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Waste Management

Reduction of High Soil Test Phosphorus by Bermudagrass and Ryegrass–Bermudagrass following the Cessation of Broiler Litter Applications

^a USDA-ARS, Waste Management and Forage Res. Unit, P.O. Box 5367, Mississippi State, MS 39762
^b USDA-ARS, Animal Waste Management Res. Unit, 230 Bennett Ln., Bowling Green, KY 42104
^c USDA-ARS, U.S. Dairy Forage Res. Center, Madison, WI 53706
^d Dep. of Plant and Soil Sciences, Mississippi State Univ., Mississippi State, MS 39762

^* Corresponding author (jjread{at}msa-msstate.ars.usda.gov)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Factors that contribute to high soil P in bermudagrass [Cynodon dactylon L. (Pers.)] pastures include continued use of broiler litter rich in P to meet forage N requirements and removal of hay in summer only. This study determined if harvesting annual ryegrass (Lolium multiflorum Lam.) in addition to bermudagrass would reduce surplus soil P, Cu, and Zn faster than bermudagrass alone after litter application ceased on a Ruston soil (fine-loamy, siliceaous, semiactive, thermic Typic Paleudults). During a 3-yr build-up, ‘Coastal’ bermudagrass was fertilized with 0, 4.5, 9, 18, and 36 Mg ha^–1 litter yr^–1. During the drawdown phase, plots were split and half was overseeded with ryegrass in fall 2001, 2002, and 2003. Whole plots were fertilized with NH₄NO₃ in spring–summer to provide 268 kg ha^–1 N yr^–1. Forage yield and P uptake increased as antecedent litter rate increased, and were greater in ryegrass–bermudagrass than bermudagrass in 2002, but not 2003. At 9 Mg ha^–1 litter, harvesting ryegrass in addition to bermudagrass increased P uptake by 10 to 55%, depending on study year. During the drawdown phase, soil Mehlich-3 P (M3P) and water-extractable P to 15-cm depth decreased by as much as 50 and 70%, respectively. Soils analysis within each sampling date found no significant effect of forage system or its interaction with litter rate. Data for 9 Mg ha^–1 litter rate indicated 2 yr of forage P removal decreased residual M3P to an acceptable agronomic level (<70 mg kg^–1). The potential to decrease surplus soil P by ryegrass–bermudagrass hay harvests was greatest when rainfall was inadequate for optimum bermudagrass yield.

Abbreviations: DM, forage dry matter • M3P, Mehlich-3 phosphorus • WEP, water-extractable phosphorus

Reduction of High Soil Test Phosphorus by Bermudagrass and Ryegrass–Bermudagrass following the Cessation of Broiler Litter Applications

^a USDA-ARS, Waste Management and Forage Res. Unit, P.O. Box 5367, Mississippi State, MS 39762
^b USDA-ARS, Animal Waste Management Res. Unit, 230 Bennett Ln., Bowling Green, KY 42104
^c USDA-ARS, U.S. Dairy Forage Res. Center, Madison, WI 53706
^d Dep. of Plant and Soil Sciences, Mississippi State Univ., Mississippi State, MS 39762

^* Corresponding author (jjread{at}msa-msstate.ars.usda.gov)

