Biogeochemie und Stickstoffkreislauf

Biogeochemistry and the Earth's Spheres

• Definition and Scope: Biogeochemistry is the study of the chemical, physical, geological, and biological processes and reactions that govern the composition of the natural environment. It operates at the intersection of several "spheres":

  • Atmosphere: The gaseous envelope surrounding the Earth.

  • Pedosphere: The outermost layer of the Earth that is composed of soil and subject to soil formation processes.

  • Biosphere: The global sum of all ecosystems; the zone of life on Earth.

  • Hydrosphere: All the waters on the Earth's surface, such as lakes and seas.

  • Lithosphere: The rigid outer part of the Earth, consisting of the crust and upper mantle.

Energy Flows vs. Material/Substance Flows

• Energy Flows (Durchflüsse):

  • Dissipative Heat: Energy is lost to the environment as heat and cannot be recovered or reused by organisms.

  • Non-recyclable: Energy cannot be recycled within an ecosystem; it must be continuously supplied (e.g., by the sun).

• Material Flows (Stoffflüsse/Biogeochemie):

  • Recyclable: Unlike energy, matter can be recycled.

  • Cycles: Because matter is preserved and transformed, material flows are typically described as "cycles" (Kreisläufe).

Methodological Approaches and Global Patterns

• Researching Biogeochemistry:

  • Stable Diffusion: Utilized in modeling cycles like the Nitrogen cycle.

  • Global Patterns and Fluxes: Understanding how elements move across the globe.

  • Nutrient Limitation: Investigating which nutrients restrict the growth of organisms in different environments (ranging from $0\%$ to $100\%$ limitation).

• Key Biodiversity Experiments:

  • Jena Experiment: Focuses on the relationship between biodiversity and nutrient cycles (Stoffkreisläufe).

• Microbial Mechanisms and the Rhizosphere:

  • Rhizosphere Research: Studying the area around plant roots where microbial activity is high.

  • Mycorrhizal Colonization: Studying how root-fungi symbioses affect carbon storage (Rillig lab).

Nutrient Cycles: Components and Input Mechanisms

• Structural Components of Nutrient Cycles:

  • Nutrient Input (Eintrag).

  • Nutrient Output (Austrag).

  • Internal Transfer: Movement and transformation within the system.

• Inputs (Einträge):

  • Weathering of Rocks (Verwitterung von Gestein): Release of minerals from the lithosphere.

  • Biological Fixation: Specifically for Nitrogen ($N$) and Carbon ($C$).

  • Fertilization (Düngung): Anthropogenic input to increase productivity.

  • Atmospheric Deposition: Nutrients falling from the atmosphere via dust or rain.

• Significance of Deposition (The Case of Phosphorus):

  • Atmospheric Contribution of P: As substrate age increases (from $1$ to $1000+$ kyr), the percentage of Phosphorus derived from atmospheric deposition increases, often reaching nearly $100\%$.

  • Long-range Transport: Dust deposition in the Pacific Ocean can originate from sources over $6,000\,km$ away (Chadwick et al. 1999, Nature).

• Nitrogen Inputs in Germany:

  • Industrial N-Fixation: $2700\,Gg\,N\,yr^{-1}$.

  • Biological N-Fixation: $270\,Gg\,N\,yr^{-1}$ (including agriculture and semi-natural ecosystems).

  • Atmospheric N-deposition: $715\,Gg\,N\,yr^{-1}$.

  • Unit Conversion: $1\,Gg = 1000\,\text{tonnes}$.

Critical Loads and Planetary Boundaries

• Critical Load for Nitrogen (N) in Germany:

  • This measures the threshold of N input below which harmful effects on sensitive elements of the environment do not occur.

  • Statistics (UBA, PINETI):

    • No exceedance: $52.4\%$.

    • Exceedance $\le 10\,kg\,N\,ha^{-1}\,a^{-1}$: $41.0\%$.

    • Exceedance $10 - 20\,kg\,N\,ha^{-1}\,a^{-1}$: $6.0\%$.

