Lignin – Not as stable as previously thought

Aswin Thirunavukkarasu has conducted experiments both in laboratory settings and in nature to try to determine in detail what happens to lignin in the soil.

"Previously, it was believed that microbes would prefer to break down the carbohydrates, such as cellulose and hemicellulose. But that’s not true. The microbes break down certain parts of the lignin to the same extent," says Aswin Thirunavukkarasu, who is defending his thesis at the Department of Forest Ecology and Management at SLU.

It is difficult to study chemical processes in the soil because a variety of processes and factors simultaneously affect what happens. The researchers conducted field experiments by placing milled aspen wood and over time studied the decomposition and changes in chemical lignin structures using NMR techniques. The reason they used aspen is that this tree species has a natural variation in the amount of lignin in its wood. This made it possible to use wood from different aspens clones with different levels of lignin.

Not all doctoral dissertations contribute to overturning long-held textbook truths. Aswin Thirunavukkarasu says that there were indications that lignin could actually be broken down in the soil, but his research provides detailed insights into what happens at the chemical level.