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Published online 1 March 2007
Published in J Environ Qual 36:469-477 (2007)
DOI: 10.2134/jeq2006.0138
© 2007 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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Phosphorus Losses through Agricultural Tile Drainage in Nova Scotia, Canada

Robert D. Kinleya,*, Robert J. Gordonc, Glenn W. Strattonb, Gary T. Pattersond and Jeff Hoyleb

a Dep. of Animal Sciences, Nova Scotia Agricultural College, Truro, NS, Canada, B2N 5E3
b Dep. of Environmental Sciences, Nova Scotia Agricultural College, Truro, NS, Canada, B2N 5E3
c Dep. of Engineering, Nova Scotia Agricultural College, Truro, NS, Canada, B2N 5E3
d Agriculture and Agri-Food Canada, P.O. Box 550 Truro, NS, Canada, B2N 5E3

* Corresponding author (rdkinley{at}nsac.ca)

Received for publication April 7, 2006.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Tile drainage water from agricultural fields commonly exceeds environmental guidelines for phosphorus (P) in rivers and streams. The loss of P through artificial drainage is spatially and temporally variable, and is related to local factors. This study characterizes variability in total P (TP) and soluble reactive P (SRP) concentrations in weekly drainage samples from 39 agricultural fields in Nova Scotia, Canada, from April 2002 through December 2003. We examined connections between P concentrations and the factors: (i) soil texture; (ii) discharge flow rate; (iii) soil test P (STP); (iv) manure type; and (v) crop cover. Generally, variability between fields and samples was great, and fields with standard deviations exceeding the mean for TP, SRP, and flow rate were 71, 54, and 79%, respectively. It was evident that poultry and swine manure contributed to high STPs, and to constantly high TP concentrations with high proportions of SRP. Concentrations varied from week to week, and particularly in April, May, October, and November when the greatest TP, SRP, and flow rate averages were measured. Mean TP concentrations exceed the USEPA (1994) TP guideline of 0.10 mg L–1 at 82% of the fields, and periodically concentrations more than 10 times, and occasionally more than 50 times higher than the guideline were found. The proportion of SRP in TP had a tendency to be higher when TP levels were high in coarse textured soils. In Nova Scotia, dairy manure is most often applied on permanent cover crops, which did not show as much P concentration variability as crop rotations. Daily or hourly observation of short-term increases in P concentrations related to the described factors would help to characterize the changes in P concentrations observed during frequent heavy drainage flow events.

Abbreviations: SRP, soluble reactive phosphorus • M3, Mehlich 3 soil test extractant • PP, particulate phosphorus • STP, soil test phosphorus • TP, total phosphorus • UP, unavailable phosphorus


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE use of subsurface tile drains to improve field drainage has provided a pathway through which surface-applied phosphorus (P) can be transported even when water supply can't support surface runoff. Phosphorus losses from agricultural fields to surface waters have mostly been attributed to surface runoff (Sharpley et al., 1993). It has long been accepted, however, that subsurface drainage can transport substantial amounts of P (Milburn and MacLeod, 1991; Dils and Heathwaite, 1999; Beauchemin et al., 2003). Heckrath et al. (1995) found that P loss from both free draining and artificially drained soils may occur at levels likely to contribute to elevation of P in receiving waters.

Some regions have implemented guidelines to define P concentrations at which P exhilarates eutrophication in rivers, streams, and lakes. No specific guidelines have been identified for drainage water. The USEPA (1994) suggests a concentration of 0.10 mg L–1 for total P (TP) in rivers and streams. In Nova Scotia, Canada, no guideline has been set. Ontario and Quebec, however, have set a guideline of 0.03 mg L–1 for TP in rivers (Environment Canada, 2004). Both the USEPA and Environment Canada have suggested that TP concentrations as low as 0.01 to 0.05 mg L–1 may initiate eutrophication in lakes.

