In situ formation of organically bound halogens during decomposition of Norway spruce needles: effects of fertilization

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Abstract
Past years of research indicate that halogenation of organic matter is a general occurrence in soil. As this is a virgin research field, little is known about, for example, the possible relation to the turnover of organic matter, influence of environmental parameters, or ecological role. The aim of the present paper was to study the influence of fertilization on in situ formation of organically bound halogens and its possible relation to decomposition of organic matter. Total amount of organically bound halogens was determined in nutrient-rich and nutrient-poor spruce litter incubated up to 4 years in a fertilized plot and a control plot. It was found that the concentration of organically bound halogens increased throughout the incubation period, and the increase was significantly larger in the control than in the fertilized plot. This resulted in an accumulated increase in the control plot that was approximately twice as large (ca. 200 μg Cl/g dry mass) as that in the fertilized plot at the end of the incubation period. The changes in absolute amounts were complexly related to litter mass loss. This strongly indicates that there are multiple underlying processes and that halogens are both incorporated into and released from organic matter during decomposition. Calculated on a daily basis, the rate at which the amounts of organohalogens changed showed a seasonal pattern, with a pronounced increase during late summer and autumn. After 1 to several years, a net decrease was observed, particularly in the summer period. This decrease appeared earlier and was more pronounced in the fertilized plot than in the control plot, strongly indicating that mineralization of organically bound halogens, i.e., release of inorganic halides, was enhanced by fertilization. In the litter incubated in the control plot, the absolute amounts increased in relation to lignin degradation. In combination with previous findings, the results of the present study suggest that formation of organically bound halogens is conducted by organisms responsible for lignin degradation. Furthermore, the findings indicate that fertilization may favour organisms that degrade organically bound halogens at the expense of organisms that cause the formation of such compounds.

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