Effects of land use on 15N natural abundance of soils in Ethiopian highlands

We used the natural abundance of N-15 in soils in forests, pastures and cultivated lands in the Menagesha and Wendo-Genet areas of Ethiopia to make inferences about the N cycles in these ecosystems. Since we have described the history of these sites based on variations in C-13 natural abundance, patterns of delta(15)N and delta(13)C values were compared to determine if shifts of N-15 correlate with shifts of vegetation. At Menagesha, a > 500-yr-old planted forest, we found delta(15)N values from -8.8 to +3.5 parts per thousand in litter, from -3.5 to +4.5 parts per thousand in 0-10 cm soil layer, and from -1.5 to +6.8 parts per thousand at > 20 cm soil depth. The low delta(15)N in litter and surface mineral soils suggests that a closed N cycle has operated for a long time. At this site, the low delta(13)C of the surface horizon and the high delta(13)C of the lower soil horizons is clear evidence of a long phase of C-4 grass dominance or cultivation of C-4 crops before the establishment of the forest > 500 years ago. In contrast, at Wendo-Genet, high delta(13)C of soils reveals that most of the land has been uncovered by forests until recently. Soil delta(15)N was high throughout (3.4-9.8 parts per thousand), and there were no major differences between forested, cultivated and pasture soils in delta(15)N values of surface mineral soils. The high delta(15)N values suggest that open N cycles operate in the Wendo-Genet area. From the points of view of soil fertility management, it is interesting that tall forest ecosystems with relatively closed N cycling could be established on the fairly steep slopes at Menagesha after a long period of grass vegetation cover or cultivation.