Microbial populations, ammonification and nitrification in soil treated with urea and inorganic salts

Soil fertilization with urea at rates of 0.2 and 0.5 mg N/g soil was toxic for total counts of bacteria and fungi except with cellulolytic fungi where growth was promoted by addition of urea after 90-d incubation. Also, the population numbers of both bacteria and fungi were significantly decreased when soil was amended with CaCl2 and K2SO4 applied at two levels (50 and 100 mumol/g soil). Some alleviation of the toxic effect of either urea or the inorganic salts was observed when they were applied in combination. Fungal species composition was found to be affected in the treated soil. The most tolerant fungi were Alternaria alternata, Aspergillus flavus, A. fumigatus, A. niger, A. terreus, Eurotium amstelodami, E. chevalieri, Nectria haematococca, Paecilomyces variotii and Stachybotrys chartarum. Other species of fungi were either sensitive to all treatments or sensitive to some and tolerant to others. Ammonium nitrogen was found to be more accumulated in soil treated with urea mixed with inorganic salts compared with soil treated only with urea whereas NO3(-)-N (and NO2(-)-N) was decreased. The overall effect of the addition of inorganic salts on total mineralized nitrogen, however was promotive. Soil pH was increased in urea-treated soil but was depressed by application of CaCl2 or K2SO4 to values lower than that of untreated soil. The results may be of agronomical interest in overcoming problems encountered with urea application as N fertilizer.