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Title: Soil N cycling and phenols accumulation under continuous rice cropping in the Grand Prairie region, Arkansas

item Olk, Daniel - Dan

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2008
Publication Date: 5/1/2009
Citation: Olk, D.C., Anders, M.M., Filley, T.R., Isbell, C. 2009. Soil N cycling and phenols accumulation under continuous rice cropping in the Grand Prairie region, Arkansas. Soil Science Society of America Journal. 73(3):952-960.

Interpretive Summary: Many soil physical properties have been severely degraded in the Grand Prairie region of eastern Arkansas where farmers have grown rice and soybean in alternate years. Continuous rice production promises to solve the soil problems, but monoculture grain yields are lower than for the rotated rice. We proved that the yield loss with continuous rice was caused by decreased plant availability of soil nutrients. Producing continuous rice resulted in the accumulation in soil of organic compounds that bind soil nutrients and make them less available to plants. By understanding the cause for lower rice yield, we identified management options for farmers so that rice can be grown every year to improve soil properties without losing grain yield. These results will improve the quality of local soils and benefit farmers who wish to grow high-yielding rice every year.

Technical Abstract: Soil C stocks in the Grand Prairie region of eastern Arkansas have declined under the prevalent two-year rotation of rice (Orzya sativa L.) soybean (Glycine max (L.) Merr.). Continuous rice cropping could promote soil C sequestration, but in previous work continuous rice averaged 19% less grain yield than rice-soybean, apparently due to N deficiency. To further study N cycling, microplots were imbedded during rice phases of a crop rotation field study in 2002 and 2004. 15N-labeled urea was applied pre-flood at the conventional 5-leaf crop growth stage. Above-ground crop biomass was smaller with continuous rice cropping than with rice following soybean (sampled both years) and rice following corn (Zea mays L.) (sampled only in 2004), although the difference varied by growth stage. Crop uptake of unlabeled native 14N, presumably mineralized from soil organic matter, was inhibited with continuous rice in both years. This trend was clearest at harvest (P=0.02) when continuous rice averaged 40 kg 14N ha-1 less uptake than the two rotations. 15N-labeled fertilizer-N averaged only 30% of total crop N and its uptake differed among cropping treatments only in 2002. At harvest, soil C with continuous rice cropping was enriched by 42% with syringyl phenols and by 83% with cinnamic phenols compared to the rotations. These results appear unrelated to estimated input rates of lignin-derived phenols. They are consistent with but do not prove our hypothesis that continuous rice cropping promotes the binding of N by lignin-derived phenols, thereby inhibiting soil N mineralization and late-season crop growth. Similar plant and soil observations were reported for tropical rice production, raising the question whether the responsible soil processes are common in continuous rice cropping.