Assessment the effect of Slope aspect and position on some soil microbial indices in rangeland and forest

 Topography is one of the effective factors in soil formation and development. Topographical features such as slope aspect and position, by affecting soil temperature, evaporation capacity, soil moisture content, soil organic matter, precipitation, movement, and accumulation of soil solution can impress soil microbial properties. For investigating the effect of land-use change on soil ecosystem functioning because of human activities, it is essential to study the soil processes in disparate land-use systems and to preserve and regenerate the capability of soil to deliver ecosystem services. This study aimed at evaluating the effect of slope aspect and position on soil microbial indices in rangeland and forest areas of Heyran neck.

This experiment was accomplished as factorial on the basis of a randomized complete block design with three treatments containing land use (forest and rangeland), slope aspects (north and south), and three slope positions (shoulder, footslope, toeslope), and four repetitions in Heyran neck, one of the functions of Astara city in Guilan province. Totally, 48 disturbed soil samples were taken from forest and rangeland at two aspects and three slope positions. From soil biological indices, soil microbial population, soil basal, and substrate-induced respiration (BR and SIR), microbial biomass carbon (MBC), and microbial quotient (qmic ) were measured. Soil microbial population was determined by the most probable number method (MPN method). The basal soil respiration was determined by placing 50g of soil in glass jars together with 10mL 0.05N NaOH in 20mL glass vials. All samples were incubated for 24h at 25 ± 1 oC, and the CO2 that evolved and was trapped during the period was determined by titration of the NaOH with 0.1N HCl. Substrate-induced respiration was determined by measuring the CO2 production from 50gr fresh soil. The soil samples included 1% glucose, were first placed in a glass jar. Then, an absorption bottle that was filled with 25ml of 0.1N NaOH was carefully put in the glass jar, and the glass jar was precisely sealed. The jar was then incubated at 25 ± 1 oC for 6h. The evolved CO2 was trapped by NaOH and determined by titration of NaOH with 0.1N HCL. Microbial biomass carbon was estimated by the chloroform fumigation and extraction method. Microbial quotient was calculated by dividing microbial biomass carbon (MBC) with soil organic carbon (SOC). Data’s normality was assessed through Kolmogorov–Smirnov test. The analysis of variance and comparison of means by Duncan test and Pearson correlations were done using SPSS software. Figures were prepared using Excel software.

The results showed that the content of all biological parameters measured in the forest soils was significantly higher than in the rangeland soils. In both land uses and in all slope positions, the highest values of basal soil and substrate-induced respiration, microbial population, and microbial biomass carbon were measured in the north-facing slope. In the studied land uses and slope aspects, all biological indicators except the microbial quotient had the highest value in the shoulder position. In the south-facing slope, the content of microbial quotient in the toeslope position was measured more than in other positions. Still, there was no significant difference from the value of this index in the shoulder position. The highest value of microbial quotient (1.95 mgCmic.g-1Corg) was observed in the north-facing slope on the shoulder, which was not significantly different from the value of this index in the footslope and toeslope positions of north-facing slope and also the shoulder and toeslope of south-facing slope. In forest land use, the content of microbial quotient in both north-facing (1.91 mgCmic.g-1Corg) and south-facing (1.9 mgCmic.g-1Corg) slopes was almost equal, which was significantly different from its value in both slope aspect of rangeland. The correlation between all biological indices at the level of one percent probability was positive and significant. There was the highest correlation between basal soil and substrate-induced respiration (r=0.94**) as well as microbial biomass carbon and substrate-induced respiration (r =0. 89**).

Overall, the results of this study showed that biological indices are dependent largely on landscape and land use, and by affecting the structure of the microbial community, these factors can affect the properties of the soil and its development. Due to the significant role of microorganisms as an integral component in the soil ecosystem and the effect of this component on the physical, chemical and nutritional properties of this ecosystem, by providing suitable conditions for the good activity of these organisms, soil quality can be increased, and thus erosion will be reduced.