Crop Nitrogen Uptake and Soil Phenols Accumulation under Continuous Rice Cropping in Arkansas

Soil C stocks in the Grand Prairie region of eastern Arkansas have declined under the prevalent 2-yr 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 following soybean, apparently due to N deficiency. To further study N cycling, microplots were imbedded during the rice phase of a crop rotation field study in 2002 and 2004. Urea labeled with (15)N was applied preflood, when all N fertilizer is conventionally applied. Crop biomass was often smaller with continuous rice 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 native (14)N, 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 (14)N ha(-1) less uptake than rice in the two rotations. Fertilizer (15)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 with the rotations. These enrichments appear unrelated to estimated input rates of lignin-derived phenols. Results support the hypothesis that continuous rice cropping promotes the binding of soil N by lignin-derived phenols, thereby inhibiting N mineralization and late-season crop growth. Similar observations were reported for tropical rice production, suggesting that the responsible soil processes might be common in continuous rice cropping.