Agricultural production generates a variety of factors that combine with natural field conditions to create a high risk environment for drainage P levels to exceed guidelines. Every soil has an inherent level of sand and clay (texture), so conditions of P sorption, natural drainage, and soil P retention will vary. Toor et al. (2005) found that much of the variation in flow rate and leaching is associated with soil properties and the extent of time between the application of P and intense rainfall. Manure and fertilizer provides the P source, and the type of crop cover and tillage practices influence surface flow patterns, particle mobility, and water infiltration. Surface runoff is reduced on grasslands and carries less particulate P (PP) than from fields under crop rotations (Andraski et al., 1985).

The rate of application of manure and fertilizer varies, and some fields may receive a mix of manure types in several applications over one growing season. The type of manure has an influence on P losses, for example, poultry manure may contain over 15 times more P than dairy, and 10 times more than swine (Manure Management Task Group, 1991). Conversely, fields receiving dairy (grassland) sometimes get 10 times more manure volume than fields receiving poultry or swine manure.

Long-term application of manure tends to promote high soil test P (STP) levels, particularly applications containing poultry manure (Daniels et al., 2004). There is some agreement among researchers that there exists a STP environmental threshold beyond which a unit increase in STP results in much more P loss in drainage and runoff than below the threshold. Consensus, however, is more difficult to find when identifying the threshold level, and 200 mg kg–1 Mehlich-3 (M3) STP has been suggested, while other lower thresholds are also supported (Sharpley et al., 1996; Daverede et al., 2003; Sharpley et al., 2003). Unfortunately, STP in agricultural fields commonly exceeds 200 mg kg–1.

There has been no previous assessment of the state of P leaching from agricultural fields in Nova Scotia, Canada. This paper characterizes the variability in P transport in tile-drainage from 39 agricultural fields in Nova Scotia. Specific objectives were to: (i) identify weekly and monthly variation in tile drainage water TP and soluble reactive P (SRP) concentrations; and (ii) characterize the variability in tile drainage P concentrations relative to flow rate, STP, manure applications, crop cover, and soil texture.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Field Site Locations
Tile drainage samples were collected from 39 agricultural fields in Nova Scotia, Canada from April 2002 through December 2003. The 39 fields were distributed throughout the counties of Antigonish (10), Hants (8), Kings (13), and Pictou (8). The regions monitored are not representative of the entire province but they provide a variety of fields under intensive farming and with variable STP, cultivation history, and soil properties. Drainage flow from the fields typically ceased in June and resumed in October.

Table 1 shows that most of the fields were of a loamy texture and some were high in sand, and only one (SiCL-1) had sufficient clay to be considered a clayey texture. Fertility management for the fields consisted of combinations of dairy or swine manure, poultry litter, or inorganic fertilizer. During the study period the soil was under permanent cover of grass or alfalfa at 16 of the fields, and under corn or mixed rotations at 23 fields. Tile drains were 10 cm in diameter, with the exception of two fields (SiCL-1 and SL-14), which had 20 cm diameter drains, all were approximately 80 cm in depth with a 20-m spacing.


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  		Table 1. Selected soil conditions measured in the top 20-cm layer, and crop and manure or fertilizer application during the sampling period of April 2002 through December 2003.{dagger}

 
Soil and Drainage Water Sampling and Analysis
In October 2003, composite soil samples from each field were prepared from approximately 50 subsample cores per ha. Soil cores were collected from the top 20 cm with a steel 2.5-cm bore sampling probe. Composites were ground and sieved to <2 mm. Soil test P was determined by the M3 method (Mehlich, 1984). A soil to extractant ratio of 1:10 (v/v) was applied, which is the standard STP method in Nova Scotia (Nova Scotia Department of Agriculture and Fisheries, 2001). Extracts were analyzed for P using inductively-coupled argon plasma (ICAP) spectrophotometry. Soil texture classification was based on sand, silt, and clay percentages determined by the hydrometer method (Day, 1965), and then classified according to the Canadian Soil Information System (Canadian Soil Information System, 1983). Soil pH was measured in a 1:1 (v/v) soil to water mixture.

