EXTRACTABILITY OF N-15-LABELED CORN-SHOOT TISSUE IN A SANDY AND A CLAY SOIL BY 0.01 MCACL(2) METHOD IN LABORATORY INCUBATION EXPERIMENTS

Management of N fertilization depends not only on the mineral N measur ed at the beginning of the growing season but also on the status of th e low-molecular-weight organic-N fraction. Our study was conducted to analyze how much of the N-15 applied in labeled corn-shoot tissue woul d be recovered in 0.01 M CaCl2-extractable N-15 fractions and wheter a decrease in the CaCl2-extractable N-15 fraction quantitatively follow ed the trend in net mineralization of the N-15 applied in corn-shoot t issue during an incubation period. The effects of adding N-15-labeled young corn-shoot tissue to a sandy soil and a clay soil were investiga ted for 46 days in an aerobic incubation experiment at 25 degrees C. T he application of 80 mg N kg(-1) soil in the form of labeled corn-shoo t tissue (24.62 mg N-15 kg(-1) soil) resulted in a significant initial increase, followed by a decrease the labeled organic-N fraction in co mparison with the untreated soils during the incubation, The labeled o rganic-N fraction was significantly higher in the sandy soil than in t he clay soil until the 4th day of incubation. The decrease in labeled organic N in the sandy soil resulted in a subsequent increase in (NO3- )-N-15 during the incubation. Ammonification of applied plant N result ed in a significant increase in the 1M HCl-extractable non-exchangeabl e (NH4+)-N-15 fraction in the clay soil, owing to the vermiculite cont ent. The N-15 recovery was analyzed by the 0.01 M CaCl2 extraction met hod; at the beginning of the incubation experiment, recovery was 37.0% in the sandy soil and 36.7% in the clay soil. After 46 days of incuba tion, recovery increased to 47.2 and 43.8% in the sandy and clay soils , respectively, Net mineralization of the N-15 applied in corn-shoot t issue determined after the 46-day incubation was 6.60 mg N-15 kg(-1) s oil (=34.9% of the applied organic N-15) and 4.37 mg N-15 kg(-1) soil (=23.1% of the applied organic N-15) in the sandy and the clay soils, respectively. The decrease in the labeled organic-N fraction extracted by 0.01 M CaCl2 over the whole incubation period was 3.14 and 2.33 mg N-15 kg(-1) soil in the sandy and clay soil, respectively. These resu lts indicate that net mineralization of as not consistent with the dec rease in the labeled organic-N fraction. This may have been due to the inability of 0.01 M CaCl2 to extract or desorb all of the applied org anic N-15 that was mineralized during the incubation period.