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 (R-a) and heterotrophic respiration (R-h) 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 R-a and R-h and relative contribution of R-h to soil respiration (R-s) 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 R-a and R-h and the response of R-h/R-s ratio to the gradient of N addition. Soil respiration components had different sensitivities to N enrichment, with R-a decreasing more than R-h, leading to a higher R-h/R-s ratio as a function of increasing N addition rates. R-a and R-h 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. R-a and R-h 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 R-a, and this effect was mainly evident in the second and third years. Nitrogen enrichment significantly changed R-h in the third year, and the decreased R-h 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 R-a in regulating R-s responses to N enrichment and the enhancement of R-h/R-s ratio with increasing N addition. We also emphasize that N-induced shifts in plant community composition play a vital role in regulating R-a instead of R-h. The changing drivers of R-a and R-h 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.