Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 29, 2002
Publication Date: September 16, 2002
Citation: OLK, D.C., DANCEL, M.C., MOSCOSO, E., JIMENEZ, R.R., DAYRIT, F.M. ACCUMULATION OF LIGNIN RESIDUES IN ORGANIC MATTER FRACTIONS OF LOWLAND RICE SOILS: A PYROLYSIS-GC-MS STUDY. S0IL SCIENCE. 2002. V. 167(9). P. 590-606.
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. At issue is whether soil nitrogen (N) cycling is impaired by changes in the chemical nature of soil organic matter under the submerged soil conditions that accompany intensive cropping. In this study, the chemical natures of several soil organic matter fractions were analyzed for long-term field experiments in the Philippines that differed by the number of annual rice crops. Confirming previous studies, we found an accumulation of phenol compounds in the most submerged soils, those that supported two or three annual rice crops. Under laboratory conditions, phenols can stabilize N into forms unavailable to plants, suggesting a potential link between cropping intensity, organic matter chemistry and N availability. The degree of phenol accumulation was greater for higher rice yields. These results will be integrated with subsequent studies of interactions between phenol levels and N cycling to evaluate whether accumulated phenols can inhibit N uptake by rice, especially at high yield levels. Successful results would enable identification of management options for maintaining the sustainability of intensive rice cropping. The methods developed here are pertinent to other cropping systems with key characteristics similar to paddy rice, such as continuous rice in the U.S. South and no-till corn in the Upper Midwest.
In tropical Asia multiple annual cropping of lowland rice and the associated submerged soil conditions have been linked to long-term changes in nitrogen (N) cycling and the chemical nature of soil organic matter. Continuing our studies on organic matter quality, we obtained two humic acid fractions and whole soil samples from field treatments of lowland rice which varied in cropping intensity, fertilizer management, and location. These samples were methylated and analyzed by pyrolysis- gas chromatography-mass spectrometry. With compounds expressed in relative abundance, whole soil was enriched in non-methoxybenzene compounds and heterocyclic N compounds compared with extracted humic acids. The young mobile humic acid (MHA) fraction had a wide diversity of methoxybenzenes, i.e. lignin residues. The more recalcitrant calcium humate (CaHA) fraction had characteristics intermediate between whole soil and MHA. Under intensified cropping, lignin residues increased in relative abundance in all three fractions. Heterocyclic N compounds decreased with intensified cropping, consistent with previous analysis by **15N nuclear magnetic resonance spectroscopy. Their parent compounds may primarily be naturally occurring heterocyclic N compounds. For whole soil and especially the MHA, submergence effects were accentuated in treatments with high fertilizer rates. The ratio of myristic acid:stearic acid varied with soil submergence, fertilizer rate, and type of fraction in patterns following the abundance of methoxybenzenes. In general, responses of the MHA and CaHA to field treatments were representative of whole soil. Pyrolysis is most useful when used in conjunction with other analytical methods.