ALBERT

Beschreibung: SUMMARYThe effects of overwinter cover cropping, delayed ploughing and method of straw disposal on the quantities of nitrate leached (averaged over three winters during 1989–93) from a chalk loam in Eastern England were examined. The recovery of ‘retained’ nitrogen (retained through cover crop uptake, delayed ploughing and immobilization by straw) in a following spring crop was also assessed. In the first two winters, the rye cover crop decreased nitrate leaching by 〉 90% (28 kg N/ha per year), as compared with bare fallow treatments. In 1992/93 this decrease was only 23% (10 kg/ha), due to the early onset of drainage before cover was well established. Delayed ploughing on bare treatments, to decrease autumn N mineralization and subsequent nitrate leaching, was ineffectual in 1989/90 but had substantial effects in 1990/91 and 1992/93; N mineralization, inferred from soil mineral nitrogen content, and nitrate leaching were decreased by 31 and 35% in 1990/91 and by 36 and 61% in 1992/93, respectively. Nitrate leaching (averaged over three winters) was unaffected by straw incorporation. There was no evidence of recovery of cover crop N in the spring sown test crops (barley or sugarbeet). In the low soil N input situation encountered in this experiment, it was unnecessary to sow cover crops before early September in years of average or below average rainfall to ensure that the average soil solution concentrations remained below the EU drinking water limit of 11 mg NO3-N/1. However, in wetter seasons substantial N leaching occurred before cover had taken up much N. In 1992/93 N retained against leaching by a rye cover crop in previous years was apparently being remobilized and lost through leaching, although if cover was grown again there was less leaching than from bare land. In the future, an increase in the extent of cover cropping might increase transpiration rates and therefore lead to a decrease in aquifer recharge.

Beschreibung: The recovery of nitrogen ‘retained’ through cover crop uptake, delayed ploughing and immobilization by straw was assessed in a spring cropping rotation on a chalk loam in Eastern England (1989–96). The effect of annual cover cropping on yield of the subsequent spring crops and on the soil N balance was also investigated. The recovery of retained N was in part dependent upon cover crop management. Late August-sown cover crops which were incorporated in February/March tended to reduce spring crop yields and crop N offtake. Adverse effects on soil N supply, seedbed conditions and soil water reserves were not in evidence and so an allelopathic effect from the decomposition of the rye cover crop, previously reported by others, may be responsible for the reduction in yield of spring crops. When the cover crops were drilled later and their early destruction was followed by a short fallow period, spring crop yields and N offtake were increased. The soil N balance indicated that over the course of the experiment there was a positive N input to the system due to continuous cover cropping. This input may be held as immobilized organic N, in which case it could be made available to subsequent crops over a number of years or lost via other routes. Nitrate concentrations in drainage water increased with the number of years under cover cropping.

Beschreibung: Although there has been some recent interest in the morphology of individual white clover plants within established mixed swards under sheep grazing, there is little information available on the morphological changes taking place during the critical first 3 years of sward development. Undamaged white clover plants were sampled from an experiment at Plas Gogerddan, Ceredigion, comparing three contrasting white clover cultivars, sown with a common companion ryegrass cultivar, under continuous stocking with sheep. Turves (250 × 250 mm), from which individual plants were carefully extracted, were taken every month during 1990–91 (years 2 and 3 after sowing). Each plant was described in detail by assessing a range of morphological characters. Information was also gathered from each quadrat on the degree of flowering and seedling recruitment. There were clear seasonal variations in plant size and complexity with an increase in the number of simple, unbranched plants over the winter/spring period, which became the dominant plant type by June. During the later part of each year, the reverse was true. Characters associated with plant size also decreased over the winter period, followed by recovery during the ensuing summer. However the recovery was not complete and there was a strong general trend towards an increased proportion of less complex plants of reduced size over the 2 years. As plant size fell, plant number increased and morphological differences between cultivars diminished. By autumn of the second year (third year after sowing) the mean plant size reached a critical level with further stolon loss resulting in large scale plant death. Over the 2-month period August–October 1991, plant number fell by 60% and resulted in a reduction in stolon abundance of 76%; a true clover crash. The seedling data also suggest that under these conditions seedling recruitment can be up to one twentieth of that expected from a conventional sowing and can play a substantial role in the regeneration of the sward.