Long-term nitrogen addition reduced soil respiration contributed by decline in the understory plant cover but not species richness

Background and aims Understory plants in forest ecosystems play key roles in carbon (C) cycling and have generally shown negative responses in their diversity and plant cover to increased nitrogen (N) deposition, yet the potential consequences of these responses on the soil C cycle are poorly understood. Here we aim to explore the effects of N addition on understory plants and its consequences on soil respiration (Rs). Methods We conducted a N addition experiment with four N levels (0, 20, 50, and 100 kg N ha −1 yr −1 ) in a boreal forest and monitored the understory plant cover, plant species richness and Rs after 9 years. Results We found that N addition decreased the species richness and plant cover of the understory plants. Rs showed a unimodal pattern during the growing season driven mainly by soil temperature, and N addition inhibited Rs when the soil temperature effect was accounted for. A structural equation model revealed that the reduction in Rs with N addition was a result of the reduction in fine root biomass linked with the decline in understory plant cover but was unrelated to the loss of species richness. Conclusion Our findings suggested that the negative effects of increased N deposition on the understory plant community in forest ecosystems would alter soil C storage with potential climate consequences.