SHORT-TERM VARIATIONS OF SOIL PHYSICAL-PROPERTIES AS A FUNCTION OF THE AMOUNT AND C N RATIO OF DECOMPOSING COTTON RESIDUES .1. SOIL AGGREGATION AND AGGREGATE TENSILE-STRENGTH/

Addition of plant residues to soil activates a heterogeneous microbial population which in turn affects soil physical properties. A model of residue decomposition rate that is hierarchical and sequential is pro posed to explain qualitatively the changes in soil structural paramete rs which are the result of these decomposition processes. Cotton (Goss ypium hirsutum L.) stalks were incubated in soil and the changes in ag gregate geometric mean diameter (GMD), geometric standard deviation, t ensile strength and characteristic tensile strength were determined fo r the following treatment combinations of residue amount added, carbon /nitrogen (C/N) ratio and time of incubation: 0, 4 and 8 Mg ha(-1) of residue added; 0 and 150 kg ha(-1) of elemental N applied as urea; inc ubation periods of 1, 3, 6 and 12 weeks. The experiment contained trea tments designated as: control (no additions); low residues, no N; low residues, +N; high residues, no N; high residues, +N. The C/N ratios w ere 12 in the control soil, and 350-400, 20-24, 350-400 and 40-50 resp ectively, in the admixtures. An increase in the GMD of aggregates was observed by the end of the first week after a normal amount of residue was applied. This was apparently caused by a flush of fungal activity . However, when a large amount of residue was added, an increase in CM D was observed only after the sixth week, and is attributed mainly to bacterial activity. The same patterns were observed for aggregate tens ile strength. It is suggested that the size and strength of aggregates apparently caused by fungi increased during the first week due to ext ernal reinforcement by hyphae, whereas the changes appearing only afte r the sixth week and attributed to bacteria are due to internal reinfo rcement by bacterial secretions. The temporal variations of GMD are co ntrolled by the amount of residue added and N availability. An immedia te effect on GMD and strength was observed upon addition of residues w ith a high C/N ratio, but this effect only persisted for a short perio d of 1-3 weeks. This response pattern may be developed into a short-te rm management option to mitigate soil crusting or sealing in a newly s own field by an anticipated irrigation or rainfall by the timely appli cation of high C/N ratio residues. Conversely, large amounts of residu e with a moderate C/N ratio may help stabilize the GMD for longer peri ods. Proper management of residue amounts and C/N ratio can thus provi de the farmer with some measure of control over soil structure as expr essed by GMD.