Evaluation of Alkaline Hydrolyzable Organic Nitrogen as an Index of Nitrogen Mineralization Potential of Some Coastal Savannah Soils of Ghana

Numerous biological and chemical methods have been proposed over the years for estimating the nitrogen (N) mineralization capacity of soils; however, none of them has found general use in soil fertility testing. The efficacy of a recently proposed alkaline hydrolysis method for assessing N availability in soils compared with the standard long-term incubation technique for determining potentially available N was evaluated. The nitrogen mineralization of 12 surface soils incubated under aerobic conditions at 25 °C for 26 weeks was determined. Field-moist soils were direct-steam distilled with 1 M KOH or 1 M NaOH; the NH₃ released was trapped in boric acid, and its concentration was determined successively every 5 min for 40 min. The cumulative N mineralized or hydrolyzed was fitted to the first-order exponential equation to determine the potentially mineralizable N (N_o) and an analogous “potentially hydrolyzable N (N_max)” for the soils. The flush of CO₂ (fCO₂) following the rewetting and incubation of air-dried soils under aerobic conditions for 3 days was also determined. The results showed that the N_max values differed considerably among the soils, indicating differences in the chemical nature and reactivity of the organic N content of the soils, and were significantly correlated with N_o and fCO₂ values. The estimated N_max and N_o values ranged from 105 to 371 mg N kg⁻¹ and 121 to 292 mg kg⁻¹, respectively. Based on the simple and inexpensive nature of the alkaline hydrolysis procedure, the reduction in the incubation time required to obtain N_o (months to minutes), and the strong association between N_max and N_o, we concluded that N_max is a good predictor of the biologically discrete and quantifiable labile pool of mineralizable soil organic N (ON), and the use of the alkaline hydrolyzable ON as a predictor of N_o merits consideration for routine use in soil testing laboratories for estimating the N-supplying capacity of soils.