Mineralization and immobilization of nitrogen in heath soil under intact Calluna, after heather beetle infestation and nitrogen fertilization

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 NH4+ and NO3- in the organic surface layer of soils from a Danish heathland using N-15 isotope techniques in laboratory incubations. The soils were sampled under intact and dead Calluna vegetation. The dead Calluna vegetation had been fertilized at rates of 0, 15 or 35 kg N ha(-1) per year and the death of vegetation had been caused by a naturally occurring heather beetle infestation. In the soil under intact Calluna, the NH4+ pool was very low with no net mineralization, while a substantial mineralization-immobilization turnover of NH4+ was found with a large capacity for short term net NH4+ immobilization (36 mu g N g(-1) during 1 h; 135 mu g N g(-1) during 24 h). The metabolic inhibitor mercury chloride completely inhibited assimilation of Nit indicating the process was biological. The immobilization of NH4+ had no short or long-term (38 days) effect on soil respiration while NH4+ immobilization stimulated net mineralization of soil N during long-term incubation. The soils sampled under dead and dead/fertilized Calluna had large pools and high net mineralization rates of NH4+ with a decrease of gross NH4+ immobilization relative to the soil under intact Calluna. 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 NH4+ and the storage capacity for N in the soil and vegetation. The ecosystem may, however, be susceptible to disruption of the tight NH4+ cycling because of the limited capacity of the ecosystem to remove excess mineral N from the soil. This may increase the risk of conversion of the heath into grassland. (C) 1999 Elsevier Science B.V. All rights reserved.