Received for publication November 3, 2006.
Factors that contribute to high soil P in bermudagrass [Cynodon dactylon L. (Pers.)] pastures include continued use of broiler litter rich in P to meet forage N requirements and removal of hay in summer only. This study determined if harvesting annual ryegrass (Lolium multiflorum Lam.) in addition to bermudagrass would reduce surplus soil P, Cu, and Zn faster than bermudagrass alone after litter application ceased on a Ruston soil (fine-loamy, siliceaous, semiactive, thermic Typic Paleudults). During a 3-yr build-up, ‘Coastal’ bermudagrass was fertilized with 0, 4.5, 9, 18, and 36 Mg ha^–1 litter yr^–1. During the drawdown phase, plots were split and half was overseeded with ryegrass in fall 2001, 2002, and 2003. Whole plots were fertilized with NH₄NO₃ in spring–summer to provide 268 kg ha^–1 N yr^–1. Forage yield and P uptake increased as antecedent litter rate increased, and were greater in ryegrass–bermudagrass than bermudagrass in 2002, but not 2003. At 9 Mg ha^–1 litter, harvesting ryegrass in addition to bermudagrass increased P uptake by 10 to 55%, depending on study year. During the drawdown phase, soil Mehlich-3 P (M3P) and water-extractable P to 15-cm depth decreased by as much as 50 and 70%, respectively. Soils analysis within each sampling date found no significant effect of forage system or its interaction with litter rate. Data for 9 Mg ha^–1 litter rate indicated 2 yr of forage P removal decreased residual M3P to an acceptable agronomic level (<70 mg kg^–1). The potential to decrease surplus soil P by ryegrass–bermudagrass hay harvests was greatest when rainfall was inadequate for optimum bermudagrass yield.

Abbreviations: DM, forage dry matter • M3P, Mehlich-3 phosphorus • WEP, water-extractable phosphorus

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
DURING THE PAST SEVERAL YEARS, environmental policy related to animal agriculture has focused on land application of manure (Pant et al., 2004). Broiler litter—a mixture of manure, wasted feed, and bedding materials—is often used as fertilizer on pastures and hay fields in south central Mississippi, where broiler chicken (Gallus gallus domesticus) production is concentrated. ‘Coastal’ hybrid bermudagrass, a warm-season perennial forage, is commonly grown in the region and responds readily to N fertilizer or broiler litter (Read et al., 2006). Litter is often broadcast-applied at rates that meet or exceed the annual N requirement of Coastal bermudagrass, about 268 kg N ha^–1 yr^–1 (Brink et al., 2002). Because the N/P ratio of litter is much lower than the ratio of N and P absorbed from the soil by bermudagrass (2:1 vs. 10:1; Evers, 2002), soil P levels on many broiler farms are substantially greater than those required for optimum forage yield (Sistani et al., 2004). Due to the potential for increased biomass and nutrient recovery by year-round forage production, double-cropping bermudagrass with cool-season annual forages has been proposed as a "best management practice" for pastures fertilized with animal manure (Rowe and Fairbrother, 2003; McLaughlin et al., 2005). Overseeding annual ryegrass in fall into dormant bermudagrass has been used successfully in grazing systems, because it provides more winter ground cover and earlier spring growth than bermudagrass alone (Evers et al., 1997). Total nutrient uptake is enhanced due to strong species difference in growth distribution and the potential for ryegrass to be productive from autumn to spring, particularly in south central Mississippi.

Harvesting high-biomass forage crops and utilizing them at a site remote to the source is an important component of soil P remediation despite the fact that P levels may be reduced slowly or remain unchanged, especially with continued manure application (Pant et al., 2004). It is widely accepted that remediation of excess soil P by crop removal is slow (McCollum, 1991; Novak and Chan, 2002). In a study of manure-treated soil, Eghball et al. (2003) reported 5 yr of corn P removal would be needed to lower soil test P from 150 mg kg^–1 to an optimal level of 69 mg kg^–1. A faster decline in soil P is expected in hybrid bermudagrass systems, due to the potential for multiple hay harvests and off-site removal of the products (McLaughlin et al., 2005). Hybrid bermudagrass can remove 50 to 60 kg P ha^–1 yr^–1, depending on biomass yield and tissue P concentration, which are influenced by broiler litter rate (Brink et al., 2002), N fertility (Read et al., 2006), and soil type (Adeli et al., 2006). The additional hay harvested from annual ryegrass would increase P removal by 20 to 25% or more, depending on fertility and harvest management (Brink et al., 2001; Gaston et al., 2003). Evers (2002) fertilized ryegrass–bermudagrass with broiler litter in fall and different combinations of N fertilizer in spring, and reported ryegrass removed about twice as much P as bermudagrass. Harvest management of annual ryegrass also appears to be critical, as Rowe and Fairbrother (2003) found changing from a one-harvest to a two-harvest system in spring in a swine effluent spray field resulted in ryegrass–bermudagrass system recovering 27 to 60% more P from soil than bermudagrass winter fallow.

