Effect of agricultural drainage on nutrient balance
in the soil and water pollution
English summary
Original report: ŽEMIŲ SAUSINIMO POVEIKIS BIOGENINIŲ MEDŽIAGŲ TRNSFORMACIJOMS
DIRVOŽEMYJE IR VANDENS TELKINIŲ TARŠAI - Mokslinių darbų apžvalgos studija
The purpose of agricultural drainage is to regulate soil moisture regime while creating
favourable conditions for land management, plant cultivation and to provide trafficability
during wet periods. In Lithuania, atmospheric supply of water along with the rather plain
topography and low permeability of prevailing glacial tills has resulted in a large area of soils
suffering from excess moisture. Therefore, in order to remove the excess artificial drainage is
installed in agricultural land areas: moisture surplus is removed by applying open/surface
(ditches, channels and/or regulated river courses) and/or underground (subsurface/tile)
drainage systems. Most often a combination of the mentioned means is applied.
Land drainage practices in Lithuanian started at the end of the 19th century, however,
the majority of work was carried out during 1965-1985. Drainage caused a decrease in
biodiversity. Marsh and bush areas decreased fast in drained land and intensive transformation
of cultivated plots took place. The patchiness of land-tenures in the landscape decreased and
their mosaic character deteriorated. Detached individual farmsteads were changed by largescale fields of strict shape and broad linear elements increased in number. In order to achieve
great drainage efficiency fast deepening, widening and straightening of small rivulets took
place, which resulted in their transformation into drainage ditches. The length of regulated
rivers reached 46 thou kilometres and at the moment it accounts for 82% of the total
Lithuanian hydrographic network together with other ditches. The overall area of drained land
in Lithuania is 2979.1 thou ha (83.3% of agricultural land area), of which 2576.3 thou ha are
under tile drainage. According to this indicator, Lithuania is one of the most extensively
drained lands in the world.
Drainage changes soil moisture regime substantially. It has been established that the
most pronounced drainage impact is a lowered (down to 1.0–1.2 m) groundwater table. It is
manifested best during March–April and October–December as well as after intensive
rainfall. Due to this reason, moisture content is substantially reduced in the upper soil layers.
The evapotranspiration is also reduced (15–23%) in drained areas as the soil is less saturated
with water. It shows up especially in spring and the beginning of summer (April–June). It has
also been established that land drainage tends to extend the duration of high flows and causes
later occurrence of annual peaks in the rivers downstream drained areas. This can be
attributed to the “sponge effectˮ of drainage. Furthermore, the extension of land drainage
results in a reduced number of low flows by creating a more pronounced hydrological
connection to groundwater.
The hydrological regime of tile drained land depends very much on the designed
drainage depth and drain spacing characteristics. It was established that the reduction of tile
drain spacing, e.g. from 15 to 9 m, causes the increase of the annual drainage discharge in
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loamy soil by 50%, and from 40 to 27 m – 35%. The increase of drainage depth from 0.9 to
1.5 increase drainage outflow by 25%. These variations are directly related with an increased
groundwater inflow into drainage.
In Lithuania under the influence of different factors the concentrations of nitrate
nitrogen (NO3-N) in drainage water range from 2 to 20 mg/l, those of ammonium nitrogen
(NH4-N) – from 0.02 to 1.5 mg/l and total phosphorus – from 0.05 to 0.25 mg/l (40-70% of its
amount consists of orthophosphate phosphorus). From 5 to 40 kg/ha (according to field and
plot scale measurements) of total nitrogen (major part of it – as NO3) is leached to surface
waters through tile drainage per year; and 0.05 to 0.40 kg/ha of total phosphorus as well.
Drainage increases oxygen content in soil while removing moisture surplus in it. That
accelerates the mineralization process during which nutrients and other soluble elements are
faster removed from soil together with the rainwater, which is moved downward vertically.
Generally, without estimating the impact of other factors the origin of drainage itself creates
conditions for a more abundant nutrient (mainly NO3-N) loss from soil.
Due to higher water abundance and crop vegetation properties (amount of mineral
compounds assimilated by plants) the outflow of these materials through drainage and the
inflow into surface waters is greater in early spring and late autumn as well as in winter at
positive air temperatures. Such periods are called “critical” in terms of the assessment of
biogenic material outflow through drainage. At that time contrary to the plant vegetation
period the amount of nutrients in soil increases 2–5 times.
