Decomposition of 14C-labelled plant material in soil: The influence of substrate location, soil compaction and earthworm numbers

Abstract

The influences of substrate location and of earthworm number on the dynamics of decomposition of 14C-labelled plant material, freshly-added to soil, were measured in laboratory studies using different conditions of soil compaction. The labelled plant material (mature leaves of Trifolium subterraneum), was added to columns of soil compacted at three levels (200, 400 and 600 kPa), and was located either as litter on the soil surface, or was mixed with the top layer of soil, or mixed with soil at depth. Earthworms were either absent, or soils contained either 4 or 7 earthworms ( Aporrectodea trapezoides) per column. The release of CO 2- 14C from these soil columns was recorded over a period of 104 days. Total recoveries of 14C in soil plus evolved CO 2 exceeded 90% in all treatments measured, except when the soil was highly compacted and the plant material was buried at depth. Under these latter conditions, earthworm addition increased the total 14C recoveries, which ranged from 73 to 88% of input 14C. Lag periods before maximal rates of 14CO 2 evolution were evident in treatments with surface-applied litter, and to a lesser extent, with plant material incorporated at depth. In this latter treatment, lag time was highest in soil of high compaction, but was decreased with increasing earthworm numbers. In all other treatments lag time appeared to be unaffected by earthworm numbers and by level of soil compaction. Maximal rates of 14CO 2 release generally increased with soil compaction but exhibited no consistent trend with earthworm numbers. A clear exception to the general pattern occurred when plant material was incorporated at depth in highly compacted soil. In this treatment maximal rates were relatively very low, but increased with earthworm numbers. The influence of the imposed treatments on the total extent of 14C evolution reflected closely their effects on maximal rates of 14CO 2 release. Effects of either soil compaction or substrate location were most demonstrable with treatments which likely limited the aerobic processes of decomposition. In such cases, the marked effects of the presence of earthworms and their number probably depended on the favourable effects of their casts and their burrows in removing physical constraints, both on aerobic decomposition per se of added plant material and also on the efficiency of measurement of evolved gaseous metabolic products. Thus, earthworm influence appeared to be indirect, through the interaction of their activities, soil compaction and substrate location.