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a Dep. of Plant and Soil Sci., Univ. of Delaware, Newark, DE 19716-2170
b Centre for Soil and Environmental Quality, P.O. Box 84, Lincoln Univ., Canterbury, New Zealand
* Corresponding author (gurpal{at}udel.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Abbreviations: DRP, dissolved reactive phosphorus • DUP, dissolved unreactive phosphorus • FDE, farm dairy effluent • IR, irrigation • NIR, nonirrigation • PRP, particulate reactive phosphorus • PUP, particulate unreactive phosphorus • TP, total phosphorus • TUP, total unreactive phosphorus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Detailed information on the amounts, forms, and mobility of P in the soil is required if we are to manage dairy farming systems in an environmentally sustainable way (Toor et al., 2003, 2004a). Phosphorus inputs in the form of mineral P fertilizer and/or animal manures may be a significant nonpoint source of pollution as a result of P leaching, especially in intensive dairy pastoral systems (Toor et al., 2004b). While it has been reported that P loss by surface pathways in grassland systems seldom exceeds 2 kg ha–1 yr–1 (Haygarth and Jarvis, 1999; Hooda et al., 2000; Sharpley and Syers, 1979), this amount can have a significant impact on water quality and use (Condron, 2004; Toor et al., 2003).
Much of the research on P losses has been on the surface pathways because of the widely held supposition that P is strongly fixed in the soil and does not leach (Sims et al., 1998; Toor et al., 2003, 2004a). However, recently there has been a proliferation of literature on P leaching losses, although many of these studies have used either small leaching columns or unsealed monoliths. Cameron et al. (1992) have documented that the pattern of water flow and solute losses can be significantly different in unsealed and sealed lysimeters, and the latter gives the best indication of what happens in the field. Hence, there is a need to validate the results from these previous studies.
Moreover, research efforts should distinguish the transport of dissolved and particulate P in soils, as this will help to identify the transport mechanisms and the fate of P in the water body. Some workers have proposed that P loss in particulate forms become dominant during dry summer months, highlighting the importance of preferential flow (e.g., Heathwaite and Dils, 2000; Simard et al., 2000). Approximately 200000 ha of grazed pasture in Canterbury, New Zealand, are currently under spray or flood IR, and this area is expanding rapidly. Most of this grassland is under intensive dairy farming and receives 70 to 120 mm of IR every 3 to 4 wk during summer to sustain plant growth. It is likely that IR during summer will affect the amounts and forms of P loss from the soil by leaching. It is therefore important to study this variation so that the management strategies could be targeted at specific P forms through which P loss may dominate during a particular season. The objective of this study was to determine the concentrations, amounts, and physicochemical forms of P in leachate from a grassland soil as influenced by seasonal water inputs.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The lysimeters were collected according to the method described by Cameron et al. (1992) and laid out in a complete randomized block design with four replicates. In brief, each lysimeter consisted of a steel cylindrical casing which was pushed into the soil to collect an undisturbed soil monolith. A cutting ring at the base of the cylinder created a 0.5-cm annular gap between the soil monolith and the casing, which was filled with liquefied petroleum jelly. Once the jelly solidified, it formed an effective seal to prevent edge flow. The bottom 4 cm of the soil profile was removed from the cylinder and replaced with a washed gravel and sand mixture to create a free-draining situation similar to that of the field soil.
