Nowadays plant residues and organic solid wastes are considered to improve soil fertility in sustainable agriculture. Decomposition of residues in forest floors is a necessary process in transformation of soil organic matter to humus. Determination of nitrogen mineralization in organic residues has been practiced by many workers. Enzymes are central to nitrogen mineralization and turnover. L-asparaginase hydrolyses l-asparagine to l-aspartic acid and ammonia and hence, plays a key role in mineralization of organic nitrogen. Soil microorganisms and plants are two main sources of soil L-asparaginase. Few studies have been conducted to evaluate enzyme activities in solid wastes and plant residues. This study aims to determine the kinetic and thermodynamic parameters of l-asparaginase in different plant residues and then to optimize the assaying method of l-asparaginase activity for solid wastes and plant residues. Wheat and barely residues, cow manure and swage sludge, oak and Leilaki litters were sampled. The organic materials were air-dried at room temperature, passed through a 1mm sieve and kept in refrigerator. The effects of pH, sample weight, substrate concentration, incubation time and temperature were studied. Linear transformations of Michaelis-Menten were used to calculate kinetic parameters. Arrhenius and Eyring equations were used for determining the activation energy and enthalpy of activation, respectively. Results showed that l-asparaginase activity were limited at pH values lower than 8 and higher than 10. The maximum activity of L-asparaginase in the organic materials was achieved at 30 to 50 mM for the different materials. An increasing trend was observed in l-asparaginase activity, as incubation temperature increased from 10 to 40 ° C. Our results implied that l-asparaginase was denatured above 50 ° C. The optimum assay conditions of l-asparaginase activity were pH 10, 0.2 g sample weight, 50mm L-asparagine concentration, 2 hours of incubation at 37 ° C. The maximum velocity of l-asparaginase activity was ranged from 43.48 to 178.57 µg NH 4 -N g -1 h -1 . The K m values varied from 14.82 to 44.92mm. Wheat residues showed the greatest K m value while, other materials were not significantly different. The activation energy ranged from 25.50 to 44.7 and the enthalpy of activation ranged from 23.03 to 44.23 kJ mol -1 . The activity of L-asparaginase was determined at 7 temperature intervals between 10 to 70 ° C and the average Q 10 values were calculated. The average values were ranged from 1.4 to 1.84. Overall, the method used in determination of l-asparaginase activity in soil samples can be used in organic residues with minor modifications. Although the responses of the materials of different origin to incubation conditions were dissimilar, similar responses were observed in each category