A complicating factor with broiler litter is that applying it to soil without incorporation can accumulate P and lower P sorption at the soil surface compared with deeper soil layers (Lucero et al., 1995; Sharpley, 2003). Increased P concentration in surface soil increases the potential for P transport by runoff or leaching that may cause eutrophication (Sharpley, 1995; Pote et al., 2003). Another environmental concern is with certain trace metals, Cu and Zn, which are normally added to poultry diets to improve weight gain and prevent disease (Han et al., 2000). Because plants use very little Cu and Zn, accumulation can occur in soils overfertilized with manure (Sistani et al., 2004), and is greatest in the top 0 to 15 cm of soil due to a strong association with soil organic matter (Novak et al., 2004). Hybrid bermudagrass removed only 1.0 to 1.5% of Cu and 7 to 13% of Zn, as compared with 15 to 27% of P, applied in litter to soil with 25-yr history of application (Brink et al., 2002). Pederson et al. (2002) noted Cu and Zn contents were generally greater in annual ryegrass than 12 other cool-season forages, and proposed nutrient removal could be increased by improving yield of ryegrass stems. The amounts supplied by litter rates typical for forages (9 Mg ha^–1) are well below the maximum annual loading rates for municipal biosolids of 75 kg ha^–1 Cu yr^–1 and 140 kg ha^–1 Zn yr^–1 (USEPA, 1993). Nevertheless, studies in Arkansas demonstrated litter rates of 2 to 9 Mg ha^–1 increased the concentration of soluble Cu in runoff water, as compared with untreated plots (Moore et al., 1998).

To ensure adequate plant nutrition and prevent unacceptable losses of P from fields receiving animal manure, Mississippi has adopted the P-Index risk assessment approach (Osmond et al., 2006). This index is a site-specific qualitative rating defined by three transport factors (erosion, runoff, and distance to water) and five P-source characteristics, one of which is soil P concentration according to Mississippi soil test method (which is based on Lancaster P extraction method) described by Cox (2001). Producer options to remediate soils that test high in P include (i) substituting fertilizer N additions for broiler litter N to enhance plant growth and uptake of N and P (Evers, 2002; Read et al., 2006), and (ii) the cessation of litter application and continued harvest and removal of forage biomass until soil test P returns to a more acceptable level (Novak and Chan, 2002). DeLaune et al. (2004) concluded that application of litter based on the P index allows more management options than application based on a soil test P threshold. But their results of significant correlation between the concentration of soluble-reactive P in runoff and M3P in soil samples, 0- to 15-cm depth, suggest knowledge of soil test P can be used to assess P runoff in pasture before manure is applied, and thereby help a grower be proactive about when to resume applications of broiler litter. Several studies show soil test P is associated positively with P losses in runoff water (Sharpley, 1995) and would therefore be useful in risk assessment (Osmond et al., 2006).

