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Rowena B. Valencia-Gica, Russell S. Yost, Guy S. Porter, and Rosalin Pattnaik. Abstract: ...... Mclaughlin, M. R., K. R. Sistani, T. E. Fairbrother, D. E Rowe. 2005.

TECHNICAL ARTICLE

Retention, Accumulation, and Movement of Phosphorus in a Mollisol Soil Irrigated With Dairy Effluent in a Tropical Environment Rowena B. Valencia-Gica, Russell S. Yost, Guy S. Porter, and Rosalin Pattnaik Abstract: Dairy operations generate large quantities of effluent, which are stored in constructed lagoons. Lagoons, however, have finite storage capacity and can overflow, potentially polluting land and associated water bodies. Alternative uses of effluent are, therefore, needed for a more sustainable and environment-friendly dairy production. This study assesses the effects of effluent irrigation on the retention, accumulation, and movement of phosphorus. Four grassesVBana (Pennisetum purpureum K. Schumach.), California (Brachiaria mutica [Forssk.] Stapf.), Star (Cynodon nlemfuensis Vanderyst), and Suerte (Paspalum atratum Swallen)Vwere subsurface (20Y25 cm) drip irrigated with effluent at two rates based on potential evapotranspiration (ETp) at the site (Waianae, HI)V2.0 ETp (16 mm dj1 in winter; 23 mm dj1 in summer) and 0.5 ETp (5 mm dj1 in winter; 6 mm dj1 in summer). Treatments were arranged in an augmented completely randomized design. Most of these grasses produced large amounts of dry matter with effluent irrigation. California grass receiving the 2.0 ETp effluent application outyielded all other grass species, producing 60 Mg haj1 yearj1. Olsen soil phosphorus (P) and soil solution P did not significantly increase despite daily irrigation for at least 2 years and when irrigated at 2.0 ETp. A relatively small amount of P was measured at deeper soil depths at the 2.0 ETp irrigation rate (131 kg haj1 yearj1). Calcium-phosphate precipitation was predicted by calculated phosphate potentials. Acid-extractable soil P increased, supporting the hypothesis of Ca-P precipitation. Key words: Phosphorus, dairy effluent, subsurface drip irrigation, tropical grasses, tropical environment. (Soil Sci 2010;175: 500Y510)

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airy producers are highly dependent on imported feeds to sustain dairy cattle operation, especially in island environments such as in Hawaii. The dairies generate large quantities of wastewater, which are often stored in open constructed lagoons. A result is a tremendous influx and accumulation of nutrients and salts from livestock feces and urine, which can pose a threat to the land receiving the effluent as well as nearby water bodies. Effluent lagoons can overflow during the rainy season. Apart from nutrients, effluents normally contain considerable amounts of pathogens and decomposition products, other microbial contaminants and suspended solids, an excess of which could result in soil and water pollution. Lagoons have finite capacity to contain the effluent and are expensive to construct and manage. If dairy production is to survive especially in island environments, sustainable and environmentally friendly uses of effluent are needed.

Department of Tropical Plants and Soil Sciences, University of Hawaii at Manoa, 3190 Maile Way, St. John 102, Honolulu, HI 96822. Dr. Rowena B. Valencia-Gica is corresponding author. E-mail: [email protected] Received March 25, 2010. Accepted for publication August 16, 2010. Copyright * 2010 by Lippincott Williams & Wilkins ISSN: 0038-075X DOI: 10.1097/SS.0b013e3181f79669

