Influence of slope positions on soil fertility index, soil evaluation factor and microbial indices in acid soil of Humid Subtropical India

The slope positions can control the movement of water and nutrients in a hillslope and contribute to the spatial variation of soil properties. A study was conducted in acid soil of humid subtropical northeastern India with the objectives to examine the characteristics and spatial differences in soil fertility parameters and microbial properties. There were significant difference in sand, silt and clay with slope positions at 025 and 2550 cm depths except the clay fraction between middle and lower slope at both the depths. There was significant differences in soil pH under different slope position at 2550 cm but not at 025 cm depth. Cation exchange capacity (CEC) values exhibited significant difference at both the depths, with values in the order lower > middle > upper slope. Soil fertility index (SFI) and soil evaluation factor (SEF) increased from upper slope to lower slope as soil nutrients leached down from the upper catchments to lower catchments due to coarse texture of soil accompanied by high rainfall. Both SFI and SEF were higher in 025 cm as compared to 2550 cm depth at all three slope positions. Slope positions significantly affected the soil biological properties like microbial biomass carbon (MBC), microbial biomass nitrogen (MBN), microbial biomass phosphorus (MBP), acid phosphatase (APase) and dehydrogenase activity (DHA). MBC (506 mg kg^-1), MBN (31.21 mg kg^-1), MBP (6.51), APase (27.6 mg PNP g^-1 h^-1) and DHA (33.12 µg TPF 24 h^-1 g^-1) was highest in upper slope at 025 cm depth due to native forest landuse. MBC, MBN and MBP values were significantly different with respect to slope position, with values in the order upper slope > middle slope > lower slope. Activities of APase and DHA decreased markedly with soil depth.