NITROGEN CONCENTRATION AND 15N OF PONDED WATER OF PADDY SOILS: THE EFFECTS OF NUTRIENT SOURCES IN AN INCUBATION EXPERIMENT

摘要

An understanding of nitrogen (N) dynamics in ponded water in paddy fi elds may be helpfulin predicting N pollution potential from paddy lands to which various nutrient inputs such assynthetic fertilizer, green manure, and livestock manure compost are applied. In this study, ina preliminary effort to estimate N contamination potential of paddy land water when differentnutrients are applied. The changes in N concentration and the corresponding 15N of NH4+,NO3-, organic N, and total N in ponded water over paddy soils on applying urea (U), hairyvetch (HV), and pig manure compost (PMC) were investigated. The investigations were donein a laboratory incubation experiment under waterlogged conditions to mimic paddy-likesoil conditions. The concentration of all N species increased due to nutrient inputs over thecontrol. The mean T-N concentration was higher in U treatments (13.8 mg N L-1) followed byHV (7.2 mg N L-1) > PMC (5.0 mg N L-1) > control (2.9 mg N L-1), suggesting that SF is moremobile than organic inputs. At 1 day after incubation (DAI), the 15N of NH4+ was higher inPMC (12.0‰) than others, refl ecting the higher 15N of applied PMC (12.5‰ for PMC, -1.4‰for U, and -0.7‰ for HV). At 30 DAI, however, the 15N of NH4+ for U increased over that ofPMC due to N isotope fractionation associated with N loss. The 15N of NO3- was consistentlyhigher in PMC than in U and HV throughout the 30 days incubation period. Therefore, ourdata show that application of organic nutrients such as livestock manure compost and greenmanure may decrease N concentration of ponded water under paddy-like conditions due todifferent N solubility and mobility. In addition, it was also suggested that the 15N of NO3- wasmore conservative indicator of N sources.