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a Agricultural Research Centre of Finland, FIN-31600 Jokioinen, Finland
b Dep. of Quaternary Geology, Univ. of Turku, FIN-20014 Turku, Finland
Corresponding author (risto.uusitalo{at}mtt.fi)
Received for publication February 28, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: AER, anion exchange resin • AER-P, anion exchange resin–extractable phosphorus • DRP, dissolved molybdate-reactive phosphorus • ER, enrichment ratio • PP, particulate phosphorus • PPi, desorbable particulate phosphorus • PUP, particulate unavailable (nondesorbable) phosphorus • TP, total phosphorus • TSS, total suspended solids
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In Finland, the production of arable crops is concentrated in the southern part of the country, an area with abundant clayey soils that, almost without exception, are artificially drained. Measures to reduce the P load from agriculture are targeted on efforts to diminish surface runoff. However, in artificially drained soils subsurface runoff is also an important pathway for water and solutes. In a 2-yr study, Paasonen-Kivekäs and Virtanen (1998) found that subsurface drainflow accounted for 45 and 57% of the annual total runoff from a field with clayey soil in southern Finland with a tile drainage system installed in 1950. Turtola (1999) reported that 8 yr after drainage improvement of a clayey soil in southwestern Finland up to 90% of the total water flow from plots plowed in autumn to a depth of 23 cm still occurred via subsurface drainage, while the respective proportion from plots under stubble cultivation (to a depth of 8 cm) was 50 to 60%. Turtola and Paajanen (1995) and Turtola (1999) also measured high P and sediment concentrations in drainflow, comparable with those in surface runoff. Recently, attention has increasingly focused on subsurface drainage flow as a contributor to the P load from agricultural fields and pastures (Øygarden et al., 1997; Laubel et al., 1999; Hooda et al., 1999).
Water quality monitoring usually only involves analyses of DRP and/or TP. When the bulk of the P in runoff comprises PP, TP is a poor predictor of the algal-available P load since algae take up only orthophosphate and desorbable PP (Ekholm, 1998). A large part of the P in surface runoff and subsurface drainflow from clayey soils is transported by suspended soil material (Turtola and Paajanen, 1995; Heathwaite and Johnes, 1996; Øygarden et al., 1997). In areas where the P loads are due mainly to the transport of PP, it is thus important to characterize the desorption tendency of PP and the properties of the transported particles. In a study conducted in southwestern Finland, Ekholm (1998) reported that about 5% of the PP in river water sediment was assimilated by test algae. Uusitalo et al. (2000) found that 5 to 10% of PP in surface runoff waters from four Finnish clayey soils was desorbable by anion exchange resin P sink. Moreover, in very turbid runoff water, the PPi concentration exceeded the DRP concentration. To our knowledge, the PPi concentration in drainflow has not been reported. To mitigate the adverse environmental effects of farming on the quality of surface waters, all significant sources of bioavailable P should, however, be identified.
Because P added to the soil surface increases the concentration of extractable P in the topsoil (e.g., Haygarth et al., 1998) and the higher the soil test P value the more PPi there is in runoff sediment (Uusitalo et al., 2000), the origin of the drainflow sediment is of importance. A way of tracing sediment origin is to seek fingerprints, for example, the chemical properties of the sediment studied, with which its source can be identified (see Walling and Woodward, 1995). For example, Cs is an element that is fixed to cation exchange sites in clay minerals (Anderson and Sposito, 1991). It has a man-made isotope, 137Cs, to which only topsoil has been exposed due to fallout from nuclear testing in the atmosphere and, in some parts of Europe, the Chernobyl accident. Mahara (1993) showed that vertical stratification of fallout 137Cs in soil remains very sharp for decades, thus making it a suitable marker for topsoil. Recently, Laubel et al. (1999), using 137Cs as a marker for topsoil, showed that in a Danish loamy soil the sediment material carried by drainflow originated from the topsoil. In addition to tracing sediment origin (Grant et al., 1996; Laubel et al., 1999), 137Cs has been used to assess erosion rates (Walling and He, 1999; Weigand et al., 1998).
