Introducing soil in NbS Monitoring Plans: in search of a compromise between feasibility and reliability

Growing interest in soil, a terrestrial compartment that has been historically neglected, is linked to the recognition of the crucial environmental services it provides to human societies. In the face of the current global crisis, soil appears as a strategic tool for mitigating climate change, for reducing risks associated with it, and for developing adaptation strategies. It is important to stress that soil does not have an autonomous functