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TECHNICAL REPORTS

Surface Water Quality

Evaluating Aeration Techniques for Decreasing Phosphorus Export from Grasslands Receiving Manure

^a Univ. of Georgia, Dep. of Crop and Soil Sciences, 3111 Miller Plant Sciences Bldg., Athens, GA 30602
^b Natural Resource Conservation Center, USDA-ARS, 1420 Experiment Station Rd., Watkinsville, GA 30677
^c Univ. of Georgia, Agricultural and Environmental Services Lab., 2400 College Station Rd., Athens, GA 30602
^d Mississippi State Chemical Lab., Mississippi State, MS 39762

^* Corresponding author (dory.franklin{at}ars.usda.gov).

Received for publication June 1, 2007.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Because surface-applied manures can contribute to phosphorus (P) in runoff, we examined mechanical aeration of grasslands for reducing P transport by increasing infiltration of rainfall and binding of P with soil minerals. The effects of three aeration treatments and a control (aeration with cores, continuous-furrow "no-till" disk aeration perpendicular to the slope, slit aeration with tines, and no aeration treatment) on the export of total suspended solids, total Kjeldahl P (TKP), total dissolved P (TDP), dissolved reactive P (DRP), and bioavailable P (BAP) in runoff from grasslands with three manure treatments (broiler litter, dairy slurry, and no manure) were examined before and after simulated compaction by cattle. Plots (0.75 x 2 m) were established on a Cecil soil series with mixed tall fescue (Festuca arundinacea Schreb.)-bermudagrass [Cynodon dactylon (L.) Pers.] vegetation on 8 to 12% slopes. Manures were applied at a target rate of 30 kg P ha^–1, and simulated rainfall was applied at a rate of 85 mm h^–1. Although the impact of aeration type on P export varied before and after simulated compaction, overall results indicated that core aeration has the greatest potential for reducing P losses. Export of TKP was reduced by 55%, TDP by 62%, DRP by 61%, total BAP by 54%, and dissolved BAP by 57% on core-aerated plots with applied broiler litter as compared with the control (p < 0.05). Core and no-till disk aeration also showed potential for reducing P export from applied dairy slurry (p < 0.10). Given that Cecil soil is common in pastures receiving broiler litter in the Southern Piedmont, our results indicate that pairing core aeration of these pastures with litter application could have a widespread impact on surface water quality.

Abbreviations: BAP, bioavailable phosphorus • DBAP, dissolved bioavailable phosphorus • DRP, dissolved reactive phosphorus • STP, soil test phosphorus • TBAP, total bioavailable phosphorus • TDP, total dissolved phosphorus • TKP, total Kjeldahl phosphorus • TSS, total suspended solids • WSP, water-soluble phosphorus

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
THE poultry and dairy industries are significant components of agricultural production in the Southern Piedmont, USA. In this region, manures associated with these industries are typically surface-applied to pastures as a fertilizer. Phosphorus in surface-applied manures can accumulate at the soil surface and can be transported to surface waters in agricultural runoff (Kuykendall et al., 1999). Additionally, P associated with agricultural nonpoint pollution has the potential to contribute to eutrophication of surface waters (Carpenter et al., 1998; Hubbard et al., 2004; Sharpley and Rekolainen, 1997).

Agricultural professionals have developed several strategies to reduce the amount of available P transported to surface waters. One strategy is incorporation of manures into the soil (Bundy et al., 2001; Little et al., 2005; Shah et al., 2004b) to facilitate binding of P with soil minerals and to reduce the amount of nutrients remaining at the soil surface where they are susceptible to transport in runoff. Nichols et al. (1994) incorporated poultry litter on tall fescue plots using shallow (2.5-cm) rotary tillage, but subsequent P export in runoff was no different than surface-applied litter, perhaps due to the shallow incorporation by tillage. Disturbance of the soil surface by tillage, regardless of depth, may negatively affect forage production and may cause a higher risk of P transport due to erosion vulnerability. A potential solution is partial soil disturbance or banding of manures into the soil, which has been reported to reduce nutrient loss (Ross et al., 1979; Thompson et al., 1987).

Mechanical aeration partially disturbs the soil surface and has generally not reduced forage productivity (Burgess et al., 2000; Chen et al., 2001; Malhi et al., 2000; Shah et al., 2004a). Several types of aeration implements have been used to disturb or puncture the soil surface in grasslands. Types of aeration created by these implements may include slit aeration by tines (Bittman et al., 2005; Franklin et al., 2006; Franklin et al., 2007; Harrigan et al., 2006; Shah et al., 2004a; van Vliet et al., 2006), disk aeration using no-till drills (Little et al., 2005), or core aeration by cylindrical cores (Callahan et al., 1998; Hartwiger and O'Brien, 2001; Kraft et al., 2004). All have the potential to partially incorporate applied manures, allow for more P adsorption to soil minerals, increase infiltration by breaking the soil surface, and slow runoff flow by increasing the roughness of the landscape.

In West Virginia, Shah et al. (2004a) examined the impacts of slit aeration (15-cm depth, 0° offset) on a well drained silt loam soil (Alfisol), but with applied dairy slurry. Over six simulated rain events, aeration reduced concentrations of total P (TP) by 13% and dissolved reactive P (DRP) by 29% in runoff at p 0.11. Similarly, mass TP export was reduced by 37% and DRP by 47%. In another study involving applied dairy slurry, van Vliet et al. (2006) reported that slit aeration (14-cm depth, rotating at 2.5° offset) reduced annual runoff volumes on grasslands by 47 to 81% and total suspended solids (TSS) export by 48 to 69% on somewhat poorly drained silt loam (alluvial Inceptisol). Total P export was reduced by 25 to 75% and DRP export was reduced by 60 to 96%.

Two studies in Georgia, one at the plot scale with simulated rainfall and one at the field scale with natural rainfall, examined the impacts of slit aeration on runoff volumes and nutrient export. At the plot scale, although only significant at p = 0.16, there was a 27% decrease in runoff volume from slit-aerated plots (6-cm depth, 0° offset) compared with non-aerated plots on a moderately well drained Ultisol (Franklin et al., 2006). Mass export of DRP and TP were unaffected by aeration treatment. This lack of impact may be because slit aeration was done parallel to the slope rather than perpendicular to the slope. At the field-scale on well drained soils (Ultisols), slit aeration (10- to 12-cm depth, 0° offset) significantly reduced runoff volume and DRP export (Franklin et al., 2007). However, this was not the case on poorly drained soils (Ultisols and Alfisols) because slit aeration increased runoff volume and P export.

Disk aeration uses methods similar to no-till or conservation tillage seeding of crops, which disrupts the soil surface in a series of parallel rows. Although there are no studies examining the impact of no-till disk aeration on grasslands, Little et al. (2005) examined different manure incorporation methods on a clay loam soil (Mollisol) in Alberta, Canada. Using a single pass of a double-disk (10- to 15-cm depth, parallel to the slope) as one of the treatments, the researchers reported a 14% reduction in TP losses from applied beef cattle manures compared with no incorporation.

On a silt loam (Ultisol) in Arkansas, Pote et al. (2003) examined the impacts of a similar continuous-furrow aeration technique (8-cm depth). Aeration furrows were created by using a steel blade to slice the soil surface, spanning the width of the plots. The researchers reported that dissolved P concentrations in runoff were lower from plots with poultry litter incorporated into aeration furrows than from plots with surface-applied poultry litter and were no greater than plots with no litter application. However, aerated plots with surface-applied poultry litter generally did not produce smaller concentrations of soluble P in runoff than did those plots without aeration.

Research is lacking on the impact of core aeration on P export in runoff. This mechanical treatment involves pulling cylindrical cores from the soil surface. There has been some research on core aeration in turf grass situations to reduce thatch and increase nutrient retention (Callahan et al., 1998; Hartwiger and O'Brien, 2001; Kraft et al., 2004). Runoff from core-aerated land may transport more sediment than undisturbed soil because the cores are removed from the soil and are placed on top of the soil surface, where bare soil of the cores is exposed to rainfall impact. However, this method may result in reduced localized compaction compared with slit aeration given that plugs are pulled from the soil rather than the soil being pressed open.

