IntroductionTop
Nitrogen is the main crop nutrition element that has some impact on its productivity (Oenema et al., 2009). Continuous fertilisation with N fertiliser ensures large and constant harvests. Nitrogen excess in agricultural production has a negative impact on the environment (leached nitrates, evaporating ammonia and N oxide) (Ahlgren et al., 2008). The abuse of N fertiliser is becoming a global problem and affects the environment in a number of ways: (1) human health problems caused by nitrates (Powlson et al., 2008); (2) livestock health problems (Lundberg et al., 2008); (3) surface water eutrophication (Smith & Schindler, 2009); (4) formation of nitrous acid, which is linked to acid rain (Menz & Seip, 2004); (5) depletion of the ozone layer in the atmosphere because of atmospheric nitrate oxides, which strengthen ultraviolet radiation (Savci, 2012; Rosenstock et al., 2013); and (6) causes the global warming effect (Martinez-Blanco et al., 2014; Payen et al., 2015).
In terms of non-point source pollution, N constitutes 52 to 61% (Klaushal et al., 2011). Agricultural pollution constitutes on average 30 to 35% of N leaching (Klaushal et al., 2011). Due to particular agricultural operations in Lithuania, 36,000 tonnes of N per year reach the Baltic Sea waterborne load (Larsson & Granstedt, 2010). Previous research has shown that leaching of elements and compounds from the soil is essentially determined by the hydrothermal regime, the amount of these element in the soil and their fertilisation (Beeston et al., 2010; Komonweeraket et al., 2011). A higher rate of Nmin leaching occurs in humid years (Sogbedji et al., 2001). Views on fertilisation systems constituting human influence on water quality are rather mixed in the scientific literature. While some authors claim that mineral fertilisation systems are more environmentally dangerous (Kokkora & Hann, 2007; Ulen et al., 2012), others claim the opposite, that organic systems are more dangerous (Hossain et al., 2007). Nonetheless, it is known that unproductive fertiliser losses and water pollution increase considerably through the irrational and unbalanced use of both mineral and organic fertiliser (Hongxu et al., 2011; Hyytiäinen et al., 2011). It was determined in Finland that the yield of crops increases substantially, while N leaching falls with balanced fertilisation with N carried out several times during the vegetation of crops (Salvioni et al., 2011). As tests have shown, NO3- N leaching from the soil can be reduced significantly by changing scattered fertilisation into local (Nakamura et al., 2004).
Current knowledge suggests that the scale of leaching of chemical elements and compounds should be inversely proportional to the agroecosystem’s total biomass synthesis. Cabrera et al. (2007) determined that a bigger biomass synthesis and dryer year limit N leaching by drainage and ground water. Securing a rich harvest (crop rotation with cover crops and continuously occupied soil, plants of high biological potential, balance and rational fertilisation and application of organic production) reduces the potential water pollution risk (Thorup-Kristensen et al., 2012; Buckley & Carney, 2013).
The quality of water which filters though the soil and in the surface runoff depends on the changes that occur in the soil when a variety of crops is grown. The fact that around 67% of the leached N is from the soil has been known for a very long time (Ulen & Johansson, 2009). Research data on N migration and its compounds’ dynamics in the soil in the agroecosystems affected by human activity is not substantial.
Based on statistical data, cereals crop areas have increased since 1991 in Lithuania and constitute 34.3 to 41.8% of the total crop area. Perennial and annual grass areas reduced from 46 to 41%. For many years, large areas of perennial grasses, which are used for a longer time without inclusion in crop rotations, are used to grow cereals successively. The farmer’s decision to grow more of one or another kind of crop, as well as the general farming culture, is affected by economic interest. Such a one-sided approach can worsen the indicators of the agro landscape and become a potential non-point pollution source, even when other agrarian measures are used correctly and rationally.
