Alice Ardichvili, Sorbonne University, discusses her article: Spatial context allows the evolution of the control of nitrification by plants
Once upon a time
The project began a long time ago, in the late 1990s, when my PhD advisor J-C was still a carefree and enthusiastic individual. He and his colleagues observed that a common grass in a West African Savanna (Lamto, Ivory Coast) exerts a special control over the nitrogen cycle: it strongly slows down soil nitrification. Nitrification is the transformation of ammonium into nitrate and occurs in both terrestrial and aquatic ecosystems due to the action of microorganisms that feed on ammonium. Interestingly, despite strong nitrogen limitation, this tropical savanna is very productive—as much as a tropical forest. In fact, the low nitrification rate reduces the leaching of nitrate, a mobile ion that easily binds to runoff water. This makes ammonium fully available to plants. In short, reducing the nitrification rate limits nitrogen losses and boosts plant productivity in this West African savanna.
Here comes the question that we addressed along with Seb and Nicolas, two theoreticians, at the Institute of Ecology and Environmental Sciences – Paris. If inhibiting nitrification is so advantageous for plants, allowing them to secure a source of immobile ammonium, why isn’t this mechanism widespread among plants? The answer is twofold. First, it is possible that many (if not all) plant species control nitrification, speeding it up or slowing it down depending on environmental conditions, but to a degree that is difficult to measure in the field (remember that it took about a decade to isolate molecules from grasses inhibiting nitrification). Second, our study shows that plants need certain characteristics, such as being long-lived and only partially sharing the soil with neighbours. This is exactly the case for Lamto grasses, which live for 50-80 years, grow in tufts, and are isolated from others by bare soil! Perhaps growing in tufts—an adaptation to seasonal fires—has enabled the selection of nitrification control. Are other perennial grasses that grow in tufts able to control nitrification? We will soon find out from a global survey of savanna grasses monitored by J-C.
Implications for agriculture
In our fields, nitrogen is the source of many torments. The industrial production of fertilisers is energy-intensive, and up to 50% of the applied fertiliser leaves the field, with some of it running off with rainwater and later causing algae blooms in aquatic systems. These blooms harm the health of both aquatic organisms and humans. Some of the nitrogen returns to the atmosphere in the form of nitrous protoxide, a greenhouse gas 20 times more potent than carbon dioxide. Many of the plants that we (and our cattle) eat are grasses: maize, rice, barley; accordingly, agronomists are seduced by grasses that control the nitrogen cycle! But would we be able to select for crops that inhibit nitrification? Given that controlling nitrification is also costly for the plant, would the control of nitrification ultimately benefit the productivity of our crops? Our theoretical exploration provides food for thought on this matter… To be continued by the works of Seb, Amed, and Sarah!