Tile drainage water samples were collected manually in 250-mL polyethylene bottles every 7 d. A total of 1358 samples were collected, and stored at 4°C for no longer than 4 mo. Subsamples for SRP were filtered (<0.45 µm) before storage and analysis, and those for TP were digested unfiltered. A control sample was collected from field SL-3 of which 12 subsamples for each of SRP and TP were subject to quality control monitoring. It was confirmed that no significant difference developed in the SRP or TP concentrations when refrigerated or frozen, or between storage for 10 d or 6 mo.

For TP, sample preparation required conversion of all P forms into orthophosphate (PO43––P) which was completed on 25-mL samples by acid digestion with HNO3 and H2SO4 as described in APHA method 4500-P B/4 (Clesceri et al., 1998). Most samples were free of visible particles and were digested a minimum of 3 h to a volume of less than 10 mL. Total P was then determined by the method of Murphy and Riley (1962) where the ammonium molybdate reaction with PO43––P forms molybdophosphoric acid, which is reduced to the molybdenum blue complex by ascorbic acid and then measured colorimetrically.

Soluble reactive P (as dissolved PO43––P) was measured directly in filtered (<0.45 µm) samples. Analysis for SRP was completed by ion exchange chromatography (IEC) with chemical suppression of eluent conductivity (Clesceri et al., 1998). The IEC results are specific to dissolved orthophosphate (PO43–) which may be overestimated by Murphy and Riley's (1962) SRP through chemical liberation of unreactive P (UP) <0.45 µm (Haygarth and Sharpley, 2000).

Tile drain discharge flow rates were measured at the time of sampling, and the mean of three timed volume discharge measurements was recorded. Accurate drainage system details on total drainage area and system functionality were not available for all fields, and consequently flow rates are expressed as m3 d–1 and are not corrected for area. Interpretation of P loss relative to flow rate is limited to concentration, and P loading cannot be calculated. When discussing flow rate we have not included data from nine of the fields (SiCL-1; L-3,7,8,11,12; LS-1,2; SL-5). Flow data were limited at these sites due to significant snow, ice, and flooding at various times during the sampling period.

Analytical results were classified as censored data for concentrations <0.04 mg L–1, the detection limit of the method. This was necessary when investigating the ratio of TP/SRP as SRP values may be below detection limit in samples throughout the observed range for TP. Censored data for SRP, therefore, were substituted with one half the detection limit (0.02 mg L–1) as recommended by the USEPA (McBean and Rovers, 1998). We compare results to some literature guidelines that are less than 0.04 mg L–1 by referring to results that exceed the detection limit as also being concentrations that exceed the guidelines, and any error, therefore, is slight under estimation.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Weekly and Monthly Patterns in Phosphorus Losses
The evolution of TP data from four representative fields of different soil texture groups and manure types is summarized in Fig. 1. These fields illustrate the variability between weekly samples and sites that was observed at 39 Nova Scotia fields. It was evident that SL-3, which had been receiving poultry litter, was consistently producing the greatest TP and SRP concentrations. The fields receiving poultry litter always had TP concentrations exceeding the USEPA (1994) guideline of 0.10 mg L–1. When the drain at SL-3 began to flow in November, TP was extreme at 5.50 mg L–1. There had been poultry litter applied in July, which may explain the average of 45% per week decline in concentration in 4 successive weeks. The fact that TP never dropped below 0.28 mg L–1 at SL-3 may be a result of several factors such as STP, soil texture, and manure history, for example, the STP at SL-3 was 418 mg kg–1. In contrast, S-1, a sandy soil without recent manure applications, had an even higher STP of 467 mg kg–1, but drainage TP and SRP concentrations were dramatically lower from S-1 than SL-3.


