Changes of chemical properties of protected field andosol during sterilization by soil reduction

The degradability of wheat bran by means of sterilization by soil reduction and the differences of chemical properties of plow layer before and after sterilization by soil reduction were investigated. 1. The nitrogen degradation rate of wheat bran during sterilization by soil reduction was 18-43% (average 32%) and the carbon degradation rate was 43-67% (average 55%). If the annual nitrogen degradation rate of wheat bran is supposed to be the same as that of rapeseed oil cake (ca. 75%), it is estimated that about 30-60% of nitrogen in wheat bran that remains after sterilization by soil reduction will decompose during successive one year. Since about 24 kg of nitrogen are contained in 1 ton of wheat bran, 7-14 kg of nitrogen in 1 ton of wheat bran presumably decompose changing into inorganic nitrogen. However, because of conversion of nitrogen into organic compounds following the degradation of remaining carbon in wheat bran, temporary suspension of the fertilizing effect of nitrogen can be considered. 2. The contents of components in plow layer such as EC, inorganic N, available N, available P2O5, and each exchangeable cation tended to increase when the contents before sterilization by soil reduction were low, and tended decrease when the contents were high. The contents of transition from the increase to decrease before sterilization by soil reduction were approximately as follows: EC = 0.25 mS/cm, N03-N = 10 mg/100g, available N = 5 mg/100g, available P2O5 = 20 mg/100g, exchangeable CaO = 600 mg/100g, exchangeable MgO = 130 mg/100g, and exchangeable K2O = 70 mg/100g. For this reason, when sterilization by soil reduction is carried out at a protected field loaded with salts, the simultaneous effect of salt removal is suggested.