长三角典型城郊土地利用变化及其土壤碳氮响应

Land use change and its influence on soil organic carbon and total nitrogen in a typical peri-urban watershed of the Yangtze River delta

城市化在改变城市周边土地利用及其空间布局的同时,也改变了城郊土壤的碳氮循环等关键生物地球化学过程。明确城郊地区土地利用变化及其对土壤碳氮储量的影响,可有效揭示城郊地区土壤肥力提供等关键生态系统服务的演变特征。以长三角典型城郊宁波樟溪流域为例,分析了1974年至2015年其土地利用变化特征,并采用DNDC模型模拟了土地利用变化所引起的土壤碳、氮储量变化。研究表明该流域内农地和林地面积均在不断减小,而园地和城镇建设用地面积在不断增加。模拟表明农地单位面积有机碳、总氮含量逐年降低,而林地则不断增加,园地呈波动变化,不同土地利用类型的单位面积有机碳和总氮含量对温度和降雨的变化有不同程度的响应。随着城郊地区土地利用变化,流域内农地土壤有机碳和总氮储量逐年降低,园地和林地土壤有机碳和总氮储量逐年增加,土地利用变化决定了流域土壤有机碳和总氮的储量变化特征。

Land use change has significant influences on many biogeochemical and ecological processes in peri-urban ecosystems, especially for soil carbon and nitrogen cycles. Identifying the relationship between land use change and soil processes in peri-urban areas is a key issue in current studies. In this study, the Zhangxi watershed was selected as the study area, which is a typical peri-urban watershed in the Yangtze River Delta. The land use changes were analyzed from 1974 to 2015 based on remote sensing images. The DNDC （DeNitrification--DeComposition） model was used to simulate soil organic carbon and soil total nitrogen changes in this period. The results showed that from 1974 to 2015, area of forestland and farmland decreased because of urbanization, and the area of urban construction land increased. Calibration of the DNDC model showed that the simulated values were consistent with observed values. This indicated that the calibrated DNDC model can accurately simulate soil organic carbon and soil total nitrogen in the study area. Results of the DNDC model showed that soil organic carbon and soil total nitrogen content per unit area in farmland decreased during the simulation period, and the content decreased with increasing temperature and the amount of precipitation. The values of soil organic carbon and soil total nitrogen content per unit area in orchards displayed no obvious changes during the past 41 years, and soil organic carbon and soil total nitrogen in orchards were sensitive to temperature changes. The soil organiccarbon and total nitrogen per unit area in forestland increased during the simulation period and increased with increasing temperature and decreased with increasing amounts of precipitation. At the watershed scale, simulation results showed that the total amount of soil organic carbon and soil total nitrogen in farmland decreased from 1974 to 2015, and the values in orchard increased. The soil organic carbon in forestland increased during simulated years, whereas the soil total nitrogen decreased in early simulated years and increased in the recent two decades. The changes in total organic carbon and total nitrogen reserves at the watershed scale were dominated by forestland because of the high content of soil organic carbon and total nitrogen in fbrestland and its large area in the watershed. This indicated that land use type and its composition determines the characteristics of soil organic carbon and total nitrogen storages at the watershed scale. This study showed that both land use, and climate factors （temperature and precipitation） have effects on soil organic carbon and soil nitrogen storage, and land use take a determining role in this process. Results of this study could provide scientific guidance on soil security and land use optimization in rapid urbanization areas.