Characterization of soil organic matter content of two sample size populations along a climatic transect

Soil organic matter (SOM) content was determined in two populations of soil samples that were taken from 0-2 soil depth. One population represented soil samples that were taken from a square of 25 cm2 in size (small-S population) and the other population represented soil samples that were taken from a square of 2500 cm2 in size (large-L population). The samples were collected on hillslopes in different climatic regions: Mediterranean (GIV), semi-arid (MAL), mildly-arid (MIS) and arid (KAL). The results of both S and L populations showed decreasing SOM mean and variance from the Mediterranean site to the arid site. Statistical and spatial characteristics of each population were compared between the climatic regions. In addition, comparison between the two populations was made for each site. The difference in sample size did not significantly affect the mean values of SOM of the two populations in sites GIV, MAL and KAL, but did affect the mean at site MIS. At all study sites, except for site MAL, the variance increased with decreasing sample size. At sites GIV and KAL the coefficient of variation of S population was higher (more than 1.5 times) than that of L population, whereas at sites MAL and MIS, the differences were negligible. The relationships between the values of S and L samples at the individual sampling points defined the background of the study sites, which reflects the effect of vegetation (type), grazing, biological crust and soil properties. It was found that at the extreme sites GIV and KAL the background was characterized by relatively low SOM content with small areas of high organic matter content. At site MIS the background was characterized by relatively high SOM with small areas of low organic matter content. At site MAL the background was not dominated by high values of SOM nor by low ones. The spatial pattern of L population became more simple with increasing aridity. At the relatively wet sites the spatial pattern did not depend on the sample size while in the more arid sites it was sample size dependent. It was indicated that the spatial structure of SOM at the semi-arid and mildly arid sites is anisotropic whereas at the Mediterranean and arid sites it is isotropic.