Short-term Uptake Of 15NH4+ Into Soil Microbes And Seedlings Of Pine, Spruce And Birch In Potted Soils

Short-term competition between soil microbes and seedlings of Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies (L.) Karst.) and silver birch (Betula pendula Roth) for N was assessed in a pot study using (15NH4)2SO4 as a tracer. Seedlings were grown in organic and mineral soil, collected from a podsol soil; 3.18 mg (15NH4)2SO4 per pot were injected into the soil, corresponding to 4 µg 15N g-1 d.m. (dry matter) mineral soil and 17 µg 15N g-1 d.m. organic soil. The amounts of N and 15N in the seedlings and in microbial biomass derived from fumigation-extraction were measured 48 h after addition of 15N. In the mineral soil, 19–30% of the added 15N was found in the plants and 14–20% in the microbial biomass. There were no statistically significant differences between the tree species. In the organic soil, 74% of the added 15N was recovered in the microbial biomass in birch soil, compared to 26% and 17% in pine and spruce soils, respectively. Correspondingly, about 70% of the 15N was recovered in pine and spruce seedlings, and only 23% in birch seedlings. In conclusion, plants generally competed more successfully for added 15NH4+ than soil microbes did. An exception was birch growing in organic soil, where the greater amount of available C from birch root exudates perhaps enabled micro-organisms to utilise more N.