Compared to all other terrestrial components, soil comprises the largest organic C stocks such that knowing the controlling factors and underlying processes can help to utilize its potential. Owing to the substantial spatio-temporal interplay at soil-plant-microbial interface, we have insufficient information to answer how different soil types and tree biomass interactions influence the surface layer soil microbial dynamics. Recognizing the uneven distribution of different soil fractions and temporal dynamics in moisture, soils of dryland agroecosystems are expected to have unknown trends in microbial biomass C. To understand how spatially variable tree biomass and temporal soil moisture trends affect soil microbial biomass, we performed periodic sampling runs in a flood irrigated Pecan orchard, located in Tornillo, Texas. The concerned study site has some unique spatial contrast developed by fluvial deposits on the regionally coarse soil fractions. The intriguing mosaic of contrasting soil textures has resulted in pecan trees with substantially variable tree biomass and created a spatial pattern of large and small trees adjoining the contrasting soil textures. In the present investigation, we have taken advantage of these features and chosen two pecan trees of contrasting biomass and collected four samples each on the north and south side of trees at a distance less than 5m from the tree base. The campaign involved surface soil sampling (0-10 cm) at every four-days interval for a period of 36 days and covering two irrigation events. Analysis of soil samples involved gravimetric soil moisture content, microbial biomass C and N. We found that soils associated with lower tree biomass (addressed as Pecan-fine) recorded significantly higher microbial biomass over the larger trees (addressed as Pecan-coarse). Furthermore, the regression analysis of soil microbial biomass versus gravimetric soil moisture content exhibited weak dependency of soil microbial biomass on the soil moisture variations. Overall, these results conclude that Tree size have substantial influence on the soil microbial biomass whereas the moisture dynamics have weakly affected the microbial biomass C.