STUDIES ON NITROGEN MINERALIZATION PROPERTIES OF TOKACHI SOILS BY KINETIC-ANALYSIS

Nitrogen mineralization properties of major soils in the Tokachi district of Hokkaido in northern Japan were investigated by the kinetic analytical method. Soil samples collected in spring and autumn from plots with tillage experiments consisting of a Brown Andosol (U.S. Soil Taxonomy: Typic Hapludands) and Gleyic Cumulic Andosol (Typic Meranaquands), as well as from a Brown Lowland soil (Typic Udifulvents) were incubated at 20, 25, and 30-degrees-C for 196-441 d and the time course of nitrogen mineralization was analyzed. In some soils, the time course of nitrogen mineralization fitted the first order reaction model, while in others it fitted the zero order reaction model. There was no distinct relationship between the nitrogen mineralization parameters (NMPs) and the soil properties. Since the cumulative amounts of mineralized nitrogen of all the studied soils showed a linear relation with the incubation time up to the number of days transformed at standard temperature (DTS) in a crop growing season, the data of the 0, 2, 4, and 8 week incubation experiments were analyzed by the zero order reaction model, which enabled to define the nitrogen mineralization properties of each soil type. The Brown Andosol showed a smaller k value (mineralization rate constant) at 25-degrees-C (0.241-0.303 mg kg-1 d-1) and an average E(a) value (apparent activation energy) (3,270-4,610 J mol-1) in conventional tillage. The Gleyic Cumulic Andosol exhibited a larger k value at 25-degrees-C (0.250-0.379 mg kg-1 d-1) and a larger E(a) value (4,350-5,040 J mol-1) in conventional tillage, while the Brown Lowland soil showed a larger k value at 25-degrees-C (0.330-0.423 mg kg-1 d-1) and a smaller E(a) value (3,010-3,300 J mol-1). Also, reduced tillage resulted in a larger k value for the Brown Andosol compared with the conventional tillage. No significant difference in the NMPs between the spring samples and autumn samples was detected.