Above- and below-ground factors determine the effect of added nitrogen

When a plant community’s environment is enriched with external nitrogen (N), taller plants are expected to be favoured because they can compete for light more effectively. However, increases in the leaf area of shorter plants could level the playing field, allowing them to better capitalize on better nitrogen nutrition despite shading.

The picture is further complicated by the effect of phosphorous (P) as a potentially limiting nutrient in this scenario, making a species’ non-nitrogen resource acquisition relevant. However, few studies have investigated the interrelated effects of both the above-ground light environment and below-ground resource acquisition strategies in the individual’s overall response to nitrogen enrichment.

In a recent article published in Annals of Botany, lead author Dianye Zhang and colleagues attempted to account for both light and nutrient acquisition as factors in relative species abundance within a community under N enrichment. Two species were compared: Stipa purpurea, which is taller and colonized by arbuscular mycorrhizal fungi, and Carex stenophylloides, which is shorter and produces cluster roots.

The authors found that the abundance of the taller species increased with N enrichment due to improved light acquisition. However, the shorter species, using cluster roots, was able to step up its P absorption through increased root exudates. The improved P nutrition bettered its light use efficiency, increasing its relative abundance even in the shading presence of taller competitors.

“Although light competition is important in regulating community structure under N enrichment, plants would cope with light competition not only by modifying their light acquisition capacity but also by altering their light use efficiency,” write the authors, noting that their results show that above- and below-ground resource acquisition strategies co-determine how plant species react to increased nitrogen. “[C]onsidering the differences in species resource acquisition strategies can help to reveal the mechanisms underlying the dynamics of community structure and ecosystem functions under [an] N enrichment scenario.”