| Popis: |
Modern agricultural techniques replace nitrogen with artificial manure. This leads to the problems we have now: high nitrate content of groundwater sources, high loss of N2 due to denitrification and adsorption of nitrogen to soil particles which makes this nitrogen unavailable for plants. Farmers in Europe are turning more to biological methods, with less use of artificial manure. The amount of N fixation on pastures that were previously fertilized and get now managed with biological methods is minimal. This research aimed to determine the contribution of N fixation by legumes on fertilized and unfertilized grazed pastures that were fertilized with 170 kgN/ha (sheep) and 240 kgN/ha (cattle) under normal farming regiemes until 1993 compared to field plots using the natural abundance method (1–5). Experiments were made on sandy loam field sites. Following experiments were conducted over 3 years from 1994 to 1996: 1. Non-randomised field plots, with Trifolium pratense L., a mixture of Trifolium pratense L. and Lolium perenne L., Vicia faba and Brassica napus. Plots were cut three times yearly. 2. Pastures, grazed daily by cows or sheep. Two grazing areas were prepared for cows, one unfertilized, one fertilized (260 kg N/ha/yr). Eight pastures were grazed by sheep, four unfertilized, four fertilized (260 kg N/ha/yr). Sheep rotated between sites if pasture was grazed down to a limit. To simulate grazing previously grazed areas were covered with wicker-baskets to protect them from further grazing so that plants could regrow until about 15 to 25 cm height and then were cut. Crops were separated into grass, clover and rest. Non fertilized pastures showed higher fixation than fertilized. Cattle grazed pastures showed a higher clover content than sheep grazed. The N fixed by clover depended on the percentage of clover (the type of animal), total N content, and dry matter yield. The N fixation on pastures calculated for 100% clover shows an increase over three years, especially for unfertilized pastures. Field plots sown only with clover showed high N fixation rates. Nitrogen content showed no difference between manured and unmanured sites. In spite of been fertilized FNdfa of clover was relative high on pastures. This may be due to competing with grass in N-uptake from soil. |