    • Exceedance > 20\,kg\,N\,ha^{-1}\,a^{-1}: $0.6\%$.

• Planetary Boundaries (Rockström et al. 2009/2023):

  • The concept identifies safe operating spaces for humanity.

  • Biogeochemical Flows: Both the Nitrogen cycle and Phosphorus cycle boundaries have been significantly crossed (the red zone).

Outputs and Internal Transfers

• Outputs (Austräge):

  • Trace-gas Emission: Release into the atmosphere (e.g., $N_2O$, $CO_2$).

  • Leaching: Movement into watercourses or groundwater.

  • Erosion: Wind and water moving soil and nutrients.

  • Fire: Combustion releasing stored nutrients.

  • Harvest: Human removal of biomass.

• Internal Transfers:

  • Transformation: Converting nutrients between organic and inorganic forms.

  • Biological Uptake: Plants and microbes absorbing nutrients.

  • Ion Exchange: Processes occurring at soil surfaces.

The Nitrogen Cycle in Detail

• Characteristics: Nitrogen ($N$) is a macroelement. It is frequently the limiting factor for Net Primary Productivity (NPP) in terrestrial systems.

• The Nitrogen Paradox:

  • $N_2$ gas makes up $78\%$ of the atmosphere.

  • Question: Why is N still limiting if it is so abundant in the air? Why don't N-fixers dominate until N is no longer limiting?

• Hypotheses for N-Limitation (Vitousek & Howarth, 1991):

  • Energy Availability: N-fixation is energy-intensive. It is limited by light, mostly occurring in early successional stages or savannas rather than closed canopies.

  • Nutrient Limitation: N-fixation requires other elements like Phosphate (for ATP) and trace metals like Molybdenum ($Mo$), Iron ($Fe$), and Sulfur ($S$).

  • Grazing Pressure: N-fixers often have high protein content, making them attractive targets for herbivores.

• Alternative Nitrogen Sources:

  • Rock Weathering: While traditionally thought to come only from the atmosphere, up to $26\%$ of global Nitrogen could come from the weathering of rocks (Houlton et al. 2018, Science).

Core Microbial Nitrogen Processes

• Perspective on Microbes: Organisms do not perform these processes as a "service" to the ecosystem; they do so to earn a "living" (energy and growth). From a human perspective, we categorize these as ecosystem services.

• Immobilization: The assimilation/uptake of inorganic N into microbial biomass. This requires energy and N for growth.

• Mineralization (Ammonification): The conversion of organic N into Ammonium ($NH_4^+$). This is an energy-yielding reaction where N is a byproduct in inorganic form.

• Nitrification: The oxidation of Ammonium ($NH_4^+$) into Nitrate ($NO_3^-$). This is performed by chemolithoautotrophic bacteria (e.g., Nitrosomonas) and Archaea (Crenarchaeota).

• Denitrification: The conversion of Nitrate ($NO_3^-$) into gaseous products (like $N_2$ or $N_2O$).

  • Mechanism: $NO_3^-$ is used as a terminal electron acceptor when Oxygen ($O_2$) is unavailable.

  • Controls (Chapin et al. 2002): Influenced by Nitrate concentration, labile carbon, oxygen levels, temperature, water, and soil texture.

• Anammox (Anaerobic Ammonium Oxidation):

  • Formula: $NH_4^+ + NO_2^- \rightarrow N_2 + 2H_2O$.

  • Discovery: Process identified in 1992; bacteria described in 1999.

  • Significance: Significant in marine environments and wastewater treatment. It is a chemolithoautotrophic process involving synproportionation (yielding energy from two different oxidation states).

Global Change and Human Impacts

• Multiple Drivers of Change: Ecosystems are exposed to cumulative pressures including Climate Change, human land use, population growth, pollution, and alien species.

• Emerging Research Areas:

  • Light Pollution: Expanding research on how artificial light affects ecosystems.

  • Antibiotic Resistance: Elevated levels of antibiotic resistance genes are now considered a factor of human-caused global environmental change (Rillig et al. 2024, Global Change Biology).