Water transport is one of the main ways of matter exchange between ecosystems. Hence, an understanding of the controls on the flux of water as the carrier and solvent of pollutants in the landscape is essential. The structure and distribution of plant cover has great bearing on the output of chemicals from watersheds. Mechanisms and factors modifying water flux across biogeochemical buffer zones play a decisive role in controlling non-point pollution of P in the rural landscape.
Buffer zones, such as riparian forests or meadows, or midfield afforestation plots, limit the spreading of chemicals in ground water by reducing their passage (Cooper et al., 1987; Muscutt et al., 1993; Naiman and Decamps, 1990; Paulukevicius, 1981; Peterjohn and Correl, 1984; Ryszkowski, 1979; Ryszkowski and Bartoszewicz, 1989; Ryszkowski et al., 1989). The kinetics of transport through buffer zones is less well known. Our long-term studies integrated information on water cycle, transport of chemical compounds by ground water below various components of agricultural landscapes, soil characteristics and meteorological conditions with the aim to analyse the changes of phosphate flux in ground water through a meadow during the year.
One of the difficulties in evaluating chemical migration is the estimation of evapotranspiration (ET) under field conditions. An approach combining the estimation of solar energy flux as the driving force for water cycling facilitates the estimation of ET from intact land segments (Ryszkowski, Kedziora, 1987; Kedziora et al., 1989; Olejnik, 1988) over time. Thus, by coupling solar energy flux with water flux through the latent heat used for ET, one can obtain estimates of ET under field conditions if heat balance components are known. Therefore, the heat balance of a meadow was also studied, which permits the quantification of solar energy used for ET, and air and soil heating (Kedziora et al., 1989; Ryszkowski and Kedziora, 1987). Estimation of the energy used for ET allowed the calculation of water losses by ET which permitted the evaluation of the role of the root system in P migration in ground water below the meadow.
Area and methods of study
A catena of field-meadow-channel, oriented in a south-north direction was studied (Figure l). The meadow belt separating the cultivated field from the channel was 80-90 m broad. The terrain is a slightly undulating ground moraine, with many drainage valleys. Light, well-drained soils cover the higher parts of the area. The valleys are mostly filled with 0.5 to 3.0 m thick organic deposits. The level of the ground water table in the moraine depends on the surface elevation and ranges from 0.5 to 3.5 m below the surface. In the valleys the ground water table varies with deposits: in organic deposits, it ranges from near the surface to 0.8 m below; in mineral intrazonal hydromorphic soils (Haplaquolls, Psammaquents) it ranges from 0.5 to 1.2 m; and in the zonal soils (Hapludalfs) it ranges from 1.2 to 3.5 m below the surface (Krygowski, 1961; Margowski et al., 1976).
Average annual temperature and precipitation are 8°C and 600 mm, with a growing season (mean air temperatures > 5°C) of H 225 days. The year of the study (1985) was moist with normal temperatures. Air (at 2 m) and soil temperatures (at 5, 10, 20, 50 cm depth), sunshine, wind speed, vapour pressure, saturation deficit, precipitation and humidity were used for the calculation of the heat balance. Soluble reactive phosphate (SRP) was determined.
Figure 1. Map and cross section of experimental
area, ground water levels in cm.
It was assumed (1) that the flux of water flowing through the root system of plants is determined by the amount of incoming subsurface water, as well as by ET, the intensity of which depends on evaporative demand of the atmosphere and availability of solar energy; (2) that, in the process of evapotranspiration, the ground water in the saturated zone which is in contact with the root zone is used first. When this is depleted, water from the unsaturated zone of the soil is absorbed by plants.
