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than 50% of the yearly contribution of P to Danish freshwaters (Kronvang et al., 1995). In spite of these Strongly sorbing compounds such as P, pesticides, and heavy metals significant reductions in discharge, P is still problematic can be transported through soils while being adsorbed to mobile colloidal particles. While the rapid leaching of nonadsorbing chemicals

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Abstract. Nitrate leaching after one year of a cut grass/clover ley was measured in two succeeding years to investigate how the postponing of ploughing leys from early to late autumn or spring, in combination with spring or winter cereals affected leaching of nitrate. The experiment was conducted as three field trials, two on a coarse sandy soil and one on a sandy loam soil. For calculation of nitrate leaching, soil water samples were taken using ceramic suction cups. The experiments started in spring in a first year ley and ended in spring three years later. Total nitrate leaching for the three year periods for each trial ranged between 160–254 and 189–254 kg N/ha on the coarse sand and 129–233 kg N/ha on the sandy loam. The results showed that winter wheat (Triticum aestivum L.) did not have the potential for taking up the mineralized N in autumn after early autumn ploughing of grass/clover leys, and that the least leaching was generally found when ploughing was postponed until spring, and when winter rye (Secale cereale L.) was grown as the second crop rather than spring barley (Hordeum vulgare L.). Nevertheless, leaching was generally high in the winter period even when winter rye was grown. On these soil types ploughing out should be postponed, whenever possible, to spring. Crop systems that maximize the utilization of mineralized N and thereby minimize nitrate leaching need to be further developed. Based on N balances, the data were further used to estimate the biological N fixation by the clover.

use of a catch crop showed that N fertilization could be reduced by 11 to 23 kg N ha Ϫ1 yr Ϫ1 without yield reduc-A 24-yr-old permanent field trial on coarse sand (Orthic Haplohution compared with no catch crop use (Hansen and mod) under temperate coastal climate conditions was used to deter-Djurhuus, 1997a). However, no information is available mine (i) the effect of introducing perennial ryegrass (Lolium perenne L.) as a catch crop on plots with a history of low input of organic on the extent of residual N effects after discontinuation matter, and (ii) the residual effect of long-term use of ryegrass as a of catch crop use. catch crop on main crop yield and N uptake. The catch crop (8-10 To optimize the use of catch crops with continuous kg ha Ϫ1 ) was undersown in spring wheat (Triticum aestivum L.). From cereal production as well as with crop rotations, it is 1993 to 1996, four treatments were included: catch crop since 1968, important that the above-mentioned issues be further catch crop since 1993, no catch crop, and catch crop until 1993. Each investigated. A long-term field experiment suitable for treatment was conducted at two previously established N rates (60 such investigations was available on a coarse sandy soil. and 120 kg N ha Ϫ1 yr Ϫ1 ), which were subdivided into four new N rates When the present experiment started, plots with or with-(0, 60, 90, and 120 kg N ha Ϫ1 yr Ϫ1 ). Two years after introduction of out ryegrass as a catch crop had existed for 24 yr. By the catch crop, yields were no longer different from yields with longchanging the treatments in 1993, the effects of catch term previous catch crop use. The residual effect of long-term catch crop use on yield persisted for more than 4 yr. With previous long-crop use could be further investigated. The aim of the term use of a catch crop compared with no previous use, N fertilization experiment was to determine (i) the effect of introduccould be reduced by 15 or 27 kg N ha Ϫ1 yr Ϫ1 at the 60 or 120 kg N ing a catch crop on plots with a history of low input of ha Ϫ1 yr Ϫ1 rate, respectively, without yield reductions. The experiment organic matter, and (ii) the residual effect of long-term shows that the use of ryegrass as a catch crop has the potential to use of a catch crop on main crop yield, N uptake and benefit main crop yield and soil fertility.nitrate leaching. This paper reports on the yield parameters; nitrate leaching is reported by Hansen et al. (2000).