Variant outflow of biogenic substances is monitored during wet and dry years. The
majority of researchers indicate a clear direct relationship between abundant wateriness and
high nitrogen runoff. The outflow caused by individual showers often coincides with the
highest runoff. Therefore, it is obvious that soil hydrological regime is very important in
assessing the drainage water quality.
The carry-over of biogenic materials through surface runoff and subsurface drainage in
drained areas is distributed very unevenly. The results of different research indicate that the
annual amounts of total phosphorus (kg/ha) in tile drainage water are 3 to 12 times lower
compared with those recorded in surface water. The nature of suspended matter carry-over is
similar – its contents in tile drainage water are 2 to 11 times lower. However, the content of
orthophosphate phosphorus in drainage water can account for 1.5 to 20 times higher
compared with surface runoff water. The annual outflow of total nitrogen (kg/ha) through tile
drainage constitutes as well 5 to 20 times higher amounts and that of nitrate nitrogen actually
10–30 times higher amounts compared with the amounts in surface runoff. The average
annual concentrations of total nitrogen in tile drainage water can be 2 to 4 times higher
compared with surface water. These consistent patterns are determined by the fact that
phosphorus in surface runoff is carried over in an adsorbed/particulate form and in tile
drainage it is in a soluble form. The highest amounts of phosphorus are present in a topsoil
layer several centimetres deep. The amounts of soluble and mobile nitrogen compounds are
higher in the very soil compared with its surface, therefore, tile drainage carries over higher
amounts of it compared with surface runoff.
The assessment of land use effect on the runoff of biogenic materials revealed that
intensive agricultural areas with varied surface cover has a higher nitrogen outflow potential
compared with that of natural vegetation areas. It is universally regarded that tile drainage
leaches out only those N and P amounts, which have not been uptaken by plants, i.e. the
amounts accumulated in soil as surplus of these nutrients. The major part of nitrate nitrogen is
leached out from the areas covered with root crops (potatoes, etc.) as well as from rapeseed
fields. Somewhat lower amounts are leached out from spring and winter cereals. Considerably
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lower losses are observed from the first and second year cultivation of perennial grasses in
arable land and the lowest inorganic nitrogen concentrations have been recorded in the
drainage water coming from perennial meadows and pasture areas. These consistent patterns
can be expressed respectively by the following rates of average nitrate nitrogen concentrations
in drainage water – 7.5 : 6.2 : 4.7 : 1.0. Unfortunately, ploughing of perennial grass areas and
turning them into arable land results in an abrupt increase of nitrogen outflow from 5 to 10
times.
On the contrary to nitrogen, the highest phosphorus losses are attributed to meadow and
pasture areas with perennial grass cover. The total phosphorus runoff in perennial grass
(pasture) areas is from 40 to 70% higher compared with the fields covered with other plants.
Perennial grasses are distinguished for the property to lift subsoil phosphates and thus provide
themselves with nutrient matter. In such way due to its root system grassy vegetation
increases the solubility of poorly soluble phosphorus compounds (especially that of calcium
phosphates) and increases the amount of its mobile forms in soil and tile drainage water. In
addition, the root system of perennial grasses generates a lot of macropores responsible for
major migration of phosphorus compounds from the upper soil layers to the lower ones.
The assessment of farming impact established that nitrogen loss is marginal, when the
amount of nitrogen fertilizer does not exceed 100 kg N/ha per year. This is conditioned by a
fairly high amount of nitrogen assimilated by plants. E.g. cereals can assimilate 60 to 120 kg
N/ha, and perennial grasses – actually 120-200 kg N/ha per year. However, the excess of
these fertilizer amounts results in a sudden increase of nitrogen losses in soil and through tile
drainage. The N leaching can increase from 2 to 4 times. This manifests the importance of
fixing correct fertilizer rates and assessing nutrient amount in soil. These processes are
determined by soil moisture conditions as well.
Organic farming conditions do not prove significant advantage in terms of nitrogen and
phosphorus leaching through tile drainage compared with conventional farming. The highest
differences between farming systems are observed only in the years of low and average water
abundance. The research carried out in Lithuania established that in wet years there are no
significant differences between nitrate nitrogen outflows from farms of organic or intensive
farming, when soils are fertile and rich in nitrogen and the optimum fertilizer rates are used;
and the leaching of total phosphorus mainly depends on soil richness in phosphorus compared
with fertilization.
The application of reduced or zero-till revealed that these practices instead of
conventional ploughing in autumn can reduce the porosity of topsoil and subsoil layers by 1.6
to 2.2%. This can also lead to 5–11% higher soil density and reduced soil permeability. Under
zero-till conditions water infiltration rate in the upper 0–20 cm soil layer is 5–8 times lower.
