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 (N 2 O) flux measurement campaigns in both wet and dry season and in situ gross soil N transformations using 15 N‐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., NO x and N 2 as well as particulate N) might play a major role in the N cycle of mature African tropical forests.