The changes of bulk density, porosity percentage and soil seedbanks in rangelands under different grazing intensities

This research was aimed to investigate the effects of three levels of grazing intensities (no grazing، moderate and heavy grazing) on bulk density، porosity percentage and soil seed bank. The study was conducted in Khabr rangelands (Kerman province، Iran) during 2005 and 2006. After determing the sites based on the same ecological conditions، soil samples were taken from two depths of 0-15 and 15-30 cm during six periods (May to October)، as split plots based on a randomized complete block design with three replications. To study the soil seed bank، intact soil samples were taken from the depth of 0-15 cm to (36 samples per year) at the end of grazing period during two years. Artifical trampling was carried out on seedbank samples through changing bulk density of each site to another site. The results of the experiment showed that soil physical characteristics (bulk density and porosity) were significantly affected by grazing intensity. At the end of the grazing season، bulk density increased and porosity decreased in moderate (11. 7 and 12. 64 percent) and heavily grazed (17. 6 and 18. 22 percent) sites compared to control، respectively. According to the significant and irreversible increase of soil bulk density in heavy grazing site at the end of the grazing period (September and October) and the reduction of soil porosity for this site in October (in comparison with other sites)، observing the proper time for livestock exit by ranchers is emphasized to reduce the harmful and damaging effects of heavy grazing. The germination percentage and other growth characteristics of seeds and seedlings from soil seed bank indicated that artificial changes in soil bulk density had a positive effect on their growth and development.