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Biogeochemistry in temperate forest of southern Chile: C and N fluxes in pristine environments versus sites with change in land use

Leitung:Prof. Dr. Roberto Godoy, Dr. Jens Boy
Förderung durch: FONDECYT (Fondo Nacional de Desarollo Cientifico y Tecnologico)

Old-growth forests in Chile are an important part of the global temperate forest reserve. They are characterized by a comparably high efficiency in nutrient cycling, particularly under extreme climatic conditions like high rainfall (> 7000 mm yr-1). In addition, they are considered relatively unpolluted compared with forest ecosystems of the Northern Hemisphere, so they can act as blueprints for preindustrial deposition regimes in biogeochemical research. The contribution of long-distance aerosol transport to nutrient inputs to Chilean forests is still little known and crucial for estimating the anthropogenic impact on- and the resulting loss of self-sustainability for- forest ecosystems in remote mountain areas of Chile. The low availability of nitrogen along with low atmospheric input both produce a strong dependence on symbiotic (e.g. mycorrhizal) and non-symbiotic N-fixation at stand level, further supporting already highly efficient nutrient cycling mechanisms. Moreover, temperate forest ecosystems store large amounts of carbon (C), which are controlled by the rates of net primary production and decomposition of soil organic matter (SOM). Recent studies indicate that especially C-accumulation in deeper soil depths contributes significantly to carbon sequestration in old-growth forests. Therefore, it is necessary to understand nutrient dynamics in the whole soil profile (topsoils and subsoils) in order to minimize losses through gas emission and leaching by better management strategies. The hypotheses of this study are:

H1. Contrasting to polluted ecosystems of the Northern Hemisphere, which loose the majority of N as dissolved inorganic nitrogen (DIN), the major pathway for N-losses in forests under low N-deposition regime is dissolved organic nitrogen (DON). After deforestation of Chilean old-growth forests, N losses will increasingly occur as DIN instead of DON, since DON production collapses and DIN release by microorganisms rise.

H2.  Anthropogenic disturbance (deforestation) causes emissions of greenhouse gases (CO2, CH4, and N2O) by changing microorganism communities and enzymatic activities in the soil. These emissions are substantially higher as to be expected by the decomposition of remaining dead biomass after deforestation. They are fed by subsoil C- and N-stocks which are irreversibly depleted by this process. In contrast, subsoil stocks are not mobilized under native vegetation in dynamic equilibrium, thus demonstrating the high value of native Chilean forests for climate protection.

H3. Nutrient depositions from distant source regions taken alone are not explaining elevated nutrient depositions to pristine Chilean forest. The majority of nitrogen and sulfur loads are received when wind directions favor transport from agriculturally used areas in the Central Valley, whereas base cations and phosphorus depositions originate from distant source regions. 

H4. This elevated nutrient deposition triggers nutrient losses from the ecosystem by spreading the N:P:base cation ratio beyond growth optimum conditions of the forest ecosystem. This leaching of unused nitrogen and sulphur species furthermore depletes alkali- and earth alkali stocks by simultaneous transport of base cations due to charge equilibrium in the soil solution. These processes undermine the self-sustainability of Chilean forest ecosystems.

H5. Luxurious growths conditions deplete mycorrhizal symbiosis, thus not only lowering nutrient cycling rates but also mycorrhiza-induced weathering. Aside atmospheric deposition, weathering is the only external source (=not bound to nutrient cycling) of base cations and P. Thus, resilience of Chilean slow growing forests is minimized by anthropogenically induced nitrogen deposition by preventing natural long-term stand amelioration.

H6. Comparing Chilean pristine and land-used forest ecosystems, nutrient losses by land use are not overcome by atmospheric deposition, also if including local anthropogenic sources. The weathering rates under land-used systems break down, since mycorrhiza is increasingly absent. Both processes taken together degrade growth conditions for the native vegetation below resilience thresholds, thus making future recovery of indigenous vegetation impossible.



The general objective of this research is to provide the scientific baseline information on the biogeochemical cycles in temperate rainforests and the quantification of nutrient fluxes as inputs from- and emissions to- the atmosphere. Furthermore, investigating into the overall nutrient budgets of pristine and land-used microcatchments aims to understand the long-term sustainability of Chilean forest ecosystems by elucidating whether atmospheric nutrient deposition and nutrient yields of mineral weathering taken together can overcome nutrient losses via outgassing and leaching from the ecosystem.

Specific objectives.

  1. Select contrasting experimental sites (control situation vs. change in land use) and to characterize the structure, composition of vegetation, and microclimatic variables.
  2. Quantify the hydrological and chemical fluxes  in the pristine ecosystem and their changes after deforestation in a whole-catchment approach including monitoring in all compartments.
  3. Assess GHG emissions of  CO2, CH4 and N2O before and after  land-use change.
  4. Describe soil profiles and determine their physico-chemical parameters under the selected microcatchments.
  5. Identify the microorganism communities associated with C- and N-cycling qualitatively and quantitatively in the soil of the experimental plots.
  6. Determine the content of C and N in biomass, dead biomass and soil organic matter and relating this to the biological activity (enzymes) in soil in order to establish potential bioindicators for nutrient flux estimation.
  7. Estimate the influence of elevated nitrogen deposition to mycorrhization, with special emphasis on its impact on mycorrhiza induced weathering by in-situ experiments comparing pristine and land-used forest ecosystems in the Cordillera de los Andes.
  8. Understand the long-term sustainability of Chilean forest ecosystems by investigating into the overall nutrient budget, thus elucidating whether atmospheric nutrient deposition and nutrient yields of mineral weathering can overcome nutrient losses via leaching from the ecosystems.
  9. Model nutrient fluxes with a proven nutrient cycling model (e.g. NuCM) and test resilience scenarios of Chilean old-growth forests after its validation.