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Background: Across the tropics, the share of secondary versus primary forests is strongly increasing. The high rate of biomass accumulation during this secondary succession relies on the availability of essential nutrients, such as nitrogen (N). Nitrogen primarily limits many young secondary forests in the tropics. However, recent studies have shown that forests of the Congo basin are subject to high inputs of atmospheric N deposition, potentially alleviating this N limitation in early succession. Methods: To address this hypothesis, we assessed the N status along a successional gradient of secondary forests in the Congo basin. In a set-up of 18 plots implemented along six successional stages, we quantified year-round N deposition, N leaching, N2O emission and the N flux of litterfall and fine root assimilation. Additionally, we determined the N content and C:N stoichiometry for canopy leaves, fine roots, and litter, as well as δ15N of canopy leaves. Results: We confirmed that these forests receive high amounts of atmospheric N deposition, with an increasing deposition as forest succession proceeds. Additionally, we noted lower C:N ratios, and higher N leaching losses, N2O emission, and foliar δ15N in older secondary forest (60 years). In contrast, higher foliar, litter and root C:N ratios, and lower foliar δ15N, N leaching, and N2O emission in young (< 20 years) secondary forest were observed. Conclusions: Altogether, we show that despite high N deposition, this early forest succession still shows conservative N cycling characteristics, which are likely indicating N limitation early on in secondary forest succession. As secondary succession advances, the N cycle gradually becomes more open. [ABSTRACT FROM AUTHOR]

Tropical montane forest store high amounts of soil organic carbon. However, global warming may affect these carbon stocks by enhancing soil organic carbon respiration. Better insight into temperature response of soil organic carbon respiration can be obtained from in and ex situ warming studies. In situ warming via translocation of intact soil mesocosms was carried out along an elevational transect ranging between ca. 1250 m a.s.l. in the Kibale Forest to ca. 3000 m a.s.l. in the Rwenzori Mountains. Samples from the same transect were also warmed ex situ to determine temperature sensitivity. The ex situ results revealed that along the natural climate gradient in the elevational transect, specific heterotrophic CO2 respiration decreased linearly by 1.01 ± 0.12 μg C h−1 g−1 SOC per 100 m of elevation increase. Similarly, the temperature sensitivity increased from 1.50 ± 0.13 in the lowest elevation clusters to 2.68 ± 0.25 in the highest elevation cluster, showing a linear decrease of 0.09 ± 0.03 per 100 m of elevation increase. Additionally, the 13C depletion factor of the respired CO2 decreased linearly by 0.23 ± 0.04 ‰ per 100 m of elevation increase. The results indicate an increased recalcitrance and decreased mineralisation of soil organic carbon with elevation driven by decreasing soil temperature and pH. Subsequently, after two years of in situ warming (0.9 to 2.8 °C), specific heterotrophic soil organic carbon respiration tends to be lower for warmed as compared to control soil. Further, in warmed soils, 13C and content of soil organic carbon relatively increased and decreased, respectively. This indicates increased mineralisation and depletion of readily available carbon during two years of warming. In conclusion, our results suggest that climate warming may trigger enhanced losses of soil organic carbon from tropical montane forests, due to a combination of a higher temperature sensitivity of mineralisation and soil organic carbon content. [ABSTRACT FROM AUTHOR]

Central African tropical forests face increasing anthropogenic pressures, particularly in the form of deforestation and land-use conversion to agriculture. The long-term effects of this transformation of pristine forests to fallow-based agroecosystems and secondary forests on biogeochemical cycles that drive forest functioning are poorly understood. Here, we show that biomass burning on the African continent results in high phosphorus (P) deposition on an equatorial forest via fire-derived atmospheric emissions. Furthermore, we show that deposition loads increase with forest regrowth age, likely due to increasing canopy complexity, ranging from 0.4 kg P ha−1 yr−1 on agricultural fields to 3.1 kg P ha−1 yr−1 on old secondary forests. In forest systems, canopy wash-off of dry P deposition increases with rainfall amount, highlighting how tropical forest canopies act as dynamic reservoirs for enhanced addition of this essential plant nutrient. Overall, the observed P deposition load at the study site is substantial and demonstrates the importance of canopy trapping as a pathway for nutrient input into forest ecosystems. Nowhere is biomass burning more abundant than on the African continent, but the biogeochemical impacts on forests are poorly understood. Here the authors show that biomass burning leads to high phosphorus deposition in the Congo basin, which scales with forest age as a result of increasing canopy complexity. [ABSTRACT FROM AUTHOR]

