C AND N MINERALIZATION IN AN ACID FOREST SOIL ALONG A GAP-STAND GRADIENT

Using aerobic long-term incubations of 174 days, carbon and nitrogen m ineralization, pools of potentially mineralizable C and N, microbial C and N, and leaching of organic CaCl2-extractable C and N were determi ned in forest floor material (O-F and O-H) and mineral soil (0-5 cm) c ollected along a gap-stand gradient. Sample locations were the centre of gaps, the northern part of gaps, the northern edge between gaps and stand, and a mature European beech stand. Conditions changed along th e gradient in (a) soil moisture, which was highest in the gap centre, (b) soil temperature, which was elevated in the northern part of the g ap and at the northern edge, and (c) active tree roots, which were abs ent from the gap centre and the northern part. Samples were collected 21 months after gaps (30 m dial had been created. Because N mineraliza tion in the field during the first year after gap creation was higher in the gap than in the stand but lower in the second year, it was hypo thesized that mineralizable substrate had been partially depleted unde r gap conditions. No differences were found between the gap and the st and in forest floor mass, or forest floor and mineral soil C and N con centrations. In the forest floor, C and N mineralization, and conseque ntly the potentially mineralizable C and N, as derived from first orde r mineralization kinetics, decreased in the order stand > northern edg e > gap centre > gap north. Amounts mineralized were 1053, 861, 761 an d 610 g N g(-1) and 24.4, 22.6, 22.5 and 20.2 mg C g(-1) respectively. In mineral soil, these values were highest in the gap centre. A subst antial decrease in microbial biomass occurred in samples from the gap centre during incubation. Other samples showed no marked decrease in m icrobial biomass during incubation. In the forest floor a shorter turn over time of mineralizable N was observed for gap locations, whereas t he turnover of mineralizable C was slower in the northern part of the gap and the northern edge. This suggests that potentially-mineralizabl e pools of C and N were affected differently by either rooting, moistu re or temperature. Similar turnover times (4-11 weeks) for potentially -mineralizable N in both forest floor and mineral soil suggested simil ar sources of mineralized N. Potentially-mineralizable C consisted of more recalcitrant fractions in mineral soil than in forest floor. Nitr ogen immobilization in the form of CaCl2-extractable organic compounds was unimportant in this soil. Organic N (N-org) in CaCl2-leachates wa s 4-12% of mineral N. Kinetic parameters indicated that N mineralizati on in the second year after gaps had been created was lower in the gap s, because mineralizable C and N pools had decreased. Copyright (C) 19 96 Elsevier Science Ltd.