D. Steffens et al., 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, Biology and fertility of soils, 22(1-2), 1996, pp. 109-115
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.