3.2 Terrestrial Carbon Cycling and Storage

Terrestrial Carbon Cycling Processes

Carbon cycles through land ecosystems through two opposing processes: photosynthesis pulls $CO_2$ out of the atmosphere, and respiration puts it back. The balance between these two fluxes, combined with how carbon behaves in soils, determines whether a terrestrial ecosystem is gaining or losing carbon over time.

Photosynthesis and Respiration

Photosynthesis is the entry point for carbon into terrestrial ecosystems. It happens in two stages:

1. Light-dependent reactions occur in the thylakoid membranes. Chlorophyll absorbs light energy, which splits water molecules and generates ATP and NADPH as energy carriers.
2. Light-independent reactions (Calvin cycle) take place in the stroma. The enzyme RuBisCO fixes atmospheric $CO_2$, and the energy from ATP and NADPH reduces it into glucose.

The net reaction:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Respiration is the reverse flux. Both plants and soil organisms break down organic compounds to release energy, returning $CO_2$ to the atmosphere. The three main stages are:

1. Glycolysis breaks glucose into pyruvate in the cytoplasm.
2. Citric acid cycle (Krebs cycle) oxidizes pyruvate in the mitochondria, generating electron carriers NADH and $FADH_2$.
3. Electron transport chain uses those carriers for oxidative phosphorylation, producing the bulk of ATP.

The net reaction:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

The difference between gross photosynthesis and total ecosystem respiration is what determines whether carbon accumulates or is lost. This difference is called net ecosystem production (NEP).

Soil Carbon in Terrestrial Cycles

Soils hold roughly two to three times more carbon than the atmosphere, making them the largest terrestrial carbon reservoir. Soil organic carbon (SOC) is a mix of living microbial biomass, dead plant material (litter), and humus, which is highly decomposed organic matter that persists for decades to millennia.

Carbon enters soils through three main pathways:

• Plant litter deposition (fallen leaves, dead roots, woody debris)
• Root exudates (sugars and organic acids that living roots release into the soil)
• Microbial biomass turnover (dead microbes become part of the soil organic matter pool)

Once in the soil, carbon exists in two broad forms:

• Particulate organic matter (POM): relatively fresh, decomposes faster
• Mineral-associated organic matter (MAOM): bound to clay and silt particles, much more stable

What determines how long carbon stays in the soil? Three stabilization mechanisms matter most:

1. Physical protection — carbon gets trapped inside soil aggregates where microbes can't easily access it
2. Chemical recalcitrance — some compounds (like lignin) are inherently harder to break down
3. Organo-mineral associations — organic molecules bind to mineral surfaces, which shields them from decomposition

Carbon leaves soils as $CO_2$ through soil respiration (microbes and roots breathing) and as dissolved organic carbon (DOC) that leaches into groundwater and streams. Soil respiration alone accounts for a massive global flux, roughly 60–80 Pg C per year.

Factors Affecting Terrestrial Carbon Storage

Land Use, Climate, and Ecosystem Responses

Several interacting factors control how much carbon terrestrial ecosystems store.

Land use change is one of the most direct drivers:

• Deforestation removes aboveground biomass rapidly and accelerates soil erosion, releasing stored carbon. Tropical deforestation alone contributes roughly 10% of global $CO_2$ emissions.
• Agricultural conversion disrupts soil structure through tillage, exposing previously protected organic matter to decomposition. Cultivated soils typically lose 20–40% of their original SOC within a few decades.
• Urbanization seals and compacts soils, cutting off carbon inputs from vegetation and altering local hydrology.

Climate change affects carbon storage through multiple pathways:

• Rising temperatures speed up microbial decomposition (releasing more $CO_2$) but can also extend growing seasons (absorbing more $CO_2$). Which effect wins depends on the ecosystem.
• Changes in precipitation alter plant productivity and soil moisture. Drier soils slow microbial activity but also stress plants; wetter soils can become anaerobic, slowing decomposition but potentially increasing methane emissions.
• Extreme events like droughts, floods, and heat waves can cause sudden carbon losses through tree mortality, erosion, and fire.

Ecosystem-specific responses vary widely:

• Forests face shifts in tree species composition and altered fire regimes. Boreal forests, for instance, are seeing more frequent and intense fires as temperatures rise.
• Grasslands are experiencing woody plant encroachment (shrubs invading grass-dominated areas) and changes in grazing pressure, both of which reshape carbon allocation between above- and belowground pools.
• Wetlands are vulnerable to peatland degradation from drainage and to coastal squeeze from sea-level rise, both of which can convert long-term carbon sinks into sources.

Ecosystems as Carbon Sinks vs. Sources

Whether an ecosystem absorbs more carbon than it releases (a sink) or releases more than it absorbs (a source) depends on the balance of several competing processes.

Sink mechanisms include:

• $CO_2$ fertilization — higher atmospheric $CO_2$ concentrations can boost photosynthesis, though this effect is often limited by nutrient availability (especially nitrogen and phosphorus)
• Increased nitrogen deposition — anthropogenic nitrogen inputs can stimulate plant growth in nitrogen-limited ecosystems
• Reforestation and afforestation — establishing new forests on previously cleared or non-forested land builds carbon stocks in both biomass and soil

Source processes include:

• Permafrost thaw — as Arctic soils warm, previously frozen organic matter decomposes, releasing $CO_2$ and $CH_4$. Permafrost soils contain an estimated 1,400–1,600 Pg of carbon globally.
• Wildfire — increasing fire frequency and intensity, particularly in boreal and Mediterranean ecosystems, releases stored carbon directly to the atmosphere
• Accelerated decomposition — warmer temperatures drive faster breakdown of soil organic matter

Ecosystem-specific sink/source potential:

• Tropical forests are highly productive (high GPP) but vulnerable to deforestation and drought
• Boreal forests store enormous carbon stocks, mostly in soils, but are increasingly threatened by fire and permafrost loss
• Temperate grasslands have high belowground carbon storage capacity because grasses allocate heavily to roots
• Peatlands accumulate carbon slowly under waterlogged, anaerobic conditions, but drainage can reverse thousands of years of accumulation in decades

Feedback loops are central to how these dynamics play out:

Carbon Budgets and Management

To quantify whether ecosystems are net sinks or sources, biogeochemists use two key metrics:

• Net Ecosystem Exchange (NEE): the net $CO_2$ flux between an ecosystem and the atmosphere, measured directly with flux towers. A negative NEE means the ecosystem is absorbing carbon.
• Net Biome Productivity (NBP): a broader measure that accounts for disturbances (fire, harvest, insect outbreaks) and lateral carbon transfers (DOC export, crop removal). NBP gives a more realistic picture of carbon balance at regional scales.

Management strategies for enhancing terrestrial carbon sequestration include:

• Agricultural practices: no-till farming reduces soil disturbance; cover crops maintain root inputs and protect soil between growing seasons
• Forest management: selective logging maintains canopy cover; prescribed burns reduce the risk of catastrophic wildfire
• Ecosystem restoration: rewetting drained peatlands, reforesting degraded land, and restoring riparian buffers all rebuild carbon stocks over time