Although some studies have specifically examined the potential of mechanical aeration to reduce runoff volume, suspended solids, and/or nutrient export from grasslands (Franklin et al., 2006; Franklin et al., 2007; Pote et al., 2003; Shah et al., 2004a; van Vliet et al., 2006), no studies have directly compared mechanical aeration implements (core, slit, disk) under purely well drained soil conditions before and after soil compaction. However, past work offers an experimental basis for analyzing various aeration treatments, which in turn will allow for the use of available nutrients while sustaining environmental quality.

One important aspect of analyzing export of P from agricultural land is an examination of the fractions of P present in runoff. Total Kjeldahl P (TKP) is a measure of the total amount of P present in runoff, including sediment and particulate-bound inorganic and organic P as well as dissolved inorganic and organic P. Total dissolved P (TDP) is a measure of the total dissolved inorganic and organic P present. Dissolved reactive P determines the fraction of P that can be determined colorimetrically under the molybdate blue reaction (Murphy and Riley, 1962), and it is largely representative of the dissolved inorganic (orthophosphate) fraction of P. As such, the difference between DRP and TDP represents dissolved organic P. The fraction of bioavailable P (BAP) in runoff is considered to be a more specific estimate of the potential environmental impact of transported P in aquatic sytems (Sharpley et al., 1992) and has been estimated using FeO-coated papers (Myers et al., 1997; Robinson et al., 1994; Sharpley, 1993). This method can be applied to unfiltered runoff samples to estimate the total BAP (TBAP) present in runoff or applied to filtered samples to estimate the amount of dissolved BAP (DBAP).

The objective of this study was to determine the effectiveness of three mechanical aeration treatments and a control treatment (aeration with cylindrical cores, continuous-furrow no-till disk aeration perpendicular to the slope, slit aeration with tines, and no aeration treatment) in reducing the loss of P in surface runoff from grasslands with three manure treatments (broiler litter, dairy slurry, and no manure) under simulated rainfall at the plot-scale before and after simulated compaction by cattle.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Plot Establishment
In the summer of 2004, 48 plots (0.75 x 2 m) were established on a Cecil coarse sandy loam soil (fine, kaolinitic, thermic, Typic Kanhapludult) with mixed tall fescue-bermudagrass vegetation on 8 to 12% slopes in Oconee County, Georgia, USA (33° 47' N, 83° 23' W, elevation 225 m). Experimental plots were delineated with galvanized sheet metal (23-cm width) placed into the ground to a depth of 18 cm. Before all rainfall simulations, forages on plots were harvested to a 10-cm stubble height to standardize forage canopy height and determine forage herbage mass (dried at 65°C for 48 h).

Rainfall Simulations to Establish Blocks
Simulated rainfall was applied to plots (all forages at 10-cm stubble height) at a rate of 85 mm h^–1 until 30 min of runoff had occurred to evaluate baseline conditions of runoff from plots. For all rainfall events, the duration of rainfall required to produce 30 min of runoff averaged 56 min. Given this time period, the rainfall rate of 85 mm h^–1 represents an approximate 1-in-50-yr return period for a 1-h rainfall in Oconee County (Hershfield, 1961). Histograms were used to identify frequency distributions of baseline runoff volumes. Using these histograms, plots were classified into four blocks, each with three aeration treatments and a control (aeration with cylindrical cores, no-till disk aeration perpendicular to the slope, slit aeration with tines, and no aeration treatment) factorially combined with three manure treatments (broiler litter, dairy manure, and no manure). The four aeration (including the control) and three manure treatments (4 x 3) were randomly applied to the 12 plots within each of the four blocks (48 plots).

