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USDA-ARS, J. Phil Campbell Sr. Nat. Resour. Conserv. Cent., 1420 Experiment Station Rd., Watkinsville, GA 30677-2373
* Corresponding author (afranz{at}uga.edu)
Received for publication September 17, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Poultry production in the Appalachian Piedmont is extensive (Census of Agriculture, 1992). Poultry manure is often mixed with bedding material at the end of the production cycle, cleared from confinement housing, and applied as litter (manure plus bedding) to nearby land as a source of valuable nutrients for crop and pasture production. Depending upon management, however, repeated application of poultry litter to the same land could become a source of excessive nutrients threatening water quality and creating an imbalance in soil fertility (Sharpley et al., 1998). Compared with the recent concern for excessive P application with poultry litter, significantly less research has focused on the long-term changes in other soil properties with poultry litter application (Moore, 1998). Long-term (21 ± 4 yr), heavy application of poultry litter (11 ± 5 Mg ha–1 yr–1) to variably managed tall fescue (Festuca arundinacea Schreb). stands has been shown to increase soil (0- to 15-cm depth) pH by 8%, extractable-soil Mg by nearly twofold, extractable-soil K and Ca by nearly threefold, and extractable-soil Cu and Zn more than threefold (Kingery et al., 1994). However, the effects of moderate poultry litter application on soil pH and nutrient cations during early pasture development under controlled management scenarios has not been thoroughly addressed. In a 2-yr study on ‘Coastal’ bermudagrass in eastern Texas, poultry litter application (9 Mg ha–1 yr–1) raised extractable-soil nutrient levels (0- to 15-cm depth) 43% for K, 32% for Ca, and 73% for Mg compared with inorganic fertilization (Evers, 1998). However, it is likely that changes in soil nutrient cations with poultry litter application will depend on climate and soil.
Grazing of a forage crop compared with haying returns much of the nutrients with manure directly to the land, which should affect nutrient distribution in soil (Follett and Wilkinson, 1995). Bermudagrass cut for hay removed 30 ± 8% of the N, 12 ± 6% of the P, and 54 ± 11% of the K applied in poultry litter (9 Mg ha–1 yr–1) (Evers, 1998). The quantity of Cu and Zn harvested from 16 plant species at maturity and fertilized with 9 Mg ha–1 of poultry litter averaged 41 and 213 g ha–1 in aboveground biomass and 10 and 24 g ha–1 in belowground biomass, respectively (Pederson et al., 2002). As a percentage of total Cu and Zn applied, aboveground harvested portions only represented 5 to 6%, suggesting that a low percentage of secondary plant nutrients may be removed even with harvested biomass.
The impact of whether forage is mechanically harvested or not on soil nutrients deserves attention, based on the extent of land currently managed under the Conservation Reserve Program offered by the government of the USA. How forage is utilized would be expected to alter the distribution of nutrients among soil depths because of the presence or absence of animal traffic, ruminant processing of forage (i.e., biological transformation of nutrients), and nutrient removal in hay. There are very few investigations that have reported comparisons of grazed vs. ungrazed forages on soil nutrient levels with time, despite the large economic impact of cattle production and extensive land area devoted to grazing lands (Phillips, 1998). At the end of 10 yr of differential stocking densities of sheep (Ovis aries), soil inorganic components were relatively unchanged in New Zealand (Scott, 2000).
We hypothesized that with equivalent amounts of total N applied, fertilization strategy (i.e., inorganic vs. organic) might affect the availability of nutrients to forage and thereby lead to an alteration in form and depth distribution in soil. We wanted to characterize soil nutrient dynamics and depth distribution in response to typical fertilization regimes based on equivalent N supply, which inherently have differences in diversity and quantity of secondary elements applied since fertilizer rates in most forage systems are still based on N requirements. Within those fertilization regimes, we wanted to ascertain the impact of forage utilization (i.e., grazed vs. ungrazed) on soil pH and extractable- and total-soil nutrient cations during the first 5 yr of grass management following conversion from long-term cultivated cropland.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Experimental Design
The experimental design was a randomized complete block with treatments in a split-plot arrangement in each of three blocks, which were delineated by landscape features (i.e., slight, moderate, and severe erosion classes). Main plots were fertilization regime (n = 3), and split plots were forage utilization regime (n = 4) for a total of 36 experimental units. Individual paddocks were 0.69 ± 0.03 ha. Spatial design of paddocks minimized runoff contamination and handling of animals through a central roadway. Each paddock contained a 3- by 4-m shade, mineral feeder, and water trough placed in a line 15 m long at the highest elevation. Exclosures (100 m2) (unharvested and hayed) were placed side by side in paired low- and high-grazing pressure paddocks of each fertilization regime.
