Influence of hill slope on biological pools of carbon, nitrogen, and phosphorus in acidic alfisols of citrus orchard

•Impact of hill slopes under uniform orchard management on soil biochemical attributes was assessed.•Higher size of biological pools of C, N, and P in soils was in order of summit > shoulder > backslope.•Higher microbial biomass- and dissolved organic-C supported higher potentially mineralizable-N.•The combination of topography and soil moisture strongly influenced the variability of soil biochemical attributes.•The uniform soil management practice may not be appropriate within an orchard on hill slopes.

This study assessed how different hill slope positions under uniform management practice within a Khasi mandarin orchard (Citrus reticulata Blanco) influence biological pools of carbon (C), nitrogen (N), and phosphorus (P), and soil quality. The orchard soils (0 to 15 and 15 to 30 cm depths) of summit, shoulder and backslope hill slopes were analysed during post-monsoon (October-November) and post-winter (March-April). Higher soil moisture content, organic carbon (SOC), pH, size of biological pools {microbial biomass-C, -N and -P; dissolved-OC (DOC), MBC:SOC, potentially mineralizable-N (pMN)}, and soil dehydrogenase and acid-phosphomonoesterase activities were in order of summit > shoulder > backslope. Principal component analysis revealed that hill slope position and soil moisture had significant influence on variability of soil biological pools. Sizes of biological pools were significantly higher in post-monsoon than post-winter. Higher MBC and DOC in summit supported significant higher pMN compared to that in shoulder and backslope. The soil quality index within the orchard differed significantly between hill slope positions and higher soil quality was in order of summit > shoulder > backslope. In conclusion, hill slope position and soil moisture had coupled control on spatio-temporal variability in soil biochemical attributes within an orchard in spite of uniform orchard management practice. We believe these findings will ignite new thoughts that the current approach of uniform orchard management within a horticultural orchard especially on hill topography may not be appropriate.