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a Dep. of Crop and Soil Sci., Michigan State Univ., 539 Plant and Soil Sciences Bldg., East Lansing, MI 48824-1325
b Dep. of Agron., 3405 Agronomy Hall, Iowa State Univ., Ames, IA 50011-1010
c USDA-ARS, 310 Natl. Soil Tilth Lab., Ames, IA 50011-3120
d Dep. of Agric. and Biosyst. Eng., 3222 Natl. Swine Res. and Inf. Cent., Iowa State Univ., Ames, IA 50011-3080
* Corresponding author (mliebman{at}iastate.edu).
Received for publication December 5, 2002.
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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Most swine manure in the USA is handled and stored as a liquid (NRCS, 2000), but the environmental and social impacts of doing so have some producers and scientists searching for alternative forms of management in which manure is handled as a solid (Honeyman, 1996). One option involves swine production in deep-bedded hoop structures. In Iowa, nearly one million head of swine are finished per year in these hoop structures (Leopold Cent. for Sustainable Agric., 2001). Swine hoop structures are typically bedded with corn stalks or cereal straw, which absorb urine and feces throughout the four- to six-month production cycle. During this time, some in situ composting occurs although the extent of this unmanaged decomposition varies widely. Swine manure from hoop structures can be spread on fields immediately after animals are removed from the buildings, or it can be piled for additional composting (Tiquia et al., 2000).
Composted manure has a number of potential advantages over fresh manure, including reductions in viable weed seed content (Wiese et al., 1998; Eghball and Lesoing, 2000), improvements in handling characteristics (by reducing manure volume and associated transportation costs), and a reduction in particle size leading to increased uniformity of field application (Rynk, 1992). Compost-amended soils can increase crop growth beyond levels explainable by nutrient effects (Valdrighi et al., 1996), provide protection from plant pathogens (Hoitink and Kuter, 1986), and suppress weed seedling emergence (Menalled et al., 2002). Phytotoxic substances contained in fresh solid swine manure, such as high concentrations of NH+4–N, decrease with time of composting (Tiquia and Tam, 1998) and time following soil application. Disadvantages of composting are potentially large losses of C and N and labor and capital costs associated with extra manure handling and space requirements for the compost piles. Losses of N measured during composting of animal manure have ranged from 20 to 70% (Martins and Dewes, 1992; Rao Bhamidimarri and Pandey, 1996; Eghball et al., 1997; Tiquia et al., 2002). Garrison et al. (2001) estimated that 41% of total N contained in fresh swine hoop manure was lost during two months of intensively managed composting.
Synchrony of plant-available soil nutrients and crop nutrient demand is essential for optimum crop performance and environmental protection (Magdoff, 1995). If plant-available N (NO–3 and NH+4) is not supplied in synchrony with crop demand, then substantial N losses can occur before or after periods of crop demand. The quantity of plant-available N is dynamic and reflects the balance between N mineralization, N immobilization, and removal of inorganic or organic N from the soil rooting zone (e.g., via leaching, volatilization, denitrification, soil erosion, and plant uptake). Soil physical conditions, including temperature, water status, and aeration, and the C/N ratio and C constituents (especially lignin quantities) of organic materials are the primary factors affecting mineralization rates (Jenny, 1980; Swift et al., 1979).
In previous investigations, corn yield responses to composted and fresh manure have been similar when these amendments were applied at the same time (Reider et al., 2000; Eghball and Power, 1999; Brinton, 1985; Ma et al., 1999; Xie and MacKenzie, 1986). However, N use efficiencies observed in these studies indicate that plant-available N from manure-derived compost is typically equal to or less than that from fresh manure. Timing of amendment application can influence crop responses but often interacts with weather conditions (Warman, 1995; Talarczyk et al., 1996; Sanchez et al., 1997).
