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a Dep. of Crop and Soil Environmental Sciences, Virginia Polytechnic and State Univ., Blacksburg, VA 24061
b Dep. of Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602
c Animal and Dairy Sciences, Univ. of Georgia, Athens, GA 30602
* Corresponding author (mcabrera{at}arches.uga.edu)
Received for publication July 7, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Abbreviations: DRP, dissolved reactive phosphorus • ln DAA, natural logarithm of days after litter application • ln RUNOFF, natural logarithm of runoff volume • STP, soil test phosphorus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Application of broiler litter to grasslands can increase N and P in surface runoff because these applications deposit the litter on the soil surface, where it is likely to interact with runoff water. With time, litter nutrients are likely to be moved into the soil by the action of rain and animals, thereby decreasing the risk of surface water contamination. The time it takes for concentrations in surface runoff to decrease, however, is not well known.
Most studies have evaluated nutrient concentrations in surface runoff shortly after application of broiler litter (1 to 68 d) (Edwards and Daniel, 1994; Sauer et al., 1999, 2000). Less data are available on the changes in NH+4–N and DRP concentrations during an extended period of time after application. In one long-term study (32 mo) with poultry litter, Edwards et al. (1996) measured the concentrations of DRP in surface runoff from two paired tall fescue fields in Arkansas (USA) that had received repeated applications of animal manure. The field with the lowest soil test P (210 mg P kg-1; Mehlich 3) received two yearly applications of broiler litter (5.6 Mg ha-1 application-1) for 2.5 yr, whereas the field with the highest soil test P (630 mg P kg-1) only received split applications of ammonium nitrate. The results showed no trend in surface runoff concentrations for the field that received poultry litter applications. In contrast, the field that received ammonium nitrate showed a significant trend toward decreasing DRP concentrations with time. Similar information is needed for fields that are initially low to medium in P availability and are fertilized with broiler litter.
In soils with high P availability, farmers are currently encouraged to apply animal manures based on crop P requirement, as opposed to N requirements (Soil Conservation Service, 1994). One of the challenges faced with this management is the difficulty of identifying soil test P levels that result in unacceptable P losses in runoff. Establishing these levels is often a highly controversial process because the database relating soil test phosphorus (STP) to runoff P is limited and, when available, is considered site specific (Sharpley, 1995; Sharpley et al., 1996; Pote et al., 1996, 1999). Clearly, the relationship between STP and runoff DRP, especially for grasslands fertilized with broiler litter, requires further investigation (Sharpley et al., 1999).
The objectives of this work were to study (i) the dynamics of NH4–N and DRP concentrations in surface runoff from grasslands fertilized with broiler litter and (ii) the relationship between STP and DRP concentration in runoff.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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100 x 75 m, 6–8% slope) fescue–common bermudagrass paddocks located at the Central Georgia Branch Station (33°24'N, 83°29'W; elevation 150 m) were used for this study. Soil series at the site are Cecil (fine, kaolinitic, thermic Typic Kanhapludults), Altavista (fine-loamy, mixed, semiactive, thermic Aquic Hapludults), Helena (fine, mixed, semiactive, thermic Aquic Hapludults), and Sedgefield (fine, mixed, active, thermic Aquultic Hapludalfs). Earthen berms (0.6 m high, 1.5 m wide) were constructed around each paddock to direct surface runoff to a 0.45-m H-flume equipped with a SENIX (Burlington, VA) ultrasonic sensor to measure depth of flow. A 0.6-m Coshocton wheel subsampled the surface runoff. The Coshocton wheel was modified so that at any point in time its pan would hold a composite sample of the last 1000 L of runoff that flowed through the flume. At predetermined runoff volumes, samples were automatically collected from the Coshocton wheel pan and stored in an ISCO (Lincoln, NE) 3700FR refrigerated sampler. The first sample was taken after 4000 L and each subsequent sample was separated from the previous sample by an increasing multiple of 4000 L (e.g., the second sample was separated from the first sample by 8000 L, the third sample was separated from the second sample by 12000 L, and so forth). Samples were kept at 4°C for up to 24 h, then brought to the laboratory, filtered, and immediately analyzed or frozen for later analysis. Precipitation and runoff volume data were recorded with CR10 dataloggers (Campbell Scientific, Logan, UT). Surface runoff from the paddocks was monitored during the winter of 1994–1995 to obtain baseline values for runoff NH4–N and DRP concentrations before broiler litter applications. Broiler litter was applied in March and September–October 1995 and 1996 and urea–ammonium nitrogen solution (UAN) was applied in March 1997 and 1998 (Table 1).
