Indexes of plant available organic nitrogen in soils

Two sets of surface soils (13 from Iowa and 7 from Chile) were used in greenhouse experiments to study the uptake of organic N by three successive croppings of corn (Zea mays, L.) at 40-day intervals or three cuttings of ryegrass (Lolium multiflorum L.) at 30-day intervals. Before planting, the soils were treated with all the macro- and micronutrients, except N. After harvest, the plant materials were dried at 65(DEGREES)C, weighed, and analyzed for total N;Three biological and 26 chemical methods were evaluated as indexes of plant response to available organic N in soils. Results showed that the values obtained by incubation of soils under aerobic or water logged conditions were significantly correlated with DM and N yields. The amount of NH(,4)-N released by the following methods were significantly correlated with N uptake by corn and ryegrass: autoclaving soil with 10 mM CaCl(,2), digestion with phosphoric acid-sodium chromate solution, digestion with 2M KCl solution at 100(DEGREES)C for 20 h, or by direct steam distillation for 5 min with 1 M NaOH, 1 M KOH, 1 M LiOH, 4 M K(,2)CO(,3), or Na(,3)PO(,4) + Na(,2)B(,4)O(,7) buffer;Nitrogen mineralization and, therefore, plant uptake of N, was significantly correlated with organic C and organic N in soils;Calculated potentially mineralizable N (N(,o)) in soils varied with incubation temperature, reagent used to hydrolyze soil N, and the N uptake by plants according with the plant used;The N(,o) values calculated by using either the exponential or reciprocal plot equation for the results obtained from the leaching columns were not correlated with dry matter or N yields by plants. However, the N(,o) values obtained for the results from hydrolysis with 2 M KCl or Na(,3)PO(,4)-Na(,2)B(,4)O(,7) buffer were significantly correlated with the N(,o) values obtained from the leaching column (30(DEGREES)C)