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USDA-ARS, J. Phil Campbell Sr. Natural Resource Conservation Center, 1420 Experiment Station Road, Watkinsville, GA 30677-2373
Corresponding author (afranz{at}arches.uga.edu)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: CRP, Conservation Reserve Program
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Restoration of eroded cropland in the southeastern USA is possible with conservation tillage systems, which minimize soil disturbance and maximize surface residue accumulation (Langdale et al., 1992). In the Southern Piedmont region, however, an increasing portion of land supports small-farm, cattle-grazing production systems (Census of Agriculture, 1992). Despite the abundance and importance of managed pastures in the southeastern USA, relatively little information is available to describe rates of soil organic C and N accumulation under pasture management systems (Schnabel et al., 2001).
Grazing of a forage crop compared with haying returns much of the manure directly to the land with a positive impact on soil organic C and N accumulation (Franzluebbers et al., 2000), but the impact of stocking density on plant productivity, soil compaction, and soil organic C and S cycling is not well understood. Further, the impact of not harvesting forage on soil organic C and S deserves attention, based on the extent of land currently managed under the Conservation Reserve Program (CRP). Harvest management would be expected to alter the distribution of C and S among surface residue and the soil profile because of the effects of animal traffic, ruminant processing of forage, and forage removal.
The effect of fertilization strategy on soil organic C dynamics in managed pastures is variable (Schnabel et al., 2001). In some cases, increased fertilization may improve forage yield but have little effect on soil organic C (Owensby et al., 1969; Jenkinson, 1988; Ross et al., 1995). In other cases, increased fertilization improves both forage yield and soil organic C in the long term (Schwab et al., 1990; Haynes and Williams, 1992; Malhi et al., 1997). The impact on soil organic C and S dynamics of whether fertilization comes from an organic or an inorganic source has received limited attention, but is a very important issue in the southeastern USA, where poultry production and associated availability of manure are abundant. Soil organic C was little affected whether grass received manure or inorganic fertilizer in a long-term experiment at Rothamsted (Jenkinson, 1988). However, greater accumulation of soil organic C was observed under fertilized ryegrass (Lolium perenne L.) than under ryegrass–white clover (Trifolium repens L.) (Hatch et al., 1991). Much more work is needed to understand the sequestration of soil organic C and S in response to organic and inorganic amendments to grazed and ungrazed pastures.
We hypothesized that with equivalent amounts of total N applied, fertilization strategy (i.e., inorganic and organic) could affect the availability of N to forage and could therefore affect the quality and quantity of forage, leading to differences in soil organic C and S sequestration rates. In addition, we wanted to ascertain the impact of forage harvest strategy (i.e., grazed and ungrazed) on soil compaction and cycling of C and S 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 METHODSNOTESRESULTS 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 strategy (n = 3) and split-plots were harvest strategy (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 near the top of the landscape. Unharvested and hayed exclosures within each paddock were 100 m2.
Fertilization strategy consisted of (i) inorganic only (
20 g N m-2 yr-1 as NH4NO3 broadcast in split applications in May and July), (ii) crimson clover cover crop plus supplemental inorganic fertilizer (20 g N m-2 yr-1 with one-half of the N assumed fixed by clover biomass and the other half as NH4NO3 broadcast in July), and (iii) broiler litter (20 g N m-2 yr-1 broadcast in split applications in May and July). Details of fertilizer applications each year are reported in Table 1. Phosphorus and K applications varied among treatments because excess P and K were applied with broiler litter (12.4 ± 4.0 g P m-2 yr-1 and 16.7 ± 4.8 g K m-2 yr-1) to meet N requirements, while diammonium phosphate and potash were applied based on soil testing recommendations (1.6 ± 1.1 g P m-2 yr-1 and 5.2 ± 4.1 g K m-2 yr-1 for inorganic fertilizer and 2.3 ± 2.0 g P m-2 yr-1 and 5.5 ± 4.1 g K m-2 yr-1 for crimson clover cover crop plus supplemental inorganic fertilizer). Crimson clover was direct drilled in clover treatments at 1 g m-2 in October each year. All paddocks were mowed in late April following soil sampling and residue allowed to decompose [i.e., clover biomass in clover plus inorganic treatment and winter annual weeds (primarily Lolium annuum L. and Bromus catharticus Vahl.) in other treatments].
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300 g m-2 of available forage), (iii) high grazing pressure (put-and-take system to maintain a target of 150 g m-2 of available forage), and (iv) hayed monthly to remove aboveground biomass at 4-cm height. Yearling Angus steers (Bos taurus) grazed paddocks during a 140-d period from mid May until early October each year, except during the first year of treatment implementation (1994) when grazing began in July due to repairs to infrastructure following a tornado. No grazing occurred in the winter. Animals were weighed, available forage determined, and paddocks restocked on a monthly basis.
