عنوان مقاله [English]
Background and objectives: The organic carbon and nitrogen contents of soils are controlled by the amounts of organic carbon and nitrogen input into soils and by the decomposition rates of organic carbon and nitrogen of plant residues. A combination of environmental and biological factors are involved in organic carbon and nitrogen mineralization. Microbes are agents responsible for litter degradation and abiotic factors such as temperature, soil type, bulk density, soil moisture and irrigation water quality influence organic carbon and nitrogen mineralization rates since microbial activities are affected by these factors. The rate of organic carbon and nitrogen mineralization is higher in warm and moist environments than in cold or dry sites. Due to the inverse relationship between soil moisture and aeration this experiment was performed to evaluate the effects of soil moisture levels on dynamics of organic carbon and nitrogen from alfalfa and barley residues.
Materials and methods: This experiment was performed to evaluate the effects of soil moisture on organic carbon and nitrogen dynamics. For this purpose, a split – split plot experiment with three replications was conducted using litter bag method. Factors examined were types of plant residue (barley and alfalfa), soil moisture levels (10, 25, 50, 75 and100% of saturation levels) and incubation periods (1, 2, 3 and 4 months). At the end of incubation periods, the litter bags were pulled of the pots and the weights of plant residues remained in bags were measured. The plant residues were also analyzed for organic carbon and nitrogen using dry combustion and kjeldahl methods respectively. Organic carbon and nitrogen losses were calculated by subtracting the remaining amounts of organic carbon and nitrogen in one incubation time interval from those of the latter incubation.
Results: The results showed that the amounts of organic carbon lost after 4 months were 12.05, 54.21, 70.59, 66.52 and 62.04% for alfalfa residue and 10.36, 48.37, 60.63, 59.38 and 55.29% for barley residue when the soil moisture levels were adjusted at 10, 25, 50, 75 and 100% of saturation percentage (sp) respectively. The corresponding amounts for organic nitrogen lost were also 20.54, 57.65, 70.44, 59.62 and 57.51% for alfalfa residue and 11.68, 56.05, 63.61, 52.27 and 47.58% for barley residue respectively.
Conclusion: The amounts of organic carbon and nitrogen lost in the first month of incubation were higher than the sum of those lost in the other months of incubation. The results also showed that soil moisture deficit is a stronger limiting factor for plant residue decomposition than aeration and in saturated soils a significant amount of plant residues is decomposed.