Changes of SRP concentration in ground water
From the beginning of the vegetation season (March 19, Table 1), SRP concentrations in ground water are very low until May 21 when a very high concentration was observed in well No 1, while SRP in the other wells remained low. This indicates the date when the concentration wave of phosphate leached from the cultivated field during the early spring reached the edge of the meadow (well No 1). High precipitation (Table 2) during the whole year resulted in high SRP concentration in ground water, and also in a high ground water table. If the meadow plants do not take up P, the wave of higher concentration should reach well No 2 after 10 days because the water flow velocity was 3 m day-1. However after 30 days the concentration of phosphate increased only to 0.13 mg l-1 in well No 2. The peak concentration in well No 2 (3.68 mg l-1) was observed 169 days after March 19 (Table 1). At the same time, a concentration peak was observed in the well No 3 amounting 0.017 mg l-1, much lower than in wells No 1 and 2. These data indicate that plants in the meadow absorbed phosphate very actively from the ground water.
Table 1. Concentration of phosphate and ground water level under meadow in the agricultural landscape of Turew, Poland, 1985.
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75.300 | 75.000 | 74.500 |
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75.050 | 74.850 | 74.500 |
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75.000 | 74.800 | 74.400 |
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74.975 | 74.775 | 74.735 |
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74.975 | 74.775 | 74.400 |
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74.975 | 74.775 | 74.425 |
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75.950 | 74.750 | 74.400 |
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74.950 | 74.750 | 74.400 |
ground-water Flux and evapotranspiration rate
The estimates of ET flux per day balanced with the input of water from rain, giving an almost stable ground-water level during the whole year which facilitated the estimation of the ground-water flux. Accordingly, the water table was very stable and was lowered only by 0.35 m in well No 1, by 0.25 m in well No 2 and by 0.125 m in well No 3 during the whole vegetation season. Under such conditions the hydraulic gradient along the width of the meadow was rather stable between 0.007 to 0.01. The flux of water traversing a plane of 2 m depth and 1 m width at the field - meadow boundary amounted to 12 l day-1 during the beginning of the vegetation season (March 19 - April 15), then decreased to a 8 l day-1 for the rest of the vegetation period, because of the change in hydraulic gradient. Evapotranspiration from the meadow increased steadily from 1.4 mm day-1 (March 19 - April 15) to 3 mm day-1 (May 21 - June 18) and then gradually decreased to 0.6 mm day-1 in October. Total ET for the whole period was equal to 421 mm. In the same period the precipitation amounted to 525 mm, which counterbalanced the loss of water by ET. The balance between precipitation and ET was observed during the whole vegetation season, with the exception of the period from July 12 to September 3 when precipitation was two times higher than ET. The water transpired by the meadow plants is taken up mainly from ground water, which is restored by percolating rain water.
Because the ET of one mm of water is equivalent to one l of water evaporated from one square meter, dividing the flux of incoming ground water by daily ET rates gives an estimate of the breadth of a 1 m meadow strip in which the incoming ground water flux would be totally absorbed by plants. For example, in the period of May 25 to June 18 (Table 2), the incoming ground water flux was 8 l day-1. Evapotranspiration was 3 l m-2day-l so the breadth of meadow required was less then 3 m (Table 2). Thus, if there was no percolation of rain water through the soil profile, the incoming ground water would be used by meadow plants growing on a 3 m strip.
One can conclude, therefore, that even a narrow band of meadow could strongly affect the passage of the ground water and thus can influence P migration in ground water. When plant growth is intensive (May - September) the 3-4 m broad meadow could control ground water flux (Table 2). For the greater fluxes of early spring and late autumn, a 10-15 m wide meadow could be a sufficient barrier.
Control of P flux by meadow vegetation
Primary production of the meadow including above and below
ground components of vegetation amounted to 1336 g m-2 dry weight (Ryszkowski,
1991), of which 40% (531 g m-2) were harvested.