Currently, no guidelines are available for when and in what form (composted or fresh) swine hoop manure should be field-applied to best utilize it as a nutrient resource and to minimize potential negative environmental impacts. The objective of this study was to determine first-year corn response to season of application (fall vs. spring) and form of swine hoop manure (composted or fresh).
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MATERIALS AND METHODS |
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Plant, Soil, and Amendment Sampling and Analysis
A 4-L composite sample of each amendment (fresh or composted manure) was collected immediately before materials were applied to plots, generating one sample per plot and four replicates per treatment. Samples were stored at –20°C in plastic freezer bags, then thawed, homogenized, separated for various analyses (total P, K, NH+4–N, NO–3–N, moisture, ash content, pH, and electrical conductivity), and then refrozen until individual parameters were analyzed. Amendment total C and N were determined after acidification with 0.5 M HCl (1:2 sample/solution ratio), air drying, grinding, and dry combustion in a Carlo-Erba NA1500 NCS elemental analyzer (Haake Buchler Instruments, Paterson, NJ) as described by Cambardella et al. (2003). Total P and K were determined on dried ground samples by USEPA method 3051 at a commercial laboratory (Midwest Laboratory, Omaha, NE) following a protocol given by Dancer et al. (1998). Ammonium N and nitrate N were determined using 2 M KCl extracts (1:80 amendment/solution ratio) and Lachat flow analysis (Lachat Instruments, Milwaukee, WI) (Keeney and Nelson, 1982). Amendment moisture content was determined by drying at 70°C for 48 h, ash content was determined by ignition at 550°C, and pH and electrical conductivity were determined using a 1:5 amendment/water slurry.
To monitor plant and soil N status throughout the growing season, late-spring soil NO–3–N concentration, ear leaf N and chlorophyll contents, and fall stalk NO–3–N concentration were measured. All plant and soil parameters were measured from the center three rows of each plot. Soil NO–3–N samples, consisting of a composite of ten 2-cm-diam. soil cores from the surface 30 cm, were collected from each plot on 3 June 2000 and 4 June 2001 and were processed according to procedures described by Blackmer et al. (1989).
Thirty leaf chlorophyll meter readings were taken in each plot using a Minolta SPAD-502 chlorophyll meter (Minolta, Ramsey, NJ) as others have done (Piekielek and Fox, 1992). Readings were taken 1.5 cm from the leaf edge of the center (lengthwise) of the topmost fully expanded leaf or the same location on the ear leaf, when developed.
Ten ear leaves were collected in each plot at growth stage R1 (Hanway, 1963) for nutrient analysis. Ear leaf samples were dried at 60°C for 4 d, ground to pass a 0.85-mm screen, and analyzed for total Kjeldahl N. Ear leaf P concentrations were determined by nitric acid plus peroxide digestion followed by inductively coupled plasma mass spectrometry (Harris Laboratory, Lincoln, NE). Grain was harvested with a combine from 9.8 and 10.7 m of the center three rows of each plot in 2000 and 2001, respectively. Reported grain yields are adjusted to a moisture content of 155 g kg–1. Fifteen stalk samples (20 cm in length) were collected 15 cm above the soil surface from each plot at grain harvest, dried at 60°C for 4 d, ground to pass a 0.85 mm screen, and analyzed for NO–3–N (Binford et al., 1992).
Statistical Analysis
Analysis of variance (ANOVA) was conducted using the PROC GLM routine of SAS (SAS Inst., 1999) to test for main and interaction effects, with blocks, years, and treatments in the model. Single degree-of-freedom contrasts were used to test specific hypotheses and main and interaction effects. Stalk nitrate concentrations were square-root–transformed before statistical analysis to meet the ANOVA assumption of homogeneity of variances. Correlations between soil and plant parameters were made on an experimental unit basis using PROC CORR in SAS. PROC REG of SAS was used to fit quadratic equations to the relationship between grain yields and urea N fertilizer rates.