Sampling and Analyses
Soil and surface residue were sampled in April prior to grazing and in October following grazing during most years. Hayed and unharvested exclosures were sampled in July, rather than May during 1994. 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 sites within a paddock varied from as few as four to as many as nine, averaging seven ± one. Two sampling locations were fixed within each hayed and unharvested exclosure. Surface residue was collected from a 0.25-m2 area at each sampling point following removal of vegetation at a height of 
4 cm. Surface residue, including plant stubble, was cut to the mineral surface with battery-powered hand shears, bagged, and dried at 70°C for several days. During 1994 and 1995, soil was sampled at depths of 0 to 2, 2 to 4, and 4 to 6 cm from the composite of two 8.5-cm-diam. cores within each sampling location. From spring 1996 until the spring of 1998, soil was sampled to the same depths from the composite of nine 4.1-cm-diam. cores within each sampling location. Soil was air dried and ground to <2 mm in a mechanical grinder in 1994 and 1995. Soil was oven dried (55°C, 72 h) and gently crushed to pass a 4.75-mm screen in all other years.
Beginning in February 1999, sampling strategy was changed (i) to collect surface residue and soil only once per year, (ii) to more directly address the zonal changes in pastures in response to animal behavior near shade and water sources, and (iii) to collect soil to deeper depths. Surface residue was collected from a composite of eight 0.04-m2 areas randomly selected within each of three zones within paddocks (i.e., 0–30, 30–70, and 70–120 m distances from livestock shades) and within each exclosure. Surface residue was processed as described previously. A single 4.1-cm-diam. soil core was collected from each of the eight residue sampling sites and composited. Soil was collected at depths of 0 to 3, 3 to 6, 6 to 12, and 12 to 20 cm, oven dried (55°C, 72 h), and gently crushed to pass a 4.75-mm screen.
Soil bulk density was calculated from the oven-dried soil weight (55°C) and pooled-core volume (2.26–8.45 x 10-4 m3, depending on depth of sampling). During 1994 and 1995, soil was collected by scooping to a particular depth by a highly experienced technician. To mechanize the process independent of experience, a tray with slots at 2, 4, and 6 cm for cutting soil sections with precision was used in 1996, 1997, and 1998. In 1999, soil was cut to depth inside the sampling tube. Surface residue was ground to <1 mm and a 20- to 30-g soil subsample from each composite sample was ground to a fine powder in a ball mill for 3 min prior to analysis of total C and S with dry combustion at 1350°C (Leco CNS-2000, St. Joseph, MI).1 It was assumed that total C was equivalent to organic C because soil pH was near 6.
Data from multiple samples within an experimental unit were averaged and not considered as a source of variation in the analysis of variance (SAS Institute, 1990). Within-depth, across-depth, within-year, and across-year analyses were conducted according to the split-plot design with three blocks. Across-depth analyses considered the bulk density of soil in calculating standing stock values of soil organic C and total S. Across-year analyses considered years as repeated measures. Effects were considered significant at P 
0.1.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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In February 1999 at the end of 5 yr of management, soil organic C concentration continued to be greater under grazed than under ungrazed management in all three fertilization strategies at a depth of 0 to 3 cm (Fig. 4). At this depth, soil organic C concentration under low grazing pressure was greater than under high grazing pressure with inorganic fertilization. Also, soil organic C concentration under unharvested management was greater than under hayed management with clover and broiler litter fertilization, but not with inorganic fertilization. Below 3 cm, soil organic C concentration was unaffected by harvest and fertilization strategies.
The standing stock of soil organic C averaged across fertilization strategies increased with time under forage management the most at a depth of 0 to 2 cm, intermediately at a depth of 2 to 4 cm, and not at all at a depth of 4 to 6 cm (Fig. 5) . Both low and high grazing pressures sequestered three to six times more soil organic C than unharvested and hayed management at depths of 0 to 2 and 2 to 4 cm. Unharvested management sequestered approximately twice the soil organic C as hayed management, in which aboveground biomass was removed. Fertilization strategy had significant, but inconsistent effects on the standing stock of soil organic C at a depth of 0 to 6 cm, depending on harvest strategy and time of sampling (Table 2).
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The standing stock of total soil S at a depth of 0 to 6 cm was greater under broiler litter than under inorganic and clover fertilization strategies beginning in April 1996 and continuing each sampling event thereafter (Table 3). Broiler litter fertilization added an average of 2.2 g m-2 yr-1 of S (Table 1), of which 76% was retained in the standing stock of total S in soil (0–6 cm) and residue when averaged across harvest management strategies (calculating the difference between broiler litter and inorganic fertilization with time). Soil organic matter formations would have sequestered a part of this added S, but unlike soil organic C dynamics with a dominant atmospheric flux, the mineralization of organic S leads to sulfate accumulation in soil solution, which can be either leached or retained in soil (Stevenson and Cole, 1999). It appears that only a small fraction of the S added in broiler litter was leached beyond the upper 6 cm of soil.