On the basis of the data obtained, we have proposed the hypothesis that non-point source pollution of the agro-landscape is inversely proportional to the synthesis of total biomass of the ecosystem, and that all of the available means ensuring high crop productivity actually reduce the potential risk of water pollution. The novelty of this paper is the evaluation of the effect of crop rotations under different fertilisation levels on the environment. The aim of the research was to evaluate the relationship between crop rotations of different intensity and biological potential with the migration of N.
Material and methodsTop
Site description
The experiment was performed in the Middle Lithuania lowland in the basin of Graisupis (area 16.6 km2) and in the village of Lipliūnai (N 55°18′, E 23°51′). A subsurface drainage site was used for the investigations. Investigations were carried out from 2006 to 2013. The experimental site included three plots (Fig. 1).
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The soil in the site is non-acid Endocalcari - Endohypogleyic Cambisol (CMg-n-w-can) sandy light loam and sandy loam on sandy loam and sandy light loam (Liekis, 2001). According to the size distribution of soil particles, the research soil is rather homogenous. The arable layer of TI and TII is sandy light loam. Its composition in deeper layers lightens until sandy loam. The soil in TIII is slightly lighter (Table 1). In the upper (0-20 cm) soil layer, the soil was average and high in humus (1.79-4.32 %). Agrochemical data analysis showed that differences in chemical elements of the soil among the treatments at the beginning of the research were not significant, except TIII, where we determined that N (tfact.=2.82>ttheor.95%=2.3) and humus (tfact.=2.47>ttheor.95%=2.3) were significantly lower than in TI. However, as we projected to use the perennial grass crop rotation in TIII, where perennial grasses fix N uses legume bacteria, we decided to start the research.
Meteorological conditions are described using the data of the Dotnuva meteorological station, which is 8 km away from the study area.
Experimental design
In each of these plots, a field crop rotation with different fertilisations using mineral fertilisers was used. This was a production experiment; therefore, in different years at the beginning of the experiment, the amount of fertiliser used depended on the financial state of the farm. The following crops were grown: sugar-beet (Beta vulgaris L. var. sacharifera) ‘Belmonte’, spring barley (Hordeum vulgare L.) ‘Ūla’, winter wheat (Triticum aestivum Host) ‘Portal’, perennial grass red clover (Trifolium pratense L.) ‘Liepsna’ + feeding timothy (Phleum pratense L.) ‘Gintaras II’, summer wheat (Triticum aestivum L.) ‘Nandu’ and winter rape (Brassica napus L.) ‘Valesca’. The long-term field experiment in Lipliūnai consisted of three crop rotations with 75, 87.5 and 50% proportions of cereals (Table 2).
Table 2. Field crop rotations (Rot) with proportions of 75, 87.5 and 50% of cereals and its N fertilisation (Fert, kg/ha a.m.) and crop yield (t/ha, dry matter)
Only mineral fertilisers were applied to crop fields. Nitrogen fertiliser (as well as phosphorus and potassium) was applied in the spring. The fertilisation schedule is presented in Table 2. The N fertiliser used was ammonium nitrate (NH4NO3). A dispersed fertilisation method was adopted, using a suspended fertiliser spreader “Amazone”. The scattered fertiliser amount in higher or lower fertilisation fields was determined by academic recommendations and the financial position of the production farm.
Soil, plant and water analyses
Before starting the research, there were two soil profiles dug out in each of the treatments, and their soil genetic horizons were determined as well as samples from each of the layer were taken to determine physical and agrochemical characteristics from 0-24, 24-55 and 55-91 cm depth. The size distribution of soil particles (or soil texture) was determined by the Fere triangle method (Liekis, 2001), and soil density by the weight method in intact structure samples (Vadiunina & Korcagina, 1986). In order to determine the agrochemical soil characteristic, samples were taken in the autumn, after the harvest. Each year, in order to determine the agrochemical properties of the soil, samples were taken in autumn after harvest. Soil samples were collected using a drill up to 60 cm depth at every 20 cm. In TI and TII there were 15 boreholes drilled, while in TIII there were 20 boreholes. In each experiment treatment there were 3 collective samples created, which were analysed in the laboratory.
