Transformations of chemically fixed liquid anhydrous ammonia by soil microorganisms

The application of liquid anhydrous NH₃ to soil leads to chemical fixation of NH₃ by organic matter and of NH_inf4^sup+by clay minerals. A laboratory study was conducted to ascertain the biological transformations of newly fixed liquid anhydrous ¹⁵NH₃ in a Drummer silty clay loam by incubation of the ¹⁵N-labelled soil with glucose for 0, 7, 30, and 90 days and by sequential extraction of organic-matter-fixed ¹⁵NH₃ with 0.15 M Na₄P₂O₇, 0.15 M KOH, 0.1 M NaOH, and acidified dimethyl sulfoxide. About 16% of the ¹⁵NH₃ injected was fixed, of which 52% was accounted for by clay fixation. The various humic fractions (fulvic acid, humic acid, and humin) were obtained, and the distribution patterns of the fixed ¹⁵NH₃-N in these fractions were compared. The potential availability of the fixed ¹⁵NH₃-N was also estimated. The percentage of the ¹⁵NH₃ recovered as organic-matter-fixed ¹⁵NH₃ decreased as the length of incubation increased (to 28% after 90 days); the decrease was attributed in part to an increase in the amount recovered as clay-fixed NH_inf4^sup+(from 52 to 64%). Changes in the distribution of the organic-matter-fixed ¹⁵NH₃-N in the humic fractions included: (1) an increase in the relative amount of the fixed ¹⁵NH₃ as humic acid in both the Na₄P₂O₇ and KOH extracts, (2) an increase in the percentage of organic-matter-fixed ¹⁵NH₃-N in the fulvic acid fractions as high-molecular-weight components (determined by dialysis) or as generic fulvic acid (determined by sorption-desorption from XAD-8 resin), and (3) an increase in the percentage of the organic-matter-fixed ¹⁵NH₃ as humin. The potential availability of the organic-matter-fixed ¹⁵NH₃-N decreased as the length of the incubation increased, from 22 to 4% over the 90-day incubation period, and was correlated significantly (0.05 level) with Na₄P₂O₇-extractable N. These results suggest that organic-matter-fixed liquid anhydrous NH₃ is initially more labile than the native soil N but becomes less labile with time.