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TECHNICAL REPORT

Waste Management

Fate of Phosphorus in Dairy Wastewater and Poultry Litter Applied on Grassland

^a Soil and Crop Sciences Department, Texas A&M University, College Station, TX 77843-2474
^b Texas A&M University Agricultural Research and Extension Center, Overton, TX 75684

^* Corresponding author (dvietor{at}tamu.edu).

Received for publication March 6, 2003.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Large and repeated manure applications can exceed the P sorption capacity of soil and increase P leaching and losses through subsurface drainage. The objective of this study was to evaluate the fate of P applied with increasing N rates in dairy wastewater or poultry litter on grassland during a 4-yr period. In addition to P recovery in forage, soil-test phosphorus (STP) was monitored at depths to 180 cm in a Darco loamy sand (loamy, siliceous, semiactive, thermic Grossarenic Paleudults) twice annually. A split-plot arrangement of a randomized complete block design comprised four annual N rates (0, 250, 500, and 1000 kg ha^–1) for each nutrient source on coastal bermudagrass [Cynodon dactylon (L.) Pers.] over-seeded with ryegrass (Lolium multiflorum L. cv. TAM90). Increasing annual rates of N and P in wastewater and poultry litter increased P removal in forage (P = 0.001). At the highest N rate of each nutrient source, less than 13% of applied P was recovered in forage. The highest N rates delivered 8 times more P in wastewater or 15 times more P in poultry litter than was removed in forage harvests during an average year. Compared with controls, annual P rates up to 188 kg ha^–1 in dairy wastewater did not increase STP concentrations at depths below 30 cm. In contrast, the highest annual P rate (590 kg ha^–1) in poultry litter increased STP above that of controls at depth intervals to 120 cm during the first year of sampling. Increases in STP at depths below 30 cm in the Darco soil were indicative of excessive P rates that could contribute to nonpoint-source pollution in outflows from subsoil through subsurface drainage.

Abbreviations: STP, soil-test phosphorus

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
DILUTION and downward movement of P from manure sources into soil can reduce potential nonpoint-source losses to surface water when subsoils limit P movement into ground water (King et al., 1990). Yet, studies of P losses through leaching and outflows of subsurface drainage have dispelled myths about the unlimited P sorption capacity of soils (Sharpley, 1996; Sims et al., 1998). Phosphorus leaching below a depth of 30 cm was observed for a range of soil textures when rates of manure and other organic wastes exceeded crop requirements (Whalen and Chang, 2001; Sims et al., 1998). In addition, preferential-flow pathways can develop through fine-textured soils and allow movement of both dissolved and particulate P to subsurface drainage pathways during heavy rain events (Simard et al., 2000). Moreover, organic forms of P in manured soils can exhibit greater downward mobility through subsoils than inorganic P (Sims et al., 1998).

Problematic outcomes of P leaching and transport through subsurface drainage include discharge through tile or drainage lines to surface waters. The discharge of P from artificially drained soils can contribute to nonpoint-source contamination of surface waters comparable with surface runoff losses (Sims et al., 1998). For example, probabilities of dissolved reactive P concentrations of >30 µg L^–1 in drainage outflow increased from 6 to 46% during three years of manure applications at average P rates of 225 kg ha^–1 yr^–1 (Hergert et al., 1981). As suggested in the example, P leaching and losses through subsurface transport to runoff are commonly associated with high rates and long-term application of animal manure (Sims et al., 1998). Similar to P losses to drainage water after high manure applications on cropland (Hergert et al., 1981), P leaching below the root zone of perennial grass could increase as N rates applied in wastewater and manure are increased.

Previous evaluations of P leaching described or classified potentially harmful events related to P losses in terms of "the risk of water contamination by soil P" (Simard et al., 2000). A P risk index for P movement in surface runoff from fields (Lemunyon and Gilbert, 1993, McFarland et al., 1998) specifies field events or characteristics that could be equally as relevant to evaluations of risk of P leaching. For example, STP and application rate, method, and timing of fertilizer and organic P sources are components of the P risk index for Texas (McFarland et al., 1998). Variation of P runoff losses has been monitored and related to variation of these site characteristics in evaluations of the P index applied on a national scale (Sharpley et al., 2001). In cases where subsurface P transport was important, preferential flow through soil macropores was added to variables considered in the P risk index in the northeastern USA. Similar to runoff losses, variation of P leaching and potential contributions to nonpoint-source pollution need to be evaluated in relation to site characteristics of agricultural fields and pastures.