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Effluent irrigation to produce forage, which can then be fed to the dairy animals, would be an attractive alternative that recycles the nutrients and water from effluent and partially Bcompletes[ the otherwise Bopen[ cycle of nutrients in island milk production systems. A recycling strategy not only allows producers to save on feed costs but also minimizes the entry of imported nutrients into an island environment reducing environmental hazards associated with the effluent, thus, creating a win-win option. However, there is a concern about the accumulation of phosphorus (P) and downward movement of nutrients when applying effluent to the soil. Phosphorus accumulation has been a major issue with concentrated and intensive animal feeding operations and where animal wastes are repeatedly applied (Sharpley et al., 1984). Repeated manure applications onto the same fields in Coastal Plain soils resulted in excessive soil test P concentrations (Sharpley and Withers, 1994; Sharpley et al., 1984). In Hawaii, P accumulation has already been reported for some soils currently used for pasture, for example, soil samples at 0 to 20 cm in a dairy pasture where effluent had been applied exhibited elevated levels of P (up to 1,055 mg kgj1 of Modified Truog P) (R. Yost, personal communication). The soil was an Oxisol with moderate P sorption capacity (Fox and Searle, 1978). A study in Texas involving dairy effluent fertilization of BAlamo[ Switchgrass (Panicum virgatum L.) planted as filter strip, indicated that soil P increased with increasing rates of effluent application (Sanderson et al., 2001). Phosphorus accumulated in systems that received both short-term (Johnson et al., 2004) and long-term (Redding, 2001) animal effluent application. Phosphorus has been generally considered immobile, being readily sorbed by the soil, yet, its downward movement has been reported in undisturbed soil core experiments (Phillips, 2002), grasslands used for effluent application (Toor et al., 2004; Barton et al., 2005), horticultural crop land fertilized with pig effluent (Redding, 2001), and in areas of high animal manure application or extensive animal feeding operations (Hansen et al., 2002; Mozaffari and Sims, 1994). Holford et al. (1997) examined the effects of dairy, pig, and sewage effluent and other materials containing P on the P sorption characteristics of two Australian soil groups. They reported that P leaching and downward movement might commence before all the P sorption sites are occupied. Various tropical grasses and crops have been used for phytoremediation of N and P from wastewater (e.g., McLaughlin et al., 2005; Woodard et al., 2002). Although some studies investigated the use of subsurface drip irrigation in applying animal effluent (Stone et al., 2008; Cantrell et al., 2009), few studies have determined the potential of tropical grasses for subsurface drip irrigation. Whereas studies on effluent application have usually been based on N and P loading or on the crop requirement, this study implemented a weather-based effluent management scheme. In this approach, the rates of effluent applied were adjusted as needed based on the changes in rainfall and ETp rates. This approach thus differs by targeting the irrigation requirement and Soil Science

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then assessing the resulting N and P status in the soil, rather than basing applications on arithmetic estimates of N and P requirements of the crops. This contrast may be important in implementing management strategies that dairy farmers in Hawaii and elsewhere can consider to pursue economic and ecological sustainability. Under conditions with high ETp, however, this strategy may lead to excessive application of N and P. This study was conducted to determine a practical effluent application rate and assess the effects of dairy effluent irrigation on the retention, accumulation, and movement of nutrients in the soil planted to highly productive tropical forage species. The focus of this article is on the accumulation and movement of P in the soil irrigated with dairy effluent.

MATERIALS AND METHODS Soil and Climate The experimental site was in Waianae (21-26¶56µN/ 158-10¶37µW), O’ahu, HI, with a mean elevation of 2.4 m (8 ft) above sea level, annual rainfall of 150 mm, and average ETp of 5 to 21 mm dj1, with the minimum occurring during the winter season of November to January and the maximum occurring in the summer months of June to August. Rainfall is very low and

Phosphorus Dynamics in Effluent-Irrigated Soil

the ETp is, thus, high compared with other locations on the island. The soil (Pulehu series) is a fine-loamy, mixed, semiactive, isohyperthermic Cumulic Haplustoll (Soil Survey Staff, 1972) and has 63 g kgj1 clay, 19 g kgj1 silt, and 18 g kgj1 sand.

Experimental Design In 12 plots, each measuring 13.4 m2, four tropical grass species that have demonstrated tolerance to saline soil in Waianae (ECspe = 11Y26 dS mj1) were planted in December 2003VBanagrass (cv. HA 5690), California grass, Stargrass, and Suerte grass. In this article, Bplots[ were the experimental units. Grasses were allowed to establish to 100% groundcover with freshwater irrigation (30 min, twice a day) until July 2004. After establishment, two rates of effluent irrigation were applied to Suerte grass and Banagrass with two replications each. Augmented treatments (single replications) were California grass (added beginning February 2005) and Stargrass (added at the beginning of the experiment) that also received two rates of effluent irrigation, patterned after the augmented block design developed by Federer (1956, 2005). The irrigation rates were based on the ETp at the site: 2.0 ETp (16 mm dj1 in winter; 23 mm dj1 in summer) and 0.5 ETp (5 mm dj1 in winter; 6 mm dj1 in summer). From August 2004 to August 2006, the plots were

FIG. 1. Solar radiation, potential evapotranspiration in Waianae, O’ahu, HI, and selected chemical properties of dairy effluent used for irrigating tropical grasses, August 2004YAugust 2006. * 2010 Lippincott Williams & Wilkins

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FIG. 2. Dry matter production of tropical grasses irrigated with dairy effluent, O’ahu, HI, August 2004YAugust 2006.

