Biomass: what it is, how it is calculated, and how it is distributed

Biomass gives us an interesting picture of how many living things there are in the world.

Bioelements, as their name suggests, are the chemical elements of the periodic table that make up the different living things on the planet. Although life is made up of about 30 elements, 96% of the cell mass of almost every taxon you can think of is made up of only six of them: carbon, oxygen, nitrogen, hydrogen, phosphorus and sulfur. These elements give rise to proteins, vitamins, nucleic acids, lipids, carbohydrates and many other compounds, so conceiving life without them is an impossible task.

The organic matter present on Earth is not fixed, but is transformed through the use of energy. For example, a plant grows thanks to light energy and inorganic compounds present in the soil, transforming minerals into carbon. This mass is consumed by a herbivorous animal, then by a carnivore and then by a superpredator, until it dies. At this point, all the accumulated matter decomposes in the soils and we restart the cycle again.

The trophic chains in ecosystems modulate this flow of energy, i.e., the "who eats whom" conditions the functioning of environments and, therefore, of all life in the environment. In any case, to understand the exchange of energy in different Biological systems, it is necessary to describe extensively a term of great interest: biomass.. Today we tell you all about it, so read on.

What is biomass?

Biomass is the mass of living biological organisms present in a given ecosystem at a given time.. The weight can be determined at the level of a particular taxon or population (species biomass) or comprising all living elements cohabiting in the environment (community biomass). Biomass is distributed in terrestrial ecosystems in a pyramidal fashion in the trophic chain, from the primary producers at the base to the superpredators at the top.

It should be noted that biomass is not 100% utilized at all ecosystem levels. Let us explain. At the ecological level, of all the biomass consumed by a cow in the form of grass (100% of the energy), only 10% will pass to the next trophic level. Mammals must burn the organic matter they consume to forage, reproduce, produce heat and ultimately live, so only a tiny part of the energy obtained from biomass passes from level to level in the chain. Fortunately, solar energy is "unlimited", so this loss should not be noticeable in a healthy ecosystem as long as there are plants performing photosynthesis.

A term interrelated with biomass is bioenergy, which refers to obtaining energy from the sun.Bioenergy refers to obtaining renewable energy in the human sector by using organic matter (either treated naturally in the ecosystem or mechanically). Biomass and bioenergy are two sides of the same coin, but the former term generally refers to a natural event, while the latter has a clear anthropogenic applicability.

Earth's biomass, in raw data

In 2018, the research The biomass distribution on Earth was published in the scientific portal PNAS, which sought to estimate the biomass on the entire Earth in the form of carbon (C), the quintessential organic component of living things.. A total of 550 gigatons of carbon were calculated, distributed among the different living taxa as follows:

• Plants were the dominant producer kingdom. Plants are responsible for storing 450 gigatons of carbon, or 80% of the total. They are the primary producers of any normal ecosystem.
• Behind them, you will be surprised to learn that bacteria are found, providing about 70 Gt, 15% of the total carbon. Although we cannot see them, these microorganisms are everywhere.
• Fungi, archaea and protists rank third, fourth and fifth respectively, with 12, 7 and 4 Gt total.
• To the shame of the evolutionary pinnacle, we animals only assume 2 gigatons of carbon: only viruses contribute less than us, with 0.2 Gt.

Moreover, this study calculated that the amount of terrestrial biomass is two orders larger than that of marine biomassbut it is estimated that biota in the aquatic environment contribute a total of about 6 gigatons of carbon, which is by no means a negligible figure. As you can see, most of the Earth's organic matter is found in microorganisms and plants.

Calculating biomass

Calculating the total biomass produced in an ecosystem is an extremely difficult task, although new technologies (such as the Laser Vegetation Imaging Sensor) help researchers to make fairly reliable estimates, at least when it comes to quantifying the vegetation carbon in an environment. Due to the intrinsic complexity of taking into account all the living elements of the biome, we have to resort to equations and regression methods, i.e. calculate the biomass produced by an individual and then extrapolate this value to the total population..

To give you an idea of how biomass can be calculated, we will take a petri dish with microorganisms, the smallest scale we can think of. To estimate the carbon, the following equation is followed:

Biomass (in micrograms of carbon/milliliter of sample): N x Bv X F.

In this equation, N represents the number of microorganisms counted in a milliliter of sample, Bv is the biovolume is what each microorganism occupies (in µm^3 scale) and F is the carbon conversion factor, in µg of C per µm^3. As you can see, quantifying the biomass in a sample is not at all simple, even when we move on microscopic scales.

Productivity and biomass

A term completely linked to biomass is ecological productivity.. This parameter is defined as the production of organic matter in a given area per unit of time, i.e., the amount of biomass generated in a natural ecosystem or human-made system.

The most common unit used to quantify productivity in an ecosystem is kilograms/hectare per year, although other weight scales can be used (tons, gigatons), surface area (square meters, square centimeters, etc.) and even time (days, hours, decades). It all depends on the usefulness and focus of the study in question that is trying to obtain specific parameters.

Let's take an example. Suppose we have an area of 40 hectares that was initially empty, but has been repopulated with plants that, on average, weigh 1 kilogram. In total we counted about 1,000 plants of the species of interest at the end of the year, which gives us, consequently, 1,000 kilograms of total mass (species biomass). If we make the relevant calculations (1,000 kg/ 40 Ha), we will obtain that, in total, the productivity has been 25 kg/Ha/year.

This hypothetical model presents a high productivity rate, but things change a lot if we talk about animals. Now think of a population of cows that, for example, need a land area of 20,000 hectares to thrive. As much as these livestock mammals weigh, they will be fewer total individuals than plants and, in addition, the foraging ground is wider, which gives us a much lower total biomass produced.

In addition to this, it is necessary to take into account the previous point: the energy that jumps from link to link in the chain is only 10% of the total biomass produced.. Cows spend 90% of their energy on living, so a primarily plant-based ecosystem is always more productive than one with abundant animals. In any case, natural selection does not "seek" to maximize productivity, but to maintain a stable long-term balance among all components. Therefore, when exotic species are introduced into an ecosystem, the outcome is usually disastrous.

Summary

To put everything we have learned into perspective, we will compare two concrete cases: plant productivity (primary) in a desert is less than 0.5 grams/square meter/day, while in a crop field the value ranges up to 10 grams/square meter/day. The more plants present in an ecosystem, the more biomass there is and, therefore, the higher the productivity rate.

To summarize, biomass reflects the amount of organic matter at a particular location and site, while productivity refers to the speed and effectiveness with which this organic matter is produced.. These parameters help us to understand the functioning of natural ecosystems, but also allow us to maximize material and economic benefits when exploiting the land for human purposes.

Bibliographical references:

• Bar-On, Y. M., Phillips, R., & Milo, R. (2018). The biomass distribution on Earth. Proceedings of the National Academy of Sciences, 115(25), 6506-6511.
• Brown, S. (1997). Estimating biomass and biomass change of tropical forests: a primer (Vol. 134). Food & Agriculture Org.
• Cai, J., He, Y., Yu, X., Banks, S. W., Yang, Y., Zhang, X., ... & Bridgwater, A. V. (2017). Review of physicochemical properties and analytical characterization of lignocellulosic biomass. Renewable and Sustainable Energy Reviews, 76, 309-322.
• Macgregor, C. J., Williams, J. H., Bell, J. R., & Thomas, C. D. (2019). Moth biomass increases and decreases over 50 years in Britain. Nature Ecology & Evolution, 3(12), 1645-1649.
• Parikka, M. (2004). Global biomass fuel resources. Biomass and bioenergy, 27(6), 613-620.