During summer periods in the cases of reduced or no-till practices the water content in the
upper soil layer is lower by 8 to 40%, in autumn – 12 to 40% and in spring – 5–7%. However,
the amounts of nitrate and total nitrogen in the soils of reduced tillage are lower compared
with conventional tillage. During the years of average wetness and dry years in the cases of
reduced tillage the mean concentrations of nitrate nitrogen in tile drainage water are 14.0%
and 17.6% respectively, and in late autumn they are actually 30% lower compared with the
conventional soil tillage areas. In dry years the outflow of nitrate nitrogen in reduced tillage
areas during autumn–winter periods is 33% lower compared with the conventional tillage
areas. In wet and average years of water abundance due to reduced tillage nitrate nitrogen
losses are 45–55% lower on average compared with conventional land tillage.
In assessing the impact of large-scale livestock husbandry farms on tile drainage water
quality it was established that, for example, in the upper reaches of the Šušvė river (Middle
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Lithuania, where an animal husbandry enterprise has been operating since 1978 on which
about 25.0 thou of fattening pigs are produced per year yielding 58.0 thou m3 of organic
fertilizer – slurry, liquid and solid manure which is spread in surrounding agricultural fields) a
negative effect of the drainage water coming from the fertilized fields on the river water
quality was observed in terms of the amounts of nitrogen, phosphorus and potassium (NPK)
carried over. The drainage water coming from these fields brings in 11.4 t of nitrogen, 0.073 t
of phosphorus and 5.53 t of potassium on average to the river annually. The calculations of
NPK amounts carried over by the Šušvė revealed that in 2002–2011 in the investigated
section of the river the increase in total nitrogen accounted for 83 t/year or 58 %, potassium –
51 t/year or 54% and the lowest amount of total phosphorus – 1.2 t/year or 48% on average.
All pollutants entering the river in the investigated section are leached during the cold period
(starting from 60% (total phosphorus) to 88% (total nitrogen), when plants are absent. The
water abundance of a certain year has influence on leached N, P and K amounts.
The maximum permissible average annual concentration of pollutants from the drainage
systems in the fields sprayed with liquid organic fertilizer cannot exceed BOD5-20 mgO2/l
(BOD7-23 mgO2/l), total phosphorus – 2 mg/l, total nitrogen – 15 mg/l, ammonium nitrogen –
5 mg/l, nitrite nitrogen – 0.3 mg/l, and the amount of total nitrogen reaching soil per year –
170 kg/ha (according to “Environmental Requirements for Manure Management” LAND
050711-5).
Having evaluated conventional crop rotations applied in Lithuanian agricultural land as
well as the average productivity of different agricultural plants, the amounts of nitrogen and
phosphorus in precipitation and soil, the number of animals, catchment tenures and
fertilization conditions (e.g. variable rates of mineral nitrogen fertilizer accepted starting from
40 kg/ha for legumes and up to 170 kg/ha for winter rapeseed, and those of phosphorus
fertilizer – from 15 kg/ha for different cereals up to 38 kg/ha for root crops) according to the
meteorological and hydrological conditions of 1997–2012 and traditional characteristics of
tile drainage (drainage depth – 1.1 m, spacing between drains – 24 m) it was established that
the highest NO3-N losses through drainage (5-18 kg/ha) in Lithuania are characteristic to the
minor tributaries of the rivers Mūša, Lielupė and Nemunas as well as certain sections of
coastal rivers. Somewhat smaller amounts (1.0–4.0 kg/ha) of nitrogen are leached out in
Middle Lithuania – the rivers Nevėžis, Dubysa as well as Šešupė, Venta and Jūra catchments.
The lowest amounts (up to 1.0–2.0 kg/ha) of nitrogen through drainage are leached out in the
Neris river catchment tributaries as well as in the small tributaries of the rivers Merkys,
Nemunas and Dauguva. The highest losses of soluble phosphorus are observed in the
catchments of the Šešupė river, Nemunas small tributaries, Bartuva, Mūša and the coastal
rivers (0.06–0.16 kg/ha). The lowest losses – in the catchments of the rivers Merkys and Neris
tributaries (up to 0.050 kg/ha).
In order to reduce the losses of nutrients in river catchments, where farming activities
are very intensive, quite often agronomical measures do not suffice. Therefore, it is
recommended to apply up-to-date water table management practices aiming to retrofit the
existing tile drainage systems or to install the new ones. The intensive drainage systems most
often remove more water than necessary leading to temporary overdrainage. Consequently, tile
drainage systems can be transformed into controlled drainage (subirrigation) systems.
