Phenol accumulation in a young humic fraction following anaerobic decomposition of rice crop residues

Authors: Olk, Daniel - Dan; JIMENEZ, R - INTN'L RICE RESEARCH INST; MOSCOSO, E - INTN'L RICE RESEARCH INST; GAPAS, P - INTN'L RICE RESEARCH INST

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2008
Publication Date: 5/1/2009
Citation: Olk, D.C., Jimenez, R.R., Moscoso, E., Gapas, P. 2009. Phenol accumulation in a young humic fraction following anaerobic decomposition of rice crop residues. Soil Science Society of America Journal. 73(3):943-951.

Interpretive Summary: Rice production in many regions has evolved to include two or three crops of paddy rice each year to supply adequate food for the increasing population. However, the long-term sustainability of this practice has been questioned, because several studies have shown decreased yields of rice grain after 20 to 30 years of continuous cropping. We suspected that nearly continuous anaerobic (without air) decomposition of the rice roots and straw resulted in the accumulation of organic molecules known as phenols, and that these molecules were binding with the soil nitrogen (N) making it unavailable to the plants. We then measured the abundance of phenols in soil during a 4-year field experiment that evaluated crop rotation, N fertilizer rate, and timing of soil tillage. As expected, the greatest accumulation of phenols occurred when continuous, highly-fertilized rice roots and straw decomposed under anaerobic conditions. Farmers in the Philippines can minimize the problem by either (i) promoting aerobic (with air) decomposition of the rice roots and straw, or (ii) rotating rice with a maize crop since this allows the soil to dry and therefore the rice residues decompose aerobically. These results are also important for scientists and producers in the U.S. where anaerobic decomposition of rice roots and straw in the Southern U.S. and no-tillage of corn roots and straw in the Upper Midwest have been reported to reduce N availability to the various crops.

Technical Abstract: Soil phenols have been implicated as inhibitors of soil nitrogen (N) cycling within many ecosystems, including irrigated lowland rice (Orzyza sativa, L.) in the Philippines. We measured soil phenol concentrations in two humic fractions at two crop growth stages in each season during a 4-year field study at the International Rice Research Institute (Philippines) to quantify the effects of crop management on soil phenol accumulation in lowland rice soils. Samples were collected from two crop rotations [continuous rice and a rice-maize (Zea mays L.) double-crop] with two N fertilizer rates (0 and non-limiting), and with either aerobic or anaerobic decomposition of incorporated crop residues. Phenols were determined by tetramethylammonium hydroxide thermochemolysis. We found a gradual enrichment of phenols in the mobile humic acid fraction during the 4 years of continuous rice cropping with non-limiting N fertilizer and anaerobic decomposition of crop residues. The level of enrichment varied among phenol compounds, developing the fastest and becoming most pronounced with the smaller molecules of molecular weight 168 or less. Anaerobic decomposition had less effect on phenol enrichment for continuous rice cropping without N fertilizer. No phenol enrichment was found with anaerobic decomposition of rice residues in the rice-maize rotation. Our results are consistent with previous studies showing inhibited N mineralization under anaerobic decomposition of continuous rice with non-limiting rates of N fertilizer. Rotation of maize with rice or other techniques to ensure aerobic decomposition of crop residues may help mitigate or prevent phenol accumulation.