590 0038-075C/02/16709-590–606 September 2002 Soil Science Vol. 167, No. 9 Copyright © 2002 by Lippincott Williams & Wilkins, Inc. Printed in U.S.A. I N recent decades, increasing population density in tropical Asia has prompted intensification of irrigated lowland rice cropping, with two or three crops planted annually. The sustainability of such intensive cropping has been called into question (Cassman et al., 1995). Attention has focused on possible changes in the chemical nature of soil or- ganic matter (SOM) and in nitrogen (N) cycling occurring under the submerged soil conditions that are associated with intensive cropping.In our previous work on two extracted humic acid (HA) fractions, incompletely decomposed compounds were found to accumulate more in intensively cropped soils than in aerated soils. The accumula- tion was demonstrated for specific compounds using a variety of chemical analyses: phenolic car- bon (C) by Fourier transform infrared (FT-IR) spectroscopy and 13 C nuclear magnetic resonance (NMR) spectroscopy (Olk et al., 1996, 1998, 2000); amide N by 15 N NMR (Mahieu et al., 2000a) and FT-IR (Olk et al., 2000); diester phos- phorus (P) forms by 31 P NMR (Mahieu et al., ACCUMULATION OF LIGNIN RESIDUES IN ORGANIC MATTER FRACTIONS OF LOWLAND RICE SOILS: A PYROLYSIS-GC-MS STUDY D.C. Olk 1 , M.C. Dancel 2 , E. Moscoso 3 , R.R. Jimenez 3 , and F.M. Dayrit 2 1 National Soil Tilth Laboratory, USDA-ARS, 2150 Pammel Drive, Ames, IA 50011. Dr. Olk is corresponding author. E-mail: olk@nstl.gov 2 National Chemistry Instrumentation Center, Department of Chemistry, Ateneo de Manila University, Loyola Heights, Quezon City 1108, Philippines. 3 International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines Received Dec. 13, 2001; accepted May 29, 2002. DOI: 10.1097/01.ss.0000031728.12196.de In tropical Asia, multiple annual cropping of lowland rice and the as- sociated submerged soil conditions have been linked to long-term changes in nitrogen (N) cycling and the chemical nature of soil organic matter. To identify changes in organic matter properties, two humic acid fractions and whole soil samples were obtained from field treatments of lowland rice that varied in cropping intensity, fertilizer management, and location. These samples were methylated and analyzed by pyrolysis-gas chromatography-mass spectrometry. With compounds expressed in rela- tive abundance, whole soil was enriched in nonmethoxybenzene com- pounds and heterocyclic N compounds compared with extracted humic acids. The young mobile humic acid (MHA) fraction had a wide diversity of methoxybenzenes that are associated with lignin residues. The more recalcitrant calcium humate (CaHA) fraction had characteristics interme- diate between whole soil and MHA. Under intensified cropping and in- creased soil submergence, lignin residues increased in relative abundance in all three fractions. Heterocyclic N compounds decreased with intensi- fied cropping, consistent with previous analysis by 15 N nuclear magnetic resonance spectroscopy. Their parent compounds may be primarily nat- urally occurring heterocyclic N compounds. For whole soil, and especially the MHA, submergence effects were accentuated in treatments with high fertilizer rates, trends that may be related to the balance between input rates and degradation rates of crop residues. The ratio of myristic acid: stearic acid varied with soil submergence, fertilizer rate, and type of frac- tion in patterns following the abundance of methoxybenzenes. In general, responses of the MHA and CaHA to field treatments were representative of whole soil. (Soil Science 2002;167:590–606) Key words: Pyrolysis, soil organic matter, phenols, rice, humification.