Figure 1
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  		Fig. 1. Sample total P (TP) evolution over 21 mo of observations and represented by four selected fields from various texture classes and manure histories.

 
Figure 2 shows that December 2002 with a mean flow rate of 22 m3 d–1 was the only month in that year that the 39 fields had an overall mean TP concentration that was lower than the corresponding month in 2003 when the flow rate was 130 m3 d–1. In both years increases in tile drainage TP concentrations began in March, and peaked in April and May coinciding with greater mean flow rates. Although only 30 and 20% of the fields had significant correlations between sample flow rate, and TP and SRP respectively, there was 73 and 60% that had positive slopes for the regression lines. This suggests generalized increases in concentration with flow, but also variable rates of increase, which are often not significant.


Figure 2
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  		Fig. 2. Monthly mean total P (TP) and soluble reactive P (SRP) concentrations and discharge flow rates from tile drainage of 39 fields in Nova Scotia, Canada from April 2002 through December 2003. Variability between fields is indicated by standard error bars and the dashed line marks the USEPA (1994) guideline for TP in rivers and streams.

 
Tile Drainage Discharge Flow Rate
Flow rate was relatively low in May 2002 (12 m3 d–1); the TP and SRP concentrations however, remained quite high (Fig. 2). This may be explained by a superseding influence of manure and fertilizer applications in the weeks prior to sampling. Table 1 indicates that 70% of the fields had received some form of P fertility applications from March through May, and 23% had received mixed applications. The applications were similar in 2003; the flow rates, however, were greater in 2002 adding to flushing of freshly applied P into drainage in May before flow rate dropped to dryness. The monthly mean TP equaled or exceeded the USEPA standard of 0.10 mg L–1 year-round.

McDowell and Sharpley (2004) observed greater P concentrations and rates of decrease with higher flow. Our results agreed in that peak concentrations occurred during heavy flow events and decreased abruptly as flow subsided. Flow rate was not significantly ({alpha} = 0.05) correlated with TP (r2 = 0.064, p = 0.18) or SRP (r2 = 0.127, p = 0.054). Figure 3, however, shows that there exists a generalized increase in mean P concentrations with mean flow rate. The regression equations have small but positive slopes, both indicating a positive response to increasing flow. The jagged lines reflect the variability in P concentration between fields as flow rate increases.


Figure 3
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  		Fig. 3. Total P (TP) and soluble reactive P (SRP) response to increasing flow rate. Fields are shown in order of increasing flow rate. The data on each of the plots represent the overall mean for the corresponding field for the study period. The regression lines have the following equations: TP = 0.304 + 0.001(Flow); SRP = 0.027 + 0.0018 (Flow).

 
Phosphorus Loss Variability
Table 2 shows that the flow rate mean for samples was approximately 4 times greater than the median, which was a result of occasional extreme measurements. The field and sample means were similar, as was the field mean and field median. This suggests that flow variation from field to field, as illustrated by field S-1 (Fig. 3), is not as variable as between samples from week to week.


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  		Table 2. Frequency distribution of total phosphorus (TP), soluble reactive phosphorus (SRP), and drainage discharge flow rates from 39 fields in Nova Scotia, Canada over 21 mo of weekly sampling.

 
For TP, there was 82% of fields and 29% of samples exceeding 0.10 mg L–1 during the duration of the study, and 90% of the fields had some sample TP concentrations ≥1.0 mg L–1. It raises concern that periodically TP concentrations are more than 10 times and occasionally more than 50 times higher than the standard set by the USEPA (1994). The Quebec and Ontario standard of 0.03 mg L–1 (Environment Canada, 2004) was exceeded by all of the fields except for the finest textured soil (SiCL-1). This standard was exceeded by 55% of sample TP measurements, and 25% of the SRPs.