Lysimeter Treatments and Management
This study was designed to quantify P loss by leaching from irrigated grassland soil as influenced by the application of a combination of treatments related to dairying. The lysimeters received different combinations of P fertilizer, FDE, N fertilizer (urea), and cow urine across a 2-yr period (Table 1). Mineral P fertilizer treatments included P45, which received superphosphate (20% potash superphosphate) at 45 kg P ha–1 yr–1 in November; and P90/N200/U, which received superphosphate at 90 kg P ha–1 yr–1 (split applications in April and November) along with urea at 200 kg N ha–1 yr–1 in four split applications (in February, May, August, and November) and one application of cow urine at 1000 kg N ha–1 yr–1 (to commensurate with a urine patch similar to the field situation). Farm dairy effluent treatments included P45/FDE200, which received superphosphate at 45 kg P ha–1 yr–1 along with FDE at 200 kg N ha–1 yr–1 (in four split applications in February, May, August, and November). Similarly, P45/FDE400/U received superphosphate at 45 kg P ha–1 yr–1 along with FDE at 400 kg N ha–1 (in four split applications) and one application of cow urine at 1000 kg N ha–1 yr–1. As this study was a part of a larger experiment on N losses from dairy pastures, some amendments (such as cow urine) were included mainly to study N loss following their application, as cow urine contains only traces of P (Haynes and Williams, 1993). The total amount of P applied to lysimeters from P fertilizer and FDE across 2 yr is shown in Table 1. The annual P fertilizer rate of 45 kg ha–1 was determined as the amount of P input required to maintain an Olsen P of 45 to 50 mg kg–1, which is the optimum level for pasture production under intensive dairy farming in this region of New Zealand.
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Leachate Collection
Leachate was collected from the lysimeters following IR and/or a significant rainfall event during a 2-yr period in a plastic container, which was connected through a flexible plastic tube to the drainage outlet at the base of each lysimeter. Leachate volume was recorded and a subsample of 250 mL was taken for analyses. The data set was split into the IR (summer–autumn: November–April) and NIR (winter–spring: May–October) seasons. During the first and second years of the experiment, leachate was collected on 26 (14 IR, 12 NIR) and 25 (10 IR, 15 NIR) occasions, respectively.
Leachate Analysis
Every leachate sample collected during the 2-yr period was analyzed for different P fractions. Dissolved reactive P (DRP) and total reactive P (TRP) were measured directly in filtered (0.45 µm) and unfiltered leachate samples, respectively, by the malachite green method (Ohno and Zibilske, 1991; van Veldhoven and Mannaerts, 1987). Similarly, total dissolved P (TDP) and TP fractions were determined on filtered (0.45 µm) and unfiltered samples, respectively, following persulfate digestion (Ebina et al., 1983). The difference in P concentration (arithmetic means) between TDP and TP was assumed to be the concentration of total particulate P (>0.45 µm). Similarly, other P fractions were calculated: TUP (total unreactive phosphorus) = TP – TRP; DUP = TDP – DRP; PUP = TUP – DUP; PRP (particulate reactive phosphorus) = TRP – DRP. Sodium inositol hexaphosphate (1 mg L–1) was used as an internal TP standard with each batch of leachate samples.
Phosphorus Loss Calculation
The P loss from the lysimeters was calculated by multiplying the concentration with the volume of leachate at each sampling time. For the calculation of seasonal losses, values from individual lysimeters were summed for that particular period (NIR or IR) and a mean for the four replicate lysimeters was generated for the different treatments.
Statistical Analysis
Descriptive statistics was performed by Genstat 4.2, 5th ed. (Lawes Agricultural Trust, Rothamsted, UK) to calculate means and standard errors. The LSD test (P < 0.05) (with two-way ANOVA in Genstat 4.2) was performed on the seasonal data to test for significance of the effects of season and P inputs (treatments) on P concentrations and losses.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Unreactive Phosphorus
Concentrations of DUP were significantly (P < 0.05) higher for all the treatments during the 1999 NIR season compared with following IR season (Table 3). However, DUP did not significantly differ during the 2000–2001 IR and NIR seasons, except for the P45/FDE200 treatment. The most abundant P fraction measured in leachate was PUP with mean concentrations up to 394 µg L–1. For the FDE treatments, concentrations of PUP were significantly (P < 0.05) higher for both IR seasons compared with NIR. Whereas, for the non-FDE treatments, PUP concentrations were only significantly higher during the first year (1999–2000). Significantly higher PUP concentrations were recorded for the FDE than P45 treatment across respective IR and NIR seasons, except for the P45/FDE200 during the 1999–2000 IR season. When averaged across treatments, seasonal concentrations of DUP and PUP were significantly (P < 0.05) higher for the IR than NIR seasons.