Mechanical Aeration
Plots were aerated using an aeration device fashioned by attaching cores, tines, or metal flashing (disk aeration) to rows on a metal plate and pushing the implement into the soil. Although commercially available cores and tines were used in the fabrication of aeration devices, they did not simulate the motion or rotation that would have occurred if actual field-scale aeration equipment were used. This experiment was done at the plot-scale using rainfall simulation where hydrologic inputs on multiple manures aerated with different implements could be controlled and replicates could be increased. The core aeration implements (BEFCO Inc., Rocky Mount, NC) were approximately 11 cm long, producing a hole with an approximate diameter of 2 cm. The tine implements (AerWay, Wylie, TX) were 20 cm long and produced a tapered, wedge-shaped slit with an approximate 1.5 by 6 cm opening at the soil surface. Galvanized 11-gauge metal flashing was used to simulate disk aeration. Each aeration implement was inserted to a depth of 8 cm and impacted a similar-sized surface area of the plot (200 cm²), with a total of nine aerated rows running perpendicular to the slope on each plot.

Manure Treatments and Phosphorus Assay
Immediately after each aeration treatment, plots were fertilized with manure treatments at a target rate of 30 kg P ha^–1 (each application) according to preliminary analysis by inductively coupled plasma atomic emission spectroscopy. Actual rates of application determined in subsequent lab analyses differed slightly from these estimates (Table 1 ). All broiler litter was dried (65°C) and ground before application to ensure consistent nutrient application to plots. Samples of applied manures were analyzed for TKP by Kjeldahl digestion (Baker and Thompson, 1992) and for molybdate reactive P by shaking 20 g of broiler litter in 4 L of deionized water for 4 h, then filtering (0.45 µm) and analyzing the filtrate according to the molybdate blue method (Murphy and Riley, 1962). Dissolved reactive P in dairy slurry was determined by filtering (0.45 µm) the slurry and similarly analyzing the filtrate. Total dissolved P in broiler litter and dairy slurry was determined by Kjeldahl digestion of the filtered extracts (USEPA, 1979). Manure pH was determined using a pH electrode, with broiler litter mixed with deionized water at a ratio of 1:5 (weight basis).

In June 2005, plots were compacted using methods described by Clary (1995) to simulate compaction resulting from cattle hoof action in grazed pastures. Ten locations on each plot were compacted, each with an area of 100 cm². Compaction was intended to simulate that which would result under light grazing by cattle, not compaction of the entire area as would be expected in heavy use areas (Butler et al., 2006). After the compaction in June 2005, plots were aerated, and manure treatments were applied before this rainfall simulation using the same methods as in January 2005. Rainfall was measured using rain gauges at the plot surface during each simulated rainfall event to verify rainfall rate applied. Herein "PreCmpct" and "PostCmpct" refer to the January 2005 and June 2005 rainfall events, respectively.

Laboratory Analysis of Runoff and Soils
Unfiltered runoff samples were analyzed colorimetrically for TKP following a Kjeldahl digestion (USEPA, 1979) and for TBAP using FeO-coated paper circles as described by Myers et al. (1997). Samples were filtered (0.45 µm) to determine concentration of TSS, and subsequent filtrate was analyzed for DRP by the molybdate blue method (Murphy and Riley, 1962), TDP by the Kjeldahl method, and DBAP as described previously for TBAP. Samples collected at 5-min intervals represented point estimates of concentrations and were plotted versus cumulative runoff volume. The points were joined with straight lines, and the areas under the lines were integrated according to the trapezoid rule to determine cumulative mass of TSS and forms of P exported during 30-min of runoff.

Immediately before rainfall simulations (after manure applications), three soil cores (1.75-cm internal diameter) extracted from a 0- to 2-cm depth and two cores extracted from a 0- to 5-cm depth were obtained from each plot. In each plot, soil cores were combined into a composite sample by depth. The sample taken at the 0- to 5-cm depth was divided into two subsamples. One sample was air-dried and sieved (<2 mm), and the other sample was placed in a soil tin and dried at 105°C for 48 h to determine gravimetric soil moisture content. The air-dried, sieved sample was analyzed for Mehlich-I soil test P (STP) using methods described by Mehlich (1953) and for water-soluble P (WSP) by shaking 1 g of soil with 25 mL of deionized water for 1 h. The sample was then filtered (0.45 µm), and the filtrate was analyzed according to the molybdate blue method (Murphy and Riley, 1962).

