Impacts of land use on soil organic matter and degree of compactness in calcareous soils of central Iran

This study was conducted to investigate the impact of land use (dryland farming, grassland and irrigated farming) on bulk density, (rho(b)) and relative bulk density (rho(b-rel)), and to study the relationships between rho(b) and rho(b-rel), respectively, and soil organic matter content (OM) and soil texture at 100 locations in calcareous soils of central Iran. The rho(b-rel) was expressed as the ratio of rho(b) to a reference bulk density, rho(bef). By considering rho(b-ref) an inherent soil property that is dependent on soil texture but not on OM, the combined effects of OM due to land use and compaction (due to agricultural machinery) on the degree of compactness could be explored. Multiple linear regression was used to derive pedotransfer functions for predicting rho(b) and rho(b-rel). It was found that rho(b-rel) is strongly affected by OM, and a strong correlation was obtained between rho(b-rel) and the ratio of OM to clay content. The predictive performance of the multiple regression models was poorest for irrigated farming, which might be explained by intensive soil disturbance by tillage in irrigated farming. The main effect of land use was on OM, and consequently, the degree of compactness was mainly controlled by OM. The greatest OM and least rho(b-rel) were measured in irrigated farming. Dryland farming had the least OM and the greatest rho(b-rel).