DIURNAL CHANGES IN MOISTURE-CONTENT AND ISOTHERMAL AND THERMALLY-INDUCED MOISTURE FLUXES UNDER N-TILLAGE AND C-TILLAGE IN NIGERIA

A study on the diurnal changes of soil moisture content and on the iso thermal and thermally induced moisture fluxes was conducted on an Alfi sol at the International Institute of Tropical Agriculture (IITA), Iba dan, Nigeria, on no-tillage and conventional-tillage plots. These stud ies were conducted during the 1980 dry season, 9 years after initiatin g the tillage treatments in 197 1. Three bare 5 X 5 m2 plots per treat ment were used to study diurnal changes in moisture content as soil dr ied from the initial moisture status of field capacity. The latter was attained by excessive and deep irrigation. Moisture content, moisture potential, and soil temperature were monitored three times a day (08: 00, 14:00 and 18:00 h) at the depths of 0-7, 7-14 and 14-21 cm for fou r 7 day periods at weekly intervals. These results, along with physica l characterization of the soil profile and changes in air temperature, were used to calculate isothermal, thermally induced liquid and vapor fluxes. Results showed that there was a general increase in soil mois ture content with sampling depth during the night (18:00 to 08:00 h), and a general decrease with depth during the day (08:00 to 18:00 h). T he amplitude of the diurnal cycle of water content changes decreased w ith depth, and was superimposed on a progressive depletion in water co ntent in the layers studied. The first layer of the conventional-tilla ge treatment dried to a lower water content than that of the no-tillag e treatment. Partition of moisture fluxes, induced by isothermal and t hermal conditions, showed that isothermal liquid flux was dominant in no-tillage, and that thermal vapor flux was very important as soil dri ed in conventional-tillage. The direction of the fluxes observed (i.e. isothermal liquid flux always being positive upwards and thermal vapo r flux positive during the night and negative downwards during the day ) was of critical importance as the soil dried. The liquid fluxes beca me less important and thermal vapor and probably isothermal vapor flux es became more important with soil moisture depletion. Vapor movement under these circumstances may have played an important role in supplyi ng water to roots both during the day (deep roots) and night (shallow roots) depending on the magnitude of the fluxes. Vapor fluxes were hig her and started earlier in conventional-tillage than in no-tillage.