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 ÿ in the organic surface layer of soils from a Danish heathland using N 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 haÿ1 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 mg N gÿ1 during 1 h; 135 mg N gÿ1 during 24 h). The metabolic inhibitor mercury chloride completely inhibited assimilation of NH4 ‡ 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 nitri®cation 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. # 1999 Elsevier Science B.V. All rights reserved.