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A composite soil sample (12 cores, 1.8 cm diameter) from the upper 15-cm layer of each paddock was collected on 12 Mar. 1995, 18 Oct. 1995, 15 Feb. 1996, 5 Mar. 1997, 3 Mar. 1998, and 20 Oct. 1998. Soil samples were dried at 35°C and analyzed for Mehlich 1 P (Mehlich, 1953).
Broiler litter samples were analyzed in quadruplicates for total Kjeldahl N and total Kjeldahl P using a micro-Kjeldahl method (Baker and Thompson, 1992). Inorganic N in broiler litter was extracted by shaking 0.2 g of litter with 40 mL 1.0 M KCL for 30 min in a reciprocating shaker at 120 oscillations per minute. The supernatant volume was analyzed for ammonium N with the salicylate–hypochlorite method (Crooke and Simpson, 1971), and for 
–N with the Griess–Ilosvay method (Keeney and Nelson, 1982) after reduction of NO-3 to NO-2 with a Cd column. Water-soluble P was determined by extracting 0.2 g broiler litter with 40 mL deionized water for 30 min (in a reciprocating shaker at 120 oscillations per minute), centrifuging, filtering through a 0.45-µm filter, and measuring orthophosphate P in the supernatant volume by the molybdate blue method (Murphy and Riley, 1962).
Runoff samples were vacuum-filtered through Whatman (Maidstone, England) 0.45-µm cellulose nitrate membranes placed on 47-mm Fisherbrand glass filter holders (Fisher Scientific, Pittsburgh, PA). Filtered samples were analyzed for DRP by the molybdate blue method (Murphy and Riley, 1962) and for NH4–N by the salicylate–hypochlorite method (Crooke and Simpson, 1971).
The masses of NH4–N and DRP exported during a runoff event were calculated by plotting nutrient concentration versus runoff volume, joining points with straight lines, and integrating the area under the straight lines with MathCad 6.0 (MathSoft, 1995). Flow-weighted concentrations of DRP and NH4–N in each runoff event were obtained by dividing the mass of nutrient lost during a runoff event by the total runoff volume for that event. The flow-weighted concentrations for each runoff event were averaged across paddocks and standard deviations were calculated.
Flow-weighted concentrations for individual runoff events were plotted against time after litter application and against runoff volume to visually study the shape of those relationships. Because both types of graphs showed a strong log-linear relationship for DRP, we regressed DRP concentration against the natural logarithm of days after litter application (ln DAA), the natural logarithm of runoff volume for a single event (ln RUNOFF), and the interaction between the two variables (SAS Institute, 1994). The same type of regression analysis was carried out for flow-weighted concentrations of NH4, with the difference that we used the natural logarithm of days after application of litter during 1995 and 1996, and the natural logarithm of days after application of urea–ammonium nitrate solution for 1997 and 1998. Correlation analysis was used to study the relationship between DRP and NH4–N concentrations.
The total mass loss in all the runoff events from a paddock during a year was divided by the volume of yearly runoff to calculate yearly flow-weighted concentrations for each plot. Yearly runoff volume, loads, and flow-weighted concentrations were averaged across plots and standard deviations were calculated. The flow-weighted concentrations of NH4–N and DRP in the total runoff observed between two soil samplings was calculated by dividing the mass loss between two soil samplings by the total volume of runoff recorded in that period.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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Regression analysis indicated that the flow-weighted concentration of NH4 in individual runoff events was weakly related (R2 = 0.27) to ln RUNOFF (p < 0.03) and ln DAA (p < 0.0001; NH4–N = 12.16 + 1.04 ln RUNOFF - 2.54 ln DAA; n = 86). The variable ln DAA explained 23% of the variability in NH4 concentration, whereas ln RUNOFF only explained 4% of the variability. Clearly, this relationship is of limited value for predictive purposes.
