Promising proxy for photosynthesis from carbonyl sulfide measurements
How much carbon is taken up forests in photosynthesis? Measuring photosynthesis is challenging, but with carbonyl sulfide we might come close.

Forests are one of the largests carbon sinks on Earth, slowing down climate change. However, it is not simple to estimate the amount of carbon taken up by trees during photosynthesis. Traditional method for estimating photosynthesis is by measuring the carbon dioxide (CO2) exchange between the forest and the atmosphere. This method, however, is problematic as the plants are respiring (emitting CO2) in the same time as they photosynthesize (uptake CO2).

There are multiple other, undirect methods for measuring photosynthesis but none of them come without restrictions. One of the newer, promising methods is using carbonyl sulfide (COS) as a proxy for photosynthetic uptake, as it shares the same transport pathway in the leaves as CO2.

Carbonyl sulfide is the most abundant sulfur gas in the atmosphere. Plants take up COS through their stomata in a similar way as CO2, but in contrast to CO2, it is not emitted back to the atmosphere. This unidirectional flux makes COS a promising proxy for photosynthesis.

The ratio of how much pine needles take up COS in relation to CO2 was determined in a measurement campaign conducted in Hyytiälä SMEAR II station in 2017. During the campaign, COS exchange was measured in three different scales: by soil chambers, branch chambers and eddy covariance technique. With these measurements, the role of environmental factors, especially humidity and radiation, affecting COS and CO2 uptake ratio could be quantified.

In addition to estimating photosynthesis, COS can be used for measuring the stomatal control of plants. Stomata control gas exchange and transpiration of plants.

This research was published on 25.1.2019 in PNAS:

Read more in Finnish: