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

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.

Temperature, nutrient availability, and species traits interact to shape elevation responses of Australian tropical trees

“…We suggest that this increase in foliar δ 15 N in the nutrientrich pots can be explained by greater opportunity for N loss from the pots by δ 15 N fractionating pathways than in the case where nutrient availability is low and available N is effectively captured by the soil-plant system. In support of this, we also observed that foliar δ 15 N varied as a function of root mass (Supplementary Figure 7), indicating the influence of larger root systems in capturing available nitrogen, thereby limiting loss pathways from the soil-plant system that would otherwise leave the residual δ 15 N in the system 15 N enriched (Baumgartner et al, 2021).…”

“…Foliar δ 15 N has been observed to decrease with increasing elevation in tropical rainforest trees (Bauters et al, 2017). This is thought to be the result of more open N cycling at warm, low elevations, and with tighter N cycling at higher elevations as a result of slower N mineralization rates caused by low temperatures and less decomposable litter (Martinelli et al, 1999;Baumgartner et al, 2021). More open N cycling feeds N loss pathways from the ecosystem that tend to leave the residual N pool relatively enriched in 15 N (Martinelli et al, 1999;Craine et al, 2015).…”

Warming and redistribution of nitrogen inputs drive an increase in terrestrial nitrous oxide emission factor

“…Geographical coverage was improved by adding a further 748 samples 85 – 88 , including unpublished data for 112 samples from Australia (BASE database; 89 ) and 392 from Africa (experimental sites and set up described in refs. 90 – 96 )—regions which were underrepresented in the Craine database. The majority of data was from near-surface soil (0–50 cm depth).…”

Soil mycobiomes in native European aspen forests and hybrid aspen plantations have a similar fungal richness but different compositions, mainly driven by edaphic and floristic factors

“…All told, interpreting soil nitrogen isotope levels to describe N availability is challenging due to the many factors that influence its value. But overall, it shows the turnover of the nitrogen cycle: higher levels are more prone to N losses and lower values refer to more closed N systems where soil holds the nitrogen more ( Baumgartner et al, 2021 ). Roughly, 15 N can be used as a measure describing nitrogen availability for plants.…”