Warming leads to significant loss of CO2 in high-altitude regions (HAR), posing threat to the carbon sink of terrestrial ecosystem. Additionally, the spatial distribution of environmental factors and underlying surfaces also determine the carbon sink pattern. Therefore, it is necessary to systematically explore the carbon sink of HAR. Based on it, choosing the Qilian Mountains (QLM) as the study area, the continuous observation data of 14 eddy covariance in different ecosystems was used to analyze the variation characteristics of carbon use efficiency (CUE) and net ecosystem primary productivity (NEP), which is helpful to systematically understand the response of carbon cycle to climate change in alpine ecosystem. The research results indicated that the QLM serves as an effective carbon sink (13 of the sites yielded a net carbon sink), owing to the combined influences of environmental factors and vegetation characteristics. Annual NEP varied across the 14 sites, ranging from-192.6 to 524.5 g C/m(2)/yr. Limited observation indicated that wetland/swamp had the highest carbon sink, followed by forest, and shrub have the lowest carbon sink in this study. Along the altitudinal gradient, both gross primary productivity (GPP) and ecosystem respiration (Re) demonstrated a declining trend ( P < 0.05), while, CUE displayed an increasing trend. Soil temperature and photosynthetically active radiation dominated the variation in carbon exchange and CUE along the altitudinal gradient. However, soil moisture was the dominant factor in drought ecosystem. This study provides basis for the assessment of carbon sink of the HAR.
The central carbon (C) metabolic network is responsible for most of the production of energy and biosynthesis in microorganisms and is therefore key to a mechanistic understanding of microbial life in soil communities. Many upland soil communities have shown a relatively high C flux through the pentose phosphate (PP) or the Entner-Doudoroff (ED) pathway, thought to be related to oxidative damage control. We tested the hypothesis that the metabolic organization of the central C metabolic network differed between two ecosystems, an anoxic marsh soil and oxic upland soil, and would be affected by altering oxygen concentrations. We expected there to be high PP/ED pathway activity under high oxygen concentrations and in oxic soils and low PP/ED activity in reduced oxygen concentrations and in marsh soil. Although we found high PP/ED activity in the upland soil and low activity in the marsh soil, lowering the oxygen concentration for the upland soil did not reduce the relative PP/ED pathway activity as hypothesized, nor did increasing the oxygen concentration in the marsh soil increase the PP/ED pathway activity. We speculate that the high PP/ED activity in the upland soil, even when exposed to low oxygen concentrations, was related to a high demand for NADPH for biosynthesis, thus reflecting higher microbial growth rates in C-rich soils than in C-poor sediments. Further studies are needed to explain the observed metabolic diversity among soil ecosystems and determine whether it is related to microbial growth rates. IMPORTANCE We observed that the organization of the central carbon (C) metabolic processes differed between oxic and anoxic soil. However, we also found that the pentose phosphate pathway/Entner-Doudoroff (PP/ED) pathway activity remained high after reducing the oxygen concentration for the upland soil and did not increase in response to an increase in oxygen concentration in the marsh soil. These observations contradicted the hypothesis that oxidative stress is a main driver for high PP/ED activity in soil communities. We suggest that the high PP/ED activity and NADPH production reflect higher anabolic activities and growth rates in the upland soil compared to the anaerobic marsh soil. A greater understanding of the molecular and biochemical processes in soil communities is needed to develop a mechanistic perspective on microbial activities and their relationship to soil C and nutrient cycling. Such an increased mechanistic perspective is ecologically relevant, given that the central carbon metabolic network is intimately tied to the energy metabolism of microbes, the efficiency of new microbial biomass production, and soil organic matter formation. We observed that the organization of the central carbon (C) metabolic processes differed between oxic and anoxic soil. However, we also found that the pentose phosphate pathway/Entner-Doudoroff (PP/ED) pathway activity remained high after reducing the oxygen concentration for the upland soil and did not increase in response to an increase in oxygen concentration in the marsh soil. These observations contradicted the hypothesis that oxidative stress is a main driver for high PP/ED activity in soil communities. We suggest that the high PP/ED activity and NADPH production reflect higher anabolic activities and growth rates in the upland soil compared to the anaerobic marsh soil. A greater understanding of the molecular and biochemical processes in soil communities is needed to develop a mechanistic perspective on microbial activities and their relationship to soil C and nutrient cycling. Such an increased mechanistic perspective is ecologically relevant, given that the central carbon metabolic network is intimately tied to the energy metabolism of microbes, the efficiency of new microbial biomass production, and soil organic matter formation.
The soil quality index (SQI) is a comprehensive indicator that reflects the agricultural productivity of soil, as well as playing important roles in understanding microbial nutrient metabolism and carbon use efficiency (CUE). However, it is unclear how drip irrigation treatments in apple orchards affect the SQI, eco-enzyme stoichiometry, and soil microbial CUE. Thus, in the present study, we tested three different treatments in orchard plots: T1 (50-60 % field water capacity (theta f)), T2 (65-75 % theta f), and T3 (80-90 % theta f), as well as control with no drip irrigation (CK). The study focused on the effects of these treatments during two key stages: bud breaking and fruit maturity. During the bud breaking stage, we observed that water availability had a more pronounced influence on the SQI when soil moisture was limited. Specifically, in the 0-20 cm soil layer, the T2 treatment showed a significantly lower SQI value compared to T3, with a decrease of 31.89 %. On the other hand, there were no significant differences among all the irrigated treatments during the maturity stage. Both vector length and angle were significantly affected by water availability during the bud breaking stage, while only the vector angle was impacted during the maturity stage. The vector length and angle were both influenced by SQI (Mantel's test: p < 0.01). During the bud breaking stage, the CUE values in 0-20 cm layer under T1, T2, and T3 were 30.27 %, 21.79 %, and 85.47 % lower, respectively, compared with CK. By contrast, in the fruit maturity stage, CUE was 27.39 % higher under T1 compared with CK. SQI and CUE had a negative correlation in the bud breaking stage (p < 0.001, R-2 = 0.26), but a positive correlation in the fruit maturity stage (p < 0.001, R-2 = 0.51). Our findings suggest that the T3 treatment consistently yields the highest Soil Quality Index (SQI) across most soil layers during the bud breaking and maturity stages. Moreover, the T3 treatment effectively alleviates early spring drought in the Weibei region and encourages deep-root development, enabling fruit trees to absorb nutrients from deeper soil levels. Overall, these findings enhance our understanding of how the SQI and enzyme stoichiometry under drip irrigation affect phosphorus and carbon metabolism in soils, and they suggest that SQI should be considered a key factor that limits microbial metabolic restrictions and microbial CUE.