The soil pH was determined potentiometrically in a soil suspension in 1 M KCl (ISO 10390:2005); the humus content was determined by the method of Tyurin (1937); the Ntotal was determined by Kjeldahl’s procedure (ISO 11261:1995); available P2O5 and K2O were determined by the AL method (soil:solution ratio – 1:20, 0.1 M ammonium lactate + 0.4 N acetic acid, pH 3.74, 2 h shaking) (Egner et al., 1960); and Nmin amount (NH4+ -N + NO3–-N) contained in the soil was measured from the extraction of 0.2 M KCl using a flow injection analyser (FIA star 5012, AN 551, ASN 65-31-84, and ASN 50-01-84) (ISO/TS 14256-1:2003).
The yield was harvested mechanically and crop samples were taken from all replications. The cereals and grasses crop yield was harvested mechanically. The yield of row crops was determined manually. The stacionary area of the test fields of crops and grass was 30 m2, row crops covered an area of 45 m2. Crop harvest was determined by 6 replications in TI and TII, and by 8 in TIII. The main and secondary production of crops were weighed separately. The samples of the main and secondary production of yield were taken at the same time. Twenty different plants of sugar beet were taken in each test field (in a diagonal way). Leaves were sampled taking 2 kg of leaf mass. During the crop harvest time 3 kg of grain and 2 kg of straw were taken from each test field. Grass samples were taken during the grass haymaking time when a certain amount of grass was removed from 10 (in TI and TII) to 15 (in TIII) places in each test field. The amount of dry matter was determined in all samples used for yield production by drying at 105°C and reweighing to constant weight. Total crop yield energy was calculated using equivalents, which were multiplied with dry matter yield (Jankauskas et al., 2000). Ntotal in crop yield was determined after burning with sulphuric acid (ISO 5983-1:2005/Cor. 1:2008).
Drainage water discharge was measured by a volumetric method at every three days. Daily drainage was calculated by linear interpolation. Samples of drainage water were taken every 10 days. Monthly Nmin and Ntotal leaching was calculated having multiplied the average monthly nutrient concentration by the monthly drainage value. N concentration in water was determined colorimetrically, NH4 + by the gas diffusion method (ISO 11732:2005), Ntotal was determined after organic matter mineralisation with potassium persulphate and NO3– by the cadmium reduction method (ISO 13395:1996).
Data analysis
The data were processed by statistical methods. Disperse analysis and correlation and regression methods were applied (Shibusawa & Hache, 2009). The statistical significance of the data was computed by a multiple-way analysis of variance (ANOVA). In addition, the LSD multiple range test was used, the means of which differed significantly using a significance level of 0.01 and 0.05%. Student’s test statistic was used to determine the reliability of differences between treatments.
ResultsTop
Data show that perennial grasses and sugar beets produced the biggest yield (3.7-10.9 and 10.3-10.4 t/ha of dry material respectively). In all years, the higher fertilisation (843 N kg/ha) cereal crop rotation yield was determined to be larger than in the lower fertilisation (540 N kg/ha) cereal crop rotation (Table 2).
As different types of crops were grown in the research field, in order to determine which crop rotation field was the most productive, each of the yields of dry material amount was converted to GJ/ha and cumulative curves were drawn. After eight years of research, the biggest productivity was found in crop rotation of cereal with higher fertilisation, which accumulated 1483 GJ/ha of total energy harvest (Fig. 2).
