Soil Carbon and Nitrogen Stock and Their Spatial Variability Along an Exclosure Chronosequence at Kewet District, Central Dry Lowlands of Ethiopia

Empirical evidence and a better explanation of the effect of exclosures on soil properties are needed to rehabilitate degraded land and properly utilize the restored ecosystem. This study was conducted to determine soil organic carbon (SOC) and total nitrogen (TN) stocks and to map their spatial distribution and aggregate stability along open grazing land, 5, 15, and 20 years exclosure, and three slope positions. To map the spatial distribution of SOC and TN stocks an ordinary kriging interpolation method was applied. The results showed that the age of exclosure had significantly (p < .05) affected SOC and TN stock. Soil organic carbon stock was the highest in the 15-year-old (18.43 Mg ha−1) and lowest (14.22 Mg ha−1) in the 5-year-old exclosures. Similarly, the 15-year-old (1.81 Mg ha−1) and 5-year-old (1.41 Mg ha−1) exclosures had the highest and the lowest TN stock, respectively. Soil organic carbon associated with macroaggregates (>250 µm) and microaggregates (<250 µm) varied significantly (p < .05) between ages of exclosures and adjacent open grazing land. Significantly (p < .05) higher SOC stock (16.99 Mg ha−1) and macroaggregate associated SOC (3.05%) were recorded in the upper slope position as compared to the middle and lower slope positions. Due to the variation in vegetation cover and density and topography of the area, both SOC and TN stock showed high spatial variability across all ages of exclosure and adjacent open grazing land. Despite its inconsistency, the age of exclosure had affected SOC and TN stock, mean weight diameter, water-stable aggregates, and aggregate associated SOC. It is suggested that exclosure as a restoration measure of degraded landscapes can sequester and stock a significant amount of atmospheric CO2. Further study on soil organisms and litterfall is suggested to understand the dynamics of SOC and TN stocks in these exclosures.