As experimental units,we used 0.15 m inner diameter,0.45 m
tall PVC-tubes that were sealed with a watertight cap at the base
and filled with soil.The soil was field-collected top soil(fine sand;
15%clay,29%silt and56%sand;pH(H2O1:2,5)6.1,C content2.1%)
that was passed through a 4-mm sieve,mixed thoroughly,and kept
for 5 days at +70-80 C to eradicate any remaining earthworms
and their cocoons.It was then re-moistened to 20% gravimetric
water content in small batches and mixed into a single batch.In
total,twenty tubes were filled with soil close to the upper margin
by a standard tapping procedure to ensure a uniform soil bulk
density through out each column