Abstract
Abstract. Nitrogen (N) availability can be highly variable in tropical forests on regional and local scales. While environmental gradients influence N cycling on a regional scale, topography is known to affect N availability on a local scale. We compared natural abundance of 15N isotopes of soil profiles in tropical lowland forest, tropical montane forest, and subtropical Miombo woodland within the Congo Basin as a proxy to assess ecosystem-level differences in N cycling. Soil δ15N profiles indicated that N cycling in the montane forest is relatively more closed and dominated by organic N turnover, whereas the lowland forest and Miombo woodland experienced a more open N cycle dominated by inorganic N. Furthermore, we examined the effect of slope gradient on soil δ15N within forest types to quantify local differences induced by topography. Our results show that slope gradient only affects the soil δ15N in the Miombo forest, which is prone to erosion due to a lower vegetation cover and intense rainfall at the onset of the wet season. Lowland forest, on the other hand, with a flat topography and protective vegetation cover, showed no influence of topography on soil δ15N in our study site. Despite the steep topography, slope angles do not affect soil δ15N in the montane forest, although stable isotope signatures exhibited higher variability within this ecosystem. A pan-tropical analysis of soil δ15N values (i.e., from our study and literature) reveals that soil δ15N in tropical forests is best explained by factors controlling erosion, namely mean annual precipitation, leaf area index, and slope gradient. Erosive forces vary immensely between different tropical forest ecosystems, and our results highlight the need for more spatial coverage of N cycling studies in tropical forests, to further elucidate the local impact of topography on N cycling in this biome.
Highlights
The nutrient status of forests is an important determinant for the allocation of sequestered carbon (C) to biomass (Vicca et al, 2012), with nitrogen (N) and/or phosphorus (P) limitation potentially constraining net primary productivity (NPP) (Alvarez-Clare et al, 2013; Fernández-Martínez et al, 2014; Townsend et al, 2011; Vitousek et al, 2010)
Spatial topsoil samples from each catchment (0–20 cm) reflected those of the soil profiles, showing the highest δ15N values in the lowland forest, while the montane forest displayed slightly lower values (6.48 ± 1.42 ‰); the Miombo woodland was most depleted in δ15N (2.93 ± 0.46 ‰) (Fig. 2b)
Our results showed that soil C had only a marginal influence on tropical soil δ15N and is only significantly influenced by MAT and not mean annual precipitation (MAP)
Summary
The nutrient status of forests is an important determinant for the allocation of sequestered carbon (C) to biomass (Vicca et al, 2012), with nitrogen (N) and/or phosphorus (P) limitation potentially constraining net primary productivity (NPP) (Alvarez-Clare et al, 2013; Fernández-Martínez et al, 2014; Townsend et al, 2011; Vitousek et al, 2010). Understanding nutrient cycling in forests globally is key to assess present and future forest productivity (Townsend et al, 2011; Wieder et al, 2011). Given their major role in the global C cycle (Lewis, 2006; Malhi and Grace, 2000), quantifying nutrient supply and availability is especially important for tropical forests. Tropical montane forests that lie on steep terrain are subjected to geomorphic processes that rejuvenate soils with bedrock nutrients through uplift and erosion. While in N-limited ecosystems (i.e., closed N cycle) the internal cycling and recycling rates of N are relatively more important than the absolute inputs and outputs (inputs: N2 fixation and N deposition; outputs: gaseous and leaching losses), the opposite is true for ecosystems that are rich in N (i.e., open N cycle) (Boeckx et al, 2005)
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