Management of plant residues and organic matters is one of the most important ways to improve soil fertility in sustainable agriculture. Decomposition of organic wastes supplie necessary elements to plants via mineralization processes. Nitrogen defficiency limits plant growth. Nitrogen Mineralization provide plants and other organisms with available N and hence plays an important role in plant nutrition, reducing deficiency symptoms and ecosystem functioning. L-glutaminase mineralizes nitrogen with catalyzing breaking of non-peptide C-N linkage i L-glutamine amino acid. This results in changing L-glutamine to L-glutamic acid and ammonium. This study aims to : 1) identify the optimum conditio of the assay of L-glutaminase activity for plant residues, forest litters and organic ammendments, 2) study the effect of air-drying and freezing of plant residues o L-glutaminase and 3) determine the kenetics and thermodynamics parameters of L-glutaminase in plant residues. For these purposes, Acer velotonium and Gleditschia caspica litters , cow manure and sewage sludge were sampled. Fresh residues were pulverised then kept at 4 °C. Freezed residues were amended with liquid nitrogen and kept at 4 °C temprature. Litters , fertilizer and a part of plant residues were air-dried at room tempratore, passed through a 1mm sieve and kept at 4 °C . The effect of incubation time, sample amount, incubation pH, incubation temperature and substrate concentratio were studied. In plant residue, we studied the effect of freezing and air-drying on L-glutaminase activity. Then, kinetics and thermodynamics parameters were determined. Linear transformatio of michaelis-menten equation including lineweaver-Burk, Eidie-Hofstee and Hones-wolf were used. Arhenius and Eyring equations were performed for determining the activation energy and activation enthalpy ,respectively. The optimum conditions of L-glutaminase activity assay were, 2 hours of incubation time at 37 °C, 0.1g sample amount, pH=10 and 50mM substrate concentration. Freezing and airdrying decreased L-glutaminase activity in plant residues and airdrying decreased L-glutaminase activity more than the freezing. The maximum velocity of L-glutaminase activity was ranged from 117.7 to 3611.6 µg NH 4 -N g -1 h -1 . The K m values varied from 11.1 to 83.3 mM. The activation energy ranged 28 to 68 kJlmol for cow manure and airdried alfalfa, respectively. The minimum and maximum enthalpy of activation found in cow manure and airdried alfalfa, respectively. The average of Q 10 values were ranged from 1.27 to 2.32. The method used in determination of L-glutaminase activity in soil samples can be used in organic materials with minimum modifications. The responses of materials of different origin to incubation conditions were dissimilar, similar responses were observed in each type including plant residues, organic fertilizers and forest litters. Keywords : L-glutaminase, Kinetics and thermodynamics, Plant residue, Forest litter, Organic fertilizers.