Abstract. As an increasing share of the human population is being clustered in cities, urban areas have swiftly become the epicentres of anthropogenic carbon (C) emissions. Understanding different parts of the biogenic C cycle in urban ecosystems is needed in order to assess the potential to enhance their C stocks as a cost-efficient means to balance the C emissions and mitigate climate change. Here, we conducted a field measurement campaign over three consecutive growing seasons to examine soil respiration carbon dioxide (CO2) fluxes and soil organic carbon (SOC) stocks at four measurement sites in Helsinki, representing different types of tree-covered urban green space commonly found in northern European cities. We expected to find variation in the main drivers of soil respiration – soil temperature, soil moisture, and SOC – as a result of the heterogeneity of urban landscape and that this variation would be reflected in the measured soil respiration rates. In the end, we could see fairly constant statistically significant differences between the sites in terms of soil temperature but only sporadic and seemingly momentary differences in soil moisture and soil respiration. There were also statistically significant differences in SOC stocks: the highest SOC stock was found in inactively managed deciduous urban forest and the lowest under managed streetside lawn with common linden trees. We studied the impacts of the urban heat island (UHI) effect and irrigation on heterotrophic soil respiration with process-based model simulations and found that the variation created by the UHI is relatively minor compared to the increase associated with active irrigation, especially during dry summers. We conclude that, within our study area, the observed variation in soil temperature alone was not enough to cause variation in soil respiration rates between the studied green space types, perhaps because the soil moisture conditions were uniform. Thus, irrigation could potentially be a key factor in altering the soil respiration dynamics in urban green space both within the urban area and in comparison to non-urban ecosystems.

中文翻译：

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芬兰赫尔辛基各种树木覆盖的城市绿地的土壤呼吸


摘要。随着越来越多的人口聚集在城市，城市地区已迅速成为人为碳（C）排放的中心。需要了解城市生态系统中生物碳循环的不同部分，以便评估增加碳储量的潜力，作为平衡碳排放和减缓气候变化的经济高效手段。在这里，我们在连续三个生长季节进行了实地测量活动，以检查赫尔辛基四个测量点的土壤呼吸二氧化碳（CO2）通量和土壤有机碳（SOC）储量，这些测量点代表了常见的不同类型的树木覆盖的城市绿地在北欧城市。我们预计由于城市景观的异质性，土壤呼吸的主要驱动因素（土壤温度、土壤湿度和有机碳）会发生变化，并且这种变化将反映在测量的土壤呼吸速率中。最后，我们可以看到不同地点之间在土壤温度方面相当稳定的统计显着差异，但在土壤湿度和土壤呼吸方面只有零星的、看似短暂的差异。 SOC 储量也存在统计上的显着差异：未积极管理的落叶城市森林中的 SOC 储量最高，而在管理下的常见椴树街边草坪中的 SOC 储量最低。我们通过基于过程的模型模拟研究了城市热岛效应和灌溉对异养土壤呼吸的影响，发现与主动灌溉相关的增加相比，城市热岛效应造成的变化相对较小，特别是在干燥的夏季。 我们的结论是，在我们的研究区域内，仅观察到的土壤温度变化不足以引起所研究的绿地类型之间土壤呼吸速率的变化，这可能是因为土壤湿度条件是均匀的。因此，灌溉可能是改变城市绿地土壤呼吸动态的关键因素，无论是在城市区域内还是与非城市生态系统相比。