Assessing Indices for Predicting Potential Nitrogen Mineralization in Soils under Different Management Systems

A reliable laboratory index of N availability would be useful for making N recommendations, but no single approach has received broad acceptance across a wide range of soils. We compared several indices over a range of soil conditions to test the possibility of combining indices for predicting potentially mineralizable N (N0). Soils (0-5 and 5-15 cm) from nine tillage studies across the southern USA were used in the evaluations. Long-term incubation data were fit to a first-order exponential equation to determine N0, k (mineralization rate), and N0* (N0 estimated with a fixed k equal to 0.054 wk-1). Out of 13 indices, five [total C (TC), total N (TN), N mineralized by hot KCl (Hot_N), anaerobic N (Ana_N), and N mineralized in 24 d (Nmin_24)] were strongly correlated to N0 (r > 0.85) and had linear regressions with r2 > 0.60. None of the indices were good predictors of k. Correlations between indices and N0* improved compared with N0, ranging from r = 0.90 to 0.95. Total N and flush of CO2 determined after 3 d (Fl_CO2) produced the best multiple regression for predicting N0 (R2 = 0.85) while the best combination for predicting N0* (R2 = 0.94) included TN, Fl_CO2, Cold_N, and NaOH_N. Combining indices appears promising for predicting potentially mineralizable N, and because TN and Fl_CO2 are rapid and simple, this approach could be easily adopted by soil testing laboratories.