Various studies indicate overseeding bermudagrass with annual ryegrass and removing forage biomass can remove more nutrients from manure-treated soils than bermudagrass alone (Brink et al., 2001, 2002; Evers, 2002; Sharma et al., 2004), but little is known about the potential of this "best management practice" to remediate excess soil P, that is, levels in excess of crop needs (Gaston et al., 2003; Sistani et al., 2004). The objective of this study was to determine if harvesting ryegrass in addition to bermudagrass would reduce surplus soil nutrients, particularly P, Cu, and Zn, faster than bermudagrass alone after broiler litter application ceased. The research was comprised of two experimental periods, each lasting approximately 3 yr. The first period used different rates of broiler litter applied to bermudagrass managed for hay to "actively" build-up manure soil nutrients. Results for bermudagrass response to litter rates are presented elsewhere (Brink et al., 2008) and will not be discussed in the present paper. The second period used two forage systems, ryegrass–bermudagrass and bermudagrass winter fallow, to "passively" remove residual soil nutrients. Results impact the long-term economic value and sustainability of using broiler litter as a fertilizer for bermudagrass hay fields, as well as fields double-cropped to ryegrass and bermudagrass.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The study site was a Coastal bermudagrass hay meadow located at the Mississippi Agricultural and Forestry Experiment Station Coastal Plain Experiment Station at Newton (32°20' N, 89°4' W). The field had no known history of broiler litter application. Soil at the site is Ruston fine sandy loam, which are well-drained, medium acid to very strongly acid soils formed in loamy marine or stream sediments. In April 1999, the existing bermudagrass sward was cleared of senesced weeds by mowing, and weed regrowth was controlled using selective herbicides. Plots (4 by 6 m) were arranged in a randomized complete block design with four replicates. Soil was sampled at 0- to 5- and 5- to 15-cm depths at several sites within the experimental area and the samples were combined by depth for subsequent chemical analysis, as described below.

During the "build-up" phase, litter rates of 0, 4.5, 9, 18, and 36 Mg ha^–1 yr^–1 were applied by hand on a wet-weight basis in April 1999, 2000, and 2001. The litter was sampled for moisture content, which averaged about 25% each year, and for nutrient concentrations (Table 1 ). Results were used to calculate the average annual nutrient application rates (Table 2 ). A litter rate of 9 Mg ha^–1 yr^–1 is typical for bermudagrass pasture in Mississippi (Brink et al., 2002). Assuming 50% of litter N was available for plant uptake in each growing season, 18 Mg litter ha^–1 should meet the annual N requirements to maintain Coastal bermudagrass yields of 9 to 13 Mg ha^–1 (Bitzer and Sims, 1988). Similarly, 4.5 Mg ha^–1 litter would meet the annual P requirement of Coastal bermudagrass, which can remove as much as 60 kg ha^–1 P yr^–1 (Read et al., 2006). Bermudagrass harvests began in late May to early June, and continued at approximately 30-d intervals depending on rainfall and plant growth patterns. Plots were harvested five times in 1999, three times in 2000, and four times in 2001 using a sickle-bar mower set at a height of 7 cm.

Forage dry matter (DM) yield was determined by cutting a 1 by 6 m swath at a 7-cm stubble height through the center of each plot using a sickle-bar mower. Plots were harvested four times in 2002, on 1 May, 19 June, 8 August, and 1 October. Plots were harvested five times in 2003, on 9 April, 22 May, 17 July, 19 August, and 3 October. Plots were harvested once in 2004, on 3 May, when annual ryegrass was in boot stage of development. Because ryegrass makes most of its growth in late winter and spring, and Marshall is late-maturing, it was harvested once in late spring when yield potential was highest (Redfearn et al., 2005). This first harvest was a mixture of ryegrass and bermudagrass and no attempt was made to separate species. Averaged across litter rates, the bermudagrass component comprised about 33, 37, and 69% of the ryegrass–bermudagrass, based on biomass harvests in spring 2002, 2003, and 2004, respectively. Subsamples (600–800 g) of forage were dried at 65°C for 48 h for determination of percentage moisture. Oven-dried forage was ground to pass a 1-mm screen, sealed in plastic containers, and subsequently analyzed for mineral nutrients (described below). Nutrient uptake was estimated as the product of DM yield and percentage nutrient concentration for each plot at each harvest, and as the summation of the values within a treatment across all harvests dates to obtain total annual nutrient uptake.

Rainfall records were obtained from a weather station at the Coastal Plain Experiment Station (Fig. 1 ). For the cool-season period from October to March, the station recorded 741, 878, and 723 mm rainfall in 2002, 2003, and 2004, respectively. For the warm-season period the station recorded 591 and 1122 mm rainfall in 2002 and 2003, respectively. Total annual rainfall was about 100 mm below the long-term average in 2001–2002. Below average precipitation also was recorded from October 2003 to March 2004. By contrast, annual rainfall in 2002–2003 was about 560 mm above the long-term average.

