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Abstract Variation in the rate of N mineralization in a Yolo soil profile was studied using in vitro incubation methods. Negligible amounts of NH 4 ‐N were recovered from leachates, indicating that the rate of N mineralization was the primary factor controlling N availability. During a 13‐week incubation at 25°C, 42% of the total estimated N mineralized was derived from the surface soil (0 – 18 cm), whereas 58% was contributed from the 18‐ to 108‐cm depths. Cumulative N mineralization in the 0‐ to 18‐cm and 18‐ to 36‐cm depth samples followed first‐order kinetics and was linear with respect to the square root of the incubation time. The mineralization rate constant differed more than twofold between the 0‐ to 18‐ and the 18‐ to 36‐cm depth samples. At lower depths (36 – 72 and 72 – 108 cm) the relationship between cumulative N mineralization and the square root of time was curvilinear. Interactive effects of soil temperature and moisture were also examined in an experiment with four incubation temperatures (15, 20, 25, and 30°C) in factorial combination with six soil moisture levels (0.1, 0.3, 0.7, 2, 4, and 10 bars). There was a significant moisture × temperature interaction; N mineralization increased above that expected from additive effects at 30°. Multiple regression was used to generate an equation that predicted net N mineralization as a function of soil temperature and moisture. The apparent effect of soil water content on N mineralization depended on experimental procdure. When soil water content was varied by adding water to air‐dry soil without complete equilibration, N mineralization declined linearly with water content. In contrast, there was a sharp decline between 0.3‐ and 2‐bar treatments and a more gradual decline at higher matric suctions when soil was equilibrated by pressure‐membrane desorption before incubation. These results indicate that both quantity and distribution of soil water affect in vitro estimates of N mineralization.
Cassman et al. (Sat,) studied this question.