Long-term nitrogen addition causes the evolution of less-cooperative mutualists

TLDR

Human activities have increased atmospheric nitrogen inputs, driving ecological changes and potentially weakening mutualistic cooperation in ecosystems. A 22‑year nitrogen‑addition experiment revealed that elevated nitrogen drives the evolution of less‑mutualistic rhizobia, reducing plant biomass and chlorophyll and threatening ecosystem nitrogen fixation.

Abstract

Human activities have altered the global nitrogen (N) cycle, and as a result, elevated N inputs are causing profound ecological changes in diverse ecosystems. The evolutionary consequences of this global change have been largely ignored even though elevated N inputs are predicted to cause mutualism breakdown and the evolution of decreased cooperation between resource mutualists. Using a long-term (22 years) N-addition experiment, we find that elevated N inputs have altered the legume-rhizobium mutualism (where rhizobial bacteria trade N in exchange for photosynthates from legumes), causing the evolution of less-mutualistic rhizobia. Plants inoculated with rhizobium strains isolated from N-fertilized treatments produced 17-30% less biomass and had reduced chlorophyll content compared to plants inoculated with strains from unfertilized control plots. Because the legume-rhizobium mutualism is the major contributor of naturally fixed N to terrestrial ecosystems, the evolution of less-cooperative rhizobia may have important environmental consequences.