Anoxic microsites in Douglas fir litter

Oxygen concentrations within forest floor litter were studied at various water contents and oxygen consumption rates. A micromanipulator moved micro-electrodes (dia 7.5 μm) downward at a velocity of 2 μm s−1. In oxygen profiles, particles and pores could be identified. The sizes of these particles agreed with particle sizes derived from a thin section of the same forest floor material. In oxygen profiles, no gradients in concentration occurred in litter macropores at water contents of 2–3.9 g g−1 dry weight, which represented the average range in field water contents. By contrast, steep oxygen gradients occurred inside organic particles. Larger organic particles in general had lower intraparticle oxygen concentrations, compared to smaller organic particles and in some particles oxygen was absent (>250 μm dia). These anoxic sites occurred at all water contents (2–3.9 g g−1 dry weight) and oxygen consumption rates (0.4–3.2 μmol g−1 h−1). The anoxic organic matter fraction of the forest floor did not significantly increase with water content nor with a stimulated oxygen consumption rate due to addition of glucose. The recognition that permanent anoxic sites exist in litter helps us understand how anaerobic processes, such as denitrification, may contribute to forest floor N2O production.