Fertilization was targeted to supply 200 kg N ha–1 yr–1 using one of the following strategies: (i) inorganically as NH4NO3 broadcast in split applications in May and July, (ii) with half assumed fixed and released by crimson clover cover crop during the winter–spring and the other half as NH4NO3 broadcast in July, and (iii) by broiler litter broadcast in split applications in May and July (Table 1). Phosphorus and K applications varied among treatments because excess P and K were applied with broiler litter to meet N requirements while inorganic phosphate and potash were applied based on soil-testing recommendations. Dolomitic limestone (2.2 Mg ha–1 event–1) was applied based on soil testing to inorganic-only and clover + inorganic treatments in February 1995 and only to the inorganic treatment in November 1996. Crimson clover was direct-drilled in clover treatments at 
10 kg ha–1 in October each year. All paddocks were mowed in late April following soil sampling and residue allowed to decompose [i.e., clover biomass in the clover + inorganic treatment and winter annual weeds (primarily Lolium multiflorum Lam. and Bromus catharticus Vahl.) in the other treatments].
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Sampling and Analyses
Soil was sampled in November 1994 (end of Year 1), February 1996 (end of Year 2), October 1996 (end of Year 3), and October 1997 (end of Year 4) at depths of 0 to 6 and 6 to 15 cm with a 4.1-cm (i.d.) hydraulic probe. Sampling locations within grazed paddocks were within a 3-m radius of points on a 30-m grid. Due to the nonuniform dimensions of paddocks, sampling frequencies within a paddock varied from four to nine, averaging 7 ± 1. Two sampling locations were fixed within each hayed and unharvested exclosure. Soil was air-dried and ground to <2 mm in a mechanical grinder. In February 1999 (end of Year 5), soil was sampled in two different manners: (i) for extractable-soil nutrient cations from a composite of three cores (4.1-cm diam.) collected at a depth of 0 to 15 cm along each of three arcs at 5, 30, and 70 m from shades within grazed paddocks and from a composite of two cores randomly collected within unharvested and hayed exclosures and (ii) for total-soil nutrient cations from a composite of eight randomly selected areas within each of three zones within paddocks (i.e., 0- to 30-, 30- to 70-, and 70- to 120-m distances from shades) and within each exclosure at depths of 0 to 3 and 3 to 6 cm following removal of surface residue. Sampling protocol in Year 5 was changed to accommodate other objectives related to spatial distribution of soil properties in response to animal behavior. Soil was oven-dried (55°C, 72 h) and gently crushed to pass a 4.75-mm screen in 1999. Subsamples within a paddock were composited before laboratory analyses in all years.
Soil bulk density was calculated from the oven-dried soil weight and coring device volume for samples at a depth of 0 to 6 cm. Bulk density at a depth of 6 to 15 cm was not determined but assumed to be 1.5 Mg m–3. Soil pH was determined in 1:2 soil/water (w/v) with a glass electrode. Extractable-soil cations at depths of 0 to 6 and 6 to 15 cm at the end of Years 1 to 4 and at a depth of 0 to 15 cm at the end of Year 5 were determined in double acid (0.05 M HCl + 0.0125 M H2SO4) with inductively coupled plasma spectroscopy (ICPS). Total elemental analysis of soil collected at depths of 0 to 3 and 3 to 6 cm at the end of Year 5 was determined with ICPS following digestion (1 g of soil in a mixture of 10 mL of concentrated nitric acid + 5 mL of concentrated perchloric acid) with heating on a hot plate for 5 min beyond the time when white, dense perchloric acid fumes appeared. The University of Georgia Agricultural and Environmental Services Laboratory conducted all pH and ICPS analyses.