Weekly variation was very large for P concentrations and flow rates, and the standard deviation (SD) often exceeded the mean. Table 3 lists the coefficient of variation (CV) for TP, SRP, and flow rate for each field. The CVs that exceed 1.00 were from fields with SDs exceeding the mean, and the proportion of sites in this category was 71, 54, and 79% for TP, SRP, and flow rate, respectively.


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  		Table 3. Proportion of total P (TP) below detection (0.04 mg L–1) and coefficient of variation (CV) for TP, soluble reactive P (SRP), and tile drainage flow rate for the 39 Nova Scotia fields (N/A = not available).

 
Unavailable Phosphorus
We measured TP and SRP, and have pooled the unmeasured contributing components of TP into a category of UP. Figure 4 shows that the proportion of UP in TP had a weak inverse regression with both texture and TP. The correlation between % UP and % sand was not significant at {alpha} = 0.05 (r2 = 0.08; p = 0.093). The correlation between % UP and TP, however, was weak but significant (r2 = 0.12; p = 0.034).


Figure 4
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  		Fig. 4. The proportion of unavailable P (UP) in total P (TP) relative to soil texture and TP. The fields are shown in order of finest to coarsest texture based on sand content; the actual value can be found in Table 1. The data on each of the plots represent the overall mean for the corresponding field for the study period. The regression lines have the following equations: %UP = 91.1 – 0.273(%Sand); %UP = 89.7 – 36.7(TP).

 
Chardon et al. (1997) measured large proportions (70–90%) of dissolved organic P (DOP) in drainage TP, and found that the proportion increased with soil depth. They suggest that DOP may be more mobile than SRP through the soil matrix, and hence play an important role in eutrophication. This may explain how mean UP at our fields averaged 75% of TP in April and May, with a high of 81% in April 2002. Particulate P (PP) may also contribute to large proportions of UP if preferential channels exist (Toor et al., 2005). However, during March, April, and May occurrence of preferential channels is much less likely in wet soil.

Soil Texture, Soil Phosphorus, Manure, and Crop Rotations
Several factors were investigated for their contribution to the variation and magnitude of drainage TP and SRP concentrations. Soil texture was combined with each of crop cover, manure history, and STP. Figure 5 shows that under crop rotation SRP was more variable than under permanent cover, TP however was always quite variable. The generally lower and more stable SRP values at most of the grassland fields is a reflection of lower M3 STP levels. Table 3 shows that many of the grassland fields had >50% of their samples below detection (0.04 mg L–1) for TP concentrations, and as such, are not included in Fig. 5. The 16 permanent cover fields were all either in the ‘low’ (6) or ‘moderate’ (10) category of STP. They had histories of either dairy manure or P fertilizer and produced large proportions of UP (Fig. 4). Conversely, poultry and swine manure dominated the ‘high’ category of STP (>200 mg kg–1), and produced generally higher proportions of SRP and lower UP. McDowell and Sharpley (2002) found a strong correlation (r2 = 0.71; p < 0.001) between STP and TP on Pennsylvania soils with a similar STP range (180–690 mg P kg–1) to our Nova Scotia soils (Table 1) which also had a significant correlation (r2 = 0.31; p < 0.001).


Figure 5
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  		Fig. 5. Tile drainage total P (TP, gray) and soluble reactive P (SRP, black) concentrations as affected by: (A) soil test P (STP), (B) manure type, and (C) crop cover. Fields are shown within each subcategory in order of finest to coarsest soil texture based on % sand. Soil test P categories were chosen based on the agronomic guideline suggested as "high" for field crops (75 mg kg–1) by NSDAF (2001), and the environmental STP threshold (200 mg kg–1) suggested by Sharpley et al. (2003). The data on each of the plots represent the overall mean for the corresponding field for the study period.

 
The greatest mean TP concentrations were from fields with an STP greater than 200 mg kg–1 (Fig. 5A). It is difficult, however, to apply a threshold such as this at any point within our STP range, because the implication of safety below that point is misleading. Throughout the range there are high TP concentration means that exceed guidelines, and there are some higher concentrations coming from fields in the low range (<75 mg kg–1) than from the high range (>200 mg kg–1).

