Biomass is essentially a physical form of carbon. When plants grow, they pull carbon dioxide out of the air and use it to build their tissues, leaves, wood, and roots. Roughly 38 to 49% of dried plant material is pure carbon by weight. So biomass and carbon are deeply linked: living things are built from carbon, and the constant exchange of carbon between biomass and the atmosphere is one of the most important cycles on Earth.
How Plants Turn CO2 Into Physical Structure
Photosynthesis is the process that connects atmospheric carbon to biomass. Plants absorb sunlight and use that energy in two stages. First, light energy splits water molecules and creates chemical energy carriers inside the cell. Then, in a second stage that doesn’t require light directly, those energy carriers power a reaction that grabs CO2 molecules and converts them into carbohydrates and other organic molecules.
Those carbohydrates become the structural building blocks of the plant. Cellulose in cell walls, lignin that makes wood rigid, starches stored in roots and seeds: all of these are carbon-based molecules assembled from CO2 that was floating in the atmosphere days or weeks earlier. A growing tree is, in a very real sense, solidified air.
Where Biomass Stores Its Carbon
Different types of biomass hold carbon in different places, and that distinction matters. Forests store most of their carbon in woody biomass and leaves, the visible parts above ground. Grasslands do the opposite, sequestering the majority of their carbon underground in root systems and soil. This difference has big consequences for how reliably that carbon stays put.
Research from UC Davis found that grasslands are actually more resilient carbon sinks than forests in scenarios involving drought and wildfire. When fire sweeps through a forest, the carbon stored in trunks and canopy burns and returns to the atmosphere. When fire burns grasslands, the carbon fixed underground tends to stay in the roots and soil. In a stable climate, trees store more carbon overall. But under the stress of a warming climate, grasslands may prove to be the more dependable long-term storage.
Soil itself is a massive carbon warehouse. As plants die and decompose, their carbon-rich material gets broken down by microbes and incorporated into soil organic matter. Microbial biomass makes up only 1 to 3% of soil organic carbon, but those microbes are the engines driving the whole process, breaking down dead plant material and helping build the stable soil structures that keep carbon locked underground for decades or centuries.
The Fast Carbon Cycle
Carbon doesn’t stay in biomass permanently. It cycles. Plants absorb CO2 as they grow, and that carbon returns to the atmosphere when organisms die, decompose, or are eaten. This is a continuous loop: photosynthesizing organisms convert CO2 to organic carbon, and other organisms, mostly microbes, convert that organic matter back to CO2 through respiration. Roughly half of Earth’s carbon cycle consists of this respiration process.
The speed of this return trip varies enormously. A leaf that falls in autumn might release its carbon within a year as it decays on the forest floor. A massive tree trunk might hold its carbon for centuries before finally rotting. Deep in ocean sediments, microbes continue degrading organic carbon at vanishingly slow rates, sometimes holding onto small fractions of it almost indefinitely. The carbon cycle looks simple on paper, but the time constants of individual carbon flows range from seasons to millennia.
How Much Carbon Is Stored in Global Biomass
The numbers are staggering. Between 1992 and 2019, roughly 35 gigatons of carbon were sequestered on land. But here’s a surprising detail: nearly all of that gain ended up in nonliving pools like soil and dead wood, not in living plants. Live biomass changed by only about 1 gigaton of carbon over that same period. Deforestation, wildfires, and land-use change have been stripping carbon from living vegetation almost as fast as regrowth adds it.
This means the planet’s soils and dead organic matter are doing more of the heavy lifting in land-based carbon storage than the visible forests and grasslands most people picture. Protecting soil carbon is just as important as planting trees.
Biomass Carbon vs. Fossil Carbon
This is where the biomass-carbon relationship becomes most relevant to climate change. There are two fundamentally different types of carbon in play, and the distinction comes down to time.
The carbon in living biomass is part of what climate scientists call the “fast domain” of the carbon cycle. It moves between the atmosphere, plants, soil, and oceans on timescales of 1 to 500 years. Burning wood or crop residues releases carbon that was in the atmosphere recently and will be reabsorbed by new growth relatively soon. Fossil fuels, by contrast, are carbon that was locked underground for millions of years in what’s called the “slow domain,” with turnover times exceeding 10,000 years. Burning coal or oil transfers ancient carbon into the fast domain, adding to the total amount of carbon circulating near Earth’s surface. That’s the core problem with fossil fuels: they introduce carbon that the biosphere hasn’t had to deal with for geological ages.
Is Burning Biomass Actually Carbon Neutral?
The traditional assumption has been straightforward: if you burn a tree for energy, the CO2 released equals what the tree absorbed while growing, so the net effect is zero. Under ideal conditions, where new growth replaces harvested biomass at the same rate, this holds in theory.
In practice, it’s more complicated. Modeling work has shown that biomass energy is roughly “half as carbon neutral” as traditionally assumed. The main reason is soil carbon depletion. When you harvest biomass for fuel, you’re removing organic material that would otherwise decompose slowly and feed the soil carbon pool. Over time, this drains carbon from the soil, a loss that doesn’t get counted if you only measure what comes out of the smokestack versus what the next generation of plants absorbs.
There’s also a timing problem. Burning biomass releases its carbon instantly, but regrowing that biomass takes years or decades. The ocean absorbs CO2 pulses only over several decades as well. So even in a sustainably managed system, there’s a temporary period where more carbon sits in the atmosphere than would otherwise be there. For annual crops like grasses, this gap is short. For forests harvested on 50- or 80-year rotations, the “carbon debt” can last a generation.
None of this means biomass energy is equivalent to fossil fuels. It operates within the fast carbon cycle rather than injecting new carbon from deep underground. But the claim of perfect carbon neutrality oversimplifies what actually happens in soils, in the atmosphere, and across the decades it takes for regrowth to close the loop.