Statistical Analysis
Runoff volume and export of TSS, TKP, TDP, DRP, TBAP, and DBAP in runoff were examined using the PROC GLM procedure (SAS Institute, 1994) with baseline runoff volume used as covariate in the analysis. If the main effect of aeration type was significant (p < 0.10), means were separated using the LSMEANS procedure with the PDIFF option. Differences between means were considered significant at p < 0.10. Considering that the focus of this manuscript is the evaluation of P export from grasslands aerated with different mechanical aeration implements and not on the variation in P export between manure treatments, differences in runoff constituents as related to aeration were examined by manure treatment. This was also important considering the variation in the amount of P applied between manure treatments (broiler litter, dairy slurry, and no manure) and, in the case of application of the liquid dairy slurry, associated addition of water to plots.

	Results and Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Soil Properties
At the 0- to 2-cm and 0- to 5-cm depths, STP averaged 36 and 22 mg P kg^–1, and WSP averaged 20 and 9 mg P kg^–1, respectively, at the baseline event (Table 2 ). For both rainfall events after manure applications (PreCmpct and PostCmpct), mean STP was not significantly related to aeration type for any manure treatment (p > 0.10). Although STP and WSP had increased substantially from the baseline event to the PostCmpct event (Table 2), values were relatively low compared with many fields with a history of manure applications, where STP levels above 100 mg kg^–1 are common (Franklin et al., 2007). Soil moisture levels were notably lower at the PostCmpct rainfall event (June) than at the PreCmpct event (January), for all manure treatments (Table 3 ). Considering the differing seasons of these two events, it is important to note that seasonal effects such as dominance of tall fescue (January) or bermudagrass (June) and a difference in soil moisture content may have played a role in the different relationships observed between aeration implements and the control at these two rainfall events. Because of this difference, PreCmpct and PostCmpct results are presented separately.

Total Suspended Solids
When broiler litter or dairy slurry was applied, there was generally no effect of aeration type on export of TSS when data from both rainfall events were combined in the analysis (Tables 4 and 5). Our findings differ from those of van Vliet et al. (2006), who reported reductions in mean TSS export of 48 to 69% with slit aeration and applied dairy slurry. The different results reported in our study may be due to differing soil types. Nevertheless, we may have made a type 2 statistical error (accepting the null hypothesis of no difference between means, when it is actually false) while testing TSS export with slit aeration after dairy slurry was applied. In this case, there was a trend of 23 to 28% lower TSS export compared with the control (Table 8 ).

Total Kjeldahl Phosphorus
With applied broiler litter, aeration type and event were significantly related to mass export of TKP when data from both rainfall events were combined in the analysis (p < 0.05; Table 4). At the PreCmpct event, core aeration reduced mean TKP export by 57% compared with no aeration (Fig. 1a ). Mean TKP export from no-till disk and slit aeration was also less than that from the control, with reductions of 25 and 28%, respectively. Relative to the control, TKP export at the PostCmpct event was 50% lower from core aeration (Fig. 1b), which was similar to the reduction by core aeration in the PreCmpct event. However, this reduction was not significant at the PostCmpct event (p = 0.13 for the main effect of aeration type).

View larger version (41K): [in this window] [in a new window]   Fig. 1. Mean export of total Kjeldahl P (TKP), total dissolved P (TDP), dissolved reactive P (DRP), total bioavailable P (TBAP), and dissolved bioavailable P (DBAP) as affected by aeration type with applied broiler litter at (a) PreCmpct (January 2005) and (b) PostCmpct (June 2005) rain events. Percentages indicate the proportion of TKP represented as TDP, DRP, TBAP, or DBAP. , , *, **, and *** indicate main effect of aeration type at p < 0.15, 0.1, 0.05, 0.01, and 0.001, respectively; within each phosphorus fraction, means and percentages represented by the same letter or no letters are not significantly different (p > 0.10).

View larger version (42K): [in this window] [in a new window]   Fig. 2. Mean export of total Kjeldahl P (TKP), total dissolved P (TDP), dissolved reactive P (DRP), total bioavailable P (TBAP), and dissolved bioavailable P (DBAP) as affected by aeration type with applied dairy slurry at (a) PreCmpct (January 2005) and (b) PostCmpct (June 2005) rain events. Percentages indicate the proportion of TKP represented as TDP, DRP, TBAP, or DBAP. , , *, **, and *** indicate main effect of aeration type at p < 0.15, 0.1, 0.05, 0.01, and 0.001, respectively; within each phosphorus fraction, means and percentages represented by the same letter or no letters are not significantly different (p > 0.10).

