Study Reveals the Effect of Scarce Phosphorus Content on Biomass Production

What amount of carbon dioxide (CO₂) can tropical rainforests consume? A new study by a deeply involved international group of scientists from the Technical University of Munich (TUM) has shown that the absorption capacity is extremely restricted by the phosphorus content of the soil.

Trees are considered as the rescuers in the period of climate change. They absorb CO₂ through their leaves and convert the greenhouse gas into biomass and oxygen. The International Panel on Climate Change (IPCC) predicted that the Amazon rainforests could absorb one-quarter of the CO₂ that is released each year from fossil fuel combustion.

Until now, worldwide climate models have presumed that there will not be any change in this absorption capacity in the future.

“But there has been no proof of this to date," stressed Dr Katrin Fleischer. “It is entirely possible that the absorption capacity will even decrease.” The ecologist from the Professorship for Land Surface-Atmosphere Interactions at the TUM jointly worked with ecologists and ecosystem modelers from 10 different countries to study how much of the nutrient supply in the Amazon region restricts the biomass production.

Comparison of 14 Models

Consequently, the scientists carried out various research experiments. According to Fleischer, to date, nobody has thoroughly explored this relationship.

Most ecosystem models which allow the future development of ecosystems to be simulated were developed for the temperate latitudes, where there is generally sufficient phosphorus. However, in many areas of the Amazon region, it is in short supply—the ecosystem is many million years old, and the soil is leached of nutrients.

Dr Katrin Fleischer, Professorship for Land Surface-Atmosphere Interactions, Technical University of Munich

The scientists chose 14 different ecosystem models to determine the reaction of the rainforest to an increase in atmospheric CO₂ concentration. Later, the 14 models were used to simulate biomass production for the following 15 years: first, for the present CO₂ concentration of 400 ppm, and second, for an increased concentration of 600 ppm.

Trees Reaching Their Limit

The findings revealed that the trees can absorb the extra CO₂ and convert it into biomass only when enough phosphorus is present. When the phosphorus content is not enough, once again, there is a decrease in the CO₂ fertilization effect.

These models, which consider various factors, estimate a 50% decrease in the theoretically feasible additional CO₂ absorption in the second case on average—where some even estimate a 100% decrease in absorption.

This would mean that the rainforest has already reached its limit and would be unable to absorb any more carbon dioxide emissions caused by human kind. If this scenario turns out to be true, the Earth’s climate would heat up significantly faster than assumed to date.

Dr Katrin Fleischer, Professorship for Land Surface-Atmosphere Interactions, Technical University of Munich

The ecologist concluded that further thorough study is needed to analyze the exact reaction of the ecosystem and the successful absorption of extra phosphorus by the trees from the soil through enzymatic processes or formation of more roots which could bind and absorb the insufficient nutrients: “What’s certain is that the tropical rainforests are not infinitely resilient CO₂ sinks.”