Seasonal Amounts of Phosphorus Leached
A pronounced seasonal variation in TP loss was observed during the 2-yr for all treatments (Table 4). Total P loss was much higher from FDE treatments (574–1311 g ha–1) compared with non-FDE (148–785 g ha–1), which was consistent with differences in TP concentrations. Leachate TP loss was significantly (P < 0.05) lower from P45 compared with FDE treatments during the different NIR and IR seasons. For non-FDE treatments, TP loss was significantly (P < 0.05) higher during both IR seasons (439–785 g ha–1) compared with the NIR seasons (148–324 g ha–1), except for the P90/N200/U treatment during 1999–2000. Similarly, for P45/FDE400/U treatment, TP loss was significantly (P < 0.05) higher during the 1999–2000 IR season compared with the previous NIR season. However, during the 2000–2001 season, TP loss significantly decreased from NIR (1300 g ha–1) to IR season (859 g ha–1) because of reduction in the amount of FDE from 400 to 200 kg N ha–1 yr–1 during the IR season. Total P loss was not significantly (P < 0.05) different between IR and NIR seasons for the P45/FDE200 treatment. It should be noted that the FDE was applied four times in a year at three monthly intervals; hence, there were two applications of FDE during the IR or NIR seasons which resulted in a similar amount of TP loss for FDE treatments.
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Losses of PRP were higher, but not significantly (P < 0.05) different, during both IR seasons compared with corresponding NIR seasons (Table 4). The PRP loss increased with the different seasons for FDE treatments; for example, PRP loss was only 23 g ha–1 from P45/FDE200 during the 1999 NIR season. This increased to 61 g ha–1 during the following IR season, followed by 103 and 116 g ha–1 during the 2000–2001 NIR and IR seasons, respectively. A similar pattern of PRP increase occurred between the 1999 and 2000 NIR seasons for the P45/FDE400/U treatment; however, PRP loss was significantly (P < 0.05) lower during the 2000–2001 IR season.
Unreactive Phosphorus
Losses of DUP were significantly (P < 0.05) higher during the 1999–2000 NIR season than the following IR season for all treatments (Table 4). However, DUP losses were not significantly different during the 2000–2001 yr, except for the P90/N200/U treatment. Consistent with the higher PUP concentrations than other P forms, PUP loss was significantly (P < 0.05) higher from non-FDE treatments for both IR seasons (246-586 g ha–1) compared with the previous NIR seasons (25–149 g ha–1). Similarly, for FDE treatments, PUP loss was two- to three-fold higher for the 1999–2000 IR than for the previous NIR season. However, losses of PUP were significantly lower during the 2000–2001 IR season (593 g ha–1) than the previous NIR season (817 g ha–1) for the P45/FDE400/U treatment. When averaged across treatments, losses of PUP were only significantly (P < 0.05) different during 1999–2000 IR and NIR season.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Seasonal Patterns
There was a clear seasonal pattern of the P losses as dissolved (DRP, DUP) and particulate (PRP, PUP) forms. The proportion of particulate P forms (mainly PUP) was significantly (P < 0.05) higher during the IR season (1999–2000, 70–75% of TP) compared with the NIR season (1999–2000, 8–47% of TP) (Table 4, Fig. 1). This indicates the potential for preferential transport of particulate P in the stony, free-draining Lismore soil under flood IR. Heathwaite and Dils (2000) observed the highest P concentrations in macropore flow during the summer months. Cooke (1976) attributed higher particulate P losses in the first drainage water after dry weather to the washing of fine material from the side of fissures and large soil aggregates. Annual losses of particulate P forms (PUP + PRP) were two-fold higher from FDE (796–1980 g ha–1) than non-FDE treatments (349–837 g ha–1). Magid et al. (1999) suggested that special attention should be given to particulate P contained in cattle manure remnants lying on the surface that may be directly transported through macropores. Losses of dissolved P forms (DUP, DRP) were significantly (P < 0.05) lower than the particulate P forms (mainly PUP) for all seasons (Fig. 1). A dominance of particulate over dissolved forms may be also because of the quick fixation of P present in the dissolved forms, whereas particulate forms escape from fixation reactions and are easily transported in the soils. Turner and Haygarth (2000) suggested that the physical detachment of particles from the walls of macropores was responsible for higher P losses in particulate forms from grassland soils in the UK. Some researchers have reported a dominance of dissolved over particulate P (e.g., Culley et al., 1983; Heckrath et al., 1995; Jordan and Smith, 1985). However, in most of the studies, particulate P forms were the dominant fractions transported through soils, especially in grasslands (e.g., Hanway and Laflen, 1974; Haygarth et al., 1998; Heathwaite and Dils, 2000; Sharpley and Syers, 1979; Simard et al., 2000). It is believed that soil under grasslands have a larger proportion of macropores than under arable farming, hence is more susceptible to preferential flow. Heathwaite and Dils (2000) noted that 68% of TP transported in macropore flow through the grassland soils was in the particulate fraction. Simard et al. (2000) also suggested that P loss in the particulate form is important in soils having large amounts of macropores because they are more prone to P loss by preferential pathways. Toor (2002) highlighted the importance of preferential flow in the Lismore soils, where 60% of TP loss occurred immediately following the application of FDE.