View larger version (22K): [in this window] [in a new window]   Fig. 1. Total monthly rainfall for the study period, October 2001 to May 2004, and 30-yr mean at Coastal Plain Experiment Station, Newton, MS.

Chemical analyses were performed as described by Sistani et al. (2004). Briefly, soil and litter pH were measured using a 10-g sample mixed in 10 mL water. Soil, plant, and litter total N was measured using an automated dry combustion analyzer (Model NA 1500 NC, Carlo Erba, Milan, Italy). Forage nutrient concentration of each harvest was determined by ashing a 0.8-g subample in a ceramic crucible at 500°C for 4 h followed by dissolution of the ash in 1.0 mL of 6 M HCl. After 1 h, 50 mL of a double-acid solution of 0.025 M H₂SO₄ and 0.05 M HCl was added to the crucible, allowed to stand for 1 h, and then filtered through Whatman no. 1 paper. The P, Cu, and Zn concentration of the filtrate was measured using emission spectroscopy on an inductively coupled argon plasma optical emission spectrometer (ICP–OES, Thermo Jarrell Ash Model 1000 ICAP, Franklin, MA). Litter N and P concentration, as well as K, Ca, Mg, Fe, Mn, Cu, and Zn concentration, were measured by the same methods used to analyze forage. Soil total P was determined by digesting 0.50 g soil with sulfuric acid, hydrogen peroxide, and hydrofluoric acid followed by determination of P using the ICP. Soil samples also were extracted using Mehlich-3 soil extractant (1:10 soil/extractant) and the filtrates were analyzed for Mehlich-3–extractable P, and exchangeable K, Cu, and Zn using the ICP (Mehlich, 1984). Water-extractable P (WEP), which corresponds to solution P and labile P forms (Tasistro et al., 2004), was determined by extracting 2 g of soil in 20 mL water for 10 min and ICP analysis of all P forms that pass through a Whatman 2V filter (8 µm particle retention).

Statistical Analysis
Experimental design was a split-plot arrangement of treatments with four replicate blocks. Litter rate was the main plot factor, forage system was the split plot factor, and treatments were repeated on the same plot area each year. Because only a single spring harvest was obtained in 2004, data for total DM yield and nutrient uptake in 2002 and 2003 were used to perform analysis of variance across years using SAS mixed model procedures with year as the repeated measure (Littell et al., 1996). Analysis of variance detected a significant (P < 0.001) effect of year for DM yield and N, P, and K uptake, a significant (P < 0.001) year x litter rate interaction for P uptake by bermudagrass winter–fallow system, and a significant (P < 0.05) year x forage system interaction for P uptake at 18 and 36 Mg ha^–1 litter. Therefore, treatment effects on yield data were compared by year using SAS general linear model (GLM) procedures (SAS Institute, 1999). A probability level of P 0.05 was considered significant and treatment means were compared using Fisher's Protected Least Significant Difference (LSD). Soil chemical data were analyzed using values from the 0- to 5-cm and 5- to 15-cm depths, and "composite" values for the 0- to 15-cm depth expressed as a weighted-sum. These data were analyzed and interpreted by antecedent litter rate using GLM procedures in SAS, with forage system as the main plot factor and soil sampling date as the split plot factor. Results from analysis of variance with sampling date as a repeated measure indicated a significant sampling date effect for most soil variables, without a significant sampling date x forage system interaction. Because analysis of variance indicated similar trends for treatment effects in 0- to 5- and 5- to 15-cm soil sampling depths, emphasis is placed on results obtained for 0- to 15-cm depth.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Effect of Broiler Litter Applications on Soil Fertility
In December 2001, following 3 yr of litter application, soil total N and Mehlich-3–extractable P, K, Mg and Cu were significantly greater at 18 and 36 Mg ha^–1 litter yr^–1 than at lower rates of litter (Table 3 ). Additionally, repeated application of 9 Mg ha^–1 litter increased Mehlich-3–extractable P, Mg, and Cu, as compared with controls. In agreement with Read et al. (2006), the use of NH₄NO₃ as a N source for bermudagrass did not significantly decrease pH in a Ruston fine sandy loam soil. Initial soil pH in 2001 ranged from 5.9 to 6.1 across antecedent litter rates (Table 3). In May 2004, soil pH averaged about 5.9 in the 36 Mg ha^–1 litter yr^–1, 6.0 in the 0, 9, and 18 Mg ha^–1 litter rates, and 6.1 in the 4.5 Mg ha^–1 litter yr^–1.