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irrigated accordingly through a subsurface (20- to 25-cm depth) drip effluent irrigation system. The experimental emphasis was on change in forage and soil properties, which were measured every 4 to 6 weeks during the 2-year experiment. The emphasis on temporal measurement and change permitted testing the hypothesis of no change in forage production, forage quality, and soil properties over time with effluent irrigation. Rainfall and evapotranspiration data were collected using the Hobo (Onset Computer Corporation, Bourne, MA) weather station installed at the site. The ETp was calculated using the Reference Evapotranspiration Calculation and Software (RefET v. 2.0), which uses the Penman equation (Allen, 2001). The monthly ETp was calculated as the average of the daily ETp for a given month, whereas the rainfall was the total of the daily rainfall for a given month. Nutrient content of dairy effluent was analyzed periodically for estimating nutrient loads (Fig. 1). At the 2.0 ETp rate, cumulative nutrient loads were estimated at 1,200 kg N haj1 yearj1, 620 kg P haj1 yearj1, 42,000 kg K haj1 yearj1, 2,900 kg Ca haj1 yearj1, and 5,900 kg Mg haj1 yearj1. At the 0.5 ETp rate, cumulative

FIG. 3. Extractable (Olsen) phosphorus at 0- to 15-cm and 15- to 30-cm depths of soils from plots irrigated with dairy effluent at the low and high rates, Waianae, O’ahu, HI. †Summer average rate was 23 mm dj1 and winter average rate was 16 mm dj1; ‡Summer average rate was 6 mm dj1 and winter average rate was 5 mm dj1; §Before irrigation installation; ¶One year after irrigation installation which is the grass establishment period.

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Phosphorus Dynamics in Effluent-Irrigated Soil

nutrient loads were estimated at 370 kg N haj1 yearj1, 190 kg P haj1 yearj1, 12,000 kg K haj1 yearj1, 850 kg Ca haj1 yearj1, and 1,700 kg Mg haj1 yearj1.

Grass Harvesting, Soil, and Soil Solution Sampling The grasses were well established for a first measured harvest (using a gas-powered lawn mower) in August 2004. A 0.3-m border area on each side of the plot was excluded in yield calculations, resulting in an effective biomass harvest area of 9.29 m2 per plot. The harvesting interval was usually 27 to 29 days, which was close to the target harvesting interval of 30 days on the first year (August 2004 to July 2005). During the second year (August 2005 to August 2006), the harvesting interval was extended to 6 weeks based on the first year’s results and occurred after 42 to 44 days of growth. Before the application of the treatments, soil samples were collected at the 0- to 15-cm and at 15- to 30-cm depths. From the first harvest onward, soil samples were taken at monthly interval at two depths (0Y15 cm and 15Y30 cm) from all 12 plots within a week after each harvesting. Soil samples were submitted to the Agricultural Diagnostic Service Center at the University of Hawaii for analysis. Assessment of the impact on soil properties involved analysis of soil samples for pH, electrical conductivity (EC), total nitrogen, organic carbon, P, K, Ca, Mg, and Na. The changes in soil properties with time at both depths were, thus, monitored to determine whether an accumulation of nutrients or salts might occur and, if so, at what depth. Results of changes in nutrients other than P will be presented in associated articles. Tension lysimeters were installed in 8 of 12 plots representing the following treatments: Suerte grass (0.5 ETp and 2.0 ETp), California grass (0.5 ETp and 2.0 ETp), Banagrass (2.0 ETp), and Stargrass (2.0 ETp). Soil solution samples were collected (at depths of 15 cm, 35 cm, 70 cm, and 100 cm) also within a week after harvesting to help assess the downward movement of nutrients from the application zone (15Y30 cm). Soil solutions were analyzed for pH, EC, P, K, Ca, Mg, Na, NO3-N, total N, and NH4-N. The flux of solute was calculated as water flux (mm moj1) multiplied by the concentration of solute (mg Lj1) in the soil solution. The water flux for each irrigation treatment was calculated using the water balance method as follows: W aterFlux ðmm mo1 Þ ¼ ðIrrigation; mm mo1 þ Rain; mm mo1 Þ  ET p; mm mo1