Ammonium load in runoff was <2 kg N ha-1 in all years, except in 1996 when it was 13.5 kg N ha-1 (Table 2). About 70% of the total NH4 loss in 1995 occurred in three events that took place within 12 d after the second litter application. The total runoff volume of those three events corresponded to 33% of the yearly runoff. Similarly, about 95% of the total loss measured in 1996 occurred in two runoff events that took place within 3 d after the third litter application. The total runoff volume in those two events corresponded to 28% of the yearly runoff. In contrast to the NH4 losses observed in 1995 and 1996 (which were the only two years with broiler applications), NH4 losses in 1997 and 1998 were more evenly distributed across runoff volumes. For example, the total NH4 loss in the three major runoff events of 1997 corresponded to 37% of the yearly NH4 loss, with the volume of runoff in those events representing 48% of the yearly runoff. In 1998, the total NH4 loss in the three major runoff events corresponded to 48% of the yearly loss, with the volume of runoff representing 52% of the yearly runoff. These results show that in pastures fertilized with broiler litter, NH4 losses could be limited if runoff events do not occur soon after application.
Dissolved Reactive Phosphorus in Runoff
The mean flow-weighted concentration of DRP during the baseline period was 0.4 mg P L-1 (Kuykendall et al., 1999). Some of this DRP is likely to have been derived from the dung deposited by grazing cattle because the soil had a relatively small initial amount of available P (13 mg P kg-1 by Mehlich 1). This assumption is consistent with results of Sauer et al. (1999), who measured 0.79 and 0.24 mg DRP L-1 (1 and 14 d after application, respectively) in runoff from fescue plots receiving 6.7 Mg ha-1 dairy feces and urine.
Dissolved reactive P concentrations increased when broiler litter applications started, and reached values as high as 19 mg P L-1 in a runoff event that occurred immediately after the third application. In general, DRP concentrations decreased more slowly than NH4 concentrations and remained above the baseline value while broiler litter was being applied (Fig. 1). These results are consistent with studies showing increases in DRP following surface application of broiler litter (Edwards and Daniel, 1994; Shreve et al., 1995; Sauer et al., 1999, 2000; Wood et al., 1999).
The concentration of DRP in runoff depended in part on the time elapsed between litter application and first runoff. For example, the DRP concentration in the first large (>2 mm) runoff event that followed the first application was <5 mg P L-1 because the event occurred almost 7 mo after litter application. In contrast, DRP concentrations were >5 mg P L-1 in runoff events that occurred within 12 d after the second and third litter applications. By 19 mo (April 1998) after the last broiler litter application, the flow-weighted concentration of DRP was near 1 mg P L-1, which is the value that has been tentatively proposed as the maximum desirable concentration in surface runoff from agricultural fields (USEPA, 1986).
Our regression analysis indicated that the flow-weighted concentration of DRP in individual runoff events was strongly related (R2 = 0.70; p < 0.0001) to ln DAA (p < 0.03), ln RUNOFF (p < 0.0001), and the interaction between both variables (p < 0.0001). The different variables explained 32% (ln DAA), 20% (ln RUNOFF), and 18% (interaction) of the variability observed (DRP = 3.31 - 0.369 ln DAA + 4.14 ln RUNOFF - 0.626 ln DAA x ln RUNOFF; Fig. 2). This equation is of relatively good predictive value (Fig. 3).
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Correlation analysis of NH4 and DRP concentrations yielded an r = 0.80 (p < 0.0001) when all data points were included. When two data points (including the two largest concentrations of NH4 and DRP) were excluded, however, the correlation between both variables decreased to r = 0.34 (p < 0.001). This low correlation was caused by the rapid decrease observed in NH4 concentrations when compared with DRP concentrations (Fig. 1).
Measurements of total P concentrations in runoff made during the first two years of the study indicated that DRP accounted for 75 and 64% of total P in 1995 and 1996, respectively (Kuykendall et al., 1999). These proportions compare well with those measured in runoff simulation studies with broiler litter applied to fescue plots in Arkansas (Nichols et al., 1994; Shreve et al., 1995).
We used a concentration of 1 mg P L-1 (as the maximum desirable DRP concentration in runoff) and a runoff of 165 mm per year (average of 1995–1998) to calculate a maximum desirable yearly load of 1.7 kg P ha-1 for these paddocks. The measured loads exceeded this value in each of the four years (Table 2), but compared well with losses measured from an Australian pasture fertilized with inorganic P fertilizer (Nash et al., 2000). The authors measured losses of 5.3 and 9.2 kg DRP ha-1 in two consecutive years that received 110 kg P ha-1 yr-1.