Similar to M3P, applying more than 4.5 Mg ha^–1 litter led to significant increases in Mehlich-3–extractable Cu and Zn at 0- to 15-cm depth (Table 3), and accumulation of these minor nutrients in the surface, 0- to 5-cm soil depth (Table 4). Because removal of Cu and Zn by Coastal bermudagrass is very slow, these metals can persist at high levels for long periods of time (Han et al., 2000). A greater concentration of Mehlich-3 Cu in subsoil, 5- to 15-cm depth, in ryegrass–bermudagrass than bermudagrass suggests increased potential for leaching by double-cropping forages (Novak et al., 2004). But unlike M3P, subsoil Cu and Zn concentrations were at least one order of magnitude lower than in the surface soil. When data for the surface, 0- to 5-cm soil depth, from 9 Mg ha^–1 litter rate were analyzed statistically, Cu and Zn concentrations did not differ between forage systems or across the five sampling dates (Table 4). This variable response of soil Cu and Zn during crop production suggests long-term studies are needed to properly manage these minor nutrients and assess their potential risk to the environment.

Nutrient Uptake and Dry Matter Yield
Forage DM yield increased linearly (P < 0.01) as antecedent litter rate increased (Fig. 2 ). This response can be credited to the additional nutrients and C provided by broiler litter, as well as repeat applications of N fertilizer in summer to meet bermudagrass N requirement (Sistani et al., 2004; Read et al., 2006). Because the Ruston soil had no history of litter application, increased forage production may have been a response to increased K nutrition. This is supported from litter-induced increases in soil K (Table 3) and forage K concentrations (data not presented). Evers (2002) also recognized the importance of K nutrition when ryegrass–bermudagrass was fertilized with combinations of broiler litter and commercial N fertilizer.

View larger version (30K): [in this window] [in a new window]   Fig. 2. Total dry matter yield and P uptake by bermudagrass winter fallow and ryegrass–bermudagrass systems after broiler litter application ceased and plots were fertilized with commercial N fertilizer (268 kg ha–1 yr–1). Values represent the mean (±1 SE) of four observations, summed across four harvests in 2002, five harvests in 2003, and a single, ryegrass harvest in May 2004.

In 2002, harvesting annual ryegrass in addition to bermudagrass increased DM yield by about 1.46, 2.15, 2.93, 3.44, and 5.03 Mg ha^–1, respectively, at the five antecedent litter rates [LSD (P = 0.05) = 0.76] (Fig. 2). The effect of forage system x litter rate was not significant for DM yield (P > 0.10). Averaged across litter rates, the additional harvest of ryegrass significantly increased DM yield by 27% (8.1 vs. 10.3 Mg ha^–1) and P uptake by 40% (24 vs. 33 kg ha^–1), as compared with bermudagrass winter fallow system. As mentioned above, greater yield in ryegrass–bermudagrass may be attributed to greater growth in spring, which depleted the soil moisture enough to slow growth of bermudagrass. A significant forage system x litter rate effect was detected for total P uptake (P < 0.01), because the difference in P uptake between forage systems was fairly similar at 9, 18, and 36 Mg ha^–1 and increased dramatically between 0 and 4.5 Mg ha^–1 litter. Averaged across forage systems, P uptake was significantly greater at 36 Mg ha^–1 than 18 Mg ha^–1 litter (46 vs. 36 kg ha^–1 P), and did not differ between 4.5 and 9 Mg ha^–1 litter (average = 24 kg ha^–1 P). In 2003, total annual DM yield and P uptake increased significantly (P < 0.01) as antecedent litter rate increased. Analysis of variance found no significant effect of forage system on DM yield (P > 0.28) or P uptake (P > 0.70), and no significant forage system x litter rate interaction effect. In 2004, a single spring harvest of annual ryegrass significantly increased DM yield by about 0.45 Mg ha^–1 and increased P uptake by about 1.25 kg ha^–1, as compared with bermudagrass winter fallow. Total P uptake was significantly greater at 36 than 18 Mg ha^–1 litter, and did not differ between 9 and 4.5 Mg ha^–1 litter. The forage system x litter rate interaction effect was significant for P uptake due to similar difference between systems of about 0.65 kg ha^–1 at 4.5 and 9 Mg ha^–1 litter that increased to 1.9 kg ha^–1 at 8 Mg ha^–1 and 2.8 kg ha^–1 at 36 Mg ha^–1 litter, as a result of increased P uptake by the annual ryegrass component.