View larger version (19K): [in this window] [in a new window]   Fig. 3. Mean export of total Kjeldahl P (TKP), total dissolved P (TDP), dissolved reactive P (DRP), total bioavailable P (TBAP), and dissolved bioavailable P (DBAP) as affected by aeration type with no manure application at (a) PreCmpct (January 2005) and (b) PostCmpct (June 2005) rain events. Percentages indicate the proportion of TKP represented as TDP, DRP, TBAP, or DBAP. , , *, **, and *** indicate main effect of aeration type at p < 0.15, 0.1, 0.05, 0.01, and 0.001, respectively; within each phosphorus fraction, means and percentages represented by the same letter or no letters are not significantly different (p > 0.10).

Total Dissolved Phosphorus
Similar to export of TKP, TDP export was most affected by aeration type with applied broiler litter (Tables 4–6). In the case of applied broiler litter, core aeration reduced mean TDP export by 66%, no-till disk aeration reduced export by 35%, and slit aeration reduced export by 27% compared with the control at the PreCmpct event (Fig. 1a). Export was similar to the control for all aeration treatments at the PostCmpct event (Fig. 1b). With applied broiler litter at the PreCmpct event, mean TDP export under core and no-till disk aeration was 69 and 78% of TKP export, respectively, both significantly less than the 90% under the aeration control with applied broiler litter (Fig. 1a). This indicates that reduction in P with mechanical aeration may be in large part due to adsorption of dissolved P to soil exposed by the aeration process, especially by the core implements. This is further supported by the increase in TSS export observed from core aeration when no manures were applied. Core aeration may also be more effective than other aeration types at slowing overland runoff flow because the extracted cores may act as flow impediments at the soil surface.

Dissolved Reactive Phosphorus
Aeration type was significantly related to DRP export with applied broiler litter (p < 0.05) (Table 4) and dairy slurry (p < 0.10) (Table 5) when data from both rainfall events were combined in the analysis. Similar to TKP export with applied broiler litter, core aeration decreased DRP export by 66% compared with the control at the PreCmpct event (Fig. 1a). Likewise, no-till disk aeration and slit aeration significantly decreased DRP export compared with the control by 34 and 28%, respectively. The greater effectiveness of core aeration may be due to the increased volume of soil particles exposed when cores are placed on the soil surface, resulting in greater adsorption of P to soil minerals. This implies that increasing STP levels may limit the effectiveness of core aeration for decreasing P losses. If the P sorption capacity of a given soil has been met for the volume of soil exposed (11 x 2 cm core), these cores could then serve as a P source rather than a sink, possibly increasing P losses from high-P soils.

With applied dairy slurry, DRP export was similar under all aeration types at the PreCmpct event (Fig. 2a). At the PostCmpct event, DRP export was reduced 47% by core aeration and 55% by no-till disk aeration (p < 0.10 for the main effect of aeration type) (Fig. 2b). The lack of an impact on P export observed with slit aeration and applied dairy slurry is somewhat surprising given that Shah et al. (2004a) and van Vliet et al. (2006) reported reductions in DRP export from applied dairy slurry when fields were slit aerated. The difference may be related to the soils present in these studies because one was conducted on an alluvial Inceptisol with a silt loam surface soil and the other conducted on a well drained Alfisol with a silt loam surface soil. The highly eroded Ultisol in our study is likely less able to accommodate a high volume of applied dairy slurry due to slower rates of infiltration and is more likely to be slightly compacted by the slit aeration procedure than the soils in the Shah et al. (2004a) or van Vliet et al. (2006) study.

