As the global climates change, elevated CO2 and soil contamination by heavy metal co-occur in natural ecosystems, which are anticipated to affect soil organic carbon fractions (SOC) and their relation to soil microbial activities, but this issue has not been extensively examined. We investigated the response of SOC and their relation with soil microorganisms and enzyme activities in rhizosphere soils of Robinia pseudoacacia L. seedlings to elevated CO2 plus cadmium (Cd) contamination. We found that elevated CO2 significantly (p < 0.05) stimulated total organic carbon (TOC) (8.6%), dissolved organic carbon (DOC) (32.6%), microbial biomass carbon (MBC) (13.5%), bacteria (11.6%), fungi (20.9%), actinomycetes (15.3%), urease (20.1%), dehydrogenase (15.8%), invertase (11.1%), and β-glucosidase (11.9%), and DOC, MBC, bacteria, actinomycetes, urease, and invertase presented smaller growth trend in the range of 500–700 μmol mol−1 CO2 than in the range of 385–500 μmol mol−1 CO2. Cd decreased DOC (30.1%), MBC (24.9%), bacteria (21.5%), actinomycetes (15.9%), and enzyme activities. Elevated CO2 offsets the negative effect of Cd on SOC and microbial activities (except for TOC and L-asparaginase). Procrustes rotation test was used to determine the drivers (elevated CO2, Cd, and CO2 + Cd) of the relation between SOC and microbial activities, revealing the correlations between SOC, soil microorganisms, and enzyme activities were higher under elevated CO2 than under elevated CO2 + Cd. Our results suggest elevated CO2 could stimulate soil fertility and microecological cycle in the rhizosphere microenvironment exposed to heavy metal by affecting the relationship between SOC and soil microbial properties.

中文翻译：

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CO2 浓度升高影响刺槐根际土壤有机碳含量及其与土壤微生物特性的关系。Cd 污染土壤中的幼苗

随着全球气候的变化，自然生态系统中同时发生二氧化碳升高和土壤重金属污染，预计这会影响土壤有机碳分数 (SOC) 及其与土壤微生物活动的关系，但这个问题尚未得到广泛研究。我们研究了 SOC 及其与刺槐幼苗根际土壤中土壤微生物和酶活性的关系对二氧化碳和镉 (Cd) 污染升高的响应。我们发现 CO2 升高显着 (p < 0.05) 刺激总有机碳 (TOC) (8.6%)、溶解有机碳 (DOC) (32.6%)、微生物生物量碳 (MBC) (13.5%)、细菌 (11.6%) 、真菌 (20.9%)、放线菌 (15.3%)、脲酶 (20.1%)、脱氢酶 (15.8%)、转化酶 (11.1%) 和 β-葡萄糖苷酶 (11.9%)，以及 DOC、MBC、细菌、放线菌、脲酶, 和转化酶在 500-700 μmol mol-1 CO2 范围内比在 385-500 μmol mol-1 CO2 范围内呈现出更小的增长趋势。Cd 降低了 DOC (30.1%)、MBC (24.9%)、细菌 (21.5%)、放线菌 (15.9%) 和酶活性。升高的 CO2 抵消了 Cd 对 SOC 和微生物活动（TOC 和 L-天冬酰胺酶除外）的负面影响。Procrustes 旋转测试用于确定 SOC 与微生物活动之间关系的驱动因素（升高的 CO2、Cd 和 CO2 + Cd），揭示 SOC、土壤微生物和酶活性之间的相关性在 CO2 升高下高于 CO2 升高下+ 镉。我们的研究结果表明，升高的 CO2 可以通过影响 SOC 与土壤微生物特性之间的关系来刺激暴露于重金属的根际微环境中的土壤肥力和微生态循环。