Nitrogen fluxes after clear-cutting. Ground vegetation uptake and stump/root immobilisation reduce N leaching after experimental liming, acidification and N fertilisation

Abstract

Clear-cutting often results in a dramatic change in the soil nitrogen (N) balance. This study analysed the fate of inorganic soil N over four years (1992–1995) in a new clear-cut containing three replicate blocks. Treatments comprised control (0), 6000kg CaCO3 ha−1 (Ca), 600kg sulphur powder ha−1 (S) and 600kg urea-Nha−1 (U), applied during 1976–1987 to a Norway spruce (Picea abies) forest. Trees were cut in March 1992, after which stems, tops and branches were removed from the 12 experimental plots. Spruce seedlings were planted in May 1992. Clear-cutting resulted in accumulation of approx. 50kg inorganic Nha−1 in the 30cm deep topsoil of the 0, S and U treatments and 25kgN ha−1 in the Ca treatment by October of 1992. During the next two years, the inorganic pools increased (S), stabilised (0 and U) or declined (Ca), but from the end of the third year all inorganic pools decreased. Plant cover gradually increased with dominance of Deschampsia flexuosa. Mean plant N uptake in grasses, forbs and spruce seedlings was estimated at 95 (0), 139 (Ca), 52 (S) and 121 (U) kgN ha−1 for the four-year period. Nitrogen leaching at 50cm depth was dominated by NO3–N and culminated during the second (Ca and U) and third (0 and S) years. Cumulative N leaching for the four-year period was lower for U and Ca (28 and 31kgN ha−1) than for 0 and S (53 and 81kgNha−1), and was inversely correlated with plant N uptake. Nitrogen immobilisation in stumps and root necromass (including spruce and grass roots) was calculated to be 35–45kgNha−1 during this period. The four-year N balance showed 5–17% higher inputs (net mineralisation and deposition) than outputs (plant uptake, leaching, immobilisation in dead stumps/roots and accumulation of inorganic N). Gaseous N losses were not studied, but high pH and high NO3−–N formation in the organic layers of the Ca treatment in 1992 might have favoured denitrification, which can partly explain the lower amount of inorganic N remaining in this treatment. A novel finding was that stump and dead root immobilisation of N was quantitatively important. A second novel finding was that lime application, although stimulating nitrification, also stimulated plant N uptake so much that nitrate leaching was reduced in comparison with the control and sulphur treatments.