昆明城市土壤有机碳氮含量及其稳定同位素耦合关系研究

Coupling relationship of urban soil organic carbon and nitrogen contents with isotopes in Kunming urban area

随着城镇化的快速推进,城市土壤已成为陆地生态系统重要组成部分,不同土地利用类型土壤有机碳氮含量、稳定同位素组成分布特征及其耦合关系是了解城市土壤碳氮循环过程的重要途径.选取昆明市区受人类活动影响程度不同的公园绿地、居住绿地、道路绿地、耕地4类土壤作为研究对象,开展0—20 cm土壤有机质碳氮元素及其同位素组成的空间分异特征与影响因素研究.结果表明,昆明城市土壤有机质含量低,有机碳(SOC)和总氮(TN)含量中值分别为6.14 g·kg^(−1)和0.55 g·kg^(−1),较全国城市土壤平均水平低27.22%(C)和28.57%(N).土壤有机碳同位素(δ^(13)C)平均值为-25.22‰±1.56‰,氮同位素(δ^(15)N)则分布范围较广,为1.22‰—10.12‰,平均值为6.55‰±1.58‰,显著高于其他自然土壤δ^(15)N值. 4类土地利用类型中,耕地土壤SOC、TN含量显著偏高,δ^(13)C和δ^(15)N值显著偏正.垂向分布上,公园绿地、居住绿地、道路绿地SOC、TN含量随土层深度的增加而降低,δ^(13)C和δ^(15)N值则呈现富集趋势,表明重同位素^(13)C、^(15)N在土壤腐殖质中富集.耕地土壤因人为耕作,理化性质分层特征不明显.昆明市较低的土壤有机质含量与城市化进程导致的土地利用类型改变、绿化年限影响的成土时间较短以及昆明市气候、土质、植被类型等多因素有关.昆明城市土壤δ^(13)C值主要受昆明C_(3)、C_(4)植被覆盖度等自然环境因素影响,而δ^(15)N值由于受城市中水灌溉影响,显著偏正于自然土壤,指示人类活动已显著改变城市土壤氮循环过程,使土壤中更富集重同位素^(15)N.不同土地利用类型土壤垂向分布特征揭示有机质的分解、转化过程会对碳、氮同位素产生分馏作用,且氮同位素分馏高于碳同位素的分馏.

With the rapid development of urbanization,urban soils have become an important part of terrestrial ecosystem.Stoichiometric and stable isotope compositions of soil organic carbon and nitrogen provide a unique insight into the carbon and nitrogen cycling of urban soils under different land uses.Here,four types of soil profiles,including park green land,residential green land,roadside green land and cultivated land,which were typical of urban soils in Kunming,were selected to uncover the spatial variation and drivers of carbon and nitrogen elements and their isotopic compositions in top soils(0-20 cm).The soil organic content was relatively low,with a median value of 6.14 g·kg^(−1) for soil organic carbon(SOC)and 0.55 g·kg^(−1) for total nitrogen(TN),which were 27.22%and 28.57%lower than the national urban average,respectively.Soil δ^(13)C was significantly depleted,with an average value of-25.22‰±1.56‰.While the variation of urban soil δ^(15)N was large,ranging from 1.22‰to 10.12‰and with an average value of 6.55‰±1.58‰,which was significantly higher than those of other natural soils.Among the four soil types,the content of SOC and TN in cultivated lands was significantly higher,with a significant enrichment in δ^(13)C and δ^(15)N signals.Over the soil profile,SOC and TN in park green land,residential green land and roadside green land all decreased with an increasing depth,meanwhile δ^(13)C and δ^(15)N values showed a trend of enrichment.Due to frequent ploughing disturbances,the stoichiometry and stable isotope of cultivated soils had no significant change in the vertical profile.Relatively low content of soil organic matter in our study region might be related to multiple anthropogenic factors,such as the change of land use type caused by urbanization,the short soil development time affected by greening practice,and the monsoon climate,soil quality,and vegetation.In comparison,soil δ^(13)C value was mainly affected by natural factors such as C_(3) and C_(4) vegetation coverage.Possibly affected by urban reclaimed water irrigation,the δ^(15)N value of Kunming urban soil was significantly higher than that of natural soils,indicating that human activities have significantly changed the nitrogen cycling of urban soils.Profile characteristics of soils across different land use types indicated that decomposition and mineralization of organic matter would induce fractionation of carbon and nitrogen isotopes,resulting in the enrichment of ^(13)C and ^(15)N signals in residual soil humus,as well as a stronger isotope fractionation for nitrogen than that for carbon.