Decomposition of plant litter and carbon turnover as a function of soil depth

Dr Brian Murphy1, Dr  Angela Newey1, Dr Richard Greene1

1Fenner School Of Environment And Society, Australian National University, Canberra, Canberra, Australia

The storage of carbon in the deeper layers of the soil offers opportunities for carbon sequestration because of potentially longer turnover times and longer storage times.  This presentation reports on an investigation into the processes controlling litter decomposition to a depth of 100cm in Red Chromosol soil and an Alluvial soil in south eastern Australia.  The aims of the investigation were (i) determine whether decomposition was slower in the deep soil and (ii), identify the mechanisms responsible.  A field based litter-bag study and a laboratory incubation experiment were both conducted to measure litter decomposition rates directly.

Litter bags of different plant and substrate materials were buried at depths of at 5, 10, 25, 50, 75 and 100cm.  Litter materials were plant materials (leaf, shallow roots and deep roots) from lucerne, Themeda, Phalaris.  Cotton tape was used as a benchmark material.  Litter bags were recovered at 75, 159, 275, 307 and 405 days after burial.

Litter decomposition was found to be slower at depth by a factor of 3.3- 6.3 depending on the plant species from which the litter was derived.  Variation in litter quality caused decomposition rates to decline by a factor of 2.2 to 4.2 between leaf litter and root materials.  Litter inputs to surface soils were higher in quality than those in deeper soils.  Soil edaphic constraints caused decomposition rates to vary by a factor of 1.5 between the surface and the deeper soils. This was primarily attributed to nitrogen deficiency and was largely ameliorated in the laboratory when soluble N was added.  Adding N did not account for all the differences between surface and subsoils.  Indirect evidence suggested that bulk density and oxygen deficiency may have been a factor.  Soil biological differences between surface and deep soils may have been were part of the deep soil constraint.

Biography: Brian Murphy has worked as a soil scientist for more than 30 years on the agricultural, conservation and environmental aspects of soil science.  His experience has included the production of several soil maps for major parts of NSW and he has been involved in research on the effects of land management practices on soil carbon and soil properties.  Brian has published a number of scientific papers and reports on a wide range of aspects concerning the measurement of soil organic carbon and the effects of land management and climate and soil type on the sequestration of carbon into soils.  He was an active member Soil Carbon Technical Working Group (TWG) for the Carbon Farming Initiative Methodology Development – Commonwealth Department of Environment.

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