NITROGEN MINERALIZATION AFTER LEYS PLOWED IN EARLY OR LATE AUTUMN

In order to study the nitrogen effects exerted on the subsequent crop by second- and third-year leys (grass, red clover, grass-clover) compa red with cereals (oats and winter wheat), eleven two-year field experi ments were performed in south and central Sweden during 1988-90. The t rials comprised two ploughing times in the first experimental year: 1) in late August or early September (''early ploughing''), whereupon wi nter wheat was sown, and 2) in late October or early November (''late ploughing''), followed by spring barley in the next year, Early plough ing of clover and grass-clover leys caused distinctly increased net N mineralization during the autumn with larger accumulation of mineral n itrogen in the soil profile during this time than after cereals. This nitrogen was largely lost during the subsequent winter. After grass, l ess mineral N accumulated than after cereals. By contrast, where leys were allowed to grow and take up nitrogen during the autumn, soil mine ral N decreased, obviously with reduced N leaching risks during the fo llowing winter, particularly after grass leys. Late ploughing after ce reals also led to less soil mineral N in late autumn than early plough ing. Compared with early ploughing, late ploughing increased the net N mineralization during the subsequent spring and growing season. Follo wing the cereal, grass, clover and grass-clover crops, the amounts of plant-utilized soil N, consisting of such released N and of the plant- available fraction of the mineral N present in the soil already in ear ly spring, were 14, 7, 18 and 14 kg ha-1 larger after late ploughing. Compared with cereals, the grass-clover and clover leys increased the amounts of plant-utilized soil N by 30-40 kg ha-1, irrespective of plo ughing time, whereas no additional nitrogen was available after grass leys. Following early ploughing, winter wheat grown without N fertiliz ation in the second year yielded ca. 1000 and 1200 kg ha-1 more grain after grass-clover and clover, respectively, than after cereals. Howev er, there was no positive yield effect of the grass. Similar results w ere obtained in spring barley sown after late ploughing. Spring fertil ization with 90 kg N ha-1 in the second year generally increased grain yields considerably. However, the additional positive influence of cl over and grass-clover remained, but to a reduced extent, especially in spring barley. With N fertilization, the content of Kjeldahl N in gra in was significantly higher after these leys than after grass and cere als. Under Swedish conditions, the N effects of leys, especially legum inous leys, will be utilized most effectively if the leys are not plou ghed in late summer or early autumn but much later. Due to the low win ter temperatures, increased N mineralization in the soil after leys se ems to be largely postponed to the subsequent year, if the leys are pl oughed in late autumn. This should reduce N losses during the winter.