Low fertiliser recoveries are often found for temperate coniferous forests and appear to be associated with the 'locking-up' of residual fertiliser N in stable organic forms. This phenomenon may well be caused by strong microbial immobilisation, out competing uptake by tree roots. This thesis investigates root/microbe competition for fertiliser-derived N (FDN) in microcosms of coniferous forest soil (supporting Sitka spruce seedlings) as well as in re-seeded blanket peat (supporting a mixed grass pasture). Combinations of selective microbial inhibitors, both with and without either 15N-labelled urea or NH4NO3, were applied to microcosms to selectively inhibit target microbial groups which may be competing with roots for FDN and determine the role of microbial immobilisation as a mechanism controlling N flow to seedlings/herbage. The dynamics of FDN novement into microcosm N pools was also studied in microcosms to which 15N-labelled urea was applied alone. At harvest, plant and soil N pools were analysed for 15N and total N (15 N &'43 14N). The impact of biocide and fertiliser N treatments on concentrations of target and non-target soil organisms were also determined. Biocidal (benlate and to a lesser extent streptomycin) inhibition of soil micro-organisms (particularly fungi) increased the uptake of both urea-derived N and NH4O3 N by Sitka spruce seedlings. Increases were associated with reduced percentages of FDA active hyphae and concentrations of FDN immobilised in the microbial biomass of LFH layer and peaty mineral soil. The results suggest that roots were competing with soil microbes for both fertiliser N forms. In conclusion this study has demonstrated the role of microbial biomass, particularly the dominant fungal component as a major competitive sink for fertiliser N and a major factor contributing to the low efficiency of fertiliser N in temperate coniferous forests. The study also has identified selective biocidal manipulation as a powerful technique for characterising competition between roots and microbes for nutrients in soil.

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