Factors controlling N mineralization, nitrification, and nitrogen losses in an Oxisol amended with sewage sludge

Laboratory incubations and a field experiment were carried out to determine the factors controlling N mineralization and nitrification, and to estimat e the N losses (leaching and volatilization) in a sewage-sludge-amended Oxi sol. Aerobically digested sludge was applied at a rate equivalent to 625 kg N/ha. The incubations were conducted as a factorial experiment of temperat ure (20 degreesC, 30 degreesC, and 40 degreesC) x soil water (-30 kPa and - 1500 kPa) x 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 fiel d study was conducted using bare microplots (4 m(2)) and consisted of a fac torial experiment of sludge type (FS and DS) x sludge placement (subsurface , I+; surface, I-). Ammonia volatilization and the profile (0-0.90 m) of mi neral N concentration were measured during 6 and 29 weeks after sludge appl ication, respectively. After 24 weeks of incubation at 40 degreesC and -30 kPa, net N mineralizati on represented 52% (FS) and 71% (DS) of the applied N. The difference betwe en sludges was due to an initial period of N immobilization in FS. Nitrific ation was more sensitive than N mineralization to changes in water potentia l and it was fully inhibited at -1500 kPa. The introduction of a large amou nt of nitrifiers with FS did not modify the rate of nitrification, which wa s principally limited by soil acidity (pH 4.9). Although N mineralization w as greatest at 30 degreesC, nitrification increased continuously with tempe rature. Nitrogen mineralization from DS was well described by the double-ex ponential equation. For FS, the equation was modified to take into account an immobilization-remineralization period. Sludge placement significantly a ffected the soil NO3-/NH4+ ratio in the field: 16 for I+ and 1.5 for I-, af ter 11 weeks. In the I- treatment, nitrification of the released NH4+ was l imited by soil moisture because of the dry soil mulch formed a few hours af ter 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 ap plied N in the I- microplots. The results showed that placement of sludge m ay be a valuable tool to decrease NO3- leaching by placing the sludge under unfavourable conditions for nitrification.