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Less fertilised cereal crop rotation productivity was 35% lower, and it was the lowest in the research range (966 GJ/ha). Perennial grass crop rotation productivity was 11% lower than the higher fertilised cereal crop rotation and constituted 1318 GJ/ha. With respect to humus and N, the soil of the site was not very homogeneous, as the variation coefficients of these indicators were 17 and 14, respectively. At the end of the research the highest humus reserves were found in the field, where perennial grasses crop rotation was applied (4.11%) (Fig. 3). A slightly lower soil humus is obtained by applying higher fertilisation cereal crop rotation (3.7%), while the lowest is obtained using a lower fertilisation cereal crop rotation (2.94%). The data analysis revealed that the effect of different crop rotations on humus accumulation was significant: between TI and TII tfact.=|–5.07|>ttheor.95%=2.11, between TI and TIII tfact.=|–8.02|>ttheor.95%=2.11, and between TII and TIII tfact.=|–2.28|>ttheor.95%=2.11. An increasing tendency in Ntotal accumulation in the soil was determined when using different fertilisation rates in the crop rotations (–0.25%, in TIII, –0.23% in TII, and –0.19% in TI). However, the differences between the treatments were not significant, between TI and TII tfact.=|–0.96|<ttheor.95%=2.11; between TI and TIII – tfact.=|–1.53|<ttheor.95%=2.11; and between TII and TIII - tfact.=|–0.53|<ttheor.95%=2.11.
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The most stable Nmin concentration and accumulated amount were in TII, where variation coefficients of this element were 22% and 18%, respectively. With TI and TIII, Nmin concentration and accumulated amount in the soil varied more (from –41 to 35% and from 51 to 57%, respectively; Table 3). The average perennial data for Nmin concentration and accumulated amount in the soil are also rather uneven. The data show that in TI Nmin concentration and its accumulation in the soil was lower than in TII, however, this difference was not statistically significant (tfact.=|–0.94|<ttheor.95%=2.14 and tfact.=|–0.79|<ttheor.95%=2.20, respectively). Statistically significant differences between Nmin concentration and its accumulation in the soil were found in TI and TIII and in TII and TIII (tfact.=2.98>ttheor.95%=2.14 and tfact.=2.23>ttheor.95%=2.14; and tfact.=5.27>ttheor.95%=2.14 and tfact.=3.52>ttheor.95%=2.14, respectively).
Table 3. The impact of different crop rotations on Nmin concentration (mg/kg)and Nmin amount (kg/ha) in the soil (0-60 cm)
Nmin concentration and its accumulated amount in the soil in higher and lower fertilisation cereal crop rotations were not statistically significant, even though the amount of this element in a lower fertilisation cereal crop rotation was determined to be lower, 13 and 9% up to 6.6 mg/kg and 69 kg/ha, respectively. The lowest Nmin concentration and its accumulated amount in the soil had the most productive perennial grasses crop rotation, and these indicators reached 3.3 mg/kg and 42 kg/ha,respectively. Correlation analysis results showed that Nmin amount was closely related to fertilisation. Also, its amount in the soil had a tendency to fall when the yield was high (Fig. 4).
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A weak reverse linear was determined between Nmin amount in the soil and crop productivity. Increasing field crop productivity reduces the amount of Nmin in the soil, whereas increasing field crop fertilisation by N fertiliser increases the amount of Nmin in the soil. Field crop productivity determined the fluctuation of Nmin quantity by around 15%, and fertilisation by around 55%.
Precipitation dispersion was uneven during the research period, and it had effect on the formation of drainage (Table 4). It can be seen from the results that four of the years were dry (2006, 2008, 2011, and 2013), when precipitation was 80, 97, 97, and 91%, respectively, of the climate normals; 2007, 2009, 2010, and 2012 were damp years, because precipitation was 113, 125, 122, and 123%, respectively, of the climate normals. During almost the entire research period, the average air temperature was higher than the climate normals, except in 2010, when it corresponded to it.
Drainage quantity depended on the precipitation level and changed according to the direct linear pattern, as a higher level of precipitation increased the drainage.
According to the research data, on average during 8 years in a more productive, higher fertilisation cereal (TII: 55 ± 23 mm) and grass crop rotation there was a falling tendency in drainage determined, while in the least productive and lower fertilisation cereal (TI: 71 ± 29 mm) crop rotation there was an increasing tendency observed.
