Plant and Soil 139: 139-143, 1992 © 1992 Kluwer Academic Publishers. Printed in the Netherlands. PLSO 8905 Effects of starch additions on N turnover in Sitka spruce forest floor JOSHUAP. SCHIMEL, STEFAN HELFER 1 and IAN J. ALEXANDER Institute of Arctic Biology, University of Alaska, Fairbanks, AK 99775, USA and Department of Plant and Soil Science, University of Aberdeen, Aberdeen, AB9 2UE, Scotland. Present address: 1Royal Botanic Garden, Edinburgh, EH3 5LR, Scotland Received 28 September 1990. Revised July 1991 Key words: carbon, catabolite repression, mineralization, N cycling, starch Abstract Starch and carboxymethyl starch were added to forest floor samples collected from a sitka spruce stand near Aberdeen, Scotland. Samples were incubated for one month and were periodically analyzed for respiration, biomass-C, net and gross N-mineralization/immobilization. Gross mineralization/immobili- zation was measured by using a 15N-isotope pool dilution technique. Starch additions did not significantly affect respiration rates or biomass-C but caused net immobilization. The mechanism of this appeared to be inhibition of the decomposition of N-containing soil organic matter by the available starch-C, which resulted in decreased gross mineralization. Carboxymethyl starch acted as a biocide, probably as a result of an osmotic effect. Introduction It has normally been found that adding available carbon to soil causes net nitrogen immobilization by soil microbes (Jones and Richards, 1978). However, Harmer and Alexander (1986) re- ported an exception to this pattern in that potato starch caused net mineralization when it was added to forest floor material from a range of conifers or to Sitka spruce (Picea sitchensis) for- est floor in the field. The data suggested that available energy may have limited the degra- dation of N-containing compounds in the forest floor. Such interactions between organic com- pounds complicates our understanding of soil organic matter dynamics and particularly the role of C vs. N limitation in them (Johnson and Edwards, 1979; Schimel and Firestone, 1989). To examine more closely the mechanism in- volved, further laboratory incubations using analytical-grade starch were performed on the same forest floor materials. In this case N im- mobilization was observed, rather than miner- alization. We subsequently discovered that the potato starch used in the original studies by Harmer and Alexander (1986), which had been supplied for a large field experiment, was in fact 'Stabilose' (Avebe Veendam Ltd., Foxhol, Netherlands), a form of carboxymethyl starch. We therefore examined the different effects of starch and carboxymethyl starch on C and N dynamics in Sitka spruce forest floor material. We measured changes in microbial biomass, res- piration, and N dynamics by using ~SN pool dilution methodology (Kirkham and Barth- olomew, 1954) to estimate gross mineralization and immobilization independently, and so sepa- rate the components of net mineralization and N turnover. Methods The study site was in a 40-year-old Sitka spruce stand in Durris forest near Aberdeen, Scotland. The soil is an acid brown forest soil (dystroch-