Nitrogen mineralization in sludge‐amended desert soil as affected by rate of sludge, salinity, and wetting and drying cycles

Three incubation experiments were conducted to determine the influence of sludge rate, N addition, salinity, and wetting and drying cycles on different N forms in sludge‐amended calcereous silt loam soil. NH+ 4‐N and NO− 2 + NO− 3‐N were analyzed at intervals of 1, 2, 4, 8, and 12 weeks incubation at 30 °C ± 1. The rate of sludge as well as N was found to significantly affect NO− 3‐N accumulation. The cumulative inorganic N at the end of 12 weeks incubation was dependent on sludge rate and slightly affected by N addition. Cumulative mineralized N at 0‐, 40‐, 80‐, and 160‐t ha−1 sludge treatments were 15.4. 83.7, 105.1, and 165.0 mg kg−1 soil, respectively. In case of sludge and N addition, these figures were 10.2, 99.6, 108.4, and 180.4 mg kg−1 soil. N‐mineralization rate constant K increased from 0.214 in the control to 0.303, and 0.439, then decreased to 0.309 week−1 in soil treated with 0, 40, 80, and 160 t ha−1 sludge, respectively. Mean K values decreased with N addition and when excluding inorganic N found in soil and sludge at the start. Nitrogen mineralization potential N 0 values greatly increased with sludge and N addition. A salt mixture of (1:1 NaCl:CaCl2) at a concentration of 7.5 mg g‘1 soil delayed nitrification in un‐mended soil, while 15 mg g−1 caused complete inhibition. In sludge‐treated soil, retardation and complete inhibition of nitrification occurred at 7.5 and 60 mg g−1 salt mixture, respectively. Complete inhibition of ammonification occurred at salt mixture of 30 mg g−1 soil. Drying of either control or amended soil led to a marked drop in NH+ 4 N and NO− 2 + NO− 3‐N content of soil. Both NH+ 4‐N and NO− 2 + NO− 3‐N increased during wetting cycles. Mineralization was greater after 4 cycles of wetting and drying than after the first cycle.