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 proposed to explain qualitatively the changes in soil structural parameters which are the result of these decomposition processes. Cotton (Gossypium hirsutum L.) stalks were incubated in soil and the changes in aggregate geometric mean diameter (GMD), geometric standard deviation, tensile strength and characteristic tensile strength were determined for 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; incubation periods of 1, 3, 6 and 12 weeks. The experiment contained treatments designated as: control (no additions); low residues, no N; low residues, +N; high residues, no N; high residues, +N. The C/N ratios were 12 in the control soil, and 350-400, 20-24, 350-400 and 40-50 respectively, 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 CMD was observed only after the sixth week, and is attributed mainly to bacterial activity. The same patterns were observed for aggregate tensile strength. It is suggested that the size and strength of aggregates apparently caused by fungi increased during the first week due to external reinforcement by hyphae, whereas the changes appearing only after the sixth week and attributed to bacteria are due to internal reinforcement by bacterial secretions. The temporal variations of GMD are controlled by the amount of residue added and N availability. An immediate effect on GMD and strength was observed upon addition of residues with a high C/N ratio, but this effect only persisted for a short period of 1-3 weeks. This response pattern may be developed into a short-term management option to mitigate soil crusting or sealing in a newly sown field by an anticipated irrigation or rainfall by the timely application of high C/N ratio residues. Conversely, large amounts of residue with a moderate C/N ratio may help stabilize the GMD for longer periods. Proper management of residue amounts and C/N ratio can thus provide the farmer with some measure of control over soil structure as expressed by GMD.