Bioavailable Phosphorus
Similar to other P constituents in runoff, BAP was generally related to aeration type only when broiler litter was applied (Table 4). At the PreCmpct event, TBAP export was 58% lower than the control when plots were aerated with cores (Fig. 1a). Likewise, DBAP was 64% lower than the control under core aeration at the PreCmpct event with applied broiler litter. Differing from the effect on TBAP export, no-till disk and slit aeration reduced DBAP export compared with the control by 29 and 27%, respectively. At the PostCmpct event, TBAP and DBAP export were increased by no-till disk aeration, although they were unaffected by core or slit aeration (Fig. 1b). For all aeration treatments with applied broiler litter, the percentage of TKP represented as TBAP and DBAP averaged 75 and 55%, respectively.

With applied dairy slurry, aeration had a significant impact on TBAP export at the PostCmpct event (p < 0.10 for the main effect of aeration type) (Fig. 2b) but not for the PreCmpct event. In this case, core and no-till disk aeration decreased mean TBAP export by 49 and 56%, respectively, which is similar to the impacts on mean DRP export. No previous studies that examined nutrient export from aeration specifically reported on BAP losses, making it difficult to gain a greater perspective of results reported here. However, the reductions observed in our study help to demonstrate the effectiveness of core aeration in reducing the export of P readily available to algae.

Forage Herbage Mass
Forage herbage production was examined by using the herbage mass from plots at the initial harvest as a covariate in comparing aeration type to herbage mass after aeration and manure application. When broiler litter was applied, aeration type was not significantly related to biomass production (p = 0.11; data not shown). Although this main effect of aeration type was not determined to be significant, mean herbage mass from core-aerated plots when broiler litter was applied did differ from the control (p < 0.10). Forage dry matter production on core-aerated plots with applied broiler litter was 2215 kg ha^–1, compared with 1633 kg ha^–1 on control plots. Aeration type was not significantly related to forage herbage production with applied dairy slurry or when no manures were applied. Results here are typical of other studies that have examined forage production with aeration in that yields were unaffected, slightly increased, or slightly decreased with aeration (Burgess et al., 2000; Chen et al., 2001; Malhi et al., 2000; Pote et al., 2003; Shah et al., 2004a). Variation in results among studies is likely due to differences in forage species and soil properties affecting the impact of mechanical aeration, such as drainage and bulk density.

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
The effectiveness of core aeration may be largely attributable to binding of P to exposed soil minerals, and, given that there is potential for increased export of TSS from core-aerated fields, core aeration should be conducted only on soils that have further P sorption capacity. Management practices that help to control TSS transport may enhance the effectiveness of core aeration. Specifically, vegetative filter strips have been shown to be very effective tools for controlling TSS exports from agricultural fields (Blanco-Canqui et al., 2006; Chaubey et al., 1995; Daniels and Gilliam, 1996). With relatively large reductions in TDP observed under core (66%) and to a lesser extent no-till disk aeration (35%) and slit aeration (27%) at the PreCmpct event, additional control of P associated with transported solids (20–30% of TKP export) could further reduce environmental risks associated with broiler litter application.

Results presented here suggest that aeration, specifically core aeration of relatively low-P soils, has the potential to reduce P export from surface-applied manures, but further research is needed to determine the effects on high-P soils. Given that Cecil soil is common in pastures receiving broiler litter in the Southern Piedmont, core aeration could have a widespread impact on water quality in the Southern Piedmont region. An important consideration of the potential impact of mechanical aeration seems to be soil type and associated drainage class, permeability, and resistance to compaction. Although studies have shown the potential of slit aeration to reduce nutrient losses from alluvial Inceptisols and well drained Alfisols with applied dairy slurry (Shah et al., 2004a; van Vliet et al., 2006) and well drained Ultisols with applied poultry litter (Franklin et al., 2007), aeration has been shown to be less effective in other situations. The efficacy of core aeration in the Southern Piedmont seems to be largely due to exposing a greater surface area of soil minerals to bind P and prevent its export in runoff as well as the lack of localized compaction when aeration implements enter the soil.

	ACKNOWLEDGMENTS

 
This research was funded by USDA-NRI grant 2003-01970. The authors gratefully acknowledge Elizabeth Barton, T.J. Holliday, and John Rema for outstanding technical support and Krystal Kerr, Tasha Mashburn, Chelly Richards, Emily Toriani, Nicolás Vaio, and Yebin Zhao for assistance with field sampling and lab analysis.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
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	REFERENCES

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