Abstract. Variations in the stable isotopic composition of carbon (δ13C)
and nitrogen (δ15N) 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
(860–4550 m a.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 δ13C and
δ15N values observed in the different ecosystems. The relative
abundance of C3 and C4 plants greatly determined the
δ13C of a given ecosystem. In contrast, δ15N
values were largely controlled by land-use intensification and climatic
conditions. The large δ13C enrichment factors
(δ13Clitter − δ13Csoil) 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 δ15N values and calculated
δ15N-based enrichment factors
(δ15Nlitter − δ15Nsoil) suggest the tightest nitrogen cycling at
high-elevation (> 3000 m a.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 δ15N as other
processes that barely discriminate against 15N (i.e. soil nitrate
leaching) have been shown to be quite significant in Mount Kilimanjaro's forest
ecosystems. The negative correlation of δ15N 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 δ15N
values. A simple function driven by soil nitrogen content and mean annual
temperature explained 68 % of the variability in soil δ15N
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.
Role of Micro-Topographic Variability on the Distribution of Inorganic Soil-Nitrogen Age in Intensively Managed Landscape