The objective of this study was to evaluate the fate of P applied in two organic nutrient sources on grassland during a 4-yr period in a humid climate. Soil-test P was monitored at depths to 180 cm for a range of P rates applied in dairy wastewater and poultry litter on a coarse-textured soil.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Four rates of two different sources of organic nutrients were arranged in a split-plot design on a Darco loamy sand established to coastal bermudagrass sod at the Texas A&M University (TAMU) Agricultural Research and Extension Center, Overton, TX. Throughout the 4-yr study, the same commercial sources of dairy wastewater and poultry litter provided the two nutrient sources for main plots (18 x 5 m) within each of four replications. Subplots (3 x 5 m) comprised four annual N rates (0, 250, 500, and 1000 kg N ha^–1) and associated P applied in each nutrient source (Table 1). A 1.9-m strip of untreated bermudagrass provided a border between adjacent subplots. In addition, a 10-m buffer zone of bermudagrass separated Blocks 1 and 2 from Blocks 3 and 4. A 16-cm-high border of metal roof flashing was installed to a 6-cm depth around the lower one-third of the plot perimeters to minimize runoff losses on a 3% slope. Rainfall was monitored at the TAMU Research Center near the experimental site (Table 2).

Wastewater and poultry litter were sampled, digested, and analyzed before each application (Nelson and Sommers, 1980). An ammonium electrode was used to quantify total Kjeldahl nitrogen (TKN) in digests during 1992 and 1993 (Estin, 1976) and a Technicon (Tarrytown, NY) Autoanalyzer II was used in 1994 and 1995. Phosphorus concentration was quantified similar to Jackson (1958). A vanadomolybdic acid solution was used for color development, which was quantified on a Bausch and Lomb Spectronic 2000 spectrophotometer (Thermo Spectronic, Madison, WI) as percent transmittance at 440 nm. Except for one date during which lagoon wastewater was diluted by a large runoff event, N and P concentrations in wastewater and poultry litter were similar among application dates over the 4-yr study. Representative mean TKN concentrations with standard deviations were 1.23 ± 0.2 g kg^–1 in wastewater and 26.6 ± 0.2 g kg^–1 in poultry litter (Johnson, 1995). Mean total P concentrations with standard deviations were 0.23 ± 0.01 g kg^–1 in wastewater and 15.6 ± 2.3 g kg^–1 in poultry litter.

During October of each year, annual ryegrass seed (40 kg ha^–1) was broadcast on all treatments. Ryegrass growth occurred during the period from November to May. Forage was harvested in February, April, and at 30- to 45-d intervals from June 1 to October 30. Forage was sampled, dried, ground, digested, and analyzed according to methods described for wastewater and poultry litter (Jackson, 1958; Nelson and Sommers, 1980). One treatment of MSMA (monosodium acid methanearsonate) was applied in June of 1993 to control an invasion of thin paspalum (Paspalum setaceum Michx.).

A power probe (Giddings Machine Company, Ft. Collins, CO) was used to sample soil twice annually from each subplot. Sampling schedules were adjusted to avoid low soil water contents that precluded deep sampling of the profile. During Years 1 and 2, a single 10-cm-diameter soil core was taken from each subplot to minimize disturbance of the profile. During Years 3 and 4, four soil cores (5-cm diameter) were taken from each subplot and composited within 30-cm increments to a depth of 180 cm. After completing the experiment, four cores were removed outside of plots to depths of 0 to 30 and 30 to 60 cm for measurement of P sorption maximum. Sample holes were backfilled with a mixture of 80% fine blasting sand and 20% bentonite clay.

Soil samples were dried in a convection oven at 60°C for at least 48 h. Soil was ground (<0.85 mm) and the NH₄OAc–EDTA method was used to extract P in samples from treated plots (Hons et al., 1990). The molybdate–SnCl method was used to analyze STP in extracts (Bray and Kurtz, 1945). The P sorption maximum of the soil sampled outside of plots was measured over P solution concentrations from 0 to 13 µmol P L^–1 (Olsen and Watanabe, 1957; Nair et al., 1984). Concentrations of P in filtered wash solutions were analyzed through inductively coupled plasma–optical emission spectroscopy (ICP–OES) after 24 h of shaking at each concentration.

Data were analyzed as a split-plot design for each year of the 4-yr study. Waste sources were main plots and N rates were subplots. All statistical analyses were performed using SAS software (SAS Institute, 1995). Tukey's Studentized range test was used to evaluate significant differences (P = 0.05) between rates.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Phosphorus Recovery in Forage
Increases in waste loading rate significantly (P = 0.001) increased mean P removal in forage over four years of dairy wastewater or poultry litter applications (Tables 3 and 4). In addition, higher P rates in poultry litter than in wastewater significantly (P = 0.05) increased forage yield and P removal in forage. Forage uptake of P was consistently greater for poultry litter than for wastewater even though increasing water volumes were applied with respective N rates of wastewater (Tables 3 and 4). The irrigation effect of wastewater would have been greatest during low summer rainfall in 1993 and 1995 (Table 2), but the total depth of May and July applications at the highest wastewater rate was only 9% of mean seasonal rainfall.

Soil Analyses
The addition of P as dairy wastewater or poultry litter significantly (P = 0.001) increased mean STP concentration in the 0- to 30-cm depth during four years of observations. Yet, no significant changes of STP within or below the depth interval of 0 to 30 cm were observed after dairy wastewater application during the initial year (data not shown). At the end of the experiment, the excess of P applied in the highest rate of dairy wastewater was evident as significantly (P = 0.05) higher STP than the control within the 0- to 30-cm depth (Fig. 1) .

