Diurnal changes in moisture content and isothermal and thermally induced moisture fluxes under n-tillage and c-tillage in Nigeria

Abstract A study on the diurnal changes of soil moisture content and on the isothermal and thermally induced moisture fluxes was conducted on an Alfisol at the International Institute of Tropical Agriculture (IITA), Ibadan, Nigeria, on no-tillage and conventional-tillage plots. These studies were conducted during the 1980 dry season, 9 years after initiating the tillage treatments in 1971. Three bare 5 × 5 m 2 plots per treatment were used to study diurnal changes in moisture content as soil dried 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 four 7 day periods at weekly intervals. These results, along with physical 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 moisture 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). The amplitude of the diurnal cycle of water content changes decreased with depth, and was superimposed on a progressive depletion in water content in the layers studied. The first layer of the conventional-tillage treatment dried to a lower water content than that of the no-tillage treatment. Partition of moisture fluxes, induced by isothermal and thermal conditions, showed that isothermal liquid flux was dominant in no-tillage, and that thermal vapor flux was very important as soil dried in conventional-tillage. The direction of the fluxes observed (i.e. isothermal liquid flux always being positive upwards and thermal vapor flux positive during the night and negative downwards during the day) was of critical importance as the soil dried. The liquid fluxes became less important and thermal vapor and probably isothermal vapor fluxes became more important with soil moisture depletion. Vapor movement under these circumstances may have played an important role in supplying water to roots both during the day (deep roots) and night (shallow roots) depending on the magnitude of the fluxes. Vapor fluxes were higher and started earlier in conventional-tillage than in no-tillage.