Soil physiochemical properties and microbial biomass carbon in Irrigated agroecosystems of mid-hills, Nepal

Several factors influence soil properties and are directly associated with agricultural productivity. This study aimed to characterize the soil physicochemical properties and soil microbial biomass carbon at three vertical profiles (0–10 cm, 10–20 cm, and 20–30 cm) of an irrigated agroecosystem in the mid-hill region of Nepal. The results revealed that the topsoil (0–10 cm) exhibited the highest levels of temperature (21.7 ± 0.4°C), electrical conductivity (106.8 ± 5.1 µS/cm), and moisture content (29.8 ± 5.4%), with a gradual decrease in these properties at deeper layers. Soil organic carbon (SOC) and organic matter (OM) contents were highest in the surface layer (2.0 ± 0.3% and 3.4 ± 0.5%, respectively) and decreased significantly at greater depths. Similarly, total nitrogen (TN), available phosphorus (AP), and available potassium (AK) also declined with soil depth, suggesting lower nutrient levels in deeper layers. Microbial biomass carbon (MBC), a key indicator of soil biological activity, decreased sharply from 527.8 ± 142.2 mg/kg in the topsoil to 110.1 ± 42.6 mg/kg at 20–30 cm. The ratios of SOC and TN, as well as SOC and MBC, were also higher in the top soil layer and decreased with depth. Correlation analysis revealed significant positive relationships among SOC, OM, TN, AP, MBC and moisture content, indicating their collective role in maintaining soil health. Principal Component Analysis (PCA) further confirmed that the topsoil and deeper layers were distinctly differentiated by nutrient and organic matter content, with the mid-layer (10–20 cm) showing intermediate characteristics. The findings indicate that the topsoil in this paddy field has higher fertility, while the deeper layers show nutrient depletion and reduced microbial activity. This pattern suggests the need for soil management strategies to sustain fertility across depths.