The objectives of this study were to compare the concentrations of different P forms in surface runoff and subsurface drainage waters from two clayey soils (a Typic Cryaquept and an Aeric Cryaquept) and, by 137Cs analysis, to investigate the origin of the particulate material transported by subsurface drainflow.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The Sjökulla field (undulating, 4% mean slope) had tile drains installed in 1950 at about 110 cm depth. Manual grab samples of surface runoff were taken from a 2.04-ha field section at the outlet of a V-notch weir located at the lowest point of the field. Subsurface drainflow was manually sampled from a collector well equipped with a V-notch weir within minutes of surface runoff sampling. The draining area of the tile drainage system was about 1.66 ha (i.e., slightly smaller than the area where the surface runoff formed). For a more detailed description of the field, see Paasonen-Kivekäs and Virtanen (1998). Sampling in Sjökulla was thus conducted whenever there was both overland flow and drainflow. Consequently, most of the samples were taken in spring and autumn (the field was then visited weekly to daily, depending on the likelihood of the occurrence of runoff); in summer, the field was visited only after heavy rainfall.
Annual crops [barley (Hordeum vulgare L.) and wheat (Triticum aestivum L.)] were grown in the fields during the time of the study (August 1997–April 1999), with ploughing (depth 23–25 cm, Sjökulla and Jokioinen Plots A and C) or stubble cultivation (depth 8 cm, Jokioinen Plots B and D) in autumn and seed bed preparation (depth 5 cm) in spring.
Precipitation data for the studied sites were obtained from the nearest observatories of the Finnish Meteorological Institute, those at Jokioinen and Vihti. The Jokioinen observatory is located about 1 km from the experimental field, but the Vihti weather station is about 20 km from the Sjökulla field.
Pairing Surface Runoff and Subsurface Drainage Water Samples
The four plots at Jokioinen differed significantly (LSD test, p < 0.05) in average concentrations of some of the P forms in runoff water, and therefore the results for Plots A through D are reported separately. Each surface runoff sample had two subsurface drainage water samples as counterparts, corresponding to the drains with two different backfills. The average concentrations of TSS and different P forms in the drainage water samples did not differ from each other (according to the Mann–Whitney U test; e.g., Johnson, 1994) as a result of the backfill material used. The mean concentration of the two drainage water samples was therefore used as a subsurface drainflow "sample" that was then paired with the surface runoff sample of the corresponding plot for comparison of TSS and P forms.
The water samples from the Jokioinen and Sjökulla fields made up 91 surface runoff–subsurface drainage water pairs (57 from Jokioinen and 34 from Sjökulla), taken simultaneously from the matching field plots. The Jokioinen samples of this study represented 52% of the total surface runoff and 43% of the total drainage flow from Plots A and C during the study. On Plots B and D, the proportions were 39 and 58%, respectively. In relation to total discharge from the field, these samples are biased toward high-turbidity waters, since they represented 62 to 74% of total TSS, TP, and PP loss from the field, calculated from total water discharge and all water samples taken from the field. Due to the sampling strategy, the Sjökulla samples represented mainly high-flow events, which tend to produce high-turbidity waters. Hence, drainflow was not sampled unless there was also surface runoff, which, in turn, requires relatively intensive rainfall. The baseflow from drainage pipes was typically low in suspended solids. The relatively clear surface runoff peak caused by snow melt in early spring was not sampled because there was no drainflow in the frozen soil.
Phosphorus Analysis
The P analysis comprised determination of TP, DRP, and AER-P. The determinations were made in triplicate, except for DRP and TP in the Jokioinen samples. These were mostly analyzed without replicates; the accuracy of the determination was then checked frequently with standard solutions.
