Soil nitrogen transformations along a successional gradient from Calluna heathland to Quercus forest at intermediate atmospheric nitrogen deposition

Abstract

Soil nitrogen transformations were studied in a Danish Calluna heathland, a 70-year-old Quercus scrub, and a 200-year-old Quercus forest. These three ecosystems were chosen to represent a secondary successional gradient from Calluna heathland to Quercus forest in an area with intermediate atmospheric N deposition. An NPK-fertilized heathland plot with Deschampsia vegetation was studied as well. In situ net mineralization and net nitrification rates were estimated during the growth season, using intact soil cores. Two additional experiments were conducted in the laboratory to show mineralization/immobilization dynamics and potential nitrification: intact soil cores were subjected to artificial rain in a percolation system; and sieved soil was incubated with combinations of mineral N, urea and CaCO 3. The morlayer (L,F,H horizon) of the Calluna heathland had a very small mineral N pool and an in situ net mineralization rate of less than 3 mg NH 4 +–N kg −1 month −1. Both the mineral N pool and the net mineralization rate was higher in the Quercus soils with highest rates (27 mg NH 4 +–N kg −1 month −1) in the old Quercus forest. Net nitrification rates were low or zero, and soil N pools were low during the growth season, which indicated a tight recycling of N. The increase in net mineralization rates along the successional gradient was confirmed by the laboratory incubations, and a low nitrification activity was seen only in the Quercus soils in the percolation system. No potential nitrification activity was observed in any of the incubations. The Calluna morlayer revealed a remarkable ability for short term immobilization of NH 4 + and NO 3 −. A mass balance of plant N uptake vs. N deposition and net mineralization in situ indicated that the heather vegetation has access to soil N pools other than the inorganic N originating from net mineralization. The NPK fertilized heath plot with Deschampsia vegetation showed an increase in mineral N pool, net mineralization and nitrification rates as compared to the Calluna soil during laboratory incubations. The results presented here are in accordance with the traditional descriptions of Calluna heath soils, but they are in contrast to recent studies, which have shown high net mineralization rates and occurrence of net nitrification in Dutch Calluna soils. These differences may relate to high atmospheric N deposition in Dutch heathlands as compared to the intermediate N deposition in the area studied here.