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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.

Information on soil properties is crucial for soil preservation, the improvement of food security, and the provision of ecosystem services. In particular, for the African continent, spatially explicit information on soils and their ability to sustain these services is still scarce. To address data gaps, infrared spectroscopy has achieved great success as a cost-effective solution to quantify soil properties in recent decades. Here, we present a mid-infrared soil spectral library (SSL) for central Africa (CSSL) that can predict key soil properties, allowing for future soil estimates with a minimal need for expensive and time-consuming wet chemistry. Currently, our CSSL contains over 1800 soil samples from 10 distinct geoclimatic regions throughout the Congo Basin and along the Albertine Rift. For the analysis, we selected six regions from the CSSL, for which we built predictive models for total carbon (TC) and total nitrogen (TN) using an existing continental SSL (African Soil Information Service, AfSIS SSL; n=1902) that does not include central African soils. Using memory-based learning (MBL), we explored three different strategies at decreasing degrees of geographic extrapolation, using models built with (1) the AfSIS SSL only, (2) AfSIS SSL combined with the five remaining central African regions, and (3) a combination of AfSIS SSL, the remaining five regions, and selected samples from the target region (spiking). For this last strategy we introduce a method for spiking MBL models. We found that when using the AfSIS SSL only to predict the six central African regions, the root mean square error of the predictions (RMSEpred) was between 3.85–8.74 and 0.40–1.66 g kg−1 for TC and TN, respectively. The ratio of performance to the interquartile distance (RPIQpred) ranged between 0.96–3.95 for TC and 0.59–2.86 for TN. While the effect of the second strategy compared to the first strategy was mixed, the third strategy, spiking with samples from the target regions, could clearly reduce the RMSEpred to 3.19–7.32 g kg−1 for TC and 0.24–0.89 g kg−1 for TN. RPIQpred values were increased to ranges of 1.43–5.48 and 1.62–4.45 for TC and TN, respectively. In general, predicted TC and TN for soils of each of the six regions were accurate; the effect of spiking and avoiding geographical extrapolation was noticeably large. We conclude that our CSSL adds valuable soil diversity that can improve predictions for the Congo Basin region compared to using the continental AfSIS SSL alone; thus, analyses of other soils in central Africa will be able to profit from a more diverse spectral feature space. Given these promising results, the library comprises an important tool to facilitate economical soil analyses and predict soil properties in an understudied yet critical region of Africa. Our SSL is openly available for application and for enlargement with more spectral and reference data to further improve soil diagnostic accuracy and cost-effectiveness.

Question: Do termitophilous and non-termitophilous trees of dry tropical woodlands show local adaptation? Location: Region of Lubumbashi, Upper Katanga, DR Congo. Methods: Three pairs of congeneric tree species showing strict edaphic specialization with respect to termite mounds, Combretum molle (termitophilous, T)/C. collinum (non-termitophilous, NT); Strychnos potatorum (T)/S. spinosa (NT), Ziziphus mucronata (T)/Z. abyssinica (NT), were used in a reciprocal transplant experiment in situ. Seedlings were reciprocally transplanted on termite mounds and in the surrounding matrix in a miombo woodland. Growth (height and number of leaves) and survival were monitored for 30 months. Soil physical and chemical properties, and available water, were assessed on and off mounds. Results: Growth was little affected by habitat; only one species showed better growth in its home habitat (Strychnos spinosa in the matrix). Survival was strongly affected by habitat, in opposite directions consistent with species' habitat specialization. Termitophilous species experienced a very high mortality rate in the matrix, especially during the dry season. Available water content was higher in termite mound soil than in the matrix soil. Conclusions: Termitophilous and non-termitophilous tree species show local adaptation at the seedling stage, expressed mostly as different patterns of mortality in the dry season. The results point to water supply as a critical factor in the edaphic specialization of termitophilous species. In contrast, the higher mortality of non-termitophilous species on termite mounds is not explained by water stress.

BackgroundAcross 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.MethodsTo 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.ResultsWe 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.ConclusionsAltogether, 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.

Context The recent expansion of commercial agriculture in the Miombo woodlands of central Africa has led to widespread levelling of termite mounds. These mounds contain significantly lower soil organic carbon (SOC) than surrounding soils, and their levelling could largely reduce SOC content in the plough layer, which remains understudied. Objectives We aim to investigate the effects of mound levelling on SOC of the plough layer in a 1.5 km 2 plot used for commercial farming and quantify the contribution of pre-existing termite mounds to SOC variation in the levelled cropland. Methods Before and after levelling, we conducted unmanned aerial vehicle (UAV) surveys with structure-from-motion (SfM) technique, and paired soil sampling (0−25 cm) in between-mound areas. Results Termite mounds were regularly distributed but morphologically heterogeneous in the plot, with volumes ranging from 7.2 m 3 to 820.9 m 3 . Large termite mounds clustered in areas with higher topographic wetness index (TWI). Three years after levelling, SOC content in the plough layer of the plot overall reduced by 26% but variability increased by 29%. In the levelled plot, mound morphology, soil texture, and TWI explained over 40% of SOC variation, with mound morphology (characterized by hypsometrical integral, HI) being most influential. Older, larger mounds (with lower HI) were associated with lower SOC after levelling. Conclusions The immediate and significant reduction of SOC content in the plough layer due to termite mound levelling in commercial farming may affect productivity. Further research is needed to assess its long-term agricultural and ecological impacts at larger scales.

Aquatic losses of nutrients are important loss vectors in the nutrient budgets of tropical forests. Traditionally, research has focused mainly on losses of inorganic nutrient forms, whereas the potential contribution of organic and particulate losses to the total nutrient export budget is much less constrained. In this study, we quantified full aquatic nitrogen (N) and phosphorus (P) exports, including inorganic, organic and particulate forms, from a moist tropical lowland forest and a semi-dry Miombo woodland forest within the Congo Basin. While particulate organic N (PON) was the highest N loss vector in the lowland stream (3.34 kg N ha−1 y−1; 44% of TN), dissolved organic N (DON) dominated the export in the Miombo stream (1.41 kg N ha−1 y.−1; 47% of TN). Aquatic P export was dominated by dissolved organic P (DOP) in both streams, with yields of 0.29 kg P ha−1 y−1 (65% of TP) in the lowland and 0.24 kg P ha−1 y−1 (69% of TP) in the Miombo. Storm events were driving those losses, exporting disproportionally high N and P loads during short periods of stormflow conditions (32% and 47% of TN and 20% and 40% of TP in the lowland and Miombo, respectively). Our results highlight the need to take particulate and organic forms into account as important loss vectors in the nutrient balance of tropical forests. This finding is of particular importance considering the projected increasing rainfall intensities in many tropical regions which might exacerbate the export of these nutrient forms in the near future.