Importance of ammonium and nitrate to the performance of herb-layer species from deciduous forests in Southern Sweden

Abstract

Experiments were performed on the effect on growth and uptake of nitrogen given as NH 4 and/or NO 3 in a nutrient solution (pH 4.2) of a composition similar to that of the rhizosphere soil solution of an acidic beech forest but lacking Al. Seven treatments (0.2 and 0.4 mM NH 4 and NO 3, respectively, and 0.1 + 0.1, 0.2 + 0.2, 0.4 + 0.4 mM of NH 4 + NO 3) were used in a flowing solution culture without recirculation. The seven species tested have ecological distributions which range from very acid soils with low nutrient levels and nitrogen mainly present as NH 4, to less acidic and nitrifying soils. In the solution experiment, species occurring in very acidic soils had small preferences for nitrogen form or combination ( Deschampsia flexuosa, Carex pilulifera, Milium effusum), while Aegopodium podagraria of less acidic soils preferred NO 3. The other three species, Poa nemoralis, Bromus benekenii and Carex sylvatica, grew best in a mixed solution. Results of a short-term (2–24 hr) exposure experiment showed that all species had a higher nitrogen uptake from NH 4 in the solo and mixed solutions. Total N uptake was, however, similar for the solo NH 4 and mixed solutions for all species except D. flexuosa and C. pilulifera, which had the highest N uptake with solo NH 4. It was concluded that none of the species studied grew best when NH 4 was supplied as the sole N source. This may restrict the distribution of species in very acid soils, where nitrification may be lacking.