Nitrogen addition reduces soil respiration but increases the relative contribution of heterotrophic component in an alpine meadow

Disentangling the relative response sensitivity of soil autotrophic (Rₐ) and heterotrophic respiration (Rₕ) to nitrogen (N) enrichment is pivotal for evaluating soil carbon (C) storage and stability in the scenario of intensified N deposition. However, the mechanisms underlying differential sensitivities of Rₐ and Rₕ and relative contribution of Rₕ to soil respiration (Rₛ) with increasing N deposition remain elusive. A manipulative field experiment with multi‐level N addition rates was conducted over 3 years (2015–2017) in an alpine meadow to explore the relative impact of N enrichment on Rₐ and Rₕ and the response of Rₕ/Rₛ ratio to the gradient of N addition. Soil respiration components had different sensitivities to N enrichment, with Rₐ decreasing more than Rₕ, leading to a higher Rₕ/Rₛ ratio as a function of increasing N addition rates. Rₐ and Rₕ decreased nonlinearly as N addition rates increased, with a critical load of 8 g N m⁻² year⁻¹ above which N enrichment significantly inhibited them. Rₐ and Rₕ were controlled by different abiotic and biotic factors, and the regulation of controlling factors on soil respiration components varied over time. N‐induced reduction in the relative abundance of forb significantly affected Rₐ, and this effect was mainly evident in the second and third years. Nitrogen enrichment significantly changed Rₕ in the third year, and the decreased Rₕ under high doses of N addition could be attributed to the changes in microbial biomass C, soil substrate quality and microbial composition. Our study highlights the leading role of Rₐ in regulating Rₛ responses to N enrichment and the enhancement of Rₕ/Rₛ ratio with increasing N addition. We also emphasize that N‐induced shifts in plant community composition play a vital role in regulating Rₐ instead of Rₕ. The changing drivers of Rₐ and Rₕ with time suggests that long‐term experiments with multiple levels of N addition are further needed to test the nonlinear responses and underlying mechanisms of soil respiration components in face to aggravating N deposition. A free Plain Language Summary can be found within the Supporting Information of this article.