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Nitrogen Mineralization Kinetics with Different Soil Pretreatments and Cropping Histories¹

ABSTRACT

A laboratory study was conducted to determine how soil pretreatment (air-drying, freezing, and field-moist) and cropping history affects the kinetics of N mineralization. Although most researchers suggest that net N mineralization behaves according to first order kinetics, some have suggested that this process is better described with zero order kinetics. It was hypothesized that the air-drying pretreatment may be largely responsible for an N mineralization pattern which may be described by first order kinetics. Experimental data showed that net N mineralization followed a first order kinetics pattern regardless of pretreatment. However, the air-drying pretreatment enhanced N mineralization during the first 7 d of incubation. The traditionally used first order kinetics model (Model I) was extended to take into account an "easily" mineralized N fraction (N_e) mostly released during the first 7 d of incubation. This amended model (Model II) generally provided a much better fit to the experimental data, especially for the air-drying pretreatment. As a result of the flush of mineralized N during the first 7 d, the total quantity of N released with air-drying was considerably greater than with the freezing or field-moist pretreatments. Organic N was released in a linear manner over the 42-d incubation period and comprised about 10% of the total N released. Since this organic N would probably be mineralized, the total N leached was used to determine mineralizable N values (N_o) and rate constants (k). Mineralizable N values for bromegrass (Bromus inermis) and alfalfa (Medicago sativum) soils were generally higher than those for corn (Zea mays L.) whereas cropping history had no apparent effect on the k value. There were no significant effects of soil pretreatment on N_o or k values produced with Model II whereas differences occurred with Model I.

¹ Contribution from the Dep. of Land Resource Science, Univ. of Guelph, Guelph, Ontario, Canada, N1G 2W1.

² Professor, Graduate Students, and Research Technician, respectively, at the Univ. of Guelph. Dr. Reynolds is on educational leave from the Land Resource Research Centre, Agriculture Canada, Ottawa, Ontario, Canada, K1A 0C6.

Received for publication September 26, 1985.

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