Plan de gobierno 1994 – 1998

Birch forest

In the three-year period, the mean NO3-N concentration was 33.1 mg l-1_{in the field and 8.4 mg l}-1_{in the}

forest (Fig. 5). The variation in NO3-N concentration was higher in the field phreatic zone (cv=44%) than in the forest (cv=32%). The annual flux of NO3-N with discharging groundwater from the field into the birch forest through the 1 m x 2 m phreatic plane was 301 g in 1984, 219 g in 1985 and 335 g in 1986 (Fig. 2).

The mean NH4-N concentration for the three years was 1.73 mg l-1 _{(cv=32%). For the same years}

discharges of NH4-N from the cultivated field into the birch forest were 14.2, 8.6 and 13.3 g respectively (Fig. 2). Total input of NH4-N during the three years was equal to 36 g while the output was 41 g.

Figure 5. Changes of NO3-N (N) concentrations in groundwater under the field (P3) and the birch forest (Z4).

Using the evapotranspiration rates (Fig. 1) and the amounts of water discharged into the birch forest from the field (Fig. 2), the length of one metre wide forest needed to evaporate the water can be estimated. It is presumed that nutrients dissolved in this water are uptaken by plants too. In these calculations it is assumed that the uptake of nutrients is characterised by mass flow and effects of the absorption by diffusion processes are neglected. The calculations of the biogeochemical barrier (BGCB) length were done in the following way.

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For example, in a ten-day period between 20 and 30 April, 246 litres of water flowed from the field into the forest through a 1 m x 2 m plane of the phreatic zone. In this ten-day period evapotranspiration was equal to 25 litres per m2_{, so the amount of water equivalent to, but not}

necessarily the same in-flow water, will be transpired by trees in an area of 1 m x 10 m. Thus at a distance of 10 m nutrients contained in influx water were also removed.

It should be emphasised that water flow is very slow in soil and in the case analysed it amounts to about 1-2.5 m per 24 hours. For greater fluxes (e.g. higher water table slope) more water will be passing and the effectiveness of a given length of the BGCB will be smaller.

In the case of the birch forest the width of BGCB changed during the plant growth period from 5 to 25 m (Fig. 6) with an average value of 10.4 m. Thus one can assume that a birch BGCB of 20 m wide will efficiently reduce the inflow of nitrate from the field. In deciduous trees, when the leaves are shed in the late autumn evapotranspiration stops, while evapotranspiration in coniferous trees either stops or is highly depressed because of unfavourable climatic conditions in the period of late autumn- winter. Thus in the winter season the evaporation of water from the earth surface is the dominating process and calculation of BGCB width makes no sense.

Figure 6. Changes of biogeochemical barrier (BGCB) length in the birch forest.

In addition to NO3-N , ammonium (NH4-N) was determined in the groundwater. In the three years the concentration of NH4-N was higher in the phreatic aquifer under the birch forest than under the adjoining cultivated field, but only in 1985 were the differences statistically significant (Table 2). One can assume that there is balance between uptake of NH4-N by plants and its release in processes of mineralisation and denitrification. Nevertheless, comparing the concentrations of both mineral forms of nitrogen (nitrate + ammonia), one finds substantially lower concentrations in groundwater beneath the forest (10.51 mg l-1_{) than in the phreatic aquifer below the cultivated field (36.64 mg l}-1_).

Table 2. Concentration of NH4-N (mg l-1_{) in groundwater under fields and forests.}

Year Field Forest Statistical

significance

19841.86 1.96 no

1985 1.28 1.77 p <0.05

1986 1.43 1.44 no

19941.27 1.07 no

The pine forest

There was a steep gradient in NO3-N concentration in groundwater between the field and the pine forest from a mean value of 52.4 mg l-1 _{to 2.7 mg l}-1 _{(Fig. 7). The coefficient of variation of NO3-N}

concentration in the phreatic aquifer below the cultivated field was 36.3%, while below the forest it was lower (27.6%), resembling the situation observed in the cultivated field and the birch forest. The annual flux of NO3-N through a 1 m x 2 m plane of the phreatic aquifer at the forest boundary was equal to 287 g. The highest rates of NO3-N inputs were detected in the period between January and April which was the result of very high precipitation in autumn 1993 and a warm winter (Table 1). In January, NO3-N discharges from the field reached the level of almost 20 g per ten-day period before decreasing (Fig. 4). After heavy rain (48 mm) in February (Fig. 3) when the unsaturated zone of soil was very moist, increased inputs of NO3-N into the pine forest reached almost 16 g N per ten-day period (Fig. 4). When evapotranspiration reached a rate of 25 mm per ten days (Fig. 3), the water flux stabilised at a level of 100 litres per ten days and NO3-N flux slightly decreased as the season progressed (Fig. 4).

Figure 7. Changes of NO3-N (N) concentrations in groundwater under the field (S2) and the pine forest (S9).

During the growing season of 1994 the width of biogeochemical barrier (BGCB) varied from 2.4 m to about 10 m with an average value of 5.8 m (Fig. 8). The mean value of BGCD is lower in the pine forest than in the birch forest; the difference is statistically significant. This may be an indication of the higher efficiency in nitrate flow control by a coniferous forest than by a deciduous one, but it should be kept in mind that the birch forest in question was a poor stand of trees.

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Figure 8. Changes of biogeochemical barrier (BGCB) length in the pine forest.

The NH4-N concentration in subsurface water inside the pine forest amounted to 1.07 mg l-1_{and was}

not statistically different from those under the field (Table 2). The annual influx of NH4-N into the forest was equal to 7.8 g and output after groundwater passed under trees was equal to 6.7.

THE DISTRIBUTION OF MINERAL FORMS OF NITROGEN ACROSS THE