|Kristensen, H - DANISH MINISTRY OF ENVIR.|
Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 8, 1999
Publication Date: N/A
Interpretive Summary: Heathland ecosystems once covered extensive areas of Western Europe, but this coverage has been greatly reduced during the last two centuries as a result of change of land use. During the last two decades much work has been done to increase understanding of heathland ecology and to enable the continued preservation of the remnant heathlands. There has been growing evidence that the heath areas are threatened by increased atmospheric deposition of anthropogenic N to the ecosystem along with pressures for natural succession of vegetation towards forest. The purpose of this study was to investigate the turnover of inorganic N (i.e., gross mineralization and immobilization processes) in heathland soil which has previously been found to have little or no net mineralization activity. In the soil under intact vegetation, the ammonium pool was maintained at very low content with no net mineralization, while a substantial mineralization-immobilization turnover of ammonium was found with a large capacity for short-term net ammonium immobilization. The soils from under dead and dead/fertilized vegetation had large pools and high net mineralization rates of ammonium with a decrease of gross ammonium immobilization relative to the soil under intact vegetation. The work demonstrated that the ecosystem maybe susceptible to the disruption of the tight N cycling because of the limited capacity of the ecosystem to remove excess mineral N from the soil.
Technical Abstract: The maintenance of low availability of mineral N in heath soils is thought to be a key factor for the stability of heathland ecosystems. We investigated the turnover of ammonium and nitrate in the organic surface layer of soils from a Danish heathland by use of 15-N isotope techniques in laboratory incubations. The soils were sampled under intact or dead Calluna vegetation of which the latter had been fertilized with N with the death of vegetation caused by a naturally occurring infestation of heather beetles. In the soil under intact Calluna, the ammonium pool was maintained at very low content with no net mineralization, while a substantial mineralization-immobilization turnover of ammonium was found with a large capacity for short-term net ammonium immobilization. The soils sampled under dead and dead/fertilized Calluna had large pools and high net mineralization rates of ammonium with a decrease of gross Nimmobil lon relative to the soil under intact Calluna. The rates of N turnover showed no relationship to N fertilization rate and neither net nor gross nitrification activity could be detected in any of the soils. The results indicate that the effects of an increased atmospheric N deposition to the heathland may be delayed because of the tight cycling of ammonium and the storage capacity for N in the soil and vegetation. The ecosystem may, however, be susceptible to the disruption of the tight N cycling because of the limited capacity of the ecosystem to remove excess mineral N from the soil which may increase the risk of conversion of the heath into grassland.