Effect of distance and depth on microbial biomass and mineral nitrogen content under Acacia senegal (L.) Willd. trees

The relations between plants and soil biota involve positive and negative feedbacks between soil organisms, their chemical environment, and plants. Then, characterization of microbial community functioning is important to understand these relations. An experiment was conducted in a field system in the north of Senegal for two years (2005 and 2006) in order to investigate the effect of depth and distance from Acacia senegal tree stem on soil microbial biomass and inorganic-N content. Soils were sampled during dry season (April, T0) and wet season (August, T1) along transects (R0, foot tree; R/2, approximately 0.50 m distance from the stem; and R, approximately 1 m distance from the stem) and at different layers: 0–25 cm, 25–50 cm and 50–75 cm of A. senegal trees rhizosphere. Total microbial biomass and inorganic-N content were negatively correlated to the distance from tree stem and the depth. The highest values of microbial biomass and mineral nitrogen were found at the foot tree (R0) and at 0–25 cm layer. Inorganic-N was mostly in nitrate form (NO3−) during the dry season. In contrast, during the wet season, inorganic-N was dominated by ammoniac form (NH4+). Soil total microbial biomass and inorganic-N (NH4++NO3−) were negatively correlated. Our results suggest a positive influence of A. senegal rhizosphere on soil microbial biomass and inorganic-N content.