methods. Phosphorus sorption experiments were conducted following the modified Fox and Kamprath (1970) method (Linquist et al., 1997). A 0.02 M potassium chloride (KCl) solution was used as the background electrolyte instead of 0.01 M calcium chloride (CaCl2) to reduce the effect of Ca from the background electrolyte and, more importantly, reduce the confounding effects of the background electrolyte on P sorption. Duplicate samples of soils (3-g oven-dry weight basis) collected from the field experiment at 6-month intervals beginning in July 2003 were equilibrated for 6 days after the addition of five different P concentrations (0, 50, 100, 250, 500 mg kgj1). The pH (using a pH meter, Oakton pH 510, Deerfield Beach, FL; Fischer Scientific Accumet 815, Hampton, NH), EC (using an EC meter, Mettler Toledo NC 226, Columbus, OH), and Ca content (using the inductively coupled plasma-atomic emission spectroscopy [ICP-AES]) of the equilibrium P solutions were also obtained. Dairy effluent and soil solution samples were analyzed for P using the ICP-AES method (Hue et al., 1997). Some studies (e.g., Phillips, 2002) usually measured the dissolved reactive P, which is a measure of orthophosphateVthe soluble inorganic (filterable) fraction of P and the form taken up by the plants. But the ICP-AES method was chosen in this study because it represents all the species of P that are present in the solutions, dissolved or particulate, that are found in the sample and was, therefore, of greater interest when studying wastewater such as dairy effluent because of its important effects on soil and plant properties. Dairy effluent and soil solutions were also analyzed for total Kjeldahl

ð1Þ

The changes in effluent properties during the period of the experiment were monitored (Fig. 2). The effluent’s sodium absorption ratio (SAR), calculated from: Naþ ðmeq L1 Þ ð2Þ SAR ¼ rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Ca2þ ðmeq L1 Þ þ M g 2þ ðmeq L1 Þ 2 and EC have both remained low (1.3Y2.2 and 3.0Y3.6 dS mj1, respectively) throughout the period, indicating the effluent’s suitability for irrigation. The exchangeable sodium percentage of the soil was also determined and reported elsewhere (Valencia-Gica, 2007) to assess the impact of effluent application on sodium accumulation.

Laboratory Analyses Total nitrogen in the soil was determined following the micro-Kjeldahl method (Bremner and Mulvaney, 1982). Extractable P in soils was analyzed following the Olsen (Olsen et al., 1954; Olsen and Sommers, 1982), Modified Truog (Hue et al., 1997), and hydrochloric acid (HCl) (Guo et al., 2000) * 2010 Lippincott Williams & Wilkins

FIG. 4. Soil solution total phosphorus collected at 15- to 100-cm depths from plots irrigated with dairy effluent at the high rate, Waianae, O’ahu, HI. †Summer average rate was 23 mm dj1 and winter average rate was 16 mm dj1; ‡Summer average rate was 6 mm dj1 and winter average rate was 5 mm dj1. www.soilsci.com

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nitrogen following the modified procedure of Technicon’s Industrial Method No. 376-75W of August 21, 1975 (Technicon, 1977).

Data Analysis The system operated continuously for more than 4 years (2004Y2008), but data collection and analysis were done only during a 2-year (2004Y2006) period because of funding constraints. This study used the SAS program in analyzing data from incomplete block designs of the augmented type (Federer, 2005) and in quantifying trends over time using repeated-measures analysis (SAS Inst., 2004). Best subset procedures were conducted using MINITAB 14.13 (Minitab Inc., 2004) and the main treatments, interaction effects, and the dynamics in yield, soil, and soil solution P (reduced model) were analyzed with repeatedmeasures analysis of variance using an autoregressive firstorder covariance structure provided by PROC MIXED SAS 9.1 software (SAS Institute, 2004). Means were compared using the least significant difference (LSD). Data were plotted using SigmaPlot 9.0 (SYSTAT Software Inc., 2004). To facilitate the data analysis and discussion, the Btime[ variable was used to refer to sampling dates that corresponded to the varying amount of effluent irrigation over time at each of the two irrigation rates.

RESULTS Dry Matter Production The tropical grasses irrigated with effluent exhibited relatively slow growth rates during the first 15 to 20 days, after

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which these species grew rapidly (data not shown). Most of these grasses produced large amounts of dry matter with effluent irrigation (Fig. 2) that compared well with or were higher than those commonly reported in the literature for these species. California grass receiving the 2.0 ETp effluent irrigation rate (57 Mg haj1 yearj1) outyielded Stargrass, receiving 0.5 and 2.0 ETp effluent irrigation rates (35Y40 Mg dry matter haj1 yearj1) (P G 0.01). Banagrass irrigated at 0.5 ETp and 2.0 ETp effluent irrigation rates produced the second highest dry matter (51Y53 Mg haj1 yearj1), which were significantly different from dry matter yields of Suerte grass irrigated at those rates (38Y40 Mg haj1 yearj1).

