Rapid abiotic transformation of nitrate in an acid forest soil

Nitrate immobilization into organic matter is thought to require catalysis by the enzymes of soil microorganisms. However, recent studies suggest that nitrate added to soil is immobilized rapidly and this process may include abiotic pathways. We amended living and sterilized soil with N-15-labeled n itrate and nitrite to investigate biotic and abiotic immobilization. We rep ort rapid transformation of nitrate in incubations of the O layer of forest soils that have been sterilized to prevent microbial activity and to denat ure microbial enzymes. Approximately 30, 40, and 60% of the N-15-labeled ni trate added to live, irradiated, or autoclaved organic horizon soil disappe ared from the extractable inorganic-N pool in less than 15 minutes. About 5 % or less of the nitrate was recovered as insoluble organic N in live and s terilized soil, and the remainder was determined to be soluble organic N. A dded N-15-nitrite, however, was either lost to gaseous N or incorporated in to an insoluble organic N form in both live and sterile organic soils. Henc e, the fate and pathway of apparent abiotic nitrate immobilization differs from the better-known mechanisms of nitrite reactions with soil organic mat ter. Nitrate and nitrite added to live A-horizon soil was largely recovered in the form added, suggesting that rapid conversion of nitrate to soluble organic-N may be limited to C-rich organic horizons. The processes by which this temperate forest soil transforms added nitrate to soluble organic-N c annot be explained by established mechanisms, but appears to be due to abio tic processes in the organic horizon.