Even though drainage was 29% higher in TI and 25% higher in TIII than in TII, the differences between these treatments were not significant: between TI and TII, tfact.=1.20<ttheor.95%=2.14; between TI and TIII, tfact.=0.12<ttheor.95%=2.14; and between TII and TIII, tfact.=–0.98<ttheor.95%=2.14.
Drainage essentially depended on precipitation (r=0.68; tfact.=4.31>ttheor.95%=2.15). More intensively grown, larger biological potential crops, which use greater amounts of water, reduce not only soil humidity, but also drainage (r=0.43); tfact.=2.23>ttheor.95%=2.15) (Fig. 5). In TIII, Nmin and Ntotal concentrations were found to be 12.1 and 12.8 mg/L, respectively, in drainage water (Table 5).
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Table 5. Different crop rotation impact on average yearly N concentration in drainage water and leaching (arithmetical mean ± average error)
Using a lower fertilised cereal crop rotation (TI) meant that Nmin and Ntotal concentrations had a tendency to increase by 13 and 24% to 13.7 and 15.9 mg/L, respectively. An increasing tendency was also determined for Nmin and Ntotal concentrations in drainage water in TII (15.6 and 18.5 mg/L). According to correlation and regression data analysis (Table 6), N concentration in water was determined by a number of factors.
Table 6. The dependence of Nmin (y1) and Ntotal (y2) concentrations (mg/L) in drainage water on environmental factors (x)
Thus, as the field crops‘ productivity increased, N concentration in the drainage water changes according to the parabolic pattern and it was at its lowest when the field crops‘ productivity was 150-170 GJ/ha. When a higher productivity is achieved by fertilising field crops, both Nmin and Ntotal concentrations in the drainage water increase. The relationship between soil Nmin and fertilisation by Nmin fertiliser with N concentration in drainage water can be expressed by this function: y = a – b / x (Fig. 6); as soil Nmin as well as fertilisation with N fertiliser increases, the concentration of both N forms in drainage water increases.
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The relationship of annual and cumulative N balance with N concentration in drainage water is of a direct linear nature and, as N balance increases, the concentration of both N forms in drainage water increases. Direct linear consistency means that Nmin concentration in drainage water increases as the soil N and humus increase. Due to the impact of field occupation coefficient N concentration in drainage water reduces.
Research data of leaching of N compounds by drainage showed (Table 5), that both Nmin and Ntotal leaching had a downward trend when using grass crop rotation (7.0 and 8.0 kg/ha respectively).
An increasing tendency was found for N leaching in lower fertilised cereal crop rotation conditions (12 and 13%) and in higher fertilised cereals crop rotation conditions, 10.0 kg Nmin/ha and 11.5 kg Ntotal/ha, respectively. Correlation and regression data analysis has shown that both Nmin and Ntotal leaching is related to the productivity of agroecosystem and factors affecting it (Fig. 6). Crop fertilisation with mineral fertiliser had a very big effect on Nmin leaching (r=0.78), while it had only average effect on Ntotal leaching (r=0.65). The yield affected Nmin leaching on average (r=0.57), while Ntotal was weakly affected (r=0.46); however, it was statistically significant.
