Seminar: Dan Frederik Lange

The Amazon rainforest is typically depleted in soil inorganic nutrients and its productivity is largely considered to be limited by phosphorus (P). However, the Amazon sustains high productivity through efficient nutrient recycling. Organic matter, particularly its dissolved fraction (DOM), thereby forms a primary nutrient reserve for plants and microorganisms. We investigated soil nutrient cycling through DOM in two forest types: terra firme forests with high soil nutrient recycling and assumed P limitation, and white-sand forests with reduced nutrient recycling and potential nitrogen (N) limitation. We analyzed the molecular composition of DOM nutrient pools employing ultrahigh resolution mass spectrometry (FT-MS) and used three markers to evidence nutrient limitations. First, we observed depletion of nutrients in DOM. Secondly, we used the transformation of plant-like to microorganism-like DOM during soil passage as a proxy for microbial processing of DOM. Thirdly, we examined the temporal variation of the DOM composition in the topsoil as a marker for nutrient cycling in the litter layer and upper 5 cm of soil. We hypothesized greater processing and temporal variation of DOM for limited nutrient pools. In line with our expectations, we found strong P depletion in terra firme soils relative to the white-sands. Terra firme forests showed highest processing of N- and P-containing DOM and highest temporal variation for P-containing DOM. In contrast, white-sand soils were not depleted in N but in sulfur (S). While nutrient processing with soil depth was strongly limited in white-sand forest, the temporal variation of nutrient pools in white-sand forests was similar to terra firme with highest variability for sulfur. The high nutrient cycling and fast molecular processing of P-containing DOM confirms the hypothesized P limitation of terra firme forests. In line with our hypothesis, white-sand forests displayed minimal nutrient cycling in the soil profile. In contrast to our expectations they did not show indications for N limitation, but our FT-MS investigation suggested for the first time an elevated S demand in white-sand ecosystems.