A NEW study has cast doubt on the ability of modern grasslands to fight climate change. An investigation into why the world’s 50 million square kilometres of grasslands aren’t soaking up more CO2 from the atmosphere has shown many such habitats reached their peak over 100 years ago. Based on UK grown samples, the study also shows that grasses are physiologically constrained from taking up further CO2 – even when nitrogen fertilisers are added to encourage their growth. In fact, grasslands that have received high levels of nitrogen fertiliser are today yielding less than they were a century ago. This latest analysis of samples from the world’s longest running ecological experiment, Park Grass, shows that some of the planet’s most productive grasslands may have reached carbon saturation sometime near the beginning of the 20th century or earlier when the concentration of atmospheric CO2 was only two thirds of its current level.