Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 29, 2004
Publication Date: April 27, 2004
Citation: Schmidt-Rohr, K., Mao, J.D., Olk, D.C. 2004. Nitrogen-bonded aromatics in soil organic matter and their implications for a yield decline in intensive rice cropping. Proceedings Of The National Academy Of Sciences. 101(17):6351-6354.
Interpretive Summary: Increasing population pressure in tropical Asia has promoted multiple annual cropping of irrigated lowland (i.e., paddy) rice as the principal mode of rice production. The sustainability of such intensification has been questioned because of impaired availability of soil nitrogen (N). One possible cause of this problem is changes in the chemical nature of soil organic matter under the submerged soil conditions that accompany intensive cropping. In this study, the chemical nature of soil N was studied by newly developed analytical techniques for a soil organic matter fraction extracted from two long-term field experiments in the Philippines. In a continually submerged, triple-cropped rice soil we found an accumulation of slowly decomposing material in the organic matter fraction. Soil nitrogen compounds in this fraction were shown to be tightly bound and accounted for about 25% of all N, representing an agronomically significant quantity (90 pounds per acre). The same organic matter fraction of a long-term field that had been maintained under aerobic conditions had a lower proportion (10%) of this stabile N. Results will help scientists improve soil N availability for crop uptake in cropping systems that involve anaerobic decomposition of crop residues, such as continuous rice in the U.S. South and no-tillage corn in the upper Midwest. Farming practices that avoid formation of this slowly available N pool, will allow rice farmers to achieve more efficient use of soil N and enhance environmental quality.
Significant amounts of nitrogen bonded to aromatic rings were detected by **14N-**13C solid-state nuclear magnetic resonance (NMR) spectroscopy in a humic acid fraction extracted from a continually submerged, triple-cropped lowland rice soil. Quantitative **13C NMR combined with spectral editing shows that this humic acid is rich in lignin derivatives (>45% of all carbon). The chemical shift of the nitrogen-bonded aromatic-carbon and the relatively high NCO:NCH signal ratio indicate that these signals are due to amide groups bonded to a significant fraction of the lignin aromatic rings. In contrast, the corresponding humic acid fraction extracted from an aerobic dryland rice soil with less lignin content showed predominantly NCO and NCH signals that are characteristic of easily degradable peptides, as well as some heterocyclic N. Nitrogen species bonded to aromatics are considered as not readily plant available, and the accumulation of N bound to lignin residues in the triple-cropped soil coincided with a substantial long-term decline in grain yield that has been attributed to decreased availability of soil N.