Agricultural soils are by far the largest source of the potent greenhouse gas nitrous oxide (N2O). Therefore, measures to reduce N2O emissions are often focused on the soil - for example through altering tillage or fertilizer use. In two recently published papers, Jan Willem van Groenigen and colleagues show that plants can regulate N2O emissions as well. This may provide the key to new mitigation strategies.
Nitrous oxide is formed in the soil through a variety of microbial processes. Its main source is nitrogen (N) applied by mineral fertilizers or manure. The microbes also need an energy source to produce N2O. Finally, soil structure and moisture content determine whether N2O gas can escape from the soil to the atmosphere.
"If you look at these controlling factors, you can already suspect that plants might be a central actor with respect to N2O emission", says Jan Willem van Groenigen. "First of all, they are the reason that N fertilizer, the ultimate source of N2O emissions, is applied to the soil at all. Moreover, they change the soil structure by root formation, thereby affecting moisture content and gas diffusion within the soil. Finally, by exuding compounds through their roots they regulate microbial processes that produce N2O."
In the first paper, in press in the journal "Plant and Soil", MSc student Bandhu Baral showed that N2O emissions are dependent on adequate P fertilization as well as N fertilization. The principle is simple: if P fertilization is not optimal, plant growth will decrease and fertilizer N will remain available for microbes in the soil , with more risk of N2O emission. In his experiment, Bandhu showed that emission decreased with 50% after adequate P fertilization. "This shows the importance of stoichiometry, the link between different nutrients, on N transformation processes" says Thom Kuyper, co-supervisor of Bandhu. "This has been well-known in agronomy for a long time, but it has been largely ignored in N2O emission studies. We predict that these effects are not limited to P availability but that deficiency of other nutrients, such as potassium, can have a strong impact on N2O emissions as well".
The second paper, published in the journal "Global Change Biology", shows that N2O emissions might be reduced by smart choice of grass species in pastures. First author Diego Abalos (a PhD student) showed that emissions from urine patches were reduced by 44% when two grass species instead of one were present. "This can partly be explained by complementarity effects: the roots of the two species are very different, making it possible to take up N more efficient from the soil and thereby reducing sources of N2O" explains Gerlinde De Deyn, co-author of the study. "Analysing such complementarity effects through a so-called trait-based approach is a rapidly developing field in ecological studies, which we now translated to the study of greenhouse gas emissions".One of the exciting aspects of both studies is that they provide a key to future mitigation strategies. "Farmers have to make decisions all the time on how to fertilize their fields with P or which grass species to sow. We show that these decisions will also affect greenhouse gas emissions from their fields", concludes Jan Willem van Groenigen.