Asymmetric responses of autotrophic and heterotrophic respiration to simulated nitrogen deposition in a subalpine grassland

The increasing anthropogenic nitrogen (N) input is expected to affect soil total respiration (Rs), whereas the mechanisms of Rs components (autotrophic respiration, Ra; heterotrophic respiration, Rh) in response to aggravated N input remains controversial, especially in the subalpine ecosystems. A multi-level simulated N deposition experiment was designed at a subalpine grassland to explore the effects of Rs components in response to N deposition and their potential mechanisms. Results showed that both Ra and Rs firstly increased and subsequently decreased at a threshold of ca. 5 g N m(-2) yr(-1), with the maximum of Ra and Rs increased by 22.98 % and 13.93 %, respectively. By comparison, Rh decreased along with N addition duration (year) at ca. 15 g N m(-2) yr(-1). Similarly, growing season average cumulative Ra and Rs also showed a bell-shaped mode in response to elevated N inputs, with a peak appearing at ca. 5 g N m(-2) yr(-1). Increased Ra with N addition rates enhanced the Ra/Rs ratio in the fourth experimental year only. The temperature sensitivity (Q(10)) of Ra, Rh and Rs spanned from 1.86 to 2.58, 1.85 to 2.31 and 1.86 to 2.45 respectively. Additionally, N addition reduced the Q(10) of Ra, Rh and Rs at ca. 10 g N m(-2) yr(-1), ca. 15 g N m(-2) yr(-1) and ca. 10 g N m(-2) yr(-1), respectively. The changes in Ra and Rh were largely affected by root TN and soil C/N, respectively. Our results address that Ra exerts a leading role in regulating Rs and Ra/Rs ratio with N enrichment. These findings supply a relatively comprehensive insight of N-induced effects on soil respirations (Ra and Rh) and their potential mechanisms, which have an important implication for evaluating soil C emission in the subalpine ecosystems under aggravating N deposition.