Analysis of variance was conducted for each depth within a year separately to identify differences due to main effects of fertilization (three levels) and forage utilization (four levels) and their interactions (SAS Inst., 1990). Across-depth analyses were from sums adjusted for bulk density and soil volume of depth increments. Across-year analyses considered year as an additional blocking criterion. Preplanned orthogonal contrasts were (i) inorganic/clover + inorganic vs. broiler litter and (ii) inorganic vs. clover + inorganic for the fertilization main effect and (i) unharvested/hayed vs. low and high grazing pressure, (ii) unharvested vs. hayed, and (iii) low vs. high grazing pressure for the forage utilization main effect. Contrasts for interactions were factorial combinations of the main-effect comparisons. Linear regression of soil properties with time was used to test for (i) significant changes with time and (ii) significant differences in soil responses with time assuming a common intercept. All effects were considered significant at p 
0.1.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Extractable-soil Ca and Mg concentrations at 0- to 15-cm depth were unaffected by fertilization regime throughout the study (Fig. 1). At a depth of 0 to 15 cm, extractable-soil Ca remained unchanged with time, but extractable-soil Mg declined significantly under all fertilization regimes (Table 3). Plant uptake, leaching, and mineral fixation were likely reasons for this decline in Mg concentration with time.
Extractable-soil Zn, Mn, and Cu concentrations at 0- to 15-cm depth were significantly greater with broiler litter than other fertilization regimes, primarily beginning at the end of 3 yr of management (Fig. 1). Broiler litter supplied a small quantity of these elements (2 to 5 kg ha–1 yr–1; Table 1). At a depth of 0 to 15 cm, extractable-soil Zn and Cu remained unchanged with time under inorganic and clover + inorganic fertilization regimes but increased significantly with time under broiler litter fertilization (Table 4). Extractable-soil Mn declined significantly with time under inorganic and clover + inorganic fertilization regimes but remained unchanged with time under broiler litter fertilization. The supply of Zn in broiler litter (1.1 mg kg–1 yr–1) was matched almost entirely with an increase in extractable-soil Zn at a depth of 0 to 15 cm (1.0 mg kg–1 yr–1) (Tables 1 and 4). As expected, accumulation of extractable-soil Mn and Cu was much less than the rates supplied in broiler litter (<20%), due likely to significant elemental transformations with soil minerals and organic matter.
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Extractable-soil Ca, Zn, and Mn concentrations were not affected by forage utilization regime throughout the 5 yr (Fig. 2). At the end of 3 yr and onwards, extractable-soil Mg and Cu were higher under grazed than under ungrazed management systems. At a depth of 0 to 15 cm when averaged across fertilization regimes, extractable-soil Ca and Cu remained relatively unchanged with time (Tables 3 and 4). However, there was a significant increase in extractable-soil Ca at the soil surface (0 to 6 cm) with unharvested and low and high grazing pressures, suggesting that redistribution of Ca toward the organic matter–enriched surface soil was occurring. Our results of similar extractable-soil Ca concentration whether forage was unharvested or grazed are supported by observations of similar extractable-soil Ca between moose-grazed plots and 40-yr-old exclosures (Pastor et al., 1993). This result contrasts with the observations of Scott (2000), who found that extractable-soil Ca tended to be reduced with higher sheep grazing intensity.
At a depth of 0 to 15 cm when averaged across fertilization regimes, extractable-soil Mg and Mn concentrations declined significantly with time under all forage utilization regimes (Tables 3 and 4). Extractable-soil Zn increased significantly with time under all forage utilization regimes with broiler litter fertilization only, but more so under unharvested and high grazing pressures. The change in extractable-soil Zn with time at a depth of 0 to 6 cm was inversely proportional to the intensity of forage utilization, i.e., largest increase with unharvested management (no utilization), moderate increase with low and high grazing pressure (low and moderate utilization), and smallest increase with haying (high utilization). This result corroborates the observation of significantly greater Zn contained in stems of forages than in leaves (Pederson et al., 2002). Bermudagrass stem production and subsequent recycling of Zn in the paddock would have been greatest with unharvested forage followed by low grazing pressure and high grazing pressure. Haying would have removed Zn from the field.
Excluding sheep and rabbit (Lepus spp.) grazing from a pasture in New Zealand for 16 yr led to significantly higher extractable Mg in the surface 7.5 cm of soil compared with grazing but no effect on extractable-soil K and Ca (McIntosh and Allen, 1998). Excluding moose browsing for 40 yr also resulted in significantly higher extractable-soil Mg compared with grazing at two of three sites in a national park in Michigan (Pastor et al., 1993). In our study, the comparison between unharvested management and the two grazing pressures produced a lack of response to grazing for extractable-soil K, Ca, Zn, Mn, and Cu (Tables 3 and 4). In contrast to the results of the aforementioned studies, extractable-soil Mg with unharvested management declined more with time compared with grazing. It is unclear why our results differed so dramatically from previous studies with regards to Mg and were more consistent with regards to other cations.