Tropical forest soils are an important source and sink of greenhouse gases (GHGs), with tropical montane forests, in particular, having been poorly studied. The understanding of this ecosystem function is of vital importance for future climate change research. In this study, we explored soil fluxes of carbon dioxide (CO 2), methane (CH 4), and nitrous oxide (N 2 O) in four tropical forest sites located on the western flanks of the Andes in northern Ecuador. The measurements were carried out during the dry season from August to September 2018 and along an altitudinal gradient from 400 to 3010 m a.s.l. (above sea level). During this short-term campaign, our measurements showed (1) an unusual but marked increase in CO 2 emissions at high altitude, possibly linked to changes in soil pH and/or root biomass, (2) a consistent atmospheric CH 4 sink over all altitudes with high temporal and spatial variability, and (3) a transition from a net N 2 O source to sink along the altitudinal gradient. Our results provide arguments and insights for future and more detailed studies on tropical montane forests. Furthermore, they stress the relevance of using altitudinal transects as a biogeochemical open-air laboratory with a steep in situ environmental gradient over a limited spatial distance. Although short-term studies of temporal variations can improve our understanding of the mechanisms behind the production and consumption of soil GHGs, the inclusion of more rigorous sampling for forest management events, forest rotation cycles, soil type, hydrological conditions and drainage status, ground vegetation composition and cover, soil microclimate, and temporal (seasonality) and spatial (topographic positions) variability is needed in order to obtain more reliable estimates of the CO 2 , CH 4 , and N 2 O source/sink strength of tropical montane forests. [ABSTRACT FROM AUTHOR]

Forests exhibit leaf‐ and ecosystem‐level responses to environmental changes. Specifically, rising carbon dioxide (CO2) levels over the past century are expected to have increased the intrinsic water‐use efficiency (iWUE) of tropical trees while the ecosystem is gradually pushed into progressive nutrient limitation. Due to the long‐term character of these changes, however, observational datasets to validate both paradigms are limited in space and time. In this study, we used a unique herbarium record to go back nearly a century and show that despite the rise in CO2 concentrations, iWUE has decreased in central African tropical trees in the Congo Basin. Although we find evidence that points to leaf‐level adaptation to increasing CO2—that is, increasing photosynthesis‐related nutrients and decreasing maximum stomatal conductance, a decrease in leaf δ13C clearly indicates a decreasing iWUE over time. Additionally, the stoichiometric carbon to nitrogen and nitrogen to phosphorus ratios in the leaves show no sign of progressive nutrient limitation as they have remained constant since 1938, which suggests that nutrients have not increasingly limited productivity in this biome. Altogether, the data suggest that other environmental factors, such as increasing temperature, might have negatively affected net photosynthesis and consequently downregulated the iWUE. Results from this study reveal that the second largest tropical forest on Earth has responded differently to recent environmental changes than expected, highlighting the need for further on‐ground monitoring in the Congo Basin. [ABSTRACT FROM AUTHOR]

The observation of high losses of bioavailable nitrogen (N) and N richness in tropical forests is paradoxical with an apparent lack of N input. Hence, the current concept asserts that biological nitrogen fixation (BNF) must be a major N input for tropical forests. However, well‐characterized N cycles are rare and geographically biased; organic N compounds are often neglected and soil gross N cycling is not well quantified. We conducted comprehensive N input and output measurements in four tropical forest types of the Congo Basin with contrasting biotic (mycorrhizal association) and abiotic (lowland–highland) environments. In 12 standardized setups, we monitored N deposition, throughfall, litterfall, leaching, and export during one hydrological year and completed this empirical N budget with nitrous oxide (N2O) flux measurement campaigns in both wet and dry season and in situ gross soil N transformations using 15N‐tracing and numerical modeling. We found that all forests showed a very tight soil N cycle, with gross mineralization to immobilization ratios (M/I) close to 1 and relatively low gross nitrification to mineralization ratios (N/M). This was in line with the observation of dissolved organic nitrogen (DON) dominating N losses for the most abundant, arbuscular mycorrhizal associated, lowland forest type, but in contrast with high losses of dissolved inorganic nitrogen (DIN) in all other forest types. Altogether, our observations show that different forest types in central Africa exhibit N fluxes of contrasting magnitudes and N‐species composition. In contrast to many Neotropical forests, our estimated N budgets of central African forests are imbalanced by a higher N input than output, with organic N contributing significantly to the input‐output balance. This suggests that important other losses that are unaccounted for (e.g., NOx and N2 as well as particulate N) might play a major role in the N cycle of mature African tropical forests. [ABSTRACT FROM AUTHOR]