higher yield than the low level of application for AS and LW. There was no significant yield difference between the low and high levels of DC. The yields from all high rate treatments are typical for well-managed silage production in the study site region (Griggs et al., 2004). The percentage of increase in yield over the control averaged over 6 yr for DC200, AS200, and LW200 was 113, 91, and 79, respectively. While DC100, AS100, and LW100 gave 115, 55, and 25% yield increase over the control, respectively.
The overall year effect was also significant. The highest yield was obtained in Year 1997 followed by Years 2002, 1999, and 2001. The yield difference between Year 2001 and 1998 and between Year 2002 and 2000 were not significant. Compost maintained corn yields even under low water availability due to a decrease in the irrigation water supply during Year 2001. Excessive early moisture and late planting decreased yields and quality in 1998. It was during these 2 yr that the most N deficiency was found. Compost can provide higher amount of inorganic N for plant growth as indicated in the preseason inorganic N section. Moreover, it mineralizes slowly but continuously, acting as a steady source of inorganic N (mainly nitrate) to the corn. Relatively, LW releases its inorganic N faster and is applied once and may not be able to supply inorganic N continuously during the growing season. Nitrogen availability from AS is probably the fastest as it is already in inorganic form and highly soluble.
Nitrogen analysis of silage resulted in values that were lower than those found in the study region except for the DC200 (Griggs et al., 2004). The 5-yr average leaf N contents were in the deficiency range for all treatments except for DC200. In Year 1997, leaf N contents from plots receiving DC100, DC200, and AS200 were not in N deficient range. Based on leaf N content, however, all the plants were N deficient for Years 1998 and 2001. Plant N contents were lowest for control, LW100, and AS100 (Table 4).
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Corn yields, plant N contents, and soil N pools suggest that LW did not supply adequate N for crop growth. For example, in 1999 the LW200 plots received 202 and 112 kg N ha–1 was recovered in the chopped corn compared with AS200 recovery of 165 kg N ha–1. The difference between the application rate estimate and recovery data suggest that significant N loss occurred in the field from the LW200 plots. There is a higher tendency for loss of the inorganic N contained in LW during application through ammonia volatilization or denitrification. Volatilization of ammonium during application was likely because the liquid dairy-waste (pH 9.3) was surface applied during warm spring days to a high pH soil (>8). A temporary anaerobic condition might also prevail immediately after LW application due to excess moisture content, encouraging denitrification. Paul and Beauchamp (1993) reported significant loss of inorganic N from liquid dairy cattle manure with only 27% of what was calculated to have been applied remaining after 1 wk. They attributed this loss to ammonia volatilization, denitrification, or immobilization of N rather than changes in manure N content between the time of sampling and application.
Unlike LW, most of the N in compost is in organic form and the amount of ammonium as a percentage of total N is very small (this study, Sikora and Szmidt, 2001). Therefore, we believe N loss through volatilization was minimal for compost treated soils. In previous work (Sikora and Szmidt, 2001), N loss from volatilization from plots receiving fresh manure was more than twice from plots receiving compost. After 4 yr of study, Eghball and Power (1999) concluded that surface application of beef cattle feedlot manure or composted manure did not result in significant N losses. They reasoned that this was because N compounds in beef cattle feedlot manure or compost are mainly in organic N forms and contain only small proportions of NH4+-N. They stressed that organic sources with large concentration of NH4+-N should be incorporated after application to minimize N loss.
Our observations on crop yield, plant N contents, and recovery suggest that the rate of compost mineralization in the field was higher than the rate we assumed based on previous laboratory studies (Shi, 1998; Shi et al., 1999) and used for the initial application calculation. The effects of soil wetting and drying can stimulate mineralization (Saetre and Stark, 2005) and this may be one factor for the underestimation of mineralization in the compost treated soil. The compost mineralization rate in the field appears to be closer to 20% of the total N, approximately double the assumed rate of 10% mineralization. Long-term field incubation studies of compost mineralization may help to determine more accurate compost application rates.
The effect of compost application on yield is mainly derived from the plant nutrients contained in compost, particularly N (Stratton and Rechcigl, 1998). Compost can also improve soil physical properties such as water holding capacity and soil aeration, and increase soil pH buffering (Smith and Van Dijk, 1987; Stukenholtz et al., 2002). Studies that compared the nutrient versus the non-nutrient contributions of animal manure with yield increase have given greater importance to nutrient than the non-nutrient effects (Magdoff and Amandon, 1980; Klausner et al., 1994). However, Stukenholtz et al. (2002) indicated that the non-nutrient contribution of compost can be more important if moisture is limiting especially in dryland systems.
In summary, our results demonstrate that repeated application of compost (DC) has significantly increased soil organic C, total N, and available P and K as well as inorganic N pool size, primarily nitrate. Dairy-waste compost treated soils maintained N supply to the plants through continuous mineralization as shown by inorganic N pools, silage corn yield, and plant N analysis. A nitrification potential increase in soils receiving dairy-waste indicates that these soils are maintaining higher nitrifier population. Nitrification potentials also increased in the AS treated soils, but to a lesser degree than the dairy-waste treatments. As reported in the companion paper (Habteselassie et al., 2006), the highly dynamic nature of the plots receiving DC in terms of N turnover rate was also indicated by higher gross N mineralization, gross nitrification, and C mineralization rates in comparison with the other treatments. The high level compost (DC200) was the only treatment that caused significant nitrate movement down the soil profile.
Silage corn yield was significantly increased by all of the treatments with the highest yield being associated with DC. Compost continues to mineralize even after plant harvest leading to nitrate production and subsequent downward movement. Considering the low C/N ratio of compost (12:1) and the fact that it significantly raised the labile organic N pool size and the decomposition rate constant (Habteselassie et al., 2006), nitrate production after corn harvest could be significant. The use of appropriate field mineralization rate estimates and the adoption of agronomic practices that match the timing of plant N demand with compost N release will be valuable tools for reducing nitrate leaching from DC treated soils.
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ACKNOWLEDGMENTS |
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Received for publication June 15, 2005.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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This article has been cited by other articles:
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  M. Y. Habteselassie, J. M. Stark, B. E. Miller, S. G. Thacker, and J. M. Norton Gross Nitrogen Transformations in an Agricultural Soil after Repeated Dairy-Waste Application Soil Sci. Soc. Am. J., June 21, 2006; 70(4): 1338 - 1348. [Abstract] [Full Text] [PDF]
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