Soil and Soil Solution Phosphorus The extractable soil P before the installation of the irrigation system (April 2003) and after the installation (July 2003) of the irrigation system was already high (93Y178 mg kgj1 and 81Y176 mg kgj1, respectively). The soil P levels did not significantly increase despite repeated effluent application between July 2003 and Aug 2006 (P 9 0.05). Extractable soil P was generally higher at the 0- to 15-cm depth (141 mg kgj1 in July 2003 and 163 mg kgj1 in Aug 2006) than at the 15- to 30-cm depth (91 mg kgj1 in July 2003; 151 mg kgj1 in August 2006) (Fig. 3). This was possibly caused by the capillary movement of water that carried with it the nutrients from the application zone (20Y25 cm depth) to the surface considering the very high ETp at the site. Relatively higher extractable soil P was obtained in March to August 2006 when higher amounts of effluent were

TABLE 1. Calculated Monthly Water and P Flux at the Tropical Grass Plots Irrigated With Dairy Effluent, Waianae, O’ahu, HI Irrigation Rate, mm moj1 Month/Year September 2004 October 2004 November 2004 December 2004 January 2005 February 2005 March 2005 April 2005 May 2005 June 2005 July 2005 August 2005 September 2005 October 2005 November 2005 December 2005 January 2006 February 2006 March 2006 April 2006 May 2006 June 2006 July 2006 August 2006

2.0 ETp†

0.5 ETp‡

270 434 420 434 434 336 372 390 1381 1336 690 690 690 690 956 531 531 637 637 637 637 617 637 637

120 155 150 155 155 112 124 98 345 334 159 159 159 159 239 133 133 159 159 159 159 154 159 159

Total Rainfall, ETp, mm moj1 mm moj1 4 4 187 125 109 21 27 15 0 0 0 0 0 0 0 0 29 19 331 19 5 0 0 0

300 200 192 171 157 200 248 296 482 499 579 641 599 538 0 374 430 355 290 328 467 499 560 620

Water Flux, mm moj1 2.0 ETp†

0.5 ETp‡

j26 238 415 388 386 157 151 109 899 837 112 50 91 152 690 157 131 302 679 328 176 118 77 17

j176 j41 145 109 107 j67 j97 j184 j137 j165 j419 j481 j440 j379 239 j241 j267 j176 201 j150 j302 j345 j401 j461

P Concentration, P Flux, mg Lj1§ kg haj1 moj1 4.7 4.1 3.3 3.9 2.4 3.2 2.7 3.3 3.2 6.1 3.2 3.2 2.0 4.5 4.5 5.4 3.2 3.2 8.1 6.4 6.1 6.1 4.5 4.9

j1 10 14 15 9 5 4 4 29 51 4 2 2 7 31 9 4 10 55 21 11 7 3 1

Summer rate was 23 mm dj1 and winter rate was 16 mm dj1. Summer rate was 6 mm dj1 and winter rate was 5 mm dj1. § Soil solution P concentration at 15-mm depth. † ‡

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applied because of the higher ETp during this period. Understandably, plots irrigated at the 2.0 ETp rate had generally higher mean extractable soil P (156 mg kgj1) than those irrigated at 0.5 ETp (129 mg kgj1) (P G 0.05). Soil solution total P had generally declined with time (P G 0.05) at all sampling depths regardless of grass species and irrigation rate (Fig. 4). During the 2-year period of effluent irrigation, the levels of soil solution P (ranging from 1.5 to 11 mg Lj1) had been mostly below those values reported for soils receiving animal wastes (7Y8 mg P Lj1) (Pierzynski et al., 2005). These levels of soil solution P may still be high from an environmental perspective because the freshwater P concentration that may cause eutrophication ranges from 0.003 to 0.3 mg Lj1 (Pierzynski et al., 2005). On an agronomic perspective, however, although many grasses have low external P requirement (Fox, 1981), some species (e.g., kikuyu grass; Fox, 1969) require higher P in soil solution for optimum growth (Pant et al., 2004). Using the moisture content of the soil (30% at the 15- to 30-cm depth from the plots irrigated at the high rate) and a dry bulk density of 1.3 g cmj3, the 3.2 mg P Lj1 was equivalent to about 1.5 kg P haj1 (or 17.0 kg P haj1 yearj1), which approximates the amount of P that might have remained in the soil profile water. However, calculations for the 2.0 ETp irrigation rate using the monthly average soil solution total P and corresponding water flux for a given month showed an average annual P flux at the 0- to 15-cm depth of 141.0

