FATE OF 15 N-LABELED UREA IN THE GROWTH CHAMBER AS AFFECTED BY ADDED ORGANIC MATTER AND N PLACEMENT

Abstract

The effects of N placement and organic matter amendments on plant uptake and immobilization of N from 15 N-labeled urea were investigated in a growth chamber employing two successive crops of rapeseed (Brassica napus L.). Immature barley (Hordeum vulgare L.) (C:N ratio 20.9) and fababean (Vicia faba L.) (C:N ratio 21.8) plant residues, and sucrose were mixed throughout the soil at 10 g material kg −1 soil. Urea enriched with 20.2 atom % 15 N was either mixed throughout the soil, or placed as a point in the center of the soil at 100 mg N kg −1 soil. The soil was incubated at field capacity for 30 d before the seeding of first crop and 60 d before the second crop. For the first crop of rapeseed, there was a net mineralization of N from the added barley and fababean materials regardless of N application. For total N uptake and dry matter yield of rapeseed, the mixed treatment of urea was superior to the point placement where barley and fababean residues were added. However, due to considerable biological interchange of N with added plant residues in the mixed treatment, 15 N uptake was greater for the point placement of added urea (47.3 vs. 26.7%). Sucrose led to considerable net immobilization of N with the result that the point placement of N was superior to mixing for N uptake and 15 N recovery. Added N remaining in the soil at harvest time after removal of the first crop's roots varied between 20.0 and 48.8% for the point placed N and between 27.5 and 79.2% for the mixed placement and was largest where sucrose was added. In spite of the large variation in 15 N remaining in the soil, plant recovery of residual 15 N in the second crop ranged from only 11.3 to 13.8%, except for the point placement of urea with sucrose where the value was 19.3%. Key words: N immobilization, organic residues, residual 15 N, tagged urea