Total P was measured after digestion with peroxodisulfate and sulfuric acid in an autoclave (120°C, 100 kPa, 30 min), and DRP after filtration of water samples through a 0.2-µm Nuclepore polycarbonate filter (Whatman International, Maidstone, UK). A slightly modified method of Sibbesen (1978) was employed in the AER extraction. One gram of Dowex 1 x 8 strongly basic AER (Fluka Chemika, Neu-Ulm, Germany) was enclosed in nylon netting bags (mesh size 0.38 mm) and converted to bicarbonate form before use by washing with NaHCO3 solutions. After overnight extraction of a 40-mL water sample, the AER bag was carefully washed with deionized water. Phosphorus was displaced from the resin by shaking the bag for 4 h in 40 mL 0.5 M NaCl. The NaCl solution was then acidified with 1 mL 6 M HCl and allowed to stand overnight before the P concentration was measured. All orthophosphate determinations were carried out by the method of Murphy and Riley (1962) using ascorbic acid as the reducing agent. The determinations were performed with a Lachat (Milwaukee, WI) QC Autoanalyzer.
The PP concentration was calculated by subtracting DRP from TP, and reversibly adsorbed particulate phosphorus (PPi) by subtracting DRP from AER-P. The PUP was calculated by subtracting AER-P from TP. The water samples were also analyzed for TSS by weighing the evaporation residue of 80-mL subsamples without replicates (Jokioinen) or 40-mL subsamples with duplicates (Sjökulla). The concentrations of different P forms and TSS in the surface runoff and drainage waters were compared by the Mann–Whitney U test.
Cesium-137 Content of Soils and Runoff Sediments
Cesium-137 was analyzed (i) on bulk soil samples taken at different depths, (ii) on clay and sand fractions of Ap-horizon samples, and (iii) on the suspended sediment carried by surface runoff and subsurface drainage waters. The Jokioinen field was sampled at eight points at depths of 0 to 10, 10 to 25, 25 to 40, and 40 to 60 cm. At Sjökulla, the samples were taken from two soil pits, one of which was dug in the location of the tile-drainage excavation and the other between the tile drains (undisturbed soil). The sampling depth in the tile-drainage excavation was 0 to 20, 20 to 30, 30 to 40, 40 to 70, 70 to 100, and 100 to 110 cm (the depth of the tile drains), and in the undisturbed soil 0 to 20, 20 to 30, 30 to 45, and 45 to 70 cm.
For separation of clay-size particles (<0.002 mm in diameter) and sand (0.2–2 mm), about 1 kg of surface soil (Ap horizon, one sample from both fields) was passed through a 2-mm sieve, mixed with 2 L deionized water and ultrasonicated for 4 h. The suspension was mixed occasionally during ultrasonication. It was then transferred to a 10-L plastic bucket, which was filled with deionized water and mixed well. To separate clay, the suspension was siphoned to 10 cm depth after 7 h 45 min sedimentation. The remaining material was then wet-sieved (0.2-mm sieve) to separate sand. The suspension containing the clay fraction was precipitated by adding about 5 mL of polyaluminium chloride solution (Kemwater PAX 18; Kemira Chemicals Ltd., Helsinki, Finland). Kemwater PAX 18 is a flocculant commonly used in water and wastewater treatment, and it had no detectable 137Cs. After the flocculant addition, the suspension was mixed and left standing for 2 d. The clear solution was then siphoned off. The remaining sludge was further concentrated by centrifuging (1500 rpm, 10 min) and dried at 60°C.
For the analysis of the 137Cs activity of suspended sediments, 15-L composite water samples were collected. At Jokioinen, we collected four surface runoff samples and eight drainflow samples; the drains with different backfills were sampled separately. At Sjökulla, we collected two surface runoff and two tile-drainage water samples. The suspended sediment was precipitated by first adding 10 mL of Kemwater PAX 18 to 10 L of water sample. Two days later the clear solution was siphoned off, and the precipitate was centrifuged and dried at 60°C. The 137Cs activity of the dry, finely ground soils and sediments was measured by a gammaspectrometer equipped with germanium semiconductor detectors cooled in liquid N.
Particle-Size Analysis of Soils and Suspended Sediments
The enrichment of particles of various size in runoff waters was calculated on the basis of particle-size analysis of soil and water samples with a Coulter Electronics LS 200 laser particle sizer (Beckman Coulter Ltd., Fullerton, CA) equipped with a small volume module. The determination was carried out without replicates on the soil samples, on composite water samples (two surface runoff and two drainage waters) from the Jokioinen field, and on one surface runoff and one tile-drainage water sample from the Sjökulla field.
