Nitrogen Mineralization Kinetics with Different Soil Pretreatments and Cropping Histories

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 (Nₑ) 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 (Nₒ) 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 Nₒ or k values produced with Model II whereas differences occurred with Model I.