Moreover, water and sediment retention ponds can be introduced as in-ditch measures by
reshaping traditional trapezoidal cross-section of the ditches.
Thus, with a view to reducing the inflow of biogenic substances from agricultural areas
conventional tile drainage can be transformed into conservation drainage by applying
controlled drainage. This practice makes use of a control structure that can be used to manage
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the level of water above the tile line. Controlled drainage can be used to raise or lower the
water table, depending on the time of year and water needs. As it was mentioned before, late
autumn, winter and early spring periods are critical in terms of nutrient losses. At those
periods the biological assimilation of soluble nitrogen and phosphorus compounds does not
take place and these substances are carried out by drainage. To mitigate this effect the water
table above the tile line can be controlled and consequently the water outflow can be reduced
by 30–60% using controlled drainage. This practice creates oxygen limited conditions in the
soil. Therefore, denitrification process starts through which nitrate nitrogen instead of staying
in water is converted into various gaseous forms of nitrogen. Such measure can reduce the
annual outflow of soluble nitrogen compounds and total phosphorus down to 50% and 90%
respectively. In addition, it is suitable as a subsoil irrigation measure during summer periods.
At present edge of field biotechnologies to remove nitrate nitrogen from drainage water
have been started to apply. They are based on the installation of bioreactors at the outlets of
tile drainage systems. Bioreactors consist of a buried trench with woodchips through which
the tile water flows before entering a surface water body. Microorganisms from the soil
colonize the woodchips. Therefore, bioreactor’s activity is based on biochemical processes,
when nitrate nitrogen in water is decomposed by denitrifying bacteria under low oxygen
conditions. The bacteria convert nitrates into dinitrogen gas. The investigations carried out
show that such technology can reduce nitrogen concentrations in water from 40 to 90%.
Saturated riparian buffer strip technology of nitrate nitrogen removal from drainage water is
also based on a similar principle.
For the retention of nutrients potentially suitable are as well those locations, where
substance retention environment is created by widening open ditch cross-sections near the
outlets. The widening of ditch mouths reduces total nitrogen flows into open natural water
bodies down to 15%, and those of total phosphorus – down to 40%.
As a separate way for reducing phosphorus flows through drainage systems a lime
drainage filter can be used. Different literature sources indicate that clayey soil with lime
admixture can reduce the migration of phosphorus into drainage systems significantly. The
investigations carried out in Lithuania showed that mixing of drainage trench backfill soil
with lime (0.6% CaO of soil mass) reduces phosphorus outflow to 2.8 times. The research
used shale ash with 16.8% CaO. This experiment also established that lime filters can
maintain the same efficiency for more than 20 years.
In summarizing it can be stated that agricultural drainage creates conditions for higher
outflow of soluble biogenic materials (N and P) from soil. Due to drainage installation the
outflow of these materials is higher early in spring and late in autumn. There is a direct
dependence between water abundance and high nitrogen outflow through tile drainage. A
runoff caused by intensive rainstorms often coincides with the largest N outflow from
drainage. The areas of intensive agriculture with variable land cover have a higher nitrogen
outflow capacity compared with the areas of natural vegetation. On the contrary to nitrogen,
the highest potential of phosphorus outflow by drainage is related with meadow and pasture
areas with perennial grasses. Due to high amounts of nitrogen taken by plants the annual
outflow of nitrogen by drainage is marginal, if the amount of fertilizer spread does not exceed
100 kg N/ha per year. The regularly spread organic fertilizer (in the form of liquid manure and
slurry) from large-scale animal husbandry farms increase the amounts of organic matter as
well as total phosphorus and total nitrogen and potassium in drainage water significantly.
To reduce nitrogen and phosphorus compound carry-overs the above mentioned water
table management practices (conventional drainage rearrangement) can be applied as well.
However, their installation requires huge financial resources, therefore, the priority for
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reducing nitrogen and phosphorus outflows from drainage systems should be given to
agronomical measures (optimum fertilizer rates, fertilizer distribution deadlines, catch crops,
reduced or zero tillage, etc.). Engineering measures are recommended only in those cases
when agronomical measures are not sufficient. Both measure groups should be harmonized
interdependently.
Arvydas Povilaitis
Water Resources Engineering Institute
Aleksandras Stulginskis University
Kaunas, Lithuania
Report and summary commissioned by Lithuanian Agricultural Advisory
Service LZUKT within CBSS Project Support Facility
Project SC 042014/7
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