Anoxic microsites in Douglas fir litter

Oxygen concentrations within forest floor litter were studied at various wa ter contents and oxygen consumption rates. A micromanipulator moved micro-e lectrodes (dia 7.5 mu m) downward at a velocity of 2 mu m s(-1). In oxygen profiles, particles and pores could be identified. The sizes of these parti cles agreed with particle sizes derived from a thin section of the same for est floor material. In oxygen profiles, no gradients in concentration occur red in litter macropores at water contents of 2-3.9 g g(-1) dry weight, whi ch represented the average range in field water contents. By contrast, stee p oxygen gradients occurred inside organic particles. Larger organic partic les in general had lower intraparticle oxygen concentrations, compared to s maller organic particles and in some particles oxygen was absent (> 250 mu m dia). These anoxic sites occurred at all water contents (2-3.9 g g(-1) dr y weight) and oxygen consumption rates (0.4-3.2 mu mol g(-1) h(-1)). The an oxic organic matter fraction of the forest floor did not significantly incr ease with water content nor with a stimulated oxygen consumption rate due t o addition of glucose. The recognition that permanent anoxic sites exist in litter helps us understand how anaerobic processes, such as denitrificatio n, may contribute to forest floor N2O production. (C) 1999 Published by Els evier Science Ltd. All rights reserved.