Variation of soil physical properties under tea gardens along an alluvial toposequence in Northern Vietnam

Soil structural and hydraulic properties are fundamental for regulating water retention, aeration, saturated hydraulic conductivity (Ks), and root development in perennial cropping systems. This study evaluated the spatial variability of soil physical properties in tea plantation systems developed along contrasting alluvial geomorphic settings in northern Vietnam. The study was conducted along a representative alluvial toposequence in Tan Cuong commune, Thai Nguyen province, including riverside lowlands, previous dikes, and older hillslope positions. Three tea garden sites (TB1, TB2, and TB3) and two adjacent forest sites planted with Acacia spp. (FB1 and FB3) were investigated along the toposequence. An additional tea garden site (TS1) located in Dong Hy commune was included as a comparative site representing a less favorable tea-growing environment developed on different parent materials. Soil samples were collected at two depths (0–10 and 20–30 cm) in triplicate using undisturbed and bulk soil samples for the determination of bulk density, three phase distribution, Ks, aggregate stability expressed as mean weight diameter (MWD), and soil water retention characteristics. The results revealed pronounced differences among sites, depths, and land-use types. Riverside alluvial soils (TB1 and FB1), developed from coarse-textured quartz-rich deposits, exhibited lower MWD values, weaker water retention, and lower structural stability. In contrast, soils located on previous dikes and weathered hillslopes (TB2, TB3, FB3, and TS1) showed higher aggregate stability, more differentiated pore systems, and greater water-holding capacity. At the 20–30 cm depth, TB3 exhibited significantly higher Ks, MWD, and plant-available water than TB1 and TB2 (p < 0.05), indicating more favorable structural and hydraulic conditions. Forest soils generally maintained higher aggregate stability and greater gaseous porosity than adjacent tea soils. The findings demonstrate that soil structural and hydraulic properties in tea plantation systems were closely associated with geomorphic position, pedogenic development, and land use along tropical alluvial toposequences.