Structural stability and near-saturated water retention of soils from four climatic regions under long-term land use versus one-time polyacrylamide treatment

Enhancing soil structure is essential for maintaining soil functions and overall soil health, and its development is strongly influenced by climate, land use, and soil type. This study evaluated near-saturation water retention—an indicator of structural condition—in soils from four climatic regions under long-term land use (>20 years) and compared these effects with a one-time application of anionic polyacrylamide (PAM). Soils from humid (USA: crop CT–conservation tillage, crop NT–no till, grass, forest), temperate (Ethiopia: crop CT, grass, bush, forest), semi-arid (Turkey: crop CT, grass, forest), and arid (Israel: crop CT, orchard, virgin) regions were analyzed (272 samples ranging from sandy loam to clay). In each region, three crop CT soils with contrasting textures were treated with PAM at 0, 25, 50, 100, and 200 mg L⁻¹ (60 samples).
Structural effects were assessed using the high-energy moisture characteristic (HEMC, 0–50 hPa). Water-retention curves were described using modified van Genuchten parameters (α and n), and structural stability was quantified as SI. Treatments produced distinct curve shapes (α = 0.036–0.099 hPa⁻¹; n = 7.1–20.6), reflecting changes in macropore domains (>250, 125–250, 60–125 μm) associated with large and small macroaggregate stability (SI = 0.002–0.060 hPa⁻¹).
Higher soil organic carbon (SOC) contents (crop CT < crop NT < grass/bush/orchard < forest/virgin) and increasing PAM rates improved α (0.054–0.096 hPa⁻¹) and SI (0.004–0.059 hPa⁻¹), while reducing n (16.0–6.3). However, the magnitude of these effects depended on soil type, texture, and climatic region. SI correlated strongly with SOC or SOC/Clay ratio in humid and temperate regions, and with SOC and clay content in arid and semi-arid regions.
Crop CT soils had the lowest SI, typically 2–4 times lower than other land-use types. Applying PAM at 25–50 mg L⁻¹ increased SI (0.007–0.033 hPa⁻¹) to levels comparable to crop NT, grass, bush, or orchard soils (0.009–0.032 hPa⁻¹). Higher PAM rates (100–200 mg L⁻¹) raised SI (0.014–0.042 hPa⁻¹) to values up to twice those of NT, grass, and orchard soils, and in some cases similar to forest or virgin soils (0.014–0.059 hPa⁻¹). PAM and SOC effects were strongest in medium- and clay-textured soils; notably, a single PAM application often improved SI more effectively than SOC, particularly in drier regions.
Across all climates, long-term NT or grass soils with SOC ≥ 2 g g⁻¹ and soils treated with 25 mg L⁻¹ PAM produced similar SI values, indicating a useful threshold for evaluating structural quality (SI ≥ 0.010–0.020 hPa⁻¹). Exponential relationships between SI and α or n (α: R² = 0.85; n: R² = 0.64, p < 0.001) can guide (I) the assessment of soil structural stability, macroporosity, and SOC; (II) the interpretation of land-use impacts on pore and aggregate-size distributions; and (III)the determination of optimal PAM rates within conservation agriculture. These relationships support the development of resilient soil structure, accelerated SOC accumulation, and site-specific management practices—particularly valuable for weakly structured, degraded soils.