Phosphorus Dynamics in Effluent-Irrigated Soil

(September 2004YAugust 2005) and 160.0 (September 2005Y August 2006) kg P haj1 yearj1. These values are much higher than the estimated 17.0 kg P haj1 yearj1 because of the fluctuations in the concentration of P in the drainage water (3.2Y6.1 mg Lj1) and the adjustments in the irrigation rates according to the estimated ETp, resulting in monthly water flux variations (Table 1). These results indicated that some downward movement of P may have occurred at the 2.0 ETp rate. At the site where this study was conducted, however, there was a dense clay layer at the 30- to 70-cm zone, which may have restricted further downward movement to deeper soil layers and groundwater, which is located 40 m to 80 m below the surface mean sea level (R. Whittier, personal communication). Calculations showed no water flux at the plots irrigated at the 0.5 ETp rate because the ETp exceeded the sum of the irrigation applied and the rainfall. The moisture content of the soil from the plots irrigated at the 0.5 ETp rate was also relatively lowVan average of 13% at the 0- to 15-cm depth and 20% at the 15- to 30-cm depth.

Phosphorus Sorption Sorption-desorption reactions control the accumulation and movement of P in the soil profile and quantitative descriptions of P sorption have commonly been made by fitting the data to Langmuir, Freundlich, or Temkin equations (Muneer and Lawrence, 2004; Villapando and Graetz, 2001). Soil samples collected at 6-month intervals between July 2003 and August

FIG. 5. Phosphorus sorption isotherm of soils at different sampling times, 0- to 15-cm and 15- to 30-cm depths, Waianae, O’ahu, HI. † Summer average rate was 23 mm dj1 and winter average rate was 16 mm dj1; ‡Summer average rate was 6 mm dj1 and winter average rate was 5 mm dj1. * 2010 Lippincott Williams & Wilkins

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2006 were used in the P sorption experiment and extraction with the recommended extractant (Olsen) as well as with acidic extractants (Modified Truog and HCl). Most of the data fit the Freundlich model better than the Langmuir model (data not shown). The soil samples taken before the effluent irrigation began (July 2003) had the highest P sorption capacity (Fig. 5). Consistent with our expectation, the P sorption capacity of the soils receiving effluent decreased in August 2004 at both 0- to 15-cm and 15- to 30-cm depths. Previous studies have shown that continuous manure and biosolids application on soils could decrease the P sorption capacity of the soil (i.e., greater increase in soil solution P per unit of added P) (Sui and Thompson, 2000; Phillips, 2002; Holford et al., 1997). In our case, however, the P sorption isotherm shifted upward in February 2005 until August 2006, indicating a slight increase in the P sorption capacity of the effluent-irrigated soil. These findings could be a result of exceeding the equilibrium concentration of an insoluble P compound, whereas further additions of P did not increase the equilibrium solution P; it only increased the amount of sorbed P.

DISCUSSION The extractable soil P values, regardless of treatment and sampling dates, were much higher than the adequate P level

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(25Y35 mg kgj1) established for heavy soils in Hawaii (Tamimi et al., 1997). This was expected for a soil with very low capacity to retain P compared with other soil orders (Fox and Searle, 1978). However, recent findings on short-term leafy vegetable P requirement showed that these soil P values were not excessive (J. Deenik, personal communication). The extractable soil P levels in the effluent-irrigated grass plots were possibly higher than those reported for Mollisol (Hawi and Waialua) kikuyu grass (Pennisetum clandestinum Hochst. ex Chiov.) pastures (110Y296 mg Modified Truog P kgj1) on the Island of Hawaii receiving dairy waste application (Mathews et al., 2005). Continuous application of animal effluent especially at high rates over a number of years is expected to eventually exhaust the P sorption sites and lead to increases in extractable P in the future. This has already been reported for areas where animal wastes have been continuously applied (Sanderson et al., 2001; Johnson et al., 2004; Redding, 2001). The previously mentioned trend in extractable soil P and soil solution total P bear an important relationship with the results of the P sorption study and the use of acidic extractants. Although the P sorption capacity of the effluent-irrigated soil increased between August 2004 and February/August 2005/ 2006, this sorbed P appeared to be not plant available, as indicated by the nearly constant or decreasing soil solution total

FIG. 6. A comparison of the extractable soil phosphorus between various extractants for soils collected from plots irrigated with dairy effluent, Waianae, O’ahu, HI. †Summer average rate was 23 mm dj1 and winter average rate was 16 mm dj1; ‡Summer average rate was 6 mm dj1 and winter average rate was 5 mm dj1.

