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The sensitivity of soil organic carbon (SOC) decomposition in seasonally frozen soils, such as alpine ecosystems, to climate warming is a major uncertainty in global carbon cycling. Here we measure soil CO2 emission during four years (2018-2021) from the whole-soil warming experiment (4 °C for the top 1 m) in an alpine grassland ecosystem. We find that whole-soil warming stimulates total and SOC-derived CO2 efflux by 26% and 37%, respectively, but has a minor effect on root-derived CO2 efflux. Moreover, experimental warming only promotes total soil CO2 efflux by 7-8% on average in the meta-analysis across all grasslands or alpine grasslands globally (none of these experiments were whole-soil warming). We show that whole-soil warming has a much stronger effect on soil carbon emission in the alpine grassland ecosystem than what was reported in previous warming experiments, most of which only heat surface soils.

The paucity of investigations of carbon (C) dynamics through the soil profile with warming makes it challenging to evaluate the terrestrial C feedback to climate change. Soil microbes are important engines driving terrestrial biogeochemical cycles; their carbon use efficiency (CUE), defined as the proportion of metabolized organic C allocated to microbial biomass, is a key regulator controlling the fate of soil C. It has been theorized that microbial CUE should decline with warming; however, empirical evidence for this response is scarce, and data from deeper soils are particularly scarce. Here, based on soil samples from a whole-soil-profile warming experiment (0 to 1 m, +4 °C) and 18O tracing approach, we examined the vertical variation of microbial CUE and its response to ~3.3-y experimental warming in an alpine grassland on the Qinghai-Tibetan Plateau. Microbial CUE decreased with soil depth, a trend that was primarily controlled by soil C availability. However, warming had limited effects on microbial CUE regardless of soil depth. Similarly, warming had no significant effect on soil C availability, as characterized by extractable organic C, enzyme-based lignocellulose index, and lignin phenol-based ratios of vanillyls, syringyls, and cinnamyls. Collectively, our work suggests that short-term warming does not alter microbial CUE in either surface or deep soils, and emphasizes the regulatory role of soil C availability on microbial CUE.

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Biological nitrogen (N) fixation is an important source of N in terrestrial ecosystems; but the response of soil microbial N fixation rate to N deposition in different forest ecosystems still remains uncertain. We conducted a field N addition experiment to simulate atmosphere N deposition in subtropical Pinus taiwanensis and Castanopsis faberi forests. We set up three levels of nitrogen addition using urea as the N source: 0 (control) ; 40 (low N) ; and 80 g N • hm-2 • a-1 ( high N) to examine the chemical properties; microbial biomass C ; enzyme activities; and nifH gene copies of top soils (0-10 cm). We also measured the microbial N fixation rate using the 15N labeling method. Results showed that N addition significantly reduced the soil microbial N fixation rate in the P. taiwanensis and C. faberi forests by 29%-33% and 10%-18%; respectively. Nitrogen addition significantly reduced N-acqui- ring enzyme (i.e.; β-1; 4-N-acetylglucosaminidase) activity and nifH gene copies in both forest soils. There was a significant positive correlation between the microbial N fixation rate and soil dissolved organic C content in the P. taiwanensis forest; but a significant negative relationship between the rate of soil microbial nitrogen fixation and NH4+-N content in the C. faberi forest. Overall' soil microbial N fixation function in the P. taiwanensis forest was more sensitive to N addition than that in the C. faberi forest; and the factors affecting microbial N fixation varied be¬tween the two forest soils. The study could provide insights into the effects of N addition on biological N fixation in forest ecosystems, and a theoretical basis for forest management. [ABSTRACT FROM AUTHOR]

Nitrogen (N) deposition rates of terrestrial ecosystems have gradually declined but are still high in some areas, and previous studies have reported that N addition elicits diverse impacts on soil organic carbon (SOC) pools. SOC can be divided into different functional fractions, namely particulate organic carbon (POC) and mineral-associated organic carbon (MAOC). The responses of these fractions to N addition should be elucidated to better understand the changes in SOC pools. We investigated the effects of N addition treatments (+0, +40, and +80 kg N ha−1 yr−1) in a subtropical Castanopsis fabri forest to simulate N deposition. The surface (0–10 cm) SOC content in different fractions, aboveground litter biomass, fine root (diameter < 2 mm) biomass, soil exchangeable cation content, and soil enzyme activity under different N addition treatments were measured. The results showed that 1) N addition exerted a positive effect on POC content but did not significantly affect MAOC content. 2) POC content was negatively correlated with pH and soil enzyme activity and positively correlated with aboveground litter biomass, suggesting that POC accumulation was influenced by aboveground litter input and microbial decomposition. 3) Root biomass was unaffected significantly by the addition of N, which could be responsible for the limited response of MAOC to N addition. Furthermore, a close negative relationship was observed between exchangeable Al3+ and Ca2+ or K+ contents, indicating the presence of a trade-off between negative effects of exchangeable cations on SOC bridging and their positive effects on SOC adsorption, thus resulting in an insignificant reaction of MAOC to N addition. Overall, N addition reduces the persistence and increases the nutrient density of SOC, and MAOC with more protection is less vulnerable to N addition than POC with less protection. By incorporating these nuances into ecosystem models, it is possible to more accurately predict SOC dynamics in response to global change. [ABSTRACT FROM AUTHOR]