Estimates of Potentially Mineralizable Soil Nitrogen Based on Short-term Incubations

Nitrogen mineralization potentials were determined for a large number of soils by a method involving determination of N mineralized after several consecutive incubations at 35C under optimum soil water conditions. The determination of N mineralization potential, No, based on the first-order equation, log (No — Nt) = log No — kt/2.303, is laborious and usually requires incubation periods of 8 weeks or more. From the present study, involving soils from major agricultural areas throughout the United States, it was demonstrated that No could be estimated reliably from the amounts of N mineralized during 2-week incubations following preliminary incubations of 1 to 2 weeks. From the above first-order equation, No = Nt / (1-10–kt/2.303). Hence, for a 2-week incubation (t = 2), No = 9.77Nt (Nt = N mineralized in 2 weeks and k is the rate constant, weeks-1). Estimates of No from short-term incubations were similar to those derived after extensive periods of incubation. Preincubation of soils is required in order to decompose plant residues and for other possible reasons noted. Estimates of No from preliminary incubations are meaningless. The implications of No as a basis for predicting amounts of soil N mineralized under fluctuating temperature and soil water conditions are discussed