The incorporation of plant residues into soil is the key of increasing and (or) maintaining soil organic matter level. Although, its effect may be controlled by the soil type, climate and other effective factors such as the quality of plant residues. On the other hand, climate on a global scale is changing and the increase of drought stress is predicted in large areas of the world. As a result of these changes, all soil processes that are in relation with soil moisture and temperature, including mineralization of soil organic matter are affected. Also, in recent years, carbon cycle has been paid special attention to because of the increased concentration of atmospheric CO 2 and the impact on global warming. Climate warming, nitrogen amount and land use change because of some driving factors are effective on ecosystem processes such as carbon cycle and soil nitrogen. The change of the nitrogen cycle by human activities has resulted in an approximate doubling of nitrogen input which this relationship has important implications on the global carbon cycle. In this regard, this study was conducted to investigate the effect of drought stress interaction and plant residues quality on nitrogen mineralization (first experiment) and plant residues degradability in nitrogen adequacy conditions (second experiment). For this purpose, four types of plant residues including the remains of alfalfa, clover, corn and wheat were picked up. For the first experiment, plant residues at three levels of constant moisture 25, 35, and 55% of water holding capacity were incubated at 25 ? C for 150 days and at the end of incubation period, net nitrification, net ammonification, net nitrogen mineralization, net soluble organic nitrogen and the activity of Urease, L-asparaginase and L-glutaminase were measured. In the second experiment, mineralization of carbon from plant residues in nitrogen non-added and added conditions (Adding nitrogen from ammonium nitrate source) in a one-month incubation test in wet condition 55% of water holding capacity at 25 ? C was measured. The results indicated that in the first experiment, the maximum amount of net nitrification, net ammonification, net nitrogen mineralization, net soluble organic nitrogen and activity of L-glutaminase refers to the treatment of the remains of alfalfa and the minimum amount refers to the treatment of the remains of the wheat and the control. Among moisture treatments in measured indexes, there was a significant difference. Net nitrification, net ammonification, net nitrogen mineralization, net soluble organic nitrogen and activity of Urease, L-asparaginase and L-glutaminase had a positive and significant correlation with nitrogen amount of residues and a negative significant correlation with the amount of Lignin and C/N and LG/N ratios of plant residues. A positive significant correlation was observed between net nitrogen mineralization and enzymatic activities; but net ammonification didn't have any significant relation with other measured indexes. In the second experiment, the impact of nitrogen levels (0 and 100 mg nitrogen from ammonium nitrate source) on all kinetics parameters of measured carbon mineralization was significant. The maximum amount of kinetics parameters of carbon mineralization in added nitrogen conditions was related to the treatment of the remains of alfalfa and in the non-added nitrogen conditions was related to the treatment of the remains of clover. Adding nitrogen caused the increase of degradation of the remains of alfalfa, corn, and wheat and decrease of the remains of clover. A significant correlation was observed between biochemical characteristics of plant residues and kinetics parameters of carbon mineralization and also among kinetics parameters of carbon mineralization. The results indicate that both indices of carbon mineralization potential (r=0.976, C0) and initial potential rate (r=0.915, KC0) are good indices for determining carbon mineralization in plant residues in added nitrogen conditions. The results of the first experiment showed that the impact of drought stress on net nitrogen mineralization from more decomposed remains (alfalfa and clover) is more evident than less decomposed remains (corn and wheat) and in drought stress conditions, net nitrification is strongly correlated with net nitrogen mineralization than net ammonification. The results of the second experiment indicated that adding nitrogen causes the increase of plant residues degradability which implies that the availability increase of nitrogen accelerates plant residues degradability and consequently may cause the decrease of carbon. Keywords: Nitrogen mineralization, drought stress, plant residues degradation, nitrogen adequacy and enzymatic activity