SOIL MATRIC POTENTIAL EFFECTS ON GROSS RATES OF NITROGEN MINERALIZATION IN AN ORTHIC FERRALSOL FROM KENYA

Gross rates of ammonification were measured over 9 days by N-15 isotope dilution techniques allied to a mathematical model in an Orthic Ferralsol from Kenya. Rates of mineralization in soils conditioned for 24 days at different constant gravimetric moisture contents increased from 0.08 mug-1 N g-1 d-1 for both ammonification and nitrification at -5.91 MPa to 2.12 mug N g-1 d-1 ammonification at -0.063 MPa. In soil which had been conditioned constantly at -5.91 MPa then wetted to -0.42 MPa, gross mineralization rates were immediately stimulated. Wetting a soil conditioned at -0.42 MPa had no effect on rates of gross mineralization over 9 days.