Prior to analysis, the soils were finely ground and organic matter was removed with H2O2, followed by dispersion with 2 h weak ultrasonication. The analyses were run with 0.5 M Na4P2O7 solution as the carrier fluid. The water samples were pretreated by ultrasonicating for 2 h, and then diluting them with Na4P2O7. The particle sums were calculated for <0.002-, 0.002- to 0.006-, 0.006- to 0.02-, 0.02- to 0.06-, and >0.06-mm particles. Enrichment ratios were then calculated for these particle size classes by dividing the count sums of the water samples by the corresponding count sums of the surface soil samples. The clay contents of the soils as presented in Table 1 are, however, based on analysis by pipette method (Elonen, 1971).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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At Jokioinen, 40 to 60% of the annual precipitation in 1987–1993 (558–704 mm) ran off the field, and after the drainage system had been improved (in 1991), 50 to 90% of the total runoff constituted subsurface drainage water (Turtola and Paajanen, 1995). At Sjökulla, total runoff was 48% of the precipitation according to Koivusalo et al. (1999), calculated for the period August 1995–December 1996. Paasonen-Kivekäs and Virtanen (1998) found that subsurface drainage waters in the Sjökulla field constituted 45% of total runoff in 1995 and 57% in 1996. In these years, precipitation at the Vihti weather station was 675 and 644 mm, respectively.
Phosphorus in Surface Runoff and Subsurface Drainage Waters
The TSS and P concentrations in surface runoff did not differ greatly from those measured in subsurface drainage waters (Table 2). Only the DRP concentrations differed significantly (p < 0.05) between surface runoff and subsurface drainage waters in two of the Jokioinen field plots and at Sjökulla. At Jokioinen, the DRP concentration tended to be higher (significantly so in field Plots C and D) in surface runoff than in drainflow. The mean DRP concentrations in surface runoff and subsurface drainage waters were, however, fairly similar (0.031–0.047 mg L-1) in all Jokioinen plots. Surprisingly, at Sjökulla we found that the mean DRP concentration in drainflow was more than twice as high as that in the surface runoff samples. Significantly higher DRP concentrations were measured in subsurface drainage waters at Sjökulla throughout the study, with only one sample pair deviating from this trend. The Olsen soil test P (Olsen and Sommers, 1982) was somewhat higher in the upper (40–45 mg kg-1) than the lower part of the field (35–40 mg kg-1), probably due to previous applications and/or storage of manure in the upper part. Increase in soil P status has been shown to cause elevated DRP concentrations in subsurface drainage waters (Heckrath et al., 1995). Surface runoff formed at the upper edge of the Sjökulla field probably percolated through the soil before reaching the sampling point at the lower edge of the field. As a result, a large part of the surface runoff sampled may have originated from an area having a lower soil test P. It is probable that there were some hot spots (probably sites where manure had been stored in heaps) at the upper end of the field with a much higher soil test P than we measured. The small difference in Olsen P values that we found would hardly explain the P concentration in drainflow that was twice as high as that in surface runoff, even though the P concentration in drainflow has been found to increase rapidly after a threshold soil test P value has been reached (Heckrath et al., 1995).
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Because we did not find any difference in PPi concentration between the surface runoff and drainflow samples, and the proportion of water flow along the subsurface pathway was 90% from the plowed plots (Turtola, 1999), the PPi loss via drainflow from plowed soil at Jokioinen was likely to be significantly higher than that via surface runoff. On the stubble cultivated plots with 50 to 60% of total discharge as drainflow (Turtola, 1999), the PPi loss via drainflow was probably almost equal to the loss in surface runoff. As the calculated DRP loss (kg ha-1) in drainflow is also larger than that in surface runoff at Jokioinen (Turtola and Paajanen, 1995), the bulk of the potentially bioavailable P load from the Jokioinen field comes via subsurface drains. At Sjökulla, there are no previous data on P losses, but on the basis of the even distribution of the water flow between surface and subsurface pathways (Paasonen-Kivekäs and Virtanen, 1998) and the P concentrations measured in our study, subsurface drainage waters may be responsible for half of the P loss from the field. Although subsurface drainage probably reduces the total losses of P from a field (Haygarth et al., 1998) by cutting down the most erosive surface runoff peaks, erosion and P losses may still be considerable (Hooda et al., 1999; Turtola and Paajanen, 1995).