The litter-induced increases in soil N, Cu, and Zn were associated with increased uptake of Cu and Zn by plants (Table 5 ). Annual removal rates were considerably greater for Zn than Cu, even though loading rates were similar (Table 2). Greater Zn uptake by forages was associated with somewhat higher Mehlich-3–extractable Zn in surface soil (Table 3), supporting evidence of higher concentration of bioavailable Zn than Cu in litter-amended soil (Han et al., 2000). Bioavailability of Cu and Zn is determined by chemical species (soil pH) and various interactions with soil organic and inorganic components, with the more easily soluble fractions being related closely to plant availability (Shuman, 1991). When data from 2002 were averaged across litter rates, harvesting annual ryegrass in addition to bermudagrass increased total N uptake by 35%, Cu uptake by 42%, and Zn uptake by 46% (Table 5). Similar results were obtained from a one-time harvest in spring 2004, suggesting strong residual effects from fertilizing a Ruston soil with broiler litter. Due to increased DM yield, the amounts of Cu and Zn removed by haying increased in 2003, but total uptake did not differ between forage systems. Brink et al. (2001) reported annual ryegrass removed more Cu and Zn than three small grains and two clover species grown on a Savannah soil (fine-loamy, siliceous, semiactive, thermic Typic Fragiudults) with high fertility due to a long history of litter application. In a swine effluent spray field, ryegrass–bermudagrass removed about 108% more Cu and 84% more Zn than bermudagrass winter fallow system (Rowe et al., 2006).

View larger version (21K): [in this window] [in a new window]   Fig. 3. Relationships between Mehlich-3–extractable P and water-extractable P in surface soil (0- to 15-cm depth) and antecedent broiler litter rate at four sampling dates in bermudagrass winter fallow and ryegrass–bermudagrass systems. Values represent the mean (±1 SE) of four observations.

View larger version (14K): [in this window] [in a new window]   Fig. 4. Evolution of Mehlich-3–extractable P in surface soil, 0- to 15-cm depth, across five sampling dates in bermudagrass winter fallow and ryegrass–bermudagrass systems following the cessation of (A) 4.5 Mg ha–1 yr–1, (B) 9 Mg ha–1 yr–1, (C) 18 Mg ha–1 yr–1, and (D) 36 Mg ha–1 yr–1 of broiler litter in 1999–2001. Bermudagrass plots were split and half was overseeded with annual ryegrass in fall 2001–2003, and whole plots were fertilized with 268 kg N ha–1 yr–1 (34–0–0) in 2002 and 2003.

