Effects of land use on soil biological parameters across ecosystems in northwestern Iran

Land use change has become a major environmental challenge in northwestern Iran, including the Namin region of Ardabil Province, where agricultural expansion has accelerated the degradation of forests and rangelands over recent decades. This study aimed to compare soil biological properties among four representative land use types (forest, rangeland, shrubland, and rainfed agriculture) and to identify the most informative indicators of soil quality in the Namin region, Ardabil Province, Iran. In July 2023, 36 soil samples (nine per land-use type) were collected and analyzed for organic carbon, pH, electrical conductivity (EC), soil texture, calcium carbonate equivalent (CCE), basal respiration (BR), substrate-induced respiration (SIR), microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), the microbial quotient (q_mic), and the metabolic quotient (qCO₂). Statistical analyses included analysis of variance (ANOVA), Tukey’s honestly significant difference (HSD) test, factor analysis (FA), principal component analysis (PCA), and permutational multivariate analysis of variance (PERMANOVA). Forest soils exhibited significantly higher BR, SIR, MBC, and MBN than the other representative land-use types. PCA identified three principal components that explained approximately 78% of the total variance, demonstrating that soil quality is governed by interacting biological, chemical, and physical processes. From the Total Data Set (TDS) of 12 soil properties, PCA identified a Minimum Data Set (MDS) consisting of four non-redundant indicators: soil pH, EC, silt, and BR. These indicators represented key chemical, physical, and biological dimensions of soil quality and collectively accounted for more than 61% of the total variance. Comparison of the soil quality indices showed that the highest and lowest IQI_TDS values were observed in the representative forest (0.792) and scrubland (0.262) sites, respectively. For IQI_MDS, the rainfed agricultural and forest sites exhibited the highest values (0.690 and 0.671, respectively), whereas the scrubland and rangeland sites exhibited lower values (0.412 and 0.479, respectively). Nevertheless, both indices consistently identified the scrubland site as having the lowest soil quality. Overall, the results indicate that differences in soil biological functioning and overall soil quality were observed among the representative land use sites. In addition, the proposed MDS retained much of the information contained in the TDS while requiring fewer indicators, suggesting that it may serve as a practical and cost-effective framework for soil quality assessment and long-term ecological monitoring in semi-arid land use systems.