RELIANCE: nutRiEnt Limitations In A ugaNdan tropiCal forEst

• Applicant: Oliver van Straaten, Ph.D.
• Subject Area: Ecology and Biodiversity of Plants and Ecosystems ; Soil Sciences ; Plant Cultivation, Plant Nutrition, Agricultural Technology
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 324629712

Tropical forests are highly productive ecosystems and play a critical role in absorbing anthropogenic carbon dioxide (CO2) from the atmosphere, accounting for up to half of the terrestrial biosphere's carbon (C) sequestration. For reasons not yet fully reconciled, there are indications that the C sink strength of these forests is slowly declining, thereby decreasing the buffering capacity these forests offer in mitigating global climate change. It is recognized that ecosystem nutrient limitations play a critical regulatory role in plant growth, therein affecting ecosystem C assimilation and specifically net primary production (NPP). However, the direction and magnitude of these limitations are poorly understood, especially on heavily weathered soils in lowland tropical forests. Although theoretical predictions indicate that NPP and decomposition processes should be limited by phosphorus (P), measurements suggest that there is more than enough labile P in the soil to satisfy ecosystem growth. Proposed but untested explanations to this P-paradox range from biogeochemical heterogeneity to (co)regulation of ecosystem functions by other elements, e.g. potassium (K). In contrast, lowland tropical forests tend to accumulate bioavailable nitrogen (N) to levels beyond growth requirements. Theoretically, biological N fixation, the ecosystem's main N source, should be down-regulated under N rich conditions, yet this does not appear to happen. A current theory is that there is a temporal and/or spatial decoupling of biological N fixation and N availability. However, only few in situ measurements of N, P and/or K availability and their potential limitations exist in tropical forests. The few existing ecosystem scale experiments indicate that diverse tropical forests do not follow Liebig's Law of the Minimum (growth restrictions by one limiting resource), but instead have multiple nutrient co-limitations. To understand how nutrient availability constrains C assimilation in understudied tropical forests in Africa on highly weathered soils, I propose to establish an ecosystem-scale nutrient manipulation experiment (NME) in a Ugandan tropical forest called the Budongo Forest. Here, I plan to set up a factorial experiment including eight nutrient addition treatments: N, P, K, and in combination as N+P, N+K, P+K and N+P+K, each replicated four times. At each of the thirty two 40 x 40 m plots I will measure the effects of the nutrient additions on changes in NPP (work package 1), changes in plant-available nutrients in the soil (work package 2), and changes in nutrient turnover in decomposing leaf litter (work package 3). This proposed experiment will only be the second NME conducted in tropical Africa, the second to include a K treatment, and the second conducted in a seasonally dry tropical forest.

• DFG Programme: Research Grants