Cesium-137 Activity and Particle-Size Distribution of Soils and Suspended Particles in Runoff
To test whether the sediment carried by drainflow derived from the topsoil, as suggested by the similar PPi contents of surface runoff and drainflow sediments, soils and runoff sediments were analyzed for 137Cs activity. In accordance with previous studies (Mahara, 1993; Walling and He, 1999), the 137Cs activity in soil decreased sharply with depth (Fig. 1)
. In suspended soil material carried by surface runoff and subsurface drainage waters, we measured about the same average 137Cs concentrations, 49 to 50 Bq kg-1 at Jokioinen and 85 to 88 Bq kg-1 at Sjökulla. The drainage system of the Jokioinen field was renewed in 1991, that is, 5 yr after the Chernobyl accident, the most recent source of fallout 137Cs in southern Finland. Surface soil was used as backfill at the upper end of the field and, therefore, we can only conclude that the sediment from those drains did not derive mainly from the subsurface soil layers. The backfill of the drains at the lower end of the field consisted of wood chips, and surface soil was prevented from falling into the excavation during the installation of the new drains. The similar 137Cs contents of the suspended sediment in surface runoff and drainflow from drains with wood-chip backfill suggest that the suspended sediment in these samples originated from the surface soil. The 137Cs activity of the suspended sediment from the drains with surface soil as backfill seemed to be slightly higher (53 ± 17 Bq kg-1, mean ± SD, n = 4) than the sediment from the drains with wood-chip backfill (44 ± 7 Bq kg-1, n = 4) but the difference was not statistically significant (two-tailed p = 0.4705, Mann–Whitney U test). At Sjökulla, the tile drains were installed in 1950 (before large-scale atmospheric nuclear testing), and the sediment carried by subsurface drainflow can be concluded to originate from the Ap horizon, since the 137Cs activity in the surface runoff sediment, 83 to 86 Bq kg-1, was similar to that measured in drainflow sediment, 83 to 92 Bq kg-1.
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The ratio of 137Cs activity in surface soil (Ap bulk sample) to that in runoff sediments, that is, the enrichment ratio (ER) for 137Cs in surface runoff and subsurface drainage waters relative to topsoil, was about 2 (range 1.5–2.4) at Jokioinen (regardless of the backfill used), indicating preferential transport of the most reactive fine-grained particles. At Sjökulla, the ER for 137Cs relative to topsoil ranged from 1.8 to 2.0. Similar ERs (1.8–2.5) were obtained with the laser particle counter for clay-sized particles in the water samples as related to surface soils (Table 4). Sharpley (1985) reported similar ERs, 1.37 to 1.86, for clay-sized particles in surface runoff. In eight small watersheds, Weigand et al. (1998) measured an average ER of 1.72 (range 0.40–4.95) for 137Cs in runoff sediment relative to surface soil. The manufacturer of the Coulter particle sizer gives an analytical range starting with 0.0004 mm, but we observed a rapid decrease in particle counts below about 0.0007 mm in all analysis runs. Hence, the ERs for particle fractions smaller than 0.002 mm have not been further subdivided. It is, however, probable (and suggested by the somewhat unreliable particle counts and the somewhat higher 137Cs content of the runoff sediments than of the clay fraction of the Ap soils) that the ER for particles smaller than, say, 0.001 mm, would be even greater than we measured here. High ERs can be expected for colloidal clay, especially in drainflow, because filtering of larger particles within the soil profile is likely to enrich the drainage waters in colloid-size particles.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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: n = 4), error bars indicate SD. At Sjökulla n = 2 (except
: n = 1, sampled just above the tile drain), error bars indicate range