The Influence of Farm Dairy Effluent on Phosphorus Leaching
The proportion of PUP losses was significantly (P < 0.05) higher than DUP for all treatments during all seasons (Fig. 1). Higher PUP concentrations for FDE treatments were because of the regular additions of FDE which contained 32% of TP as PUP and only 1% as DUP (Toor, 2002; Toor et al., 2004a), and provided a long term source of particulate P. During the IR seasons, the concentrations and losses of PUP were significantly (P < 0.05) higher from FDE than non-FDE treatments, but the relative proportion of particulate P forms was only slightly different between FDE and non-FDE treatments (Tables 3 and 4; Fig. 1). This shows the susceptibility of these soils to preferential flow during the IR seasons and the importance of high intensity IR that can dislocate soil particles. Additionally, regular inputs of water, together with the higher temperatures prevailing during the summer months (IR season), may promote wetting and drying cycles which may result in flushes of microbial P through lysis of desiccated cells by rewetting (Kieft et al., 1987; Salema et al., 1982; Turner and Haygarth, 2000, 2001). Then, this pool of microbially released P may have been intercepted by mobilized particles (added with FDE) in the soil profile that culminated in higher losses as PUP. However, during the NIR seasons, the relative proportion of PUP was much higher for FDE treatments (69–75% of TP) compared with non-FDE treatments (8–54%). As FDE was applied at regular intervals throughout the year, it was a potential source of particulates even during NIR seasons, which resulted in higher PUP losses from FDE treatments. Lower P losses as PUP from the non-FDE treatments during NIR seasons are because of the lower amounts of natural rainfall (usually <20 mm per event), which restricted the dislocation and movement of soil particles. This resulted in higher losses as DUP during the NIR seasons compared with IR seasons (Fig. 1).
Proportion of DRP and PRP fractions was <10% of TP during the first year (1999–2000) and between 11 and 23% of TP during the second year of experiment (Fig. 1). Concentrations of DRP were very similar for the FDE and non-FDE treatments for the first year despite the regular addition of DRP from FDE in the FDE treatments. On the other hand, PRP concentrations were higher from the FDE treatments than non-FDE treatments. Lower losses of DRP from FDE treatments confirm the relative ability of these soils to quickly sorb P present as DRP; however, PRP, being in the particulate phase (>0.45 µm), escaped from sorption reactions. In England, Chalmers and Withers (1998) reported that concentrations of DRP in leachate samples collected from a 0.3-m soil depth with Teflon (DuPont, Wilmington, DE) suction cups over three winter seasons decreased with the increasing depth because of P sorption effects. Higher concentrations of PRP during the IR seasons compared with the NIR seasons may be attributed to the rapid transport of PRP in soil profile through macropores during the IR season.
This study demonstrated a clear difference in the amounts and forms of P loss from soil under IR compared with natural rainfall. Significantly (P < 0.05) higher P losses were observed for the FDE treatments compared with non-FDE (e.g., P45 treatment). For all treatments, higher P losses were recorded in particulate P forms (mainly PUP) during the IR season because of the addition of high volume artificial IR (92 mm per event), while dissolved P forms (mainly DUP) dominated during NIR season in non-FDE treatments. The consistent lower concentrations of DRP measured from different treatments suggest the higher capacity of these soils to rapidly retain P present as DRP.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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