Within each category in Fig. 5, the finest textured soils generally had lower proportions of SRP. Poultry and swine manure applications, however, seem to have a superseding effect over texture. Since these manure types had elevated the STP, and in all cases were applied on crop rotations, this may bias a connection with dairy manure and lower STPs under permanent cover. Nova Scotia grasslands tend to receive dairy manure applications as opposed to poultry or swine.

The King's Co. region of Nova Scotia is intensively farmed with diverse crop systems, and many poultry and swine producers. King's Co. supplied 33% of our fields, 90% of the fields in the high STP category, 100% of poultry and swine applications, and 60% of the fields under crop rotations. With the exception of SiCL-1 all the fields in Kings Co. had sandy textures. The fields from the other counties were mostly dairy farms and did not show equivalent TP and SRP losses to the King's Co. fields, even with similar textures, crop rotations, and relatively high STPs. The difference, therefore, seems connected to manure type, particularly poultry and swine, and as we saw earlier, applications proximate to heavy flow events. Unfortunately, in spite of that difference, most fields periodically have drainage that exceeds environmental guidelines for surface waters.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Large numbers of samples had concentrations lower than the detection limit, which was above some stringent guidelines. The frequency of TP concentrations below the detection limit (<0.04 mg L–1) was often >50% of the measurements from the dairy and swine manured fields. This shortcoming may underestimate the number of samples exceeding the lowest guidelines.

Within a few weeks of sampling it was evident that the fields with poultry and swine manure histories produced constantly high TP concentrations that were rarely lower than suggested environmental guidelines, and contained the greatest proportions of SRP. As months of data were compiled the concentrations were found to vary dramatically from week to week, and particularly in April, May, October, and November. These months are also those with the highest TP, SRP, and flow rate averages, and in 2002 the averages were lower than in 2003.

Flow rate had the effect of increasing TP concentrations but not significantly. This may be a result of the superseding effect that manure applications have over the other factors, especially when applications are proximate to heavy flow events (Schroeder et al., 2004; Toor et al., 2005). Some fields would generally produce TP concentrations below guidelines but occasionally, or frequently in some cases, exceeded guidelines. Variation between months, fields, and samples seems related to soil texture, STP, and manure and crop histories.

Manure history is a dominant part of cultivation that impacts variability in both STP and the relationship between STP and P losses; the influence, however, is reduced with time after manure application (Schroeder et al., 2004). Unfortunately, at our fields the variability often includes concentrations over 10 times and occasionally more than 50 times higher than existing guidelines. The proportion of UP in TP had a tendency to be lower in coarse textured soils and when TP levels were high. Conversely, % SRP was higher in coarse textured soils.

In Nova Scotia, farming practices tend toward most permanent cover crops receiving dairy manure while crop rotations often receive a mix of manure types. This situation is partially responsible for higher STPs and P losses under crop rotations because they often receive a mix of poultry, swine, and dairy manure applications. Since the poultry manure fields were all under crop rotations it seems to bias these fields toward higher drainage SRP concentrations. On crop rotation fields where dairy was the only manure, the SRP was relatively low. Poultry manure applications, all of which were on sandy loam soils, stands out as producing the highest TPs, SRPs, and TP/SRP ratios.

Since P losses are so variable, and relative to many factors, it would improve our understanding of the interactions to intensively sample a few fields during April, May, October, and November in Nova Scotia. We need to observe the short-term (at least daily) changes in P transport relative to field condition changes, and associated with manure applications and drainage flow rates under various soils and crop systems. This would help to characterize the rise to peak and drop-off of P concentrations during the frequently observed flushing events. Knowledge of the components of UP would also help characterize pathways, sources, and how the factors impact various P components in tile drainage water in Nova Scotia.


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





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