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 Introduction: Among the collection of natural resources in the world, SOIL is considered as one of the most important components of the environment. Protect and improve the properties of this precious resource, requires a comprehensive and coordinated action that only through a deep understanding of quantitative (not only recognition of the quality) the origin, distribution and functionality in a natural ecosystem is possible. Many researchers believe that due to the quick reactions of SOIL organisms to environmental changes, SOIL biological survey to estimate SOIL quality is more important than the chemical and physical properties. For this reason, in many studies the NITROGEN MINERALIZATION and MICROBIAL RESPIRATION indices are regarded. The aim of the present study were to study the direct and indirect effects of SOIL physicochemical characteristics on the most important BIOLOGICAL INDICATORS (nitrogen mineralization and MICROBIAL RESPIRATION), which has not been carefully considered up to now. This research is the first study to provide evidence to the future planning and management of SOIL sciences.Materials and Methods: For this, a limitation of 20 ha area of Experimental Forest Station of Tarbiat Modares University was considered. Fifty five SOIL samples, from the top 15 cm of SOIL, were taken, from which bulk density, texture, organic C, total N, cation exchange capacity (CEC), NITROGEN MINERALIZATION and MICROBIAL RESPIRATION were determined at the laboratory. The data stored in Excel as a database. To determine the relationship between biological indices and SOIL physicochemical characteristics, correlation analysis and factor analysis using principal component analysis (PCA) were employed. To investigate all direct and indirect relationships between biological indices and different SOIL characteristics, PATH ANALYSIS (path analysis) was used.Results and Discussion: Results showed significant positive relations between biological indices and clay, organic carbon and total nitrogen, whereas the correlations of the other SOIL properties (bulk density, silt, sand and CEC) were insignificant. Factor analysis using of principle component analysis showed that the behavior of these two biological indices in the same territory and controlled by the same factors. PATH ANALYSIS was employed to study the relationship among SOIL biological indices and the other SOIL properties. According to results, SOIL NITROGEN MINERALIZATION is more imposed by nitrogen (0.98) and organic carbon (0.91) properties as direct and indirect effects respectively. Whereas the values of SOIL MICROBIAL RESPIRATION were affected by organic carbon (0.89) and total nitrogen (0.81). It can be claimed that total nitrogen and organic carbon are the most important SOIL properties in relation to NITROGEN MINERALIZATION and MICROBIAL RESPIRATION, respectively. Regarding to the strong relationship between SOIL organic carbon and nitrogen and also similarly strong relationship between nitrogen and organic carbon mineralization, enhancing NITROGEN MINERALIZATION is expected by the increase in organic carbon. In this regard, Nourbakhsh, et al. (2002) claimed that NITROGEN MINERALIZATION is depended to SOIL organic nitrogen and derived from total nitrogen. In addition, there is a strong relationship between total nitrogen and SOIL organic carbon. So, the greater amounts of NITROGEN MINERALIZATION can be related to more accumulation of organic carbon and nitrogen in topsoil (23). This result is in accordance with Wood, et al. (1990) and Norton, et al. (2003) findings (21, 30). Ebrahimi, et al. (2005) stated that if the C/N ratio is more than 30, the process immobility or NITROGEN MINERALIZATION stopwill be occurred. The ratios between 20 and 30 usually settle and release of mineral nitrogen does not take place, and the balance remains. If the C/N ratio is less than 20 net release of nitrogen in the SOIL will increase (9).In the present study, the values of SOIL C/N ratio were less than 20 (mean 15.80), so the process of NITROGEN MINERALIZATION occurred in the study area. Suitable conditions for microbial activity of SOIL microorganism's especially adequate supply of organic carbon increased the MICROBIAL RESPIRATION in the study area. High correlation between the amount of organic carbon and MICROBIAL RESPIRATION confirmed this claim. However; it seems that the SOIL organic carbon is driver of MICROBIAL RESPIRATION rate. This finding is reported by different researchers (6, 7, 15, and 20).Conclusion: PATH ANALYSIS as a complementary method of REGRESSION analysis and factor analysis using principal component analysis showed that the biological activity of the SOIL characteristics are directly affected by SOIL nitrogen (for NITROGEN MINERALIZATION index) and organic carbon (for MICROBIAL RESPIRATION index) and other useful features influence them indirectly through strong correlation with the characteristics of nitrogen and organic carbon in SOIL.


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