Both tree and soil carbon need to be quantified for carbon assessments of cities

Carbon storage is a valuable ecosystem service in a world facing anthropogenic climate change. Although studies have shown that urban areas have significant carbon stocks, these areas are often neglected in carbon budgets. The majority of published studies examining urban carbon storage so far focused on carbon storage in trees, neglecting organic carbon storage soils. However, the few studies including carbon storage in soils found high values. We measured carbon stored in trees and mineral topsoils in Hamburg in a joint study design. Tree carbon was calculated with the aid of allometric equations, while soil organic carbon was analyzed with taken soil samples to a depth of 30 cm. Additionally, amounts of organic carbon in trees and mineral topsoils were compared within and between different biotope types. Subsequently, carbon storage in different degrees of urbanization was compared. In total, about 6 Mt of organic carbon are stored within the political boundaries of Hamburg, with 2 Mt in trees and 4 Mt in mineral topsoils. Results from the city of Hamburg show an underestimation of urban carbon storage within its political boundaries in national carbon budgets. Carbon storage in trees showed no correlation to carbon storage in mineral topsoils. Further, stored amounts in both differed in analyzed biotope types. This underlines the need for quantification of both compartments independently. Comparison of the different urbanization degrees revealed that carbon storage is concentrated in the less urbanized areas. Results might be transferable to other cities via the degree of urbanization, as this classification is based on easy to compute data. Results show a significant amount of carbon stored in a large European metropolis. Global carbon budgets will benefit from the incorporation of found values, to result in exhaustive figures for an increasingly urbanized world. To provide stakeholders with reliable data on urban carbon storage, we show the need for future quantification studies to assess carbon stored in soils as well as in trees. (C) 2015 Elsevier GmbH. All rights reserved.