|Mao, J - CHEMISTRY DEPT.,ISU|
|Schmidt-Rohr, K - CHEMISTRY DEPT., ISU|
Submitted to: ASA-CSSA-SSSA Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: November 6, 2003
Publication Date: November 6, 2003
Citation: MAO, J., OLK, D.C., SCHMIDT-ROHR, K. DETECTION OF AROMATIC-BONDED N AND ASSOCIATED LIGNIN RESIDUES IN A HUMIC FRACTION BY ADVANCED SOLID-STATE NMR. ASA-CSSA-SSSA PROCEEDINGS. 2003. CD-ROM. MADISON, WI. ASA-CSSA-SSSA. Technical Abstract: In tropical Asia irrigated lowland rice is cropped multiple times each year, yet in those field trials where initial yield levels approached the yield potential ceiling, grain yield has declined significantly after several years of such intensive cropping. Crop data indicate that the most likely cause is decreased availability of soil organic nitrogen (N), but the reasons for this have remained unclear. Through advanced solid-state nuclear magnetic resonance (NMR) spectroscopy, we detected agronomically significant amounts of organic N (150 kg N ha-1) bound covalently to aromatic rings in a humic acid fraction extracted from a continually submerged, triple-cropped rice soil. Quantitative 13C NMR combined with advanced spectral editing showed that this humic acid is rich in lignin derivatives (more than 45% of all carbon). The chemical shift of the N-bonded aromatic carbons and the peak intensities indicate that these signals are due to amide groups bonded to lignin aromatic rings. In contrast, the same humic acid fraction extracted from an aerobic rice soil with less lignin content showed peak intensities more characteristic of easily degradable peptides, and less N bonded to aromatic carbons. Results are the first direct evidence for covalent binding of soil organic N by lignin residues under field conditions, and they provide a mechanistic explanation for the yield decline.