Nitrogen leaching through the drainage relationship between the fertilisation of an agroecosystem with mineral fertiliser and its productivity is parabolic. Fertilisation with N fertiliser until 66-71 kg/ha a.m. does not increase N leaching losses. Increasing the harvest by fertilisation leads to the increase in N leaching. The lowest Nmin leaching losses were found when productivity reached 213 GJ/ha, whereas the lowest Ntotal leaching losses were when productivity reached 212 GJ/ha. The relationship between Nmin and Ntotal leaching by drainage with the balance of agroecosystem can be expressed by the formula y = a – b / x. As N balance changes from –300 to –20 kg N/ha, Nmin and Ntotal leaching barely increases. It increases more significantly when the N balance becomes zero and positive. The relationship of Nmin leaching (kg/ha) with soil N % (x1) and humus % (x2) is as follows:
The usage of all crop rotations did not ensure a positive N balance in the agroecosystem. The lowest deficit of N was gained with the higher fertilised cereal crop rotation (–14 ± 48.1 kg N/ha/yr). Under less fertilised cereal crop rotation conditions, the N balance deficit increased by 2.2 times and reached -31±56.7 kg N/ha/yr. It was found to be the highest with perennial grass crop rotation (–100 ± 37.4 kg N/ha/yr). Significant N balance differences were found between TI and TIII as well as between TII and TIII (tfact.=2.60>ttheor.95%=2.11 and tfact.=3.02>ttheor.95%=2.07, respectively). Nitrogen balance was not statistically significant between TI and TII (tfact.=–1.27<ttheor.95%=2.07).
An average annual N balance over the eight research years was estimated, excluding N fixation with legume crops and denitrification losses in the soil, as we did not have the tools to record these indicators. An average correlation was determined between fertilisation with N fertiliser (x) and N balance (y):
According to average data, in order to create 1 GJ energy, TIII outstood due to high costs (23.5). Slightly lower costs were reported using TII (18.1), while the lowest costs were using TI (13.9). This is related to the productivity of field crops (dry matter), expressed as GJ/ha. Costs to create vegetative produce include fertiliser, fertilisation, field crops’ agricultural engineering, pesticides, etc. estimated as GJ/ha. LSD05 of named indicators reached 7.6 GJ/ha.
DiscussionTop
During the 2006-2013 period, we were analysing several research questions, such as how Nmin affects the yield, how fertilisation affects Nmin in the soil, the effect of different crop rotations on the annual N balance, and the differences in mineral and Ntotal concentrations in drainage water with fertilisation, as well as how they depend on the soil.
The study showed that a higher fertilised cereal crop rotation had the highest cumulative productivity (1483 GJ/ha), while cereals with less fertilisation and grass crop rotations had a productivity which tended to be lower by 11 to 35%, respectively, suggesting that the crop yield depends on fertilisation rate. Similar results were found by Křen et al. (2014), who analysed spring barley yields applying different N fertilisation rates. The yield increase was determined to be higher using N rate from 0 kg N/ha to 45 kg N/ha, rather than the rate from 45 kg N/ha to 90 kg N/ha. Grover et al. (2009) claimed that crop rotation is a prerequisite for a large crop yield, especially when legume crops are included in the rotation. However, the cultivation of corn monoculture produced the same yield as the crop rotation (Grover et al., 2009). Babuliková (2014) contributed to the findings using wheat in the crop rotation. It was found that in order to obtain the highest wheat yield the best N rate was 96-168 kg N/ha. There was a small difference by fertilising with a rate of 150-200 kg N/ha; however, by increasing the N rate to 240 kg N/ha, the grain yield and N effectiveness dropped. In addition to this, a long-term research revealed that crop rotation and different fertilisation had only a slight effect on yield and soil fertility parameters (Cuvardic et al., 2004). On the other hand, according to Coulter et al. (2011), a high yield in the “extended crop rotations for corn and soybean” can be obtained, even by reducing the fertilisation rates. Additionally, Øgaard (2014) found that excessive fertilisation with N leads to a fall in crop yields, an increase in N losses, and the formation of high N surplus.