Because 9 Mg ha^–1 litter is often applied to meet bermudagrass N requirement (Table 2), further analysis emphasized the decline in soil P when 9 Mg ha^–1 litter rate ceased (Table 4). Averaged across forage systems, M3P at the 0- to 5-cm depth decreased linearly by about 125 mg kg^–1 across sampling dates. Averaged across sampling dates, M3P at 0- to 5-cm depth was slightly lower (P < 0.12) in ryegrass–bermudagrass than bermudagrass (169 vs. 180 mg kg^–1); however, M3P at the 5- to 15-cm depth was similar in both forage systems. Indeed, decreases in M3P were almost exclusively confined to 0 to 5 cm, with decreases at 5- to 15-cm soil depth observed only at 0 and 4.5 Mg ha^–1 litter. While decreases in total P were less consistent with the repeated hay harvests, values were significantly lower in May 2004 than initial values in December 2001 (Table 4). We observed a sharp increase in total P between April and October 2003 that approached total P found in the soil in December 2001 after litter application ceased. Although this is difficult to explain it was associated with high P uptake by forages in 2003 that led to relatively large reductions in M3P and WEP. Short-term decreases in WEP suggest management of high soil P (levels in excess of crop needs) through double-cropping forages will decrease the potential for P export in surface runoff (Pote et al., 2003). Measurement of WEP is important because it includes forms of soil P that can be immediately bioavailable and thus have high potential to contribute to environmental contamination (Tasistro et al., 2004). Similarly, a decrease in M3P concentration at the 0- to 5-cm depth has important water quality implications, because the soil surface is the most active zone of P detachment or P solubilization to cause increased runoff P losses to the environment. Sharpley (1995) found M3P at 0- to 1-cm soil depth was related uniquely and linearly (r² > 0.90) to dissolved P concentration in runoff water from 10 different soils when broiler litter was incorporated to a depth of 5 cm.

At 9 Mg ha^–1 litter yr^–1, values for M3P generally decreased across sampling dates. Based on average hay yields in 2003, the removal of about 42 kg ha^–1 P in forage decreased M3P by about 25 g kg^–1 (Fig. 4) and WEP by about 3 g kg^–1. Based on these results and conditions in the present study, and providing P uptake and soil P are related linearly, harvesting hay for 2 yr would lower M3P to an acceptable agronomic level (i.e., slightly below 70 mg kg^–1) above which the likelihood that environmental problems from high P soils may occur. Interestingly, soil test P determined in December 2001 after the 9 Mg ha^–1 litter rate ceased, was very similar to the amount of P supplied by litter each year, about 120 kg ha^–1 (Tables 2 and 3). Because maximum P uptake by bermudagrass in Ruston soil is about 60 kg ha^–1 (Read et al., 2006), some of the litter P was apparently absorbed by the soil and converted to total P. This may be related to the short history of litter application to this bermudagrass hay field.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
A 3-yr study was conducted on Ruston soil with no history of broiler litter application. Repeated applications of 4.5 to 36 Mg ha^–1 litter yr^–1 led to increases in soil fertility, biomass yield, and nutrient uptake. During the drawdown phase, the concentration of M3P in surface soil, 0- to 15-cm depth, decreased the most at antecedent litter rates of 18 and 36 Mg ha^–1 yr^–1. Harvesting annual ryegrass in addition to bermudagrass did not result in significantly lower M3P or WEP in soil either at the 0- to 15-cm depth, a zone of high root activity, or the 0- to 5-cm depth, a zone of high P accumulation. Ryegrass–bermudagrass removed more P than bermudagrass in 2001–2002, when rainfall was below average. This result suggests greater remediation potential by double-cropping these forages when rainfall distribution during summer is less than adequate for optimum bermudagrass yield. Because N fertilizer was provided to optimize warm-season bermudagrass hay yields, further studies should determine if improved N fertility and harvest management of annual ryegrass enhances removal of soil P by the cool-season ryegrass component. Results quantify the removal rate of P from soils of common forage crops like bermudagrass and annual ryegrass, which may be useful in developing forage management strategies for reducing or avoiding high soil P levels.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Journal no. J-11036 of the Mississippi Agric. and Forestry Exp. Stn. Mention of a trademark, proprietary product, or vendor does not constitute a guarantee or warranty of the product by the USDA and does not imply its approval to the exclusion of other products or vendors that also may be suitable.

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TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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