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RESULTS AND DISCUSSION |
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Amendment Composition and Application
Carbon/N ratios of the applied amendments ranged from 10.7:1 to 14.2:1 with means of 12.5:1 and 11.6:1 for fresh and composted manures, respectively (Table 2). Materials with C/N ratios of less than 20:1 are generally thought not to immobilize soil N (Mathur et al., 1993) although short-term immobilization with partially composted hoop manure (C/N ratios of 12:1 to 15:1) has been observed (Cambardella et al., 2003). The amendments applied in the spring of 2001 had the highest C/N ratios, perhaps due to the cool and wet conditions of the fall–winter–spring period of 2000–2001, which may have slowed decomposition in the compost windrows. These weather conditions also likely increased the bedding requirement and/or altered the bedding management on the commercial farm from which the fresh manure applied in the spring of 2001 was obtained.
The ratio of NH+4–N to NO–3–N has been used as an indicator of compost maturity (Mathur et al., 1993), with lower ratios indicative of greater decomposition. The NH+4–N to NO–3–N ratios observed here suggest that the composted manure generally was more decomposed than the fresh manure; the exception being the manure applied in the spring of 2001, which had a more similar NH+4–N/NO–3–N ratio than at all other application times (Table 2).
Each of the applied amendments contained a substantial quantity of total P (Table 2). Annual applications of livestock manure to fields in corn–soybean rotations at rates sufficient to meet corn N requirements have the potential to accumulate soil P (Jackson et al., 2000) due to higher P application rates than grain P removal rates. In our study, the P application rate ranged from 79 to 242 kg P ha–1 (Table 3), with mean P application rates of 121 and 188 kg P ha–1 for fresh and composted hoop manure, respectively, and 167 and 142 kg P ha–1 for fall- and spring-applied amendments, respectively (Table 3). During 2000–2001, corn and soybean yields in Boone County, IA, averaged 9.7 and 2.7 Mg ha–1 (NASS, 2002), respectively, which would have removed an estimated 28 kg P ha–1 yr–1 for corn and 16 kg P ha–1 yr–1 for soybean (Voss et al., 1999). The combined P removal rate from one cycle of a corn–soybean rotation therefore would have been 44 kg P ha–1. A comparison of the P applied in this study with the estimated P grain removal indicates that one application of either fresh or composted hoop manure per rotation cycle would lead to soil P accumulation. It should be noted, however, that fresh hoop manure had a higher N/P ratio (Table 3), which would slow soil P accumulation compared with composted hoop manure if P removal rates for grain were equal in the two management systems.
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Late-Spring Soil Nitrate Concentration
The NO–3–N concentration in the surface 30 cm of soil when corn is 20 to 30 cm tall has been used in the midwest and northeast USA to predict corn yield response to N fertilizer (Blackmer et al., 1989; Magdoff, 1991). Although this method has been calibrated for synthetic N fertilizer sources and to a limited extent for soils amended with liquid swine manure (Hansen, 1999), it has not been calibrated for soils receiving solid livestock manure. In an evaluation of corn yield responses to variations in soil NO–3–N concentration, Blackmer et al. (1989) set the maximum soil NO–3–N concentration in the surface 30 cm at which to expect a yield response from applications of synthetic N fertilizer at 25 µg g–1 for unmanured soils in years with normal or below-normal spring precipitation, at 20 to 22 µg g–1 for unmanured soils in years with wet springs, and at 11 to 15 µg g–1 for manured soils.
In both years of our study, soil NO–3–N concentrations were higher in plots receiving manure than in the unamended fertilizer-free control (Table 4). A significant manure form x application time interaction was detected for soil NO–3–N concentrations in 2000 (Table 4), with the highest soil NO–3–N concentrations found in plots treated with spring-applied composted manure and the lowest found in plots amended with spring-applied fresh manure. The lower soil NO–3–N concentrations observed in 2001 compared with 2000 (Table 4) may have reflected the high soil moisture conditions before sampling (Fig. 1b), which could have caused nitrate leaching or denitrification losses.
