Elevated ozone (eO) and atmospheric nitrogen (N) deposition are important climate change components that can affect plant growth and plant-soil-microbe interactions. However, the understanding of how eO and its interaction with N deposition affect soil microbially mediated carbon (C) cycling and the fate of soil C stocks is limited. This study aimed to test how eO and N deposition affected soil microbial metrics (i.e., respiration, enzyme activities, biomass, necromass, and community composition) and resulting soil organic C (SOC) fractions in the rhizosphere of poplar plantations with different sensitivity to O. Exposure to O and/or N deposition for four years was conducted within a free-air O concentration-enrichment facility. Elevated O reduced soil microbial respiration and biomass C but enhanced the enzymatic acquisition of C (i.e., potential soil hydrolase and oxidase activity) and shifted to a fungi-dominated community composition. These responses suggest that microbial C availability decreased and microbes allocated more energy to obtain C and nutrients from biochemically resistant substrates under eO. Elevated O decreased bacterial necromass C and total necromass C, which could explain the observed decreases in mineral-associated organic C and SOC. The effects of eO on soil microbial C availability and community composition were strengthened by N addition, whereas there were no differences in the below-ground effects of eO between the two poplar clones. Taken together, the increased soil extracellular enzyme activities and slightly increased particulate organic C content suggest that the microbial C pump pathway via microbial modification was strengthened by eO, whereas the pathway via microbial turnover was weakened, as suggested by the decreases in soil microbial respiration, biomass, necromass, and mineral-associated organic C. Our study provides evidence that aboveground eO effects on trees may affect belowground microbial processing of organic matter and ultimately the persistence of SOC.

中文翻译：

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臭氧增强了杨树人工林中微生物碳泵的离体途径，但削弱了体内途径


臭氧（eO）和大气氮（N）沉降升高是气候变化的重要组成部分，可以影响植物生长和植物-土壤-微生物相互作用。然而，对于 eO 及其与氮沉降的相互作用如何影响土壤微生物介导的碳 (C) 循环和土壤碳库的命运的了解有限。本研究旨在测试 eO 和 N 沉降如何影响土壤微生物指标（即呼吸、酶活性、生物量、坏死物和群落组成）以及对 O 敏感度不同的杨树人工林根际土壤有机碳 (SOC) 分数暴露于 O 和/或 N 沉积四年是在自由空气 O 浓缩设施内进行的。升高的 O 减少了土壤微生物呼吸和生物量 C，但增强了 C 的酶促获取（即潜在的土壤水解酶和氧化酶活性），并转变为以真菌为主的群落组成。这些反应表明，微生物 C 的可用性下降，微生物分配更多的能量来从 eO 下的生化抗性底物中获取 C 和营养物质。升高的 O 降低了细菌坏死物 C 和总坏死物 C，这可以解释观察到的与矿物质相关的有机 C 和 SOC 的下降。添加氮加强了 eO 对土壤微生物碳有效性和群落组成的影响，而两个杨树无性系之间 eO 的地下影响没有差异。 综上所述，土壤细胞外酶活性的增加和颗粒有机碳含量的略微增加表明，eO 增强了通过微生物修饰的微生物碳泵途径，而通过微生物周转的途径则被削弱，如土壤微生物呼吸的减少所表明的那样，我们的研究提供了证据，表明地上 eO 对树木的影响可能会影响地下微生物对有机物的处理，并最终影响 SOC 的持久性。