Stabilization of fertilizer nitrogen-15 into humic substances in aerobic vs. waterlogged soil following straw incorporation

This study was undertaken to investigate and quantify the interactive effects of flooding and straw incorporation on key microbial processes, principally stabilization of fertilizer N into various soil organic matter (SOM) pools. The fate of fertilizer 15N in a paddy soil was examined at 5, 15, and 25 degrees C, with and without rice (Oryza sativa L.) straw added, and under flooded and nonflooded conditions. After a 160-d incubation, three fractions of the SOM were separated and defined as directly alkali-extractable humic substances (DAEHS), reducible metal-bound humic substances (RMBHS), and non-alkali-extractable organic matter (NAEOM). The DAEHS had the highest percentage, up to 50%, of fertilizer 15N recovered at 160 d, indicating that this SOM fraction was the most dynamic fraction of the SOM. On the other hand, the RMBHS is considered the least dynamic pool, containing up to 12% fertilizer 15N after 160 d. The NAEOM was surprisingly highly enriched, up to 28% fertilizer 15N, and showed a significant treatment effect, suggesting that some active components of N cycling were present in this SOM fraction. The addition of rice straw increased the recovery of fertilizer 15N in the above SOM fractions. Flooding significantly reduced the stabilization of fertilizer N compared with the nonflooded treatment. Indices of recalcitrance of the stabilized N confirm that the soil N supply capacity does not decrease with flooding. The total alkali-extractable organic matter (AEOM = DAEHS + RMBHS), as the NAEOM, appears to be a complex and dynamic mixture of potentially mineralizable and recalcitrant forms of N. Our data show that long-term N availability and stabilization into humic fractions is a function of rice residue input and temperature; however, the effects of residue and temperature are inversely related. With increase in temperature of incubation, less fertilizer N becomes stabilized into humic fractions, presumably from increased microbial activity, microbial consumption of potential humic precursors (N-containing precursors of humic substances turned over faster at higher temperatures), and/or formation of different end-products with less humification potential.