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

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.

Ideas and perspectives: patterns of soil CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub>O fluxes along an altitudinal gradient – a pilot study from an Ecuadorian neotropical montane forest

“…Net N 2 O consumption is often related to N-limited ecosystems, and it is presumably the cause in our case. At low NO − 3 concentrations, atmospheric and/or soil gaseous N 2 O may be the only electron acceptor left for denitrification (Chapui-Lardy et al, 2007;Goossens et al, 2001). P_3010 had the lowest content of NO − 3 along with the lowest soil δ 15 N (Table 2), which clearly reflects the shift towards a more closed N cycle at higher elevations (Bauters et al, 2017;Gerschlauer et al, 2019).…”

Stable isotope signatures of soil nitrogen on an environmental–geomorphic gradient within the Congo Basin

“…Soil δ 15 N is related to the residence time of ecosystem N. Higher rates of mineralization, nitrification, and denitrification leave enriched N in the remaining soil organic matter (SOM), while the more depleted mineral N is removed via plant uptake or denitrification and transported down into deeper soil layers by leaching (Handley et al, 1999;Hobbie and Ouimette, 2009). Over the whole profile, the Miombo woodland showed the highest depletion of 15 N (Fig.…”

“…N 2 fixation depletes soil δ 15 N values, as it imports atmospheric N that has an isotopic value of 0 ‰ by default (Högberg, 1997). Aside from these processes, siteand soil-specific characteristics can control soil δ 15 N. In addition, climatic conditions such as rainfall (Austin and Vitousek, 1998;Boeckx et al, 2005), mean annual temperature (Boeckx et al, 2005), and soil moisture (Handley et al, 1999) all influence soil δ 15 N. Generally, soil δ 15 N signals the openness of the N cycle, with more enriched values in an open system that is more prone to N losses, compared to a more closed system (with the assumption that input δ 15 N are similar in both systems) (Boeckx et al, 2005). On a local scale, soil erosion alters the soil δ 15 N and leads to more depleted values (Hilton et al, 2013;Weintraub et al, 2015).…”

Consequences of removal of exotic species (eucalyptus) on carbon and nitrogen cycles in the soil-plant system in a secondary tropical Atlantic forest in Brazil with a dual-isotope approach

“…The highest soil δ 13 C value, in comparison with the remaining analyzed compartments, may be explained by the fast mineralization of the components with lower δ 13 C values (Gerschlauer et al, 2018). Differences between the δ 13 C from the distinct litter fractions and the δ 13 C from the soil are interpreted as consequences of the litter organic matter decomposition (Gerschlauer et al, 2018). During decomposition of litter, fractionation occurs due to the decomposers that preferentially use 12 C resulting in 13 C-enrichment in the remaining soil organic matter (Lichtfouse et al, 1995).…”

“…A similar pattern was observed in the eucalyptus removal sites, except for the rest litter fraction in the M3 site given the reason aforementioned. The highest soil δ 13 C value, in comparison with the remaining analyzed compartments, may be explained by the fast mineralization of the components with lower δ 13 C values (Gerschlauer et al, 2018). Differences between the δ 13 C from the distinct litter fractions and the δ 13 C from the soil are interpreted as consequences of the litter organic matter decomposition (Gerschlauer et al, 2018).…”