impact of dominant shrub species on soil organic matter content in dry grassland habitats

the effects of dominant shrub species on soil organic matter (som), including total soil carbon (sc), total nitrogen (n) and particulate organic matter (pom) were studied in three stands differing in the type of shrub species, i.e. astragalus microcephalus (shrub), pteropyrum aucheri (shrub) and prangus uloptera (non-shrub).the stands were located in arid grasslands of north-western iran. for this purpose, 18 soil samples from each stand were systematic-randomly collected (by auger) from each depth of 0-15cm and15-30cm, along 6 transect in each stand.the results showed that the stands with the highest abundance of p. aucheriand a. microcephalushad the highest values for sc (0.99% and 0.98%) in both depths, and the highest values for n (0.13% and 0.12%), respectively. the highest pom percentage for carbon (0.24%) and nitrogen (0.03%) were observed in the stands with the highest abundance of a. microcephalus. the proportion of micro-aggregates (28.48%) was significantly higher than macro-aggregates (20.46%) in the upper soil layer of the stand with the highest abundance of a. microcephalus as compared to the others, while no significant difference detected in micro- and macro-aggregate contents of the lower soil layer between the stands. therefore, the type of shrub species in the grassland communities had important effect on soil organic matter.