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

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 NH 4 + and NO 3 − in the organic surface layer of soils from a Danish heathland using 15N 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 NH 4 + pool was very low with no net mineralization, while a substantial mineralization-immobilization turnover of NH 4 + was found with a large capacity for short term net NH 4 + immobilization (36 μg N g −1 during 1 h; 135 μg N g −1 during 24 h). The metabolic inhibitor mercury chloride completely inhibited assimilation of NH 4 + indicating the process was biological. The immobilization of NH 4 + had no short or long-term (38 days) effect on soil respiration while NH 4 + 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 NH 4 + with a decrease of gross NH 4 + 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 NH 4 + and the storage capacity for N in the soil and vegetation. The ecosystem may, however, be susceptible to disruption of the tight NH 4 + 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.