Mineralkväve i marken och kväveutlakning i olika odlingssystem: resultat från ett fältförsök vid Lanna försöksstation i Västergötland

Soil mineral nitrogen (ammonium and nitrate nitrogen, SMN) in different seasons and leaching of nitrate nitrogen were studied in 1978-86 in a long-term cropping system experiment with seven plots on a clay soil in south-west Sweden. The trial included seven systems of 6- year crop rotations with the following cultivation measures: I) spring barley (undersown) – 4-year grass ley – spring rape; high rates of fertiliser nitrogen: 200 kg N/ha to the grass, II) spring barley (undersown) – 4-year lucern ley – spring rape; no nitrogen fertilisation of the leys but cattle slurry to spring rape and barley, III) spring barley – spring rape – winter wheat – oats – spring rape – winter wheat; high rates of fertiliser nitrogen, e.g., 150 kg N/ha to winter wheat; IV) as III but 50 % of the nitrogen rates, V) spring barley (undersown) – red clover – winter wheat – broad beans – oats – peas; organic farming without application of mineral fertiliser nitrogen, VI) only spring barley (monoculture), ca. 100 kg N/ha of mineral fertiliser nitrogen but 50 % in years with application of pig slurry, and VII) only spring barley (monoculture); nitrogen fertilisation: ca. 120 kg N/ha. Each cropping system (plot) was equipped with a drainage system, with measurement of runoff and sampling of drainage water for determination of nitrate nitrogen leaching. Soil samples for SMN analysis were taken in spring and autumn (mean date: 11/4 and 13/9) down to 200 cm. In 1979-85, SMN was also determined within 0-100 cm soil generally monthly in systems III and VI.In system I, the DM yield of the intensively fertilised grass averaged 8.820 kg/ha. The yield of the unfertilised lucern ley in system II corresponded to 90 % of the grass yield. On average, the grain yields of the cereal crops in system III amounted to 5.990 kg/ha. In systems IV, V, VI and VII, the yields of cereals corresponded to 76, 77, 75 and 80 % of the level in system III. Obviously no fertiliser nitrogen applied in spring was leached during the following summer, with a few exceptions. The average annual leaching of nitrate nitrogen in systems I, II, III, IV, V, VI and VII amounted to 4.1, 1.4, 7.9, 2.6, 5.9, 7.0 and 11.8 kg N/ha, respectively. In 1980, the winter wheat in system III was damages by a fungus attack causing lower yield and increased leaching. In system V (organic farming), the nitrogen leaching was larger after leguminous crops than after cereals. The nitrogen fertilisation in system VII obviously exceeded the optimum level. Calculations showed that an increase in the annual runoff by 100 mm due to climate change would increase leaching from 7 to 10 kg N/ha as a means of systems III-VII (without ley production).Most SMN was found shortly after fertilisation in spring. Then the amounts decreased, obviously due to crop uptake of nitrogen, until late summer or early autumn, when the lowest amounts were found, for cereal crops 27 kg N/ha in 0-100 cm soil, on average, in systems III and VI. At that time the crops normally had taken up almost all nitrate nitrogen down to 100-150 cm. During the ley years in systems I and II, only very small amounts of SMN were found in early autumn. In systems III, VI and VII, frequently with too large nitrogen supply, the crops took up nitrogen mainly from the topsoil and the upper subsoil in these years, leaving SMN largely unused in deeper layers. SMN increased again during autumn due to continued nitrogen release. Obviously only a minor part of the SMN present in the autumn was leached during the following cold season. Also during the winter period (late Novembermid March) additions of mineralised nitrogen were registered, in spite of long periods of frost in the soil. Normally, this overwintering nitrogen was largely utilised within root depth in the following growing season.