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Department of Botany and Microbiology, University of Kuwait

ABSTRACT

The influence of varying soil moisture content (70, 90, and 120%of the water holding capacity (WHC)) and the application of starch (l % C) on nitrogen transformations in alluvial clay soil was studied. Ammonium nitrate (2000 ppm N) was added and the soil samples were incubated at 30°C for 30 days. NO₃^--N showed progressive increase with time at 70% WHC, but as moisture content increased the nitrate decreased. Added starch increased the rate of disappearance of NO₃^--N at all moisture levels, and NO₃^--N disappeared completely from the samples receiving starch at 120% WHC after 15 days. NO₂^--N values fluctuated during incubation, tending generally to increase at higher soil moisture contents and on adding organic carbon. NH₃-N generally decreased with the time of incubation. Increased moisture and added starch slowed down NH⁺₄-N disappearance. Organic nitrogen accumulation was greatest after 7-15 days of incubation. Highest accumulation occurred at 70% WHC in the presence of starch. Nitrogen losses increased with increasing the moisture content and were stimulated by the addition of organic carbon at moisture levels of 90 and 120% WHC, but were slightly reduced at 70% WHC. A maximum loss of 40.8% of the total amount initially present occurred after 30 days in soils of 120% WHC with added starch. The numbers of denitrifying bacteria increased during incubation and reached their maximum levels mostly after 15 days. They were greatest at 120% WHC in the presence of starch.