Sulfur fractions in particle-size separates of the sub-humid Ethiopian highlands as influenced by land use changes

Extensive deforestation of natural forests in the Ethiopian highlands and their conversion to low-input agriculture and plantations have caused severe soil degradation and significant changes in the ecosystem. We investigated the influence of land use changes on the amount, form and distribution of sulfur (S) in bulk soils and size separates of the sub-humid highlands of southern Ethiopian. Surface soil samples (0–10 cm) were collected from natural forest, tea plantation and 25 years cultivated fields at Wushwush and from Podocarpus dominated natural forest, Cupressus plantation and 30 years cultivated fields at Munesa sites. The total S contents ranged from 635 to 1082 mg kg−1 soil and from 520 to 1040 mg kg−1 soil at Wushwush and Munesa, respectively. Organic S represented on the average 98% of the total S, while inorganic SO4–S accounted for only 2%. C-bonded S was the dominant organic S fraction at both sites comprising 77–84% of the total organic S pool, whereas ester SO4–S constituted merely for 16–23%. Total S and C-bonded S were highly significantly correlated (Psilt>clay. Continuous cropping resulted in 41% and 50% depletion of total S at Wushwush and Munesa, respectively. In contrast, losses from the tea (34%) and Cupressus (13%) plantations were low. Sulfur depletions due to cultivation were lower than losses from SOC (55%, Wushwush; 63%, Munesa) and N (52%, Wushwush; 60%, Munesa), suggesting that S was more resistant to mineralization compared to SOC and N. C-bonded S accounted for 88% and 73% of the total S depletion, whereas ester SO4–S accounted only for 11% and 26% at Wushwush and Munesa, respectively. These results indicate that most of the S depletion occurred from C-bonded S. Based on our results, it is possible to conclude that S deficiency might occur in a foreseeable future, if the current depletion rate of SOM continues. Therefore, integrated soil and crop management practices have to be developed, which involve the use of organic materials of farm and non-farm origins and inorganic fertilizers to combat the ongoing nutrient depletion in the sub-humid highlands agroecosystems of southern Ethiopia.