Soil physicochemical properties and microbial biomass carbon in irrigated agroecosystem of mid-hills, Nepal

Assessments of soil physicochemical and microbial properties represent an approach to identify the soil status of agroecosystem through which sustainable soil management can be achieved. This study aimed to characterize the soil physicochemical properties and soil microbial biomass carbon at three soil depths (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. The range of soil organic carbon (SOC) and organic matter (OM) contents ranged from 2.0 to 0.9% and 3.4 to 1.5%, respectively, with the highest concentrations observed in the surface layer (SOC: 2.0 ± 0.3%; OM: 3.4 ± 0.5%). Both parameters declined significantly with depth, indicating a moderate level of SOC and OM in the topsoil and markedly lower levels in deeper layers. 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− 1 in the topsoil to 110.1 ± 42.6 mg kg− 1 at 20–30 cm soil depth. 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 highlights the urgent need for targeted soil management strategies to maintain soil fertility throughout the entire soil profile.