AVAILABILITY TO SUBSEQUENT CROPS AND LEACHING OF NITROGEN IN N-15-LABELED SUGAR-BEET TOPS AND OILSEED RAPE RESIDUES

In autumn 1991, sugarbeet tops (Beta vulgaris L.) and different compon ents of oilseed rape residues (Brassica napus L.), both labelled with N-15, were incorporated into the soil under field conditions at Askov Experimental Station, Denmark, using stainless steel cylinders to cont ain the treatments. The availability of this labelled N to a subsequen t crop was measured, using as test crops autumn-sown rye (Secale cerea le L.), wheat (Triticum aestivum L.) and spring barley (Hordeum vulgar e L.). In spring 1992, cylinders with N-15-residues received NH4NO3 an d those without (NH4NO3)-N-15-N-15. I,.parallel experiment, N-15-label led beet tops were incorporated in lysimeters. A four-course rotation of sugarbeet, spring barley (undersown with perennial ryegrass Lolium perenne L.), perennial ryegrass and winter wheat at two rates of calci um ammonium nitrate (CAN) or animal slurry was grown in these lysimete rs. Leaching and the availability of beet top N to successive crops we re followed for 2 years. The soil in the cylinders and lysimeters was a light sandy loam (similar to 10% clay). Of the 7.10 g N/m(2) added i n beet tops, 10-15% was harvested in two subsequent crops of barley an d ryegrass and 13-19% was lost by nitrate leaching. Beet top N account ed for 3-7% of the total N offtake in 1992. In 1993 < 1.5% of the tota l N offtake in ryegrass was from the beet tops applied in 1991. Combin ing results from mineral fertilized treatments, it was found that 9% o f the beet top N was removed in the first cereal crop, 9% was lost by nitrate leaching and 68% remained in the 0-20 cm soil layer (including roots), suggesting that the denitrification loss was < 15%. Incorpora tion of oilseed rape stubble (1.35 g N/m(2)), two rates of pods (6.25 and 18.75 g N/m(2)) or mixed residues (12.25 g N/m(2)) contributed 0.5 , 2.3, 7.4 and 4.6%, respectively, to the total N harvested in the fol lowing crop of winter wheat. The percentage of the added labelled N ta ken up by the wheat ranged from 4.9 to 6.1%, with 60-79% remaining in the 0-20 cm layer after harvest. For beet tops it was calculated that 100 kg N/ha in residues incorporated in the autumn could replace 18 kg N/ha given in the following spring as mineral fertilizer. For oilseed residues, the corresponding average value was 9 kg N/ha.In fertilized cropping systems, oilseed rape residues had minor effects on the subs equent crop, so that an uneven return of residues, as often occurs wit h combined crops, would do little harm. A considerable proportion of t he N applied in sugarbeet tops was lost by leaching and the residual v alue of the sugarbeet tops to subsequent crops was low.