View larger version (24K): [in this window] [in a new window]   Fig. 1. Extractable soil P on the final sampling date (October 1996) for control and three annual P rates (kg ha–1) applied as dairy wastewater over four years on coastal bermudagrass over-seeded with annual ryegrass on a Darco loamy sand. Means followed by the same letter within depths are not significantly different (P = 0.05).

View larger version (27K): [in this window] [in a new window]   Fig. 2. Extractable soil P after the initial year (October 1993) of a 4-yr study for control and three annual P rates (kg ha–1) applied as poultry litter on coastal bermudagrass over-seeded with annual ryegrass on a Darco loamy sand. Means followed by the same letter within depths are not significantly different (P = 0.05).

View larger version (28K): [in this window] [in a new window]   Fig. 3. Extractable soil P on the final sampling date (October 1996) of a 4-yr study for control and three annual P rates (kg ha–1) applied as poultry litter on coastal bermudagrass over-seeded with annual ryegrass on a Darco loamy sand. Means followed by the same letter within depths are not significantly different (P = 0.05).

In addition to the high P rate applied in poultry litter, relatively high rainfall periods during 1992 through 1994 (Table 2) and the low mean P sorption maximum (0.748 mmol P kg^–1) of the loamy sand soil (0–60 cm) could have contributed to STP increases down to 120 cm (Fig. 2 and 3). A previous report of NO₃–N leaching to the 180-cm depth during 1992 through 1994 of the present study indicated that water percolated to the deepest depth sampled in the Darco soil (Johnson, 1995; Johnson et al., 1995). Similar to NO₃–N leaching, the percolation of water through the coarse-textured Darco soil would have favored transport of soluble P forms to the 120-cm depth after application in the highest poultry litter rate.

In contrast to years with above-average rainfall, low rainfall periods during 1995 and 1996 (Table 2), combined with high P rates, could have contributed to 50% increases in STP observed within sampling depths down to 60 cm for the highest poultry litter rate. Below-average rainfall after poultry litter applications could have limited P mineralization, percolation of water, and increases in STP at depths below 60 cm during the latter two years of the experiment.

The STP within the 0- to 30-cm depth, alone or as a source component of a multivariate P index, could be used to characterize the environmental risk to water quality of P applied in wastewater or poultry litter (Sims et al., 2000). The acidified NH₄OAc–EDTA method used to quantify STP, similar to other methods for measuring plant-available P, replaces or desorbs P in aluminum phosphates (Hons et al., 1990). In the present study, a STP concentration near 65 mg P kg^–1 or average annual P rate of 590 kg P ha^–1 in poultry litter was needed before STP was greater than an unfertilized control at depths below 30 cm (Fig. 2 and 3). Although this STP value can be interpreted in relation to P rates and changes of STP with depth in this experiment, any broader use for environmental purposes is constrained by variation of relationships between STP and P loss in water among soil types and P extraction methods (Pote et al., 1999).

Additional research is needed to correlate STP determined through the acidified NH₄OAc–EDTA method with water-soluble soil P and leaching losses of P for a range of soil and management conditions (Sims et al., 2000). The NH₄OAc–EDTA method provides a measure of STP that can correlate with STP determined through other methods (Hons et al., 1990). Yet, interpretation of STP from the NH₄OAc–EDTA or other extraction methods in relation to environmental risk of P leaching can be expected to vary with respect to other site-specific characteristics. For example, previous studies of diverse soils have identified a wide range of soil P concentrations (10–119 mg kg^–1 soil) above which P in drainage water was linearly related to soil P released from aluminum phosphates in 0.5 M NaHCO₃ (Hesketh and Brookes, 2000; Olsen et al., 1954).

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Observations of the fate of P raised three concerns about the sustainability of high rates of dairy wastewater or poultry litter on perennial grassland. First, P amounts applied with high N rates in wastewater and manure far exceeded P requirements and removal in bermudagrass and ryegrass forage. Second, the P sorption maximum of the Darco loamy sand was relatively low and excess P leached to depths greater than typically reported for large manure rates over long periods. Similar to previous evaluations of P index components in relation to runoff losses of P, increasing P rates in wastewater and poultry litter on the Darco loamy sand increased STP and potential P loss through leaching from the 0- to 30-cm depth. Downward movement in the Darco soil can diminish STP increases in the 0- to 30-cm depth and reduce the magnitude of STP ratings used in computations of P risk index for runoff losses. Yet, increases in STP to depths below 150 cm and losses to subsurface drainage under long-term applications of manure and wastewater need to be evaluated. This study related increases in STP with depth to rates of two organic nutrient sources for a coarse-textured soil in a humid climate, but a third concern is the present lack of information correlating STP to measures of environmental P for diverse soils and site conditions on a national scale.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Related articles in JEQ Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (9) Citing Articles via Google Scholar Google Scholar Articles by Johnson, A. F. Articles by Haby, V. A. Search for Related Content PubMed PubMed Citation Articles by Johnson, A. F. Articles by Haby, V. A. Agricola Articles by Johnson, A. F. Articles by Haby, V. A. Related Collections Animal Waste Other Forage Crops Nutrient Management Phosphorus