Soil nutrients, carbon and nitrogen stocks dynamics following exclosure in the North-eastern highlands of Ethiopia

Restoration through exclosure is an effective approach in the rehabilitation of degraded land and improves the resilience of ecosystems. Thus, the study aimed to analyze the effects of ages of exclosure on soil properties, evaluate associations between soil variables, and estimate the soil carbon and nitrogen storages in the North-eastern highlands of Ethiopia. Soil samples were collected from four forest exclosure ages, viz., exclosure for 30 years (EX30), 20 years (EX20), and 10 years (EX10), and open woodland (OWL) as a control group across three slope positions (3–8%, 10–15%, and 15–30%). Seventy-two disturbed composite and undisturbed core soil samples were collected from two soil depths (0–25 cm and 20–50 cm) from all ages of exclosure. The variation and relationship in soil properties between ages of exclosure within slope positions and soil depths were analyzed using a General Linear Model (GLM) of multivariate analysis, principal component analysis (PCA), and Pearson correlation. We found significant (p < 0.001) differences in clay fraction and moisture content (MC %) among exclosures, slope positions, and soil depths, while silt content varied only by exclosure ages. However, sand content and bulk density (BD) decreased significantly with exclosure ages and slope position, while soil depth affected sand content. The study found significant (p < 0.001) differences in soil pH, organic carbon (SOC), total nitrogen (TN), available phosphorous (Av. P), exchangeable bases (Ex. Ca²⁺, K⁺, Na⁺, and Mg²⁺), and cation exchange capacity (CEC) across exclosure ages, slope positions, and soil depths, but no effect of soil depth on pH and Av. P. The correlation and principal component analyses revealed a strong and significant relationship between the increasing exclosure ages and critical soil parameters. Positive loadings were found for moisture content, soil organic carbon, available phosphorous, pH, total nitrogen, and basic cations, particularly over twenty years of exclosure. The present study revealed significant differences (p ≤ 0.001) in soil organic carbon (SOCs) and total nitrogen (TNs) content between exclosures, slope positions, and soil depths, with exclosure over 20 years had increased soil organic carbon stocks by over 30% and nitrogen stocks by 65% compared to open woodland. In general, it can be concluded that area exclosure results in better soil quality and carbon and nitrogen stocks than open woodland. Therefore, this assessment can provide essential insights into the effectiveness of conservation efforts and land management strategies in mitigating deforestation and land degradation, and serve as a benchmark for assessing progress towards national forest restoration targets by 2030.