© 1998 Heron Publishing—Victoria, Canada
Nitrogen-related root variables of trees along an N-deposition gradient in Europe
Peter Högberg (1), Lars Högbom (1) and Helga Schinkel (1, 2)
1. Section of Soil Science, Department of Forest Ecology, SLU, S-901 83 Umeå, Sweden / 2. Department of Forest Genetics and Plant Physiology, SLU, S-901 83 Umeå, Sweden / Received February 25, 1998
Summary
We measured fine root N concentration, root in vivo nitrate reductase activity (NRA) and root uptake capacity for 15NH4+ and 15NO3– along an N-deposition gradient from northern Sweden to central Europe, encompassing a variation in N deposition rates of
< 5 to about 40 kg N ha–1 year–1. The focus was on Picea abies (L.) Karst., but Fagus sylvatica L. in central Europe and Pinus sylvestris L. and Betula spp. in northern Sweden were also studied. We assumed that, with an increased supply of N, root N concentration would increase,
activity of the inducible enzyme nitrate reductase (NR) in roots would increase, particularly with an increasing supply of
NO3–, and root uptake capacity for inorganic N would decline, reflecting a lower demand for N. As expected, fine root N concentration
in P. abies increased along the gradient from 1.1% (d.w. basis) at the northern site to 2.1% at central European sites. This variation
compared with an amplitude of 0.7–1.5% for foliage. Root in vivo NRA was low in northern Sweden, and higher in central Europe. Picea abies and broad-leaved species had similar root NRA. At one location in Denmark and one in France, however, root NRA in the spring
was very high in F. sylvatica. Root uptake capacity for NO3–, as measured in excised roots, was low throughout the transect, but in P. abies, it was high for NH4+ in northern Sweden and decreased by a factor of 4 with increasing N deposition. A similar pattern was found in the broad-leaved
species. Unless the higher availability of NO3– and lower specific root uptake capacity per unit root mass for inorganic N in central Europe (compared with northern Sweden)
is balanced by a higher root biomass, the central European forests will be a weaker sink for N.
Keywords:
Betula, Fagus sylvatica, N nutrition, nitrate reductase, Picea abies, Pinus sylvestris, root uptake capacity, tree roots.
