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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. [ABSTRACT FROM AUTHOR]

• There was a diurnal hysteresis between GPP and CH 4 flux in an alpine meadow. • Similar hysteresis was observed between CH 4 and PAR, and between PAR and Ta. • The hysteresis between CH 4 and GPP attributes to the hysteresis of PAR and Ta. The relationship between methane (CH 4) and gross primary productivity (GPP) remains elusive and the underlying mechanism is even far from clear. To explore this relationship, we continually monitored CH 4 flux and CO 2 flux by eddy covariance technique in an alpine meadow on the Qinghai–Tibetan Plateau, China, during June 2015–December 2017. We found a parabolic-like relationship of net CH 4 flux with GPP change in both the morning and afternoon at diurnal scale. However, with similar GPP, CH 4 uptake was stronger in the morning than that in the afternoon, forming an anti-clockwise hysteretic relationship between CH 4 and GPP in all the three years. Meanwhile, similar hysteresis loop was also observed between CH 4 flux and photosynthetically active radiation (PAR), and between PAR and air temperature (Ta) during all the three years, with stronger CH 4 uptake (or lower Ta) in the morning than that in the afternoon at the same PAR. Results from our study suggested that the hysteretic relationship between CH 4 flux and GPP might be mainly induced by the hysteresis between PAR and Ta. The similar hysteresis pattern between CH 4 flux (or Ta) and PAR in the non-growing season further confirmed our hypothesis. The findings provide new insights into the coupling of plant photosynthetic activity and net CH 4 flux and help to improve model parameterization for simulating CO 2 –CH 4 interaction processes. [ABSTRACT FROM AUTHOR]

• R h and R a are highly sensitive to warming and clipping. • Increased allocation of C toward below-ground cause the increasing in R a and R s. • Due to the changes in ANPP/BNPP and ST, warming effect vary largely with years. • Plant C allocation is an emerging property in determining R s under warming. Global warming and land-use change have profound impacts on soil respiration (R s), with consequent changes in carbon exchange between terrestrial ecosystem and atmosphere and its feedback to climate change. However, it is not clear how soil respiration and its components (heterotrophic respiration, R h ; and autotrophic respiration, R a) respond to long-term warming and clipping in alpine meadow, and what are the controlling factors for the responses of soil respiration. To answer these questions, we conducted a field experiment of warming and clipping in an alpine meadow on the Qinghai-Tibet Plateau from 2014 to 2019. Infrared heaters were used to simulate climate warming and plots were clipped once a year to mimic hay or biofuel harvest. The results showed that warming decreased R h by 5.06% but stimulated R a by 10.08% across years, while clipping enhanced R s , R h and R a by 12.31%, 18.65% and 1.29%, respectively, and marginally interacted with warming in impacting R a across the years. Furthermore, the warming effect on soil respiration and its components varied largely over years, which was mostly attributable to the changes in the ratio of above-ground to below-ground net primary productivity (ANPP/BNPP, A/B) and soil temperature. Increased allocation of photosynthate toward below-ground caused the gradual increase in R a and R s under long-term warming. While soil temperature changes explained the variation of warming effects on R h across years. The results demonstrated that soil respiration and its components in the alpine meadow are highly sensitive to climate warming and hay harvest but with large interannual variations in their responses. We highlight that plant carbon allocation is an emerging property in determining soil respiration under climate warming. [ABSTRACT FROM AUTHOR]

• Alleviation of light limitation increased species richness under N enrichment. • Alleviation of light limitation stimulated NEP under N enrichment. • The promotion of species richness by weakening light limitation stimulated NEP. Global nitrogen (N) deposition generally reduces plant biodiversity and changes ecosystem carbon (C) cycle. Light limitation caused by accumulated litter under N enrichment is proposed as a main reason for the decreases of biodiversity and ecosystem C sequestration. However, mechanisms underlying this light limitation effect are not well understood, especially under different N input rates. We conducted a field manipulation experiment with N addition as the primary factor nested with litter manipulation as the secondary factor in an alpine meadow in 2017, measured ecosystem C fluxes and investigated the plant communities about 10 days after the N addition treatment, to examine the responses of biodiversity and ecosystem C sequestration to litter manipulation and N addition. Our results showed that alleviation of light limitation by removing litter increased plant species richness under N enrichment, which principally caused by higher species gains and lower losses. Common and rare species contributed most of the increased species as light limitation was relieved. N addition had no significant impact on net ecosystem productivity (NEP), whereas alleviation of light limitation and the subsequent increase in plant coverage and density under N enrichment stimulated gross primary productivity (GPP) but did not affect ecosystem respiration (ER), causing positive responses of NEP. This study highlights the promoting effect of the enhanced light availability through litter removal on species diversity and ecosystem C sequestration under N enrichment. These findings provide new insights into the mechanisms underlying light limitation influence on biodiversity and ecosystem C sequestration under N enrichment. We suggest that grassland management by removing the accumulated litter is an adaptive way to enhance light availability, mitigate diversity loss and thus increase C sequestration under eutrophication. [ABSTRACT FROM AUTHOR]