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Phosphorus Dynamics in Effluent-Irrigated Soil

TABLE 2. Calculated Phosphate Potential‡ for Various Calcium-Phosphate Compounds That May Have Formed in a Soil Planted With Tropical Grasses and Irrigated With Dairy Effluent, Waianae, O’ahu, HI

Irrigation Rate/Sampling Depth

Monocalcium Phosphate Soil pH

Dicalcium Phosphate Dihydrate

Octacalcium Phosphate

Tricalcium Phosphate

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Log H2PO4j - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

ETp†,

2.0 15Y30 cm August 2004 February 2005 August 2005 February 2006 August 2006 0.5 ETp‡, 15Y30 cm August 2004 February 2005 August 2005 February 2006 August 2006 2.0 ETp†, 15Y30 cm August 2004 February 2005 August 2005 February 2006 August 2006 0.5 ETp‡, 15Y30 cm August 2004 February 2005 August 2005 February 2006 August 2006 † ‡

8.2 8.4 8.3 8.4 8.7

0.9 1.0 1.0 1.0 0.9

j3.2 j3.2 j3.0 j3.1 j3.5

j3.4 j3.4 j3.2 j3.4 j3.9

j4.1 j4.1 j3.9 j4.1 j4.7

8.2 8.3 8.3 8.2 8.4

0.9 0.9 1.0 0.8 0.8

j3.2 j3.2 j3.1 j3.3 j3.4

j3.4 j3.5 j3.3 j3.6 j3.7

j4.1 j4.2 j3.9 j4.3 j4.4

7.9 8.2 8.2 8.3 8.5

0.8 1.0 1.0 0.9 0.9

j3.1 j2.9 j3.0 j3.1 j3.4

j3.2 j3.0 j3.1 j3.4 j3.8

j3.8 j3.6 j3.8 j4.0 j4.5

8.0 8.2 8.2 8.2 8.4

0.9 1.0 1.0 0.9 0.8

j2.9 j3.0 j3.0 j3.1 j3.4

j3.1 j3.2 j3.1 j3.4 j3.8

j3.7 j3.8 j3.7 j4.0 j4.5

Summer rate was 23 mm dj1 and winter rate was 16 mm dj1. Summer rate was 6 mm dj1 and winter rate was 5 mm dj1.

P with time. It is, thus, hypothesized that the some of the P in the soil may be precipitating with Ca, given the very high amounts of Ca added to the soil from effluent (Fig. 1) plus the already high Ca content of the soil in the site. In addition, the pH of the soil

was already initially high (7.4Y7.6 in July 2003), and with continuous irrigation of high pH effluent (7.9Y8.8), the soil pH further increased up to 8.5 after 2 years. The pH of the equilibrium P solutions in the P sorption experiment was also high

TABLE 3. Average Tissue P Concentration, Annual P Uptake, and P Removal of Tropical Grasses Irrigated With Dairy Effluent, Waianae, O’ahu, HI, August 2004YAugust 2006 Irrigation Rate 2.0 ETp†

0.5 ETp‡

Grass Species

Plant Tissue P, %

Plant P Uptake, kg haj1 Yearj1

P Removal, % of Applied Effluent P

California Grass (B. mutica) Stargrass (C. nlemfuensis) Suerte Grass (P. atratum) Banagrass (P. purpureum) California Grass (B. mutica) Stargrass (C. nlemfuensis) Suerte Grass (P. atratum) Banagrass (P. purpureum)

0.29 0.32 0.20 0.30 0.27 0.40 0.24 0.34 0.04

157 124 85 153 116 133 98 158 26

22 19 13 21 53 65 50 86 13

LSD0.05 † ‡

Summer rate was 23 mm dj1 and winter rate was 16 mm dj1. Summer rate was 6 mm dj1 and winter rate was 5 mm dj1.

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TABLE 4. Average Micronutrient Concentration in the Tissues of Grasses Irrigated With Dairy Effluent and Average Micronutrient Content of Dairy Effluent, Waianae, O’ahu, HI, August 2004YAugust 2006 Irrigation Rate ETp†

2.0

0.5 ETp‡

Grass

Fe

Mn

Zn, mg kgj1

Cu

B

California Grass (B. mutica) Stargrass (C. nlemfuensis) Suerte Grass (P. atratum) Banagrass (P. purpureum) California Grass (B. mutica) Stargrass (C. nlemfuensis) Suerte Grass (P. atratum) Banagrass (P. purpureum)

207 405 1244 416 266 387 1108 526 424 0.46

81 105 107 56 64 66 98 51 15 0.06

66 53 88 68 62 56 96 56 30 0.01

11 9 11 13 10 11 10 10 2 0.16

15 11 32 20 12 12 25 15 4 0.71

LSD0.05 Average micronutrient content of dairy effluent, mg Lj1 Summer rate was 23 mm dj1 and winter rate was 16 mm dj1. Summer rate was 6 mm dj1 and winter rate was 5 mm dj1.

