The challenges of the 21st century, namely, climate change and loss of biodiversity, especially present in heavily populated areas, should be addressed. Nature-based Solutions (NBS) seem to offer a suitable answer to these challenges. However, this new concept is not always easy to implement in a sustainable manner. In an effort to identify the barriers and levers for the implementation in cities of these sustainable NBS, several professionals working on them in France were interviewed. The first analysis with the most quoted words shows that the constraints would be mainly related to technique, management, biodiversity and people. The levers would be related to projects, roofs, people, legislation and services. Further analysis shows that most of the interviewees feel that the main barriers are the lack of knowledge, political will, financial resources and regulations. There are also cultural limitations, such as the use of exotic horticultural plants rather than wild local species. According to them, the technical problems should be easy to solve. To address these issues, the interviewees propose the development of transdisciplinary research disciplines, as well as on-field collaboration between all NBS actors in cities. They also recommend specific funds for NBS and their implication in related regulations. Demonstrative examples of urban NBS highlighting their multiple benefits are also needed to encourage their replication or upscaling. Education and communication are essential to shift the traditional points of view on nature in the city.

Urban agriculture is sprouting throughout the world nowadays. New forms of urban agriculture are observed such as rooftop farming. In the case of low-tech rooftop farming projects, based on recycled urban waste, one of the key issues is the type of substrate used, as it determines the functions and ecosystem services delivered by the green roof. Using a five year experimental trial, we quantified the food production potential of Technosols created only with urban wastes (green waste compost, crushed wood, spent mushroom), as well as the soil fertility and the potential contamination of food products. Regarding food production, our cropping system showed promising results across the five years, in relation with the high fertility of the Technosols. This fertility was maintained, as well as the nutrients stocks after five cropping years. Most of the edible crops had trace metals contents below existing norms for toxic trace metals with nevertheless a concern regarding certain some trace metals such as Zn and Cu. There was no trace metal accumulation in the Technosols over time except for Zn. This study confirmed that constructing Technosols only from urban wastes is a suitable and efficient solution to design rooftops for edible production.

Scientific research on the mitigation hierarchy has steadily increased over the past few years at the international level. While some seek to improve the application of this public action instrument, others point out its shortcomings and risks. This opinion paper—which focuses on the French context—does not provide an exhaustive overview of existing research but instead targets specific issues considered to be a “priority”. We mainly investigate the relevance and implementation of the mitigation hierarchy, especially from an ecological point of view. Part of this paper thus questions the very principle of biodiversity offsetting (BO)—the last resort of the mitigation hierarchy that brings together numerous controversies—and the adequacy of the mitigation hierarchy with the objective of no net loss (NNL) of biodiversity. The general idea underlying this paper is to show how the mitigation hierarchy has been built and based on what values (mainly economic and legal, which leads us to conclude about the lack of ecology in the policy itself). In doing so, we provide a few perspectives as to what should be done to (better) integrate ecology into land use planning and development.

Research highlights: Funding forest management with subsidies from carbon offsetters is a well-documented mechanism in tropical regions. This article provides complementary insights into the use of voluntary offset contracts in temperate forests. Background and objectives: The mitigation of greenhouse emissions has become a major global issue, leading to changes in forest management to increase the capacity of forests to store carbon. This can lead to conflicts of use with other forest ecosystem services such as timber production or biodiversity conservation. Our main goal is to describe collective actions to fund carbon-oriented forestry with subsidies from carbon offsetters and to analyze how their governance and functioning prevent conflicts pertaining to multi-functionality. Materials and methods: We assembled an interdisciplinary research team comprising two ecologists, a social scientist, and an economist. Drawing on a conceptual framework of ecosystem services, social interdependencies, and collective action, we based our qualitative analysis on semi-structured interviews from two French case studies. Results: Carbon-oriented intermediary forest organizations offer offset contracts to private firms and public bodies. Communication is geared toward the mitigation outcomes of the contracts as well as their beneficial side effects in providing the ecosystem services of interest to the offsetters. Subsidies then act as a financial lever to fund carbon-oriented forestry operations. Scientific committees and reporting methodologies serve as environmental, social, and economic safeguards. Conclusions: These new intermediary forest organizations use efficient forest operations and evaluation methodologies to improve forest carbon storage. Their main innovation lies in their collective governance rooted in regional forest social-ecological systems. Their consideration of multi-functionality and socioeconomic issues can be seen as an obstacle to rapid development, but they ensure sustainability and avoid conflicts between producers and beneficiaries of forest ecosystem services. Attention must be paid to interactions with broader spatial and temporal carbon policies.