Stable carbon and nitrogen isotopic composition of leaves, litter, and soils of various ecosystems along an elevational and land-use gradient at Mount Kilimanjaro, Tanzania

Abstract

Variations in the stable isotopic composition of carbon (δ 13 C) and nitrogen (δ 15 N) of fresh leaves, litter, and topsoils were used to characterize soil organic matter dynamics of 12 tropical ecosystems in the Mount Kilimanjaro region, Tanzania. We studied a total of 60 sites distributed along five individual elevational transects (860- 4550ma.s.l.), which define a strong climatic and land-use gradient encompassing semi-natural and managed ecosystems. The combined effects of contrasting environmental conditions, vegetation, soil, and management practices had a strong impact on the δ 13 C and δ 15 N values observed in the different ecosystems. The relative abundance of C 3 and C 4 plants greatly determined the δ 13 C of a given ecosystem. In contrast, δ 15 N values were largely controlled by land-use intensification and climatic conditions. The large δ 13 C enrichment factors (δ 13 C litter -δ 13 Csoil) and low soil C=N ratios observed in managed and disturbed systems agree well with the notion of altered SOM dynamics. Besides the systematic removal of the plant biomass characteristic of agricultural systems, annual litterfall patterns may also explain the comparatively lower contents of C and N observed in the topsoils of these intensively managed sites. Both δ 15 N values and calculated δ 15 N-based enrichment factors (δ 15 N litter -δ 15 Nsoil/ suggest the tightest nitrogen cycling at high-elevation (>3000ma.s.l.) ecosystems and more open nitrogen cycling both in grass-dominated and intensively managed cropping systems. However, claims about the nature of the N cycle (i.e. open or closed) should not be made solely on the basis of soil δ 15 N as other processes that barely discriminate against 15 N (i.e. soil nitrate leaching) have been shown to be quite significant in Mount Kilimanjaro's forest ecosystems. The negative correlation of δ 15 N values with soil nitrogen content and the positive correlation with mean annual temperature suggest reduced mineralization rates and thus limited nitrogen availability, at least in high-elevation ecosystems. By contrast, intensively managed systems are characterized by lower soil nitrogen contents and warmer conditions, leading together with nitrogen fertilizer inputs to lower nitrogen retention and thus significantly higher soil δ 15 N values. A simple function driven by soil nitrogen content and mean annual temperature explained 68% of the variability in soil δ 15 N values across all sites. Based on our results, we suggest that in addition to land-use intensification, increasing temperatures in a changing climate may promote soil carbon and nitrogen losses, thus altering the otherwise stable soil organic matter dynamics of Mount Kilimanjaro's forest ecosystems.