C and N mineralization in an acid forest soil along a gap-stand gradient

Abstract

Using aerobic long-term incubations of 174 days, carbon and nitrogen mineralization, pools of potentially mineralizable C and N, microbial C and N, and leaching of organic CaCl 2-extractable C and N were determined in forest floor material (O F and O H) and mineral soil (0–5 cm) collected 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 the gradient in (a) soil moisture, which was highest in the gap centre, (b) soil temperature, which was elevated in the northern part of the gap and at the northern edge, and (c) active tree roots, which were absent from the gap centre and the northern part. Samples were collected 21 months after gaps (30 m dia) had been created. Because N mineralization 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 hypothesized that mineralizable substrate had been partially depleted under gap conditions. No differences were found between the gap and the stand in forest floor mass, or forest floor and mineral soil C and N concentrations. In the forest floor, C and N mineralization, and consequently the potentially mineralizable C and N, as derived from first order mineralization kinetics, decreased in the order stand > northern edge > gap centre > gap north. Amounts mineralized were 1053, 861, 761 and 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 substantial decrease in microbial biomass occurred in samples from the gap centre during incubation. Other samples showed no marked decrease in microbial biomass during incubation. In the forest floor a shorter turnover time of mineralizable N was observed for gap locations, whereas the turnover of mineralizable C was slower in the northern part of the gap and the northern edge. This suggests that potentially-mineralizable pools of C and N were affected differently by either rooting, moisture or temperature. Similar turnover times (4–11 weeks) for potentially-mineralizable N in both forest floor and mineral soil suggested similar sources of mineralized N. Potentially-mineralizable C consisted of more recalcitrant fractions in mineral soil than in forest floor. Nitrogen immobilization in the form of CaCl 2-extractable organic compounds was unimportant in this soil. Organic N (N org) in CaCl 2-leachates was 4–12% of mineral N. Kinetic parameters indicated that N mineralization in the second year after gaps had been created was lower in the gaps, because mineralizable C and N pools had decreased.