Kvävemineralisering under olika årstider och utlakning på en mojord i Västergötland

During a seven-year period (1993-1999) studies were made on the effct on soil nitrogen dynamics, utilization ration of nitrogen in crops and leaching losses by different soil tillage times, application of pig slurry and undersown perennial ryegrass as a catch crop. The experimental field was situated on a loamy sand soil in Västergötland, in western Sweden. The field was divided into eight separate tile-drained plots, each with an area of 840 m². The amount of drainage water was measured and water samples were collected twice a month. The water samples were analyzed for pH, electrical conductivity, nitrogen, phosphorus and potassium. Soil samples were taken periodically from the 0-90 cm soil layer for analysis of mineral nitrogen. Plant material was sampled in late summer, late sutumn and early spring to measure plant growth and nutrient in crops, weeds and plat residues. The crop sequence was: oats (1993 and 1994), spring barley (1995), potatoes (1996), oats (1997) and spring barley (1998). The fertilizer regimes were commercial fertilizer according to normal reccommendations (90 kg N/ha) or half the dose of commercial fertilizer (45 kg N/ha) supplemented with pig slurry (on average, 94 kg tot-N/ha) applied before sowing in spring. Treatments with catch crop were ploughed either in April-May or in early November, whereas other treatments without a catch crop were stubble cultivated in August-September and plouged in November, or only plouged in April-May. The catch crop didn't impact negatively on the yield; instead incorporation if catch crop material seemes to increase the amount of nitrogen available for the next crop, due to increased nitrogen mineralization. Further, the catch crop reduced nitrogen leaching during the cold seasons to half when the catch crop was left to grow until spring ploughing. In late autumn, the amount of nitrogen in catch crop, including other green vegetation, was 15-17 kg N/ha, which were in the same range as the decrease of nitrogen leaching in treatments with spring ploughing and an undersown ryegrass. When tillage was delayed from early autumn to spring (without a catch crop), nitrogen leaching decreased with 25%, due to lower mineralization during autumn and winter. Ploughing in spring did not result in increased nitrogen delivery from the soil during growing season, compared with tillage in early autumn. When pig slurry was used, nitrogen leaching increased somewhat, compared with treatments with commercial fertilizer nitrogen, due to increased nitrogen mineralization. The amount of plant available soil nitrogen during the growing season was also larger in treatments with pig slurry applications. Balances of nitrogen were positive in all treatments, especially in treatments with catch crop and spring ploughing. By spring ploughing and using a catch crop it seems to be possible to manipulate the mineralization to be lower wintertime and increse during the growing season. This seemed to conserve nitrogen in soil organic matter