Phosphorus concentration in hay was 1.78 mg g-1 which was equal
to 1 g m-2 of P removed. The P uptake by plants during the growing
season was estimated using values of primary production and P
concentration in plant tissue. Uptake of P by plants in the meadow
was 1.81 g
m-2 per growing season. The grass in the complete meadow strip
of 80 m (by 1 m) absorbed 145 g per growing season. The
total SRP entering the meadow from the field with ground water
was 5.1 g over the growing season, while 0.5 g was discharged
into the canal. The difference (4.6 g per growing season)
was attributed to absorption by plants. Thus, the plant P uptake
from ground water is 3% of the total amount of P absorbed by
plants. Although this P uptake from ground water is a small contribution
to the plants’ needs, the vegetation controls P migration
very efficiently. The effective control of the incoming P flux
is indicated by the very low concentration in well No 3, and
the small increase of P concentration in this well only after
high concentrations are observed in wells 1 and 2 (Table 1).
Such efficient P removal indicates that plants can take up P
from ground water much more easily than from unsaturated soil,
from which, nevertheless, the majority of P supply is obtained.
If this is true, then even higher concentrations of P in in-flowing
ground water could be efficiently controlled by a strip of meadow.
Table 2. Evapotranspiration (ET), precipitation (PP), ground water flux (J) and width of meadow band totally absorbing inflowing water (L) in the agricultural landscape of Turew, Poland, 1985.
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| 19.3-15.4 (28) | 40 | 1.4 | 42 | 1.5 | 12 | 9 |
| 16.4-20.5 (35) | 72 | 2.1 | 66 | 1.9 | 8 | 4 |
| 21.5-18.6 (29) | 87 | 3.0 | 87 | 3.0 | 8 | 3 |
| 19.6-11.7 (23) | 59 | 2.6 | 49 | 2.1 | 8 | 3 |
| 12.7-3.9 (54) | 118 | 2.2 | 238 | 4.4 | 8 | 4 |
| 4.9-25.9 (22) | 29 | 1.3 | 19 | 0.9 | 8 | 6 |
| 26.9-24.10 (29) | 16 | 0.6 | 24 | 0.8 | 8 | 14 |
| 19.3-24.10 (220) | 421 | 1.9 | 525 | 2.4 | - | - |
The complicated interplay of various factors determining migration of P in ground water is indicated by a moving wave of peak P concentration across the meadow. The flush of P from the cultivated field reached the edge of the meadow about 60 days after the vegetation season started (Table 1, compare the readings of P concentration in well No 1 on March 19 and May 25). This was the result of heavy rains between April 16 and May 20 (66 mm) and especially of showers on April 30 (34 mm) and May 1 (21 mm). Because the soil was not frozen and the infiltration rate of this soil is rather high (Marcinek et al., 1990), most P leaching from field to meadow occurred in a brief 20 day period. High precipitation during the whole vegetation season probably was the cause for continuing high P input to the ground water below the field. Heavy rains in the first ten days of August (130 mm) caused an increase of P concentration from 2.61 mg l-1 to 4.0 mg l-1.
The analysis of ET rates showed that the meadow can control ground water flux. Ryszkowski and Kedziora (1993) found that the reduction of ground water flux by a 10 m wide shelterbelt (midfield trees) or a stretch of meadow on 1° sloping ground can be as much as 100 percent on a sunny day. These results are confirmed by the analysis carried out in this study. Even a 5 m wide strip of meadow could influence the flux of ground water in summer (Table 2). The root system of grasses absorbs P from ground water very efficiently. Despite the high plant available P level (63 mg kg-1, Ryszkowski et al., 1989) in the meadow soil, ground water SRP was absorbed almost completely. Thus, one can assume that P dissolved in ground water is more available to plant roots than that in soil of the unsaturated zone.
If this hypothesis is true, barriers of phreatophytes could control high ground water P loads, absorbing P from ground water instead of removing it from the unsaturated zone of soil.
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Phosphorus in the Global Environment. Edited by H. Tiessen © 1995 SCOPE. Published in 1995 by John Wiley & Sons Ltd. |
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Last updated: 30.06.2001