Turnover of N-15 in undisturbed root systems and plant materials added to three 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-derived 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 different extents at the first sampling with more N-15 mineralized from N derived from roots. However, differences among residues were insignificant after 22 weeks incubation. The soil type had a significant influence 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 clay content. More N-15 was retained in the microbial biomass at the first sampling when large wheat roots and tops had been applied compared to soil with undisturbed roots and root derived N. Within each soil, the 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 correlation with clay content. Contrary to N-15 mineralization in the pots without plant growth, N-15 uptake in the ryegrass was significantly affected by type of residue. This may be explained partly by the distribution of N-15 between the ryegrass tops and the roots. Copyright (C) 1996 Elsevier Science Ltd