🪨Biogeochemistry Unit 9 – Biogeochemistry of Terrestrial Ecosystems

Key Concepts and Definitions

• Biogeochemistry studies the interactions between biological, geological, and chemical processes in ecosystems
• Terrestrial ecosystems include land-based habitats (forests, grasslands, deserts, and tundra)
• Biotic components consist of living organisms (plants, animals, and microorganisms)
• Abiotic components encompass non-living elements (soil, water, air, and minerals)
• Biogeochemical cycles describe the movement of essential elements (carbon, nitrogen, phosphorus, and water) through ecosystems
• Primary producers convert inorganic compounds into organic matter through photosynthesis
• Consumers obtain energy and nutrients by feeding on other organisms
• Decomposers break down dead organic matter, releasing nutrients back into the ecosystem

Ecosystem Components and Interactions

• Producers, consumers, and decomposers interact through complex food webs
  • Producers (plants) form the base of the food web, converting solar energy into chemical energy
  • Primary consumers (herbivores) feed on producers, while secondary and tertiary consumers (carnivores) feed on other consumers
  • Decomposers (bacteria and fungi) break down dead organic matter, recycling nutrients
• Symbiotic relationships, such as mutualism and commensalism, facilitate nutrient exchange and survival
• Competition for resources (light, water, and nutrients) shapes community structure and diversity
• Trophic cascades occur when changes in one trophic level affect multiple levels of the food web
• Ecosystem engineers (beavers and earthworms) modify their environment, creating habitats for other species
• Disturbances (fires, storms, and human activities) alter ecosystem composition and function
  • Succession is the gradual process of ecosystem recovery following a disturbance

Biogeochemical Cycles

• Carbon cycle involves the exchange of carbon between the atmosphere, biosphere, hydrosphere, and geosphere
  • Photosynthesis fixes atmospheric carbon dioxide into organic compounds
  • Respiration and decomposition release carbon back into the atmosphere
  • Carbon storage occurs in biomass, soil organic matter, and fossil fuels
• Nitrogen cycle includes nitrogen fixation, nitrification, denitrification, and ammonification
  • Nitrogen-fixing bacteria convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$)
  • Nitrifying bacteria convert ammonia into nitrite ($NO_2^-$) and nitrate ($NO_3^-$)
  • Denitrifying bacteria convert nitrate back into atmospheric nitrogen
• Phosphorus cycle is sedimentary, with weathering and erosion releasing phosphorus from rocks
  • Plants absorb phosphorus as phosphate ions ($PO_4^{3-}$)
  • Decomposition of organic matter returns phosphorus to the soil
• Water cycle (hydrologic cycle) involves evaporation, transpiration, condensation, and precipitation
  • Transpiration is the process by which plants release water vapor through their leaves

Energy Flow in Terrestrial Ecosystems

• Solar energy is the primary source of energy for terrestrial ecosystems
• Gross primary productivity (GPP) is the total amount of energy captured by producers through photosynthesis
• Net primary productivity (NPP) is the energy remaining after accounting for plant respiration
  • NPP = GPP - Respiration
• Secondary productivity refers to the energy transfer from producers to consumers
• Ecological efficiency is the percentage of energy transferred from one trophic level to the next
  • Typically, only 10% of the energy is transferred to the next trophic level
• Energy is lost through heat dissipation, metabolic processes, and incomplete digestion
• Biomass pyramids represent the distribution of energy and matter across trophic levels

Nutrient Cycling and Availability

• Nutrient availability is a key factor limiting plant growth and ecosystem productivity
• Soil properties (texture, pH, and organic matter content) influence nutrient retention and availability
  • Clay soils have a higher cation exchange capacity (CEC), allowing them to retain more nutrients
  • Soil pH affects nutrient solubility and microbial activity
• Mycorrhizal fungi form symbiotic relationships with plant roots, enhancing nutrient uptake
• Litterfall and root turnover contribute to soil organic matter formation and nutrient cycling
• Nutrient limitation occurs when one or more essential nutrients are scarce, limiting plant growth
  • Liebig's Law of the Minimum states that plant growth is limited by the nutrient in shortest supply
• Anthropogenic activities (fertilizer application and acid deposition) alter nutrient balances in ecosystems

Human Impacts on Terrestrial Biogeochemistry

• Land-use change (deforestation and urbanization) alters ecosystem structure and function
  • Deforestation reduces carbon storage, biodiversity, and water regulation
  • Urbanization increases impervious surfaces, altering hydrologic processes and nutrient runoff
• Agriculture intensification affects nutrient cycling and soil health
  • Overuse of fertilizers can lead to eutrophication of nearby water bodies
  • Tillage practices accelerate soil erosion and carbon loss
• Fossil fuel combustion increases atmospheric carbon dioxide concentrations, contributing to climate change
• Acid deposition (acid rain) alters soil pH and nutrient availability, impacting plant growth
• Invasive species disrupt native ecosystem processes and nutrient cycling
• Ecosystem restoration and sustainable land management practices aim to mitigate human impacts

Research Methods and Tools

• Field observations and experiments provide insights into ecosystem processes and interactions
  • Litter traps measure litterfall and nutrient inputs to the soil
  • Soil cores are used to assess soil properties and nutrient content
• Remote sensing techniques (satellite imagery and aerial photography) monitor land cover change and vegetation dynamics
• Stable isotope analysis tracks the movement of elements through ecosystems
  • Carbon isotopes ($^{12}C$ and $^{13}C$) are used to study carbon cycling and sources
  • Nitrogen isotopes ($^{14}N$ and $^{15}N$) help trace nitrogen transformations and origins
• Ecosystem models simulate biogeochemical processes and predict ecosystem responses to environmental changes
  • Process-based models incorporate mechanistic understanding of ecosystem functions
  • Data-driven models rely on empirical relationships derived from observations
• Long-term ecological research (LTER) sites provide valuable data on ecosystem dynamics over extended periods

Case Studies and Real-World Applications

• Hubbard Brook Experimental Forest (New Hampshire, USA) has been a pioneer in studying nutrient cycling and the effects of forest disturbances
  • The discovery of acid rain's impact on forest ecosystems led to changes in environmental policies
• Yellowstone National Park (Wyoming, USA) showcases the importance of large predators in shaping ecosystem structure and function
  • The reintroduction of wolves triggered a trophic cascade, altering elk behavior and promoting riparian vegetation growth
• The Amazon rainforest is a critical carbon sink and biodiversity hotspot, but deforestation threatens its ecological integrity
  • Deforestation alters regional climate patterns, reduces carbon storage, and impacts global biogeochemical cycles
• The Sahel region (Africa) has experienced desertification due to overgrazing, drought, and unsustainable land management practices
  • Restoration efforts focus on planting trees, implementing sustainable grazing practices, and improving soil health
• Permafrost thaw in the Arctic releases stored carbon and methane, amplifying global warming
  • Thawing permafrost alters local hydrology, vegetation patterns, and nutrient cycling