EFFECT OF NITROGEN-FERTILIZER ON TEMPORAL AND SPATIAL VARIATION OF MINERAL NITROGEN AND MICROBIAL BIOMASS IN A SILVOPASTORAL SYSTEM

This paper describes a field study to assess the effect of increasing the frequency of split applications of N fertiliser on the pattern of plant uptake, soil N availability, and microbial biomass C and N. Meas urements were taken during the growing season in different positions r elative to young trees (Prunus avium L.) in an upland silvopastoral sy stem in its first year after establishment. At fertiliser rates of 72 and 144 kg ha(-1) N applied as NH4NO3, increasing the number of split applications increased N uptake by the pasture. Mineral forms of soil N measured 2 weeks after application indicated that residual NH4+-N an d total mineral N were also greater in this treatment on certain dates . Soil NO3-N was positively correlated with the soil moisture content, and nitrification reached a maximum in early May and declined rapidly thereafter except within the herbicide-treated areas around the trees where soil moisture had been conserved. Results of the study suggest that high NO;-N in herbicide-treated areas was probably caused by mine ralisation of grass residues and low uptake by the tree rather than by preferential urine excretion by sheep sheltering beside the trees. Me an microbial biomass C and N values of 894 and 213 kg ha(-1), respecti vely, were obtained. Microbial C was slightly increased by the higher frequency of split applications at 144 kg ha(-1) N and was probably re lated to the greater herbage production with this treatment. Microbial N was not significantly affected by the N treatments. Both microbial biomass C and N increased during the growing season, resulting in the net immobilisation of at least 45 kg ha(-1) N which was later released during the autumn.