Ear Leaf Nitrogen and Phosphorus Concentrations and Chlorophyll Meter Readings
Chlorophyll meter readings of corn ear leaves at growth stage R1 responded positively to urea application in both years (Table 5). A significant quadratic response to increasing rates of urea fertilizers (p < 0.001) was found in 2000, suggesting that N was not limiting in the higher urea application rates (120 and 180 kg N ha–1) at this point in the season (Table 5). However, because chlorophyll meters are useful for indicating N deficiencies, but not for determining excessive soil N availability (Schepers et al., 1992), this issue remains unresolved.
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Corn ear leaf N concentration at growth stage R1 responded positively to urea application in both years (Table 5) although the intensity of the response was greater in 2000 than in 2001. The mean ear leaf N concentration of all manure treatments was higher than that of the control in 2000, but no difference between manure treatments and the control was detected in 2001 (Table 5). The season of manure application was important for the 2001 corn crop; fall-applied manure generated higher ear leaf N concentrations than did spring-applied manure (Table 5).
Both the corn ear leaf N concentrations and chlorophyll meter readings at growth stage R1 correlated well with final corn grain yield (Table 5). Eghball and Power (1999) also found a strong correlation (r > 0.71) between chlorophyll meter readings and grain yield, except in a season of low precipitation. In our study, ear leaf N concentration and chlorophyll readings at R1 were also well correlated with each other (2000: r = 0.54, P < 0.01; 2001: r = 0.64, P < 0.0001).
Corn ear leaf P concentrations increased linearly with increasing rates of urea application in both years (Table 5). This may indicate that plants in the higher urea treatments foraged for soil P more efficiently and/or that the hydrolysis of urea lowered soil pH, thus making more soil P available to plants (Miller and Ohlrogge, 1958; Olson and Dreier, 1956). Differences in ear leaf P between years may have been due to differences in early-season soil moisture although many fertility and environmental factors can interact to influence ear leaf P concentrations (Voss et al., 1970). In 2001, there were minimal differences between treatments with regard to ear leaf P concentration (Table 5).
Corn Grain Yield
Corn grain yields increased in both years in response to increasing rates of urea application (Fig. 2; Table 6). The highest yields in response to urea application were similar in both years, but the yield of the control treatment was lower in 2000 than in 2001. This pattern was similar to that observed for the ear leaf N concentration at plant growth stage R1 and may reflect the influence of the previous year's crop on the quantity and quality of organic matter added to the soil and its N mineralization rate (Green and Blackmer, 1995). At 0 kg N ha–1, the 2000 corn crop, which followed oat, had a lower yield than the 2001 corn crop, which followed soybean (6.7 vs. 8.1 Mg ha–1).
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The poor yield response to spring-applied fresh manure was more pronounced in 2001 when early-season soil conditions were moist and cool relative to 2000. In Wisconsin, similar results were found in wet-cool springs if fresh solid dairy manure was applied immediately before corn planting (Talarczyk et al., 1996). Talarczyk et al. (1996) attributed this result to a pattern of manure N mineralization that was slower than normal. Fall application of solid manure in their study and in our study resulted in more consistent yield benefits than did spring applications. This may be due to more timely net N mineralization relative to plant N demand with fall application vs. spring application.
Nitrogen Fertilizer Equivalency and Nitrogen Supply Efficiency
A quadratic equation was fit to the yield data of urea N treatments for each year (Fig. 2). Although only the linear trend was statistically significant (Table 6), the quadratic function produced a better fit to the data and thus allowed for a more realistic extrapolation between the yield data of urea N fertilizer and manure treatments (see Blevins et al., 1990). Based on each quadratic urea response curve, N fertilizer equivalency values were calculated for each manure treatment mean (Table 7). Nitrogen supply efficiencies for the different manure treatments were calculated by dividing N fertilizer equivalency values by the total amount of N applied in each manure (Table 7). On average, fall application of manure gave higher N fertilizer equivalency values and higher N supply efficiencies than did spring application, and composted manure provided more consistent N benefits than did fresh manure. At the application rate used in this experiment, spring-applied fresh manure produced inconsistent N benefits.
