Agroforestry systems leveraging indigenous trees like Croton macrostachyus hold untapped potential for sustainable soil fertility management, yet their spatial biogeochemical impacts remain under-explored. In Ethiopia, several indigenous tree species are managed as parkland trees on the croplands, including Faidherbia, Ziziphus spina, Acacia sieberiana, Cordia africana, Croton macrostachyus, and Balanites aegyptiaca. This study analyses soil physicochemical properties, exchangeable cations and micronutrients under Croton macrostachyus agroforestry systems. Using a 32 composite soil sample of sixteen trees collected at four radial distances (1.5 m, 3 m, 6 m, 15 m from trunks) and two depths (0–20 cm, 20–40 cm), replicated four times. Results demonstrated significant (p < 0.05) distance and depth-dependent gradients: soil organic carbon (2.1–1.0%), total nitrogen (0.19–0.08%), and available phosphorus (18.2–8.6 mg/kg) declined steeply with increasing distance from trunks, paralleled by reductions in soil moisture (14.5–9.1%), pH (6.8–5.9), and cation exchange capacity (32–20 cmol(+)/kg). Bulk density increased by 25% (1.2–1.5 g/cm3, p < 0.001) at 15 m, while subsoil (20–40 cm) nutrient retention was 25–48% lower than topsoil. Micronutrients exhibited divergent trends, with iron (15–5 ppm), zinc (1.5–0.5 ppm), copper (3.5–1.0 ppm), and manganese decreasing with distance. These nutrients significantly decreased with depth and distance (p < 0.001). Exchangeable potassium and magnesium declined with distance from the tree trunk (p < 0.01), but calcium and sodium showed no spatial variation (p > 0.05). Depth significantly reduced SOC (p = 0.029), and iron (p = 0.0001). CEC was found to be significantly different due to the main effect of distance (p = 0.007) and soil depth (p = 0.019), with interactive distance × depth effects for zinc (p = 0.042). These findings establish C. macrostachyus as a keystone species for localised topsoil enrichment, creating nutrient-rich zones within 6 m of trunks. These insights also advocate agroecological scaling to harmonise soil health, climate resilience, and smallholder productivity, aligning with the UN Decade on Ecosystem Restoration (2021–2030) and SDGs for transformative, sustainable land-use transitions.

Spatial soil nutrient enrichment under Croton macrostachyus-based parkland agroforestry: Implications for sustainable soil fertility management

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发表日期：

2025年6月11日

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Tadele Amdemariam Kidane,
Alebel Melaku,
Yilkal Gebeyehu Mekonnen,
Birhanie Alemayehu

摘要

Agroforestry systems leveraging indigenous trees like Croton macrostachyus hold untapped potential for sustainable soil fertility management, yet their spatial biogeochemical impacts remain under-explored. In Ethiopia, several indigenous tree species are managed as parkland trees on the croplands, including Faidherbia, Ziziphus spina, Acacia sieberiana, Cordia africana, Croton macrostachyus, and Balanites aegyptiaca. This study analyses soil physicochemical properties, exchangeable cations and micronutrients under Croton macrostachyus agroforestry systems. Using a 32 composite soil sample of sixteen trees collected at four radial distances (1.5 m, 3 m, 6 m, 15 m from trunks) and two depths (0–20 cm, 20–40 cm), replicated four times. Results demonstrated significant (p < 0.05) distance and depth-dependent gradients: soil organic carbon (2.1–1.0%), total nitrogen (0.19–0.08%), and available phosphorus (18.2–8.6 mg/kg) declined steeply with increasing distance from trunks, paralleled by reductions in soil moisture (14.5–9.1%), pH (6.8–5.9), and cation exchange capacity (32–20 cmol₍₊₎/kg). Bulk density increased by 25% (1.2–1.5 g/cm³, p < 0.001) at 15 m, while subsoil (20–40 cm) nutrient retention was 25–48% lower than topsoil. Micronutrients exhibited divergent trends, with iron (15–5 ppm), zinc (1.5–0.5 ppm), copper (3.5–1.0 ppm), and manganese decreasing with distance. These nutrients significantly decreased with depth and distance (p < 0.001). Exchangeable potassium and magnesium declined with distance from the tree trunk (p < 0.01), but calcium and sodium showed no spatial variation (p > 0.05). Depth significantly reduced SOC (p = 0.029), and iron (p = 0.0001). CEC was found to be significantly different due to the main effect of distance (p = 0.007) and soil depth (p = 0.019), with interactive distance × depth effects for zinc (p = 0.042). These findings establish C. macrostachyus as a keystone species for localised topsoil enrichment, creating nutrient-rich zones within 6 m of trunks. These insights also advocate agroecological scaling to harmonise soil health, climate resilience, and smallholder productivity, aligning with the UN Decade on Ecosystem Restoration (2021–2030) and SDGs for transformative, sustainable land-use transitions.