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 Ž . distribution of sulfur S in bulk soils and size separates of the sub-humid highlands of southern Ž . 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 soil and from 520 to 1040 mg kg soil at Wushwush and Munesa, respectively. Organic S represented on the average 98% of the total S, while inorganic SO –S accounted for 4 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 SO –S constituted merely for 16–23%. Total S and C-bonded 4 Ž . Ž . S were highly significantly correlated P-0.001 with soil organic C SOC and N. However, correlation between ester SO –S and total S, SOC and N were weak. The CrS and NrS ratios 4 ranged from 58 to 99 and from 5.7 to 7.9, respectively, and decreased generally in the order: sand)silt)clay. Continuous cropping resulted in 41% and 50% depletion of total S at Ž . Ž . Wushwush and Munesa, respectively. In contrast, losses from the tea 34% and Cupressus 13% Ž plantations were low. Sulfur depletions due to cultivation were lower than losses from SOC 55%, . Ž . Wushwush; 63%, Munesa and N 52%, Wushwush; 60%, Munesa , suggesting that S was more ) Corresponding author. Tel.: q49-921-552248; fax: q49-921-552246. Ž . E-mail address: [email protected] D. Solomon . 0016-7061r01r$ see front matter q2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0016-7061 00 00103-8 ( ) D. Solomon et al.rGeoderma 102 2001 41–59 42 resistant to mineralization compared to SOC and N. C-bonded S accounted for 88% and 73% of the total S depletion, whereas ester SO –S accounted only for 11% and 26% at Wushwush and 4 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. q 2001 Elsevier Science B.V. All rights reserved.