In general, the results suggest that higher fertilisation increases mineral N concentration and accumulated amount in the soil. This is evident from our research, which showed that the highest mineral N concentration and accumulated amount in the soil was found in the higher fertilisation cereal crop rotation (respectively 7.6 mg/kg and 76 kg/ha), while the lowestamounts and concentrations were found in the grass crop rotation (3.3 mg/kg and 42 kg/ha), with values that were 57 and 45%, respectively, lowerthan for the higher fertilised cereal crop rotation. Similar results were found by Cuvardic et al. (2004), who observed an increasing trend in soil N (from 0.32 to 0.36%). Additionally, our research showed an increase in humus (4.11%) in perennial grasses crop rotation (Fig. 3). Other researchers found that using N-fixing legumes and red clover increase mineral N concentrations in the soil and might even accumulate the amount of 200 kg/ha of biological N for further crop rotation (Lapinskas, 1998; Riedell et al., 2009). In addition, Röing et al. (2005) claimed that 50% more N mineralises in crop rotations with ley rather than in crop rotations where only cereals are grown. According to Sonneveld & Bouma (2003), on old pastures, reducing N application levels can lower the probability of exceeding the environmental threshold for nitrate by up 20%, whilst hardly affecting N uptake. Köhler et al. (2006), in sand soils, showed that nitrate leaching depended on N fertilisation and different crop rotations as well as mineralisation from soil organic matter. Catch crops were the most efficient way to reduce the NO3– concentrations in the groundwater recharge of sandy soils. These results suggest that the Nmin concentrations in the soil depend not only on the fertilisation rate, but also on the type of crop rotation.
We found that the application of all crop rotations generally resulted in a negative N balance. However, the use of different cereals crop rotations showed a lower N balance deficit (–31 and –14 kg N/ha), while the grass crop rotation resulted in a negative N balance, which increased by 3.2 and 7.1 times and reached -100 kg N/ha. According to Kutra & Aksomaitiene (2003), the most negative N balance (-426 kg N/ha) was found in the perennial grass fields; the most positive balance (+ 174 kg N/ha) was in fields with sugar beets rotation.
The results from a number of countries show that the main environmental problem related to agriculture is N leaching. According to Arregui & Quemada (2006), the amount of leached N highly depends on the amount of it found in drainage and soil before planting crops. When N balance is positive, there is a high possibility that N concentrations in drainage water will be above the maximum allowable values. When N balance is negative, its concentrations are determined to be low. Crop rotation where perennial grasses are grown is regarded to be the best in terms of the impact to the environment, as Ntotal concentrations in drainage water were determined to be the lowest(12.8 mg/L) and not to exceed the required environmental regulations (VZ, 2011). This was also observed in our research, as the Nmin and Ntotal leaching had a tendency to fall in grass crop rotation (7.0 and 8.0 kg/ha, respectively), while in cereal crop rotations with lower and higher fertilisation there was an increasing tendency in N leaching (by 13 to 42%) observed. Additionally, Kutra & Aksomaitiene (2003) showed that in fields where cereals were dominating, N surplus was + 74 kg N/ha. Weber & Kubiniok (2013) also found that in the soils sown with perennial grasses, chemical elements’ leaching losses were lower than in cereals. Thus, due to the fact that grass crop rotations reduce N concentrations in the soil, they also reduce the risk of it leaching into the drainage water. However, there are more factors that affect N leaching, as fertilization rate not higher than plant need and minimized soil tillage systems (Kutra et al., 2006).