† ‡

(data not shown). These conditions are very favorable for the formation of various Ca-P compounds. In his review on soil phosphorus, Larsen (1967) suggested that at equilibrium concentrations of 20 mg P Lj1 or higher, it is more likely for precipitation reactions than usual sorption reactions to explain the removal of P from the soil solution (Sui and Thompson, 2000). Some researchers who studied the impact of animal manure or effluent application on soil properties attributed the increased P sorption of the soil to the formation of Ca-P compounds (Robinson and Sharpley, 1996), particularly in soils with very high Ca content and high pH (Eghball et al., 1996; Wang et al., 1995). Higher extractable soil P was obtained when acidic extractants such as Modified Truog and HCl were used, indicating that some of the soil P pools are in a form, possibly as calcium precipitates, which is not extractable by the Olsen extractant (Fig. 6). Assuming 20% free Ca (N. Hue, personal communication) and using 70% average inorganic P measured in soil solution, calculations of the phosphate potential of the soil at the 0- to 15-cm and 15- to 30-cm depths (Table 2) and plotting the values in the solubility diagram for various Ca-P compounds (Lindsay, 1979) indicated that the soil is supersaturated with respect to the following Ca-P compoundsVdicalcium phosphate, octocalcium phosphate, and tricalcium phosphate. Results also suggest possible simultaneous formation of these different compounds that may also explain for a steady or no increases in Olsen-extractable soil P despite continuing P additions from effluent. It is also interesting to note that the P concentrations in tissues of the different grasses were mostly below the usual sufficient levels (Mills and Jones, 1996), despite being given a large cumulative effluent P application with time (Table 3). This suggests that not all of the P is plant available, further supporting the hypothesis that some of the P applied may be precipitated by Ca. The change in the availability of micronutrients is one of the important impacts of effluent application. Given the relatively high extractable P in the soil, the possibility of pronounced antagonistic interactions of Zn with P (Alloway, 2004) may not be ruled out. The growth of the grasses improved, and micronutrient concentrations in the forage tissues increased when given supplemental fertilization with micronutrients such as Zn, Fe, and Mn, in sulfate forms. Later results suggested that Cu

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may also become deficient. Micronutrient deficiency may also have been a consequence of the high soil pH, which resulted in micronutrient low plant availability. In addition, dairy effluent supplied only minimal quantities of micronutrients (Table 4). All grasses produced very high yields, but given the high soil pH, supplemental applications of micronutrients such as Zn, Fe, Cu, and Mn were necessary to maintain acceptable tissue levels of these nutrients (Table 4).

CONCLUSIONS No significant increase in extractable soil P with time, and especially with the high biomass productivity of effluent-irrigated grasses, was observed. The high Ca content and pH of both the soil and effluent may have resulted in the precipitation of P with Ca according to calculations from the solubility diagrams. This may have been reflected in the higher extractable soil P with the use of acidic extractants, the increase in P sorption capacity of the effluent-irrigated soil between August 2004 and February/ August 2005/2006, the low soil solution P, the generally low P concentrations in plant tissues, and the low P removal by some of the grasses. The duration of effluent application was relatively short, thus, additional monitoring is necessary to determine the longer-term impacts of dairy effluent application on plant and soil properties, especially P. Specifically, longer-term monitoring is needed to ensure that P will not accumulate in the surface soil or leach in the soil profile to become a threat to associated land and water bodies. ACKNOWLEDGMENTS The authors thank the USDA T-Star Program of the University of Hawaii at Manoa for the project funding and research assistantship support to the lead author. The authors also thank the CSREES/USDA 406 Water Quality Initiative Competitive Grants Program for partial funding. The authors also acknowledge Zhijun Zhou for the design and establishment of the irrigation system, Veronica Wilcox for grass planting and establishment, and fellows in the laboratory for their invaluable help in soil sample collection and grass harvesting. Note that reference herein to any specific product, by trade name, trademark, manufacturer, or distributor does not necessarily constitute or imply its endorsement or recommendation by the authors and publishers. * 2010 Lippincott Williams & Wilkins

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