Turnover of 15N in undisturbed root systems and plant materials added to three soils

Turnover of 15N in undisturbed wheat (Triticum aestivum) root systems was compared with turnover of 15N 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 15N turnover was determined by measuring inorganic 15N and biomass 15N after 11 and 22 weeks of incubation, respectively, and uptake of 15N in ryegrass (Lolium rigidus). The residues had decomposed to different extents at the first sampling with more 15N 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 15N and soil N was negatively correlated with clay content. The amount of N and 15N in the microbial biomass was positively correlated with clay content. More 15N 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 15N in the biomass after 22 weeks. Uptake of 15N and soil N in ryegrass also showed a negative correlation with clay content. Contrary to 15N mineralization in the pots without plant growth, 15N uptake in the ryegrass was significantly affected by type of residue. This may be explained partly by the distribution of 15N between the ryegrass tops and the roots.