Laboratory incubations and a field experiment were carried out to determine
the factors controlling N mineralization and nitrification, and to estimate
the N losses (leaching and volatilization) in a sewage-sludge-amended Oxisol.
Aerobically digested sludge was applied at a rate equivalent to 625 kg
N/ha. The incubations were conducted as a factorial experiment of
temperature (20˚C, 30˚C, and 40˚C) soil water (–30 kPa
and –1500 kPa) sludge type [fresh (FS) water content 6230 g/kg;
dry (DS) water content 50 g/kg]. The amount of nitrifiers was
determined at the beginning and at the end of the experiment. The incubation
lasted 24 weeks. The field study was conducted using bare microplots (4 m) and
consisted of a factorial experiment of sludge type (FS and DS) sludge
placement (subsurface, I+; surface, I–). Ammonia volatilization and
the profile (0–0.90 m) of mineral N concentration were measured during 6
and 29 weeks after sludge application, respectively.
After 24 weeks of incubation at 40˚C and –30 kPa, net N
mineralization represented 52% (FS) and 71% (DS) of the applied
N. The difference between sludges was due to an initial period of N
immobilization in FS. Nitrification was more sensitive than N mineralization
to changes in water potential and it was fully inhibited at –1500 kPa.
The introduction of a large amount of nitrifiers with FS did not modify the
rate of nitrification, which was principally limited by soil acidity (pH 4.9).
Although N mineralization was greatest at 30˚C, nitrification increased
continuously with temperature. Nitrogen mineralization from DS was well
described by the double-exponential equation. For FS, the equation was
modified to take into account an immobilization-remineralization period.
Sludge placement significantly affected the soil
NO-3/NH+4 ratio in the
field: 16 for I+ and 1.5 for I–, after 11 weeks. In the I–
treatment, nitrification of the released NH+4 was
limited by soil moisture because of the dry soil mulch formed a few hours
after rain. At the end of the field experiment, the estimated losses of N by
leaching were 432 kg N/ha for I+ and 356 kg N/ha for I–.
Volatilization was not detectable in the I+ microplots and it represented
only 0.5% of the applied N in the I– microplots. The results
showed that placement of sludge may be a valuable tool to decrease
NO-3 leaching by placing the sludge under unfavourable
conditions for nitrification.