干旱区城市人工不透水面对土壤有机碳过程的影响研究

Urbanization is one of the most prominent land-use change in the 21st century. In one hand, most of the ecosystem carbon of cities stores in the soil. In the other hand, the rapid expansion of the artificial impervious surface area (ISA), which accounted for more than half of the urban area, removes the topsoil, disturbs the deep soil, seals the land surface, seriously disturbing the soil organic carbon processes. Until today, there were only a few studies on ISA soil carbon processes, focusing on field soil sampling in a few cities. With limited soil samplings and no studies on the soil carbon dynamics, it is difficult to reveal the distributions patterns of soil organic carbon (SOC) pools and impossible to understand their controlling mechanisms underneath the ISA, seriously limiting our abilities to evaluate the urban ecosystem carbon pools and the urbanization effects on regional carbon dynamic. By combining field investigation with site experiments, this proposed study aims to reveal the spatial distribution pattern, the characteristic composition, and the dynamics of SOC underneath the ISA (SOCISA). In addition, the changes in soil environmental as well as microbial and soil enzyme activities in response to the disturbances and sealing effects from ISA will be investigated. The impacts on SOC decomposition process will be studied. The carbon in/out of ISA soil will be investigated. Based on the findings, we aim to reveal the controlling mechanisms over the soil carbon processes in ISA and further develop an urban ecosystem model to simulate the impacts of urbanization on regional SOC dynamics in an typical urban agglomeration in arid area. The findings from this study will fill a major gap in the carbon study and urban ecosystem research, improving urban ecosystem services evaluation, and investigating the potential capacity of carbon sealing underneath ISA.

城市化是21世纪最显著的地表动态之一。城市生态碳库大部存于地下。占城市面积过半的不透水面（ISA）在急速扩张中会清除表土、扰动深层土、封闭地面，强烈干扰土壤碳过程。迄今对ISA碳过程的研究仅限于个别城市的土壤调查，数据稀少，更缺乏对ISA下碳动态的观测，导致ISA下土壤碳分布格局不清，碳过程及其控制机制不明，困扰着对城市生态碳库和城市化效应评估。本项目拟结合城市群尺度ISA土壤调查和定位实验，刻画ISA土壤碳的分布、组成和动态特征；探究在ISA建设干扰和封闭效应下，土壤环境和微生物/胞外酶活性发生的变化，及其对土壤碳分解的影响，并分析ISA下土壤同外界的碳交换过程，从而揭示ISA土壤碳动态的控制机制。在此基础上改进城市生态系统模型，模拟ISA扩张对干旱区典型城市群的土壤碳库影响。研究成果可弥补碳循环和城市生态学领域的薄弱环节，且对完善城市生态服务价值评估、拓展碳封存潜力具有实践意义。

针对不透水面（ISA）下土壤碳过程数据缺乏、机制不清的问题，本项目开展定位对比实验，刻画ISA土壤碳的分布、组成和动态特征；探究在ISA封闭效应下土壤环境和微生物/胞外酶活性发生的变化及其对土壤碳分解的影响，从而揭示了ISA土壤碳动态的控制机制。研究表明与裸土对照样（PSA）地相比，不透水面地表显著（p<0.01）提高了土壤湿度，ISA下土壤湿度明显高于PSA，15cm深度的土壤湿度差异明显大于5cm深度。但对于土壤温度没有显著影响，只有夏季ISA温度略高于PSA。三种土壤气体（CO2、CH4、N2O）的含量在ISA和PSA两处理中无显著性差异。在PSA处理中凋落物分解速率常数高于ISA。凋落物在初期经理了一个迅速的分解过程，后期分解速度逐渐缓慢。21个月时PSA个别样点的凋落物以及完全分解，ISA样点的凋落物余量仍较多但分解速率仍缓慢，表明在不透水面的封闭阻碍了土壤分解过程。清除植被后，土壤pH值缓慢增长，且从第3个月后PSA增量显著大于ISA，导致第12个月后ISA土壤pH值显著低于PSA。ISA的土壤有机碳含量各时期ISA与PSA差异均不显著。 活性碳总体呈减少趋势，而惰性碳呈增加趋势。土壤总氮在PSA总体呈先增加后减少趋势，而在ISA呈持续减少趋势。土壤有效磷在ISA和PSA均呈增加趋势，而ISA的增加量明显高于PSA，并在试验末期（21个月）ISA有效磷含量显著高于PSA的值。总之，各项土壤指标的变化量在ISA与PSA之间的差距呈现波动式变化，多数指标的PSA的变化量明显高于ISA，其中部分指标在部分时期存在显著性差异，说明土壤过程极其复杂，而ISA下土壤环境相对较为稳定。同时，在取样初期，ISA与PSA土壤指标均有较大的无规律波动且后期并未出现恢复趋势，表明城市工程建设中剥蚀表土、压实土壤对城市土壤微循环系统具有极大扰动，扰动造成的后果在较长时间内无法恢复。这同Hamed等人（Hamed et a., 2018. doi:10.2136/sssaj2017.11.0381）在美国的试验结论相似。本研究成果可以未改进城市生态系统模型提供理论依据，进而模拟ISA扩张对干旱区典型城市群的土壤碳库影响。这些发现也弥补了碳循环和城市生态学领域的薄弱环节，且对完善城市生态服务价值评估、拓展碳封存有参考价值。