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Fall Stalk Nitrate Concentration
Nitrate concentration in the lower portion of a corn stalk (the section between 15 and 35 cm above the soil surface) at plant maturity has been used as an indicator of late-season soil NO–3–N concentrations and/or environmental stress (Binford et al., 1992). A stalk NO–3–N concentration of >2000 µg g–1 indicates excessive soil NO–3 or stress whereas concentrations <200 µg g–1 indicate insufficient inorganic soil N for maximum economic grain yield (Binford et al., 1992).
In our study, urea application resulted in positive stalk NO–3 responses in both years (Table 5). The significant quadratic responses that were observed typically occur as plant-available soil N becomes greater than the plant's ability to assimilate NO–3 into amino acids (Binford et al., 1992). In both years, all manure treatments resulted in stalk NO–3–N concentrations <500 µg g–1, and the mean stalk NO–3–N concentration of manure treatments was not different from the control treatment (Table 5). In 2001, fresh-manure applications resulted in higher stalk NO–3–N concentrations than composted-manure applications, and fall applications gave higher stalk NO–3–N concentrations than did spring-applied manure. The relationship of stalk NO–3–N concentration to grain yield in 2000 followed closely the relationship described by Binford et al. (1992), but this pattern was not as distinct in 2001 (figure not shown). It is unclear if this was due to limited available soil N or increased NO–3 assimilation efficiencies. For example, in 2001, despite having similar yields, the fall-applied composted-manure treatment resulted in lower stalk NO–3–N concentrations than did the 120 and 180 kg N ha–1 urea N treatments. This suggests that factors other than N effects may have contributed to the grain yield response to manure.
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SUMMARY |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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In 2001, stalk NO–3–N concentrations in the manure treatments were low (<500 µg g–1) compared with the stalk NO–3–N concentrations of urea N treatments despite similar grain yields (Tables 5 and 6; Fig. 2). A similar pattern was observed in the soil NO–3–N concentrations in the late spring of 2001 relative to grain yield where manure treatments resulted in soil NO–3–N concentrations below levels predicted to provide for optimal yield despite similar yields to urea N treatments. This finding supports the concept that soils freshly amended with biologically active organic materials have different N dynamics than those amended with mineral N fertilizers (Magdoff, 1991; Cambardella et al., 2003). A more detailed examination of the seasonal N mineralization and crop N uptake patterns in response to fresh or composted hoop manure is needed to determine when and if supplemental N fertilizers may increase N use efficiencies.
Although we observed similar mean N supply efficiencies for fall-applied fresh manure (24.3%) and spring-applied compost (25.0%) (Table 7), the potential for large N losses during composting of fresh hoop manure (Garrison et al., 2001) suggests that fall-applied fresh manure may be more desirable than spring-applied compost for whole-farm N conservation. However, nitrate leaching potential could be relatively high with fall-applied fresh manure, which might result in negative impacts on water quality. The multiple pathways through which N may be lost following fall application of manure need to be studied for a more complete whole-farm N budget that considers both production and environmental endpoints.
In cases where producers remove fresh manure from hoop structures in the spring, composting the material for subsequent fall application appears to be a better strategy than spreading it immediately before planting corn since mean N supply efficiency was higher for the former management system (34.7%) than for the latter (10.9%) (Table 7). However, economic comparisons of manure management alternatives are needed to examine possible tradeoffs between composting costs, hauling distance to the field with the associated reduction in compost volume, and crop yield benefits. Economic and environmental analyses will complement the agronomic results presented here as all play critical roles in assessing the suitability and sustainability of solid manure management alternatives.
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARYREFERENCES |
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