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Surface Residue Carbon and Sulfur
Surface residue C and S contents were relatively small, but significant components of the total standing stock of C and S (Tables 2 and 3). Surface residue C content averaged 10, 31, 19, 13, 7, and 9% of total standing stock of C in 1994, 1995, 1996, 1997, 1998, and 1999, respectively. Surface residue S content averaged 7, 13, 15, 10, 7, and 5% of total standing stock of S in those same years, respectively.
At the beginning of forage management in April 1994, surface residue C and S contents were little affected by fertilization and harvest management strategies (Tables 2 and 3). Beginning in April 1995, surface residue C and S contents were inversely proportional to the level of forage utilization; that is, surface residue C and S (i) were highest with unharvested management in which forage was cut and left on the soil surface to decompose, (ii) decreased with increasing level of grazing pressure, and (iii) were lowest with hay removal (Tables 2 and 3; Fig. 6) . The lower surface residue C and S contents under grazing compared with unharvested management and under high grazing pressure compared with low grazing pressure implies that cattle processed a large amount of forage and reduced the residence time of C and S flowing from forage to residue to soil. Combined with the observation of greater soil organic C and total S accumulation under grazing than under unharvested management, it can be concluded that cattle grazing shunted C more directly from forage to the soil organic C pool compared with nonutilization of forage (Fig. 6). Grazing animals consume and utilize digestible components of forage and excrete more resistant fractions of forage (Fisher et al., 1995). Cattle grazing appears to have benefitted the storage of C in soil, at least in the first 5 yr of forage management. The positive effect of cattle grazing compared with unharvested management on soil organic C storage was also observed on a mixed-grass prairie at the end of 11 yr in Wyoming (Manley et al., 1995). At the end of 15 to 19 yr of bermudagrass management in Georgia, grazed pastures had 0.18 kg surface residue C m-2 and hayed fields had 0.12 kg surface residue C m-2 (Franzluebbers et al., 2000), similar in magnitude and effect to our observations at the end of 5 yr of bermudagrass management (Table 2).
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Soil Organic Carbon Sequestration
The rate of accumulation in soil organic C during the first 5 yr under hayed management (29 g m-2 yr-1; Fig. 6) was very similar to the estimated rate under hayed bermudagrass in a chronosequence study at a nearby location (35 g m-2 yr-1 interpolated from the first 5 yr; Franzluebbers et al., 2000). In addition, our results of greater soil C sequestration under grazing compared with haying (a difference of 111 g m-2 yr-1; Fig. 6) are somewhat higher than from observations between grazed and hayed bermudagrass at a nearby location (42 g m-2 yr-1 during a 15–19 yr comparison; Franzluebbers et al., 2000). It could be expected that C sequestration rates in this previous study were higher during the initial 5 yr than those 5 to 10 yr later.
Soil organic C sequestration under unharvested management (65 g m-2 yr-1; Fig. 6) was similar to the estimated rate of soil organic C sequestration during 5 yr of unharvested grass management under CRP at six locations in Kansas, Nebraska, and Texas (58 ± 66 g m-2 yr-1 at a depth of 0 to 20 cm, 39 ± 47 g m-2 yr-1 at a depth of 0 to 10 cm, and 25 ± 30 g m-2 yr-1 at a depth of 0 to 5 cm; Gebhart et al., 1994). We fertilized the unharvested management system to obtain a more direct comparison with other harvest management strategies, but most landowners are unlikely to fertilize unharvested grass in CRP on a yearly basis. Fertilization may have increased the rate of soil organic C sequestration by allowing more plant biomass to accumulate. However, our observation of greater soil organic C sequestration under grazing compared with unharvested management is consistent with observations in a semiarid mixed grass prairie in Wyoming (Manley et al., 1995).
For the most part, broiler litter application did not affect soil organic C accumulation compared with inorganic and clover plus inorganic fertilization strategies. This was probably due to the relatively low rate of application (i.e., 0.54 ± 0.06 kg dry mass m-2 yr-1; Table 1). However, no change in soil organic matter was observed at the end of 2 yr following a single application of either 2.2 or 13.4 kg m-2 of broiler litter on a similar Cecil sandy loam (Jackson et al., 1977). In contrast, broiler litter application (1.09 ± 0.54 kg dry mass m-2 yr-1) resulted in greater soil organic C concentration at a depth of 0 to 15 cm than without broiler litter in a survey of 12 paired pastures in northern Alabama at the end of 21 ± 4 yr (Kingery et al., 1994). The estimated mean rate of soil organic C accumulation due to broiler litter application in this Alabama survey was 30 g m-2 yr-1, suggesting a retention rate in soil of 8% of applied C in broiler litter.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Received for publication August 22, 2000.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSNOTESRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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