Search

• Cold environment suppressed the accumulation of microbial-derived C in the soil. • Warming increased microbial residues in the soil, but N addition decreased it. • Warming weakened the N-driven negative effect on soil C stability. Microbial-derived soil organic carbon has attracted increased attention as an important source of soil organic matter that may affect soil carbon persistence. Ecosystems at high elevation and latitude contain massive amounts of soil organic carbon that is potentially vulnerable to rapid climate change. However, little is known about the mechanisms that govern the production and accumulation of carbon derived from microbes in the context of global changes for these regions. Here, we investigate soil biomarkers from membrane lipids and microbial residues in an experiment with six-year warming and nitrogen (N) addition treatments in an alpine meadow of the Tibetan Plateau. The results showed low accumulation of amino sugars in soil (167.51 to 297.36 μg·g−1) for this alpine meadow. After the six-year manipulative experiment, warming promoted accumulation of microbial-derived carbon in soil with both increased bacterial (increased by 53.56%) and fungal carbon (increased by 25.60%). Nitrogen addition inhibited microbial-derived carbon accumulation (decreased by 25.37%) and significantly decreased the fungal carbon content relative to bacterial carbon content. Climate warming tended to weaken this N-driven negative effect. As microbial residues relate closely with soil organic matter stability, these results highlight the potential of global change factors (warming and N addition) to alter the structure and persistence of soil organic matter, with important implications in refining carbon cycle-climate models. [ABSTRACT FROM AUTHOR]

• Atmospheric and soil droughts together greatly depressed GPP. • Atmospheric water deficits had stronger instant inhibited effects on GPP. • Drought-induced H enhancements provided positive feedback and aggravated GPP loss. Under global climate change, warmer temperatures and changing precipitation patterns will increase the relative importance of soil and atmospheric droughts in limiting productivities across different ecosystems, especially in the fragile and sensitive ecosystem on the Tibetan Plateau. Therefore, the present study examined the effects of atmospheric drought, soil drought and compound drought on alpine meadow productivity based on seven years of continuous observational data. The results demonstrated that water conditions dominated the variations in alpine meadow productivity in northern Tibet. Gross prime productivity (GPP) in the growing season was significantly inhibited during soil drought (defined as soil water content (SWC) < 0.11 m3 m−3) or atmospheric drought (defined as vapor pressure deficit (VPD) > 0.61 kPa) conditions. Pooling of the moisture data according to these thresholds revealed that soil drought occurred more frequently and accounted for 20.4% of all observed days. Atmospheric drought occurred in 2.4% of all days, and the frequency of compound drought (when SWC < 0.11 m3 m−3 and VPD > 0.61 kPa) was 8.7%. Compound drought reduced GPP by the largest extent to as much as 1.47 g C m−2 d−1. Atmospheric drought reduced GPP by 1.10 g C m−2 d−1. However, the more frequent soil drought reduced GPP by the smallest extent, with a value of 0.69 g C m−2 d−1. In spite of this, soil drought played an important role in restricting the accumulated GPP of this alpine meadow via its higher occurrence frequency and longer duration. All three types of drought significantly increased sensible heat flux (H), which provided positive feedbacks to droughts and led to further decreases in GPP. The results of the present study suggest that drought greatly threatens the alpine meadow ecosystem in northern Tibet. Therefore, more attention should be paid to water conditions under the global climate change context. [ABSTRACT FROM AUTHOR]

• Droughts enhanced the control effects of biotic factors on carbon and water fluxes. • Droughts induced changes in the driving paths of biotic and abiotic factors on RUE. • RUE responded differently to droughts in different periods in the growing season. • Droughts in the mid-growing season reduced the RUE. Drought is a threat universally faced by global terrestrial ecosystems under global climate change. The habitat of alpine ecosystems is harsh, and they necessitate to use the limited resources efficiently to survive. However, drought occurred frequently during the short growing season, which greatly affected their resource use efficiency (RUE). In this study, based on long-term flux and microclimate observations in a typical alpine meadow ecosystem in northern Tibet, the effects of drought on RUE, including water use efficiency (WUE), light use efficiency (LUE), and carbon use efficiency (CUE), were studied. The results indicate that droughts could change the driving paths of abiotic and biotic factors on carbon and water fluxes and hence affect RUE. When vegetation exhibits robust physiological activity, droughts could enhance the control effects of biotic factors. The droughts that occurred at different periods of the growing season showed divergent effects on RUE. In the early-growing season, the strong photosynthetic capacity of the nascent leaves facilitated enhancements in WUE and LUE in response to drought conditions. However, the CUE did not increase significantly due to the heightened consumption of organic matter by respiration. The mid-growing season droughts led to a sharp decrease in gross primary productivity (GPP), consequently leading to a decline in WUE, LUE and CUE. During the late-growing season, droughts together with the decomposition of chlorophyll reduced the photosynthetic capacity and led to a significant decrease in LUE. The diminished physiological activity of the senescent leaves led to insensitive responses of WUE and CUE to droughts. These findings indicate that RUE might primarily be regulated by factors affecting GPP rather than others. This study would be beneficial for assessing the responses and adaptation characteristics of alpine meadow ecosystems to future climate change and could help accurately predict their possible evolution trends. [ABSTRACT FROM AUTHOR]