Contrasting nitrogen fluxes in African tropical forests of the Congo Basin

Abstract

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.

Resource acquisition strategies facilitate Gilbertiodendron dewevrei monodominance in African lowland forests

“…In a litter manipulation experiment, they found leaf litter from both forest types to decompose at the same rate, but the actual rate of decomposition was twice as fast in mixed forest as compared to Gilbertiodendron forest. Both these findings support the suggestion that ectomycorrhizal fungi promote N limitation in Gilbertiodendron forest by depleting organic N and short circuiting the N cycle before free living decomposers can convert it to inorganic forms required by arbuscular mycorrhizal fungi‐associated species (but see Bauters et al, for different results regarding nitrate).…”

“…Letters denote statistical differences for two-way ANOVA (see . Total mass was log transformed for statistical analysis (see Tables S4.1-S4.4 for statistical details) forest by depleting organic N and short circuiting the N cycle before free living decomposers can convert it to inorganic forms required by arbuscular mycorrhizal fungi-associated species (but see Bauters et al, 2019 for different results regarding nitrate).…”

“…Research assessing the role of the association between G. dewevrei and ectomycorrhizal fungi has been inconclusive (e.g. Bauters et al, ; Kearsley et al, ; Peh, Lewis, et al, ; Peh, Sonké, et al, ; Torti et al, ). We hypothesized that (2) soils below G. dewevrei have higher carbon to nitrogen (C:N) and carbon to phosphorus (C:P) ratios than mixed species forest soils, indicators of ectomycorrhizal fungi facilitating its ability to uptake organic forms of these nutrients (Averill, Turner, & Finzi, ; Corrales et al, ; Zhu, McCormack, Lankau, Egan, & Wurzburger, ).…”

Long‐term recovery of the functional community assembly and carbon pools in an African tropical forest succession

“…The Gilbertiodendron forest, as an important other climax forest situation in Central Africa, showed marginally lower diversity indices, with yet another set of unique species. This special climax situation is not yet fully explained (Corrales et al., ; Kearsley et al., ; Peh, Sonke, Lloyd, Quesada & Lewis, ), but it is likely that association with ectomycorrhiza and edaphic factors is at the basis of the monodominance, while N cycling is unlikely to be a determining factor (Bauters, Verbeeck et al., ).…”

Seasonality, drivers, and isotopic composition of soil CO<sub>2</sub> fluxes from tropical forests of the Congo Basin

“…Old-growth forest sites in the Democratic Republic of Congo (DRC), contrasting in altitude, were selected to conduct longterm static manual chamber CO 2 flux measurements. The first site (KB) is situated in the Kahuzi-Biéga National Park (S 02.215 • , E 28.759 • ) northwest of the city of Bukavu in the South-Kivu province and represents a montane tropical forest at an altitude of 2120 m a.s.l with an annual mean temperature of 15°C and an average annual rainfall of 1500 mm (Bauters et al, 2019). Rainfall peaks in both April and October, with a dry season from June to September in between (Alsdorf et al, 2016).…”