SHORT-TERM VARIATIONS OF SOIL PHYSICAL-PROPERTIES AS A FUNCTION OF THE AMOUNTS AND C N RATIO OF DECOMPOSING COTTON RESIDUES .2. SOIL COMPRESSIBILITY, WATER-RETENTION AND HYDRAULIC CONDUCTIVITY/

There are conflicting reports on the role of soil microfauna and flora on the decomposition of incorporated crop residues and its effects on soil structure. This study examined the temporal changes in soil bulk density, compressibility and hydraulic properties as affected by the amounts of added cotton (Gossypium hirsutum L.) residues and carbon/ n itrogen (C/N) ratios. The work was carried out at constant temperature (25 degrees C) and soil water content (0.18 +/- 0.01 g g(-1)). All th e measured properties showed the same trend indicating a transient ''f lush'' of activity between the first and sixth week ofincubation, with a sharp maximum (or minimum) in the third week. After 6 weeks these p roperties again changed direction, and tended towards their initial va lue. The maxima (minima) are indicative of pore clogging or slime accu mulation lowering the hydraulic conductivity and increasing water rete ntion in the 20-75 cm suction range, increasing compressibility and lo wering tensile strength. These results are consistent with the model o f Hadas et al. (1994, Soil Tillage Res., 32:183-198). Most of the effe ct dissipated by Week 6 and was followed by some recovery, which may b e due to the activity of a microbial population that either acts more slowly than that responsible for the initial flush, or may have had it s initiation inhibited by the first population. Differences between tr eatments were very minor compared to the above temporal changes found in all the treatments. The sharp changes in properties within a short period after the start of incubation may explain why they were not det ected in the field by Rawitz et al. (1989, Final Rep. on BARD project 812, Hebrew Uni. Jerusalem; 1994, Soil Tillage Res., 32:347-366).