Nutrient supply as driver of biomass production and associated ecosystem water fluxes along a land-use and climate gradient

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The Research Unit RESPECT has combined a hydrological, a dynamic vegetation, and a climatological model into the novel “biodiversified” Land Surface Model HUMBOL-TD. HUMBOL-TD allows for addressing the effect of trait diversity of trees on biomass production and associated evapotranspiration in land-use and climate change scenarios for the humid tropical montane forest in Ecuador. Subproject A3 has helped implementing N and P cycle modules, which improved projections of the dynamic vegetation module considerably. In Phase II of RESPECT, the study area will be expanded from the humid to the dry tropical montane forest. Subproject A3 will keep its focus on nutrient supply by mineralization and weathering to provide necessary data from the dry forest for model forcing (N and P deposition, soil texture, soil depth, soil water-holding capacity, C, N, and P availability) and for model evaluation (C and N concentrations, N mineralization rates, nutrient leaching). Subproject A3 will investigate the influence of land use (native montane dry forest and agroforestry) and climate (approximated by the elevations 600 and 1200 m above sea level) on N mineralization, nutrient release by weathering, microbial P retention and nutrient leaching on the 12 core plots of RESPECT in the dry forest. We will determine nutrient deposition and leaching in the dry forest with the help of ion-exchange resin cores and provide basic soil properties including C, N and P availability. We will determine N mineralization rates with a short-term incubation at the beginning and during the high rainy season and by measuring the vertical distribution of delta13C values and C concentrations in soil. We will quantify weathering rates by using open-system mass balances, with a pHstat approach (i.e., accelerated weathering at constant pH in the laboratory), and by measuring the vertical distribution of stable Ca isotope ratios in soil. We will assess the depth of nutrient uptake by the trees at the beginning and during the high rainy season with tracer experiments. Moreover, we will use a novel 18O-labeling approach to determine the degree of microbial P incorporation on all 30 core plots of RESPECT in the humid and dry study regions. All data will be provided at the core plot and plant functional type levels and part of the data even for individual target plants thereby contributing to the joint Response-Effect Framework analysis. Overall, Subproject A3 will deepen our understanding of the effects of climate and land-use change on major nutrient supply and loss processes in the humid and seasonal forms of a highly biodiverse, remote tropical montane forest and its anthropogenic replacement systems.