Summary• Soil microorganisms are considered C-limited, while plant productivity is frequently N-limited. Large stores of organic C in boreal forest soils are attributed to negative effects of low temperature, soil acidity and plant residue recalcitrance upon microbial activity.• We examined microbial activity, biomass and community composition along a natural 90-m-long soil N supply gradient, where plant species composition varies profoundly, forest productivity three-fold and soil pH by three units.• There was, however, no significant variation in soil respiration in the field across the gradient. Neither did microbial biomass C determined by fumigation-extraction vary, while other estimates of activity and biomass showed a weak increase with increasing N supply and soil pH. Simultaneously, a phospholipid fatty acid attributed mainly to mycorrhizal fungi declined drastically, while bacterial biomass increased.• We hypothesize that low N supply and plant productivity, and hence low litter C supply to saprotrophs is associated with a high plant C supply to mycorrhizal fungi, while the reverse occurs under high N supply. This should mean that effects of N availability on C supply to these functional groups of microbes acts in opposing directions.

Carbon content and indices of microbial biomass and activity were determined in 1985 in different soil horizons of two nitrogen-fertilized pine forests in Sweden. The Kroksbo site was fertilized in 1974 with 150 and 600 kg N•ha−1 using NH4NO3 or urea, while the Nissafors site was fertilized in 1977 and 1984 with 150 kg NH4NO3-N•ha−1 The absolute amount of C per square metre of forest floor increased in fertilizer treatments compared with the control (by 10–26%, depending on the site or fertilizer treatment). Respiration rate, ATP content, and microbial biomass C, measured with the substrate-induced respiration technique, decreased in all horizons when expressed per gram of C. The decrease was most evident with NH4NO3, and at the highest level of fertilization. However, on an area basis there were no differences between fertilizer and control treatments. A calculated increase in litter fall in the fertilized plots could not explain all the increase in the amount of C per square metre compared with the control. Decreased microbial activity per gram of C therefore appeared to be an important factor in the increase in C content in fertilized plots.

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SUMMARY Uptake and translocation of nitrogen was studied in laboratory microcosms consisting of Alnus glutinosa (L.) Gaertn., Frankia sp., Paxillus involutus (Fr.) Fr. and Pinus contorta Dougl. ex Loud. P. involutus was shown to form a fully functional ectomycorrhizal association with alder as well as pine, and the seedlings thus became interconnected by a common mycelium. When microcosms were exposed to 15N2 gas, interplant translocation of 15N was observed in two out of three experiments. 15N2 was fixed by Frankia and translocated to all other parts of the system. In the two experiments in which interplant translocation occurred, between 5 and 15% of the 15N recovered was found in the pine seedlings. Within seven days, fixed N2 was incorporated into amino acids in the Frankia nodules, translocated to both the A. glutinosa and P. contorta seedlings and incorporated into macromolecules. In alder seedlings, citrulline and ornithine were the free amino acids that had both the highest 15N enrichment levels and concentrations. In pine, glutamine and citrulline had the highest 15N concentrations, and glutamine had the highest level of 15N enrichment. 15N enrichment levels were greatest in the nodules, at between 5.5 and 29% in the different amino acids and 12% in the macromolecular fraction. Enrichment levels decreased with increasing distance from the nodules. The uptake and translocation of 15N applied as 15NH4Cl to the mycelium was also studied. 15N was incorporated into amino acids in the mycelium and translocated further in this form. Generally, free amino acids had high 15N enrichment levels in the mycelium, decreasing along the translocation pathway. Citrulline and glutamine were the amino acids with highest 15N concentrations in all parts of the system. 15N was also found in the macromolecular fraction.

SUMMARYThe influence of pH on the jjrowth and assimilation of ''N-labelled ammonium and nitrate was studied in intact ectomycorrhizal systems consistinj^ of Betula pendula Roth and Picea ahies (L.) Karst. colonized with a common mycelium of I'axillus imolutus (Batsch) Pr, The plants were grown together in Plexiglass observation chambers containing non-sterile peat with three diflVrent pH values. 4-0, 5-1 and 61. The mycorrhiza! m>celium was allowed to grow over a barrier into an area of peat from which plant roots were excluded. Laheiled XH^NOj was supplied. either as '"XH^NO., or as NH^'^NOg, exclusively to the fungal myceiium. Shoots and roots were analyzed for '"N in total nitrogen while the mycelium was analyzed for '•''N' in XH^"^, XO,^" and free amino acids. The '"''X labelling pattern indicated that ammonium was immediately assimilated into amino acids, primarily glutamine, by the fungal mycelium at the uptake site. The amino acids were then translocated to the mycorrhiza] roots. In contrast, nitrate-X was not assimilated in the mycelium but rather transferred to the mycorrhizal roots a.s nitrate, Mycelial uptake and transfer of X to the spruce and birch seedlings were significantly higher for XHj-X than for XO,-X. No firm conclusions about pH effects on the preferential uptake of ammonium and nitrate could be drawn. However. pH had a pronounced effect on the myceliai growth t>f P. ivrolutus which was hampered severely at p}i 6-1 and to a lesser extent at pli 51,