About 60% of the DRP loss occurred in three runoff events that took place within 12 d of the second litter application (Table 2), similar to what was observed for NH4 in 1995. In 1996, 61% of the yearly loss occurred in two events that took place within 3 d of the third litter application. The volume of these runoff events accounted for 33 and 28% of the yearly runoff for 1995 and 1996, respectively. As for NH+4, DRP losses in 1997 and 1998 were more evenly distributed across runoff volumes. In 1997, losses in the three major runoff events accounted for 43% of the yearly DRP loss, with the volume of runoff in those events accounting for 48% of the yearly runoff. In 1998, losses in the three major runoff events accounted for 53% of the yearly loss, with the volume of runoff representing 52% of the yearly runoff. Thus, losses of DRP in pastures that are being fertilized with broiler litter could be limited by avoiding runoff events soon after application.
Soil Test Phosphorus and Dissolved Reactive Phosphorus Concentration in Runoff
Soil test P in the upper 15 cm of soil increased from 13 mg P kg-1 in March 1995 to 81 mg P kg-1 in March 1997 (Fig. 4). Soil test P subsequently decreased to 47 mg P kg-1 by October 1998. Thus, there was a large decrease of 34 mg P kg-1 in 19 mo. We used the initial (13 mg P kg-1) and final (47 mg P kg-1) STP values together with the total P added (471 kg P ha-1 in 4 yr), as well as estimates of crop P removal (120 kg P ha-1 in 4 yr) and total P loss in runoff (36 kg P ha-1 in 4 yr, assuming DRP is 70% of TP), to estimate the rate of STP increase per unit of net P added. Assuming a bulk density of 1.5 Mg m-3 in the upper 15 cm, we estimated that it took the net addition of 4.1 mg P kg-1 to increase STP by 1 mg P kg-1 (by October 1998). A similar, average value of 4.5 mg P kg-1 per STP unit has been reported for Alabama (USA) soils (Cope, 1983). Consequently, if we assume that the removal of 4.1 mg P kg-1 would decrease STP by 1 mg P kg-1, we can estimate that the decrease in STP that occurred from March 1997 to October 1998 (34 mg P kg-1) should have been caused by the removal of 139 mg P kg-1 (or 314 kg P ha-1). Clearly, such a removal of P is very unlikely in 19 mo. Therefore, the fast decrease observed in STP suggests that the large STP value measured after the last broiler litter application (81 mg P kg-1) was obtained through the sampling of not only soil, but also broiler litter on the soil surface. It is also likely that the previous two STP values were affected in the same manner (Fig. 4). Once litter applications ceased and most of the litter P reacted with soil, STP values decreased rapidly because soil samples no longer included surface litter P. This is an important observation that should be considered when interpreting STP values from grasslands recently fertilized with broiler litter.
To study the relationship between STP and the flow-weighted DRP concentration corresponding to an extended period of time (>120 d), we calculated the flow-weighted concentration of DRP between two soil samplings and plotted it (together with STP) as a function of time (Fig. 4). The concentration of DRP increased as STP increased, and decreased as STP decreased. These results suggest that it may be possible to develop a relationship between STP and DRP concentration in runoff in pastures fertilized with broiler litter. Careful observation of Fig. 4, however, suggests that this relationship would vary depending on how recent the latest broiler application was. For example, an average STP value of 60 mg P kg-1 corresponded to a DRP concentration > 8 mg P L-1 while broiler litter was being applied, but only to a DRP concentration < 2 mg P L-1 when broiler litter was no longer being applied. The apparent reason for this difference is that at least the last STP measurement made while broiler litter was being applied was inflated by the presence of litter in the soil sample. Therefore, to use STP as an index of the risk of DRP loss, separate relationships would have to be developed for fields with and without recent litter applications. Furthermore, guidelines would have to be developed to decide when to switch from one relationship to the other as time after a litter application increased. Because of these obstacles, it may be difficult to use STP as an indicator of potential DRP loss from grasslands fertilized with broiler litter.
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SUMMARY AND CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONSUMMARY AND CONCLUSIONSREFERENCES |
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