Lysimetric research revealed that nitrate concentrations in lysimeter water were highly dependent on fertilisation rates. Moreover, according to a 30-year research work (Adomaitis et al., 2008), by fertilising crop rotation plants (winter wheat, sugar beet, spring barley, annual grasses and perennial grasses), using the fertilisation rates 112 and 224 kg N/ha, the nitrate concentrations observed in lysimeter water at a depth of 0-80 cm were 112.1 and 187.2 mg/L, respectively. Tripolskaja & Verbylienė (2014) determined that by fertilising with N fertiliser (on average 120 kg/ha), compared to unfertilised soil, the average nitrate concentration in lysimeter water was increased by 37.5 to 54.3%. However drainage, due to more dense and more exuberant crops, which reduce precipitation filtration, fell during the year on average by 13.7-16.2%. This is also supported by our research results, which showed that larger biological potential crops reduced drainage by 23% compared to lower biological potential crops. Tiemeyer et al. (2006) determined that N concentrations in drainage water were increasing by increasing flow rates, high loss rates always occurred at high flow rates, and higher loss rates are to be expected in wetter years. This was also confirmed by our research: when the precipitation level was higher, the drainage was also higher. Drainage essentiallydepended on meteorological conditions. Previous research showed that agroecosystem´s hidrological regime is determined not only by natural, but also by human farming activity factors (the crops grown, the farming level). More intensively grown, higher biological potential crops use up more water and thus reduce not only the soil moisture but also drainage (Aksomaitienė et al., 2002). The problematic nature of the research is that the majority of results were obtained from small drainage sites or even by lysimetric research methods. To determine similar patterns by performing tests in large drainage systems is also difficult, not only because of their large amortisation effect (Cornelese et al., 2001), but also because of the research background promiscuity (soil cover variegation, ground water stratification depth, etc.); however, under some circumstances, patterns remain (Sileika & Guzys, 2003).
Both Nmin and Ntotal leaching by drainage, due to the crop productivity and their fertilisation with N fertiliser, change according to y = a – bx / cx2. The smallest N leaching losses were obtained by fertilising 66-71 kg N/ha and crop productivity of 212 GJ/ha. Furthermore, Nmin and Ntotal concentration in drainage water depended on crop rotation, its productivity and the factors determining productivity. Both Nmin and Ntotal concentration in drainage water had a tendency to fall in grass crop rotation (12.1 and 12.8 mg/L respectively). The use of cereal crop rotations with higher and lower fertilisation increased Nmin concentration by 13 and 29%, while total N by 24 and 44%. According to Adomaitis et al. (2008), a larger amount of nitrates is leached when annual crops are grown, compared with perennial grasses. Regular grass sown in arable land and not fertilised with N reduced nitrate concentration to the level of perennial pasture in just one year. The risk of exceeding the maximum allowable limit of nitrates in crop rotation in arable land was significant and did not depend on the level of fertilisation with N (Mašauskas et al., 2006).
Moreover, Beaudoin et al. (2005), determined that nitrate concentrations change subject to the year, crops and soil type; in a deep loamy soil, concentrations fluctuated between 31 mg/L and 92 mg/L in shallow sand. Correa et al. (2005) and Tonitto et al. (2006) observed that the biggest leaching losses occurred in the bare, fallow soil. The lowest concentrations were determined in crop rotation with sugar beet/wheat (38 mg/L), whereas the highest concentrations were found in the pea/wheat crop rotation (66 mg/L) (Beaudoin et al., 2005). Kutra et al. (2006), found that the majority of N was leached from row crops (22.4 kg/ha), slightly less from cereal fields (16.6-18.9 kg/ha) and least of all from grasslands (10.5 kg/ha). On the contrary, Bohm et al. (2009) showed that legume crop cultivation can considerably increase Nmin concentration in the soil solution and leaching as well.
The data imply that the amount of Nmin in the soil is a very labile indicator. Loosening during the mineralisation period and spreading during fertilisation ensure that Nmin is quickly used up by crops. The longer the crop vegetation period and the greater their biological potential, the stronger this preventative effect (Kutra et al., 2006; Askegaard et al., 2011; Doltra et al., 2011).
Finally, as the data collected by many researchers show, the advanced agricultural technologies and efficient application (not the rates) of mineral fertilisers are among the essential preconditions for reduced non-point source water pollution with N (Goulding et al., 2000; Sileika & Guzys, 2003).
The obtained results showed that the application of perennial grasses crop rotation in agricultural fields was the best environmental tool, reducing N migration to drainage. By fertilising with other treatments using higher N rates, mineral N accummulation in the soil increased, stimulating a higher yield and reducing N leaching by drainage. By fertilising with higher N rates than it is neccesary for plants, both mineral and total N leaching to drainage had a tendency to increase.