Abstract A toposequence is a sequence of soils located on different parts of a hillslope with different topographic characteristics. Hence, topography directly or indirectly plays an important role as a differential soil forming factor along the slope. Soil biological processes which are normally mediated by soil microbial biomass are functionally involved in the soil ecosystem services. Nutrient cycling, soil fertility, soil C storage, and organic C turnover rate are among the key features predominantly controlled by soil microbial activities. Due to the central role of soil enzymes in the eva‎luation of soil biological activities and their relative simplicity of determination and also their quick responses to the chemical application and management practices, their measurements are considered as valuable tools for soil biological quality assessment. Investigating the variability of physical, chemical and biological properties of the soils along a toposequence may shed light on the underlying mechanisms involved in soil development on a hillslope. The objective of the study were to investigate i) variation of soil biological properties along a hillslope that is considered as a eroded toposequence and ii) identifying the intracellular and extracellular fractions of soil urease along the hill slope. For the first purpose, soil microbial biomass C (MBC), soil microbial biomass N (MBN) soil microbial biomass K (MBK), metabolic quotient, microbial C percentage, overall as well as intracellular and extracellular urease activities were measured in the various positions of an eroded toposequence located on a semiarid grassland. For the purpose of the second research, the effects of sodium caseinate as an energy-, C-, and N-rich compound on the variability index of urease fractions were studied. Soil samples (0-15 cm) were collected from a toposequence which was located on a semiarid grassland, Kouhrang region. The samples were air-dried and passed through a 2-mm sieve. Results indicated that the biological properties varied widely in the various positions of the toposequence. The greatest values of metabolic quotient (0/21 mg CO2-C mg -1mic d -1) and microbial C percentage (%3/2) were observed in toe slope whereas, the position possessed contrarily the least significant amounts of soil organic C (7/02 g kg -1soil), MBC (224/74 mg Cmic kg -1soil), MBN-FI (12/96 mgNmic kg -1soil), MBN-MwE (45/23 mg Nmic kg -1soil), intracellular (12/81 mg NH+4 kg -1soil h-1) fraction of urease activity and urease quotient (0/06 mg NH4+ mg -1 Cmic h-1). Metabolic quotient revealed a significant negative correlation with soil electrical conductivity (r=-0.75**). MBN either measured by fumigation-incubation or microwave irradiation-extraction showed similar patterns along the slope, and also it was observed that the values obtained by the two measurement techniques are significantly associated (r=0.78**). This finding indicates that the less time consuming and more environmental friendly method (microwave irradiated-extraction) can be used successfully instead of the timely and less environmental friendly (fumigation-incubation) procedure. Application of sodium caseinate, in the second part of the research as a rich source of energy, C and N, clearly caused significant reducttion in the variability indices of the overall and interacellular urease activity. So their coefficient of variation respectively reduced to 89 and 71 percent. This finding well indicates that most of the variations among the hillslope positions are due to the shortage of organic sources which was adequately compensated by the application of sodium caseinate.

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شمس الله ايوبي

حسين خادمي، آقا فخر ميرلوحي