Effects of Rice-Husk Biochar on Nitrogen Retention, Nitrification, and Plant Nitrogen Uptake in Decontaminated Sandy Soils in Fukushima, Japan

Decontaminated sandy soils in Fukushima are characterized by low fertility and high nitrogen (N) loss, requiring effective nutrient management strategies. This study evaluated the effects of rice husk biochar on N dynamics, focusing on ammonium (NH4+) retention, nitrification, and plant N availability, using column, incubation, and pot experiments with decontaminated Fukushima soil. A significant interaction between biochar application and time indicated that biochar-applied soils showed different patterns in NH4+ and nitrate leaching over the experimental period. Incubation results showed that biochar reduced net nitrification rates (−18.1%) and tended to reduce the abundance of ammonia-oxidizing bacteria DNA. These effects may be attributed to the porous structure and adsorption properties of biochar. In the pot experiment, co-application of biochar with organic amendments (manure and kudzu) reduced plant N uptake by 9.6% and 9.0%, respectively, compared with their sole application. This indicates a trade-off between N retention and plant availability, particularly during the initial stage after biochar application. These findings highlight the importance of carefully balancing N retention and availability when applying biochar and organic amendments in low-fertility soils.