TURNOVER OF N-15 IN UNDISTURBED ROOT SYSTEMS AND PLANT MATERIALS ADDED TO 3 SOILS

Turnover of N-15 in undisturbed wheat (Triticum aestivum) root systems was compared with turnover of N-15 in applied wheat tops, large roots of wheat and N left in soil after removal of larger roots (root-deriv ed N) in three soils of different texture (14, 22 and 27% clay). The N -15 turnover was determined by measuring inorganic N-15 and biomass N- 15 after 11 and 22 weeks of incubation, respectively, and uptake of N- 15 in ryegrass (Lolium rigidus). The residues had decomposed to differ ent extents at the first sampling with more N-15 mineralized from N de rived from roots. However, differences among residues were insignifica nt after 22 weeks incubation. The soil type had a significant influenc e on N turnover throughout the incubation. Mineralization of both N-15 and soil N was negatively correlated with clay content. The amount of N and N-15 in the microbial biomass was positively correlated with cl ay content. More N-15 was retained in the microbial biomass at the fir st sampling when large wheat roots and tops had been applied compared to soil with undisturbed roots and root derived N. Within each soil, t he type of residue had no effect on N-15 in the biomass after 22 weeks . Uptake of N-15 and soil N in ryegrass also showed a negative correla tion with clay content. Contrary to N-15 mineralization in the pots wi thout plant growth, N-15 uptake in the ryegrass was significantly affe cted by type of residue. This may be explained partly by the distribut ion of N-15 between the ryegrass tops and the roots. Copyright (C) 199 6 Elsevier Science Ltd