Methane production and carbon mineralisation of size and density fractions of peat soils

For the purpose of characterisation of soil organic matter breakdown, soil organic matter is often divided into different fractions, each with its own decomposition rate. Thus far, no attempts had been made to quantify the methane production capacity of individual soil fractions. We aimed to improve our understanding of CH4 emissions from peat soils by studying the CH4 production capacity of individual soil fractions in parallel with their carbon mineralisation capacity. Samples from two wet grasslands on peat soil (0-60 cm) were fractionated into different size and density fractions (> 2.0 mm; 0.25-2.0 mm, light; 0.25-2.0 mm, heavy; 150-250 mu m, light; 150-250 IJ-m, heavy; <150 mu m) using sieves and Ludox, an aqueous colloidal dispersion of silica particles. The individual fractions were rather similar with respect to C mineralisation capacity and C-to-N ratio, but not with respect to CH4 production capacity. C mineralisation capacity ranged from 10 to 100 mu mol CO2-C g(-1) C d(-1) and decreased with depth. CH4 production capacity ranged from 0 to 150 nmol CH4-C g(-1) C d(-1). We found that significant CH4 production only occurred for fractions with a large particle size; the fraction >2.0 mm contributed 90% to total CH4 production capacity. Furthermore, CH4 production capacity strongly decreased with depth; the layer 0-5 cm contributed 70% to total CH4 production capacity. This indicates that in the wet peat soils recent plant residues are a major substrate for methanogens. (C) 1999 Elsevier Science Ltd. All rights reserved.