Total-Soil Nutrient Cations
At the end of 5 yr of bermudagrass management, total-soil nutrient cations were variably affected by fertilization and forage utilization regimes (Table 5). The primary effect of fertilization regime was for greater quantity of total-soil Fe, Ca, and Mg under inorganic than under clover + inorganic or broiler litter fertilization. This effect was significant at a depth of 0 to 3 cm for all three elements and also at a depth of 3 to 6 cm for total-soil Fe. Concentration of total-soil Al and Fe were highly related to the concentration of the clay-sized fraction in soil (Fig. 3) while total-soil K, Ca, Mg, and Na were not closely related to clay concentration (r2 0.23). Soils with high clay concentration tend to have high element concentrations other than Si (Helmke, 2000). Although three of the highest total-soil Fe concentrations occurred with inorganic fertilization, the distribution of total-soil Fe concentration within the inorganic fertilization treatment was similar to that of other fertilization treatments (Fig. 3), such that clay concentration (as a proxy for landscape position and soil erosion effect) was not likely the only explanation for the differences observed among treatments although an adequate explanation remains elusive. The wide distribution of clay, Al, and Fe concentrations among samples illustrates (i) the diversity of soil characteristics within a relatively small area from the variably eroded, sloping region of the Southern Piedmont USA and (ii) the landscape complexity that soil research in this region should be addressing.
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The other primary fertilization effect was for greater total-soil Na under broiler litter than under inorganic or clover + inorganic fertilization regimes at both soil depths (Table 5). Broiler litter supplied the equivalent of 13 mg Na kg–1 (0 to 15 cm) yr–1, but accumulation of total-soil Na in the surface 6 cm was only about 15% of that applied, assuming the concentration under broiler litter was equal to that under other fertilization regimes and that no change in total-soil Na had occurred with time under inorganic or clover + inorganic fertilization. Leaching of Na below the soil surface likely occurred.
There were no significant fertilization effects on total-soil Al and K. Application of Al with broiler litter was minor, amounting to only 2% of the least significant difference (Tables 1 and 5). Despite three times higher application rate of K with broiler litter than with other fertilization treatments, total-soil K was not affected by fertilization regime, and this lack of difference may have been due to the high demand for K by bermudagrass forage and its susceptibility to leaching. The lack of difference in total-soil K also contrasts with the positive effect of broiler litter fertilization on extractable-soil K.
The primary effect of forage utilization on total-soil nutrient cations was for higher levels with high grazing pressure compared with other treatments (Table 5). Total-soil Al, Fe, K, and Na at both 0- to 3- and 3- to 6-cm depths were greater with high grazing pressure than with other forage utilization regimes when averaged across fertilization regimes. Total-soil Ca was greater with high grazing pressure than with other forage utilization regimes only at a depth of 0 to 3 cm. Interestingly, total-soil Mg was not different among forage utilization regimes at either depth. These results suggest that continuous close grazing with high grazing pressure by cattle (but not overgrazing) kept soil nutrients concentrated near the soil surface through a closely integrated cycling of nutrients from soil to plant to animal to soil. The concomitant increase in organic matter at the immediate soil surface with high grazing pressure (Franzluebbers et al., 2001) appears to have acted as a sponge or filter that may have become increasingly reactive for cation transformations into stable organo-mineral complexes (Stevenson and Fitch, 1986).
The only other significant forage utilization effect on total-soil nutrient cations when averaged across fertilization regimes was for greater total-soil K under low grazing pressure than under haying at a depth of 0 to 3 cm (Table 5). This effect contributed to the only significant interaction between fertilization and forage utilization regimes on total-soil nutrient cations and was primarily due to the significantly positive grazing pressure response of total-soil K under clover + inorganic fertilization, mildly positive response under broiler litter fertilization, and no response under inorganic fertilization. The heavy demand for K with hay removal and the continuous recycling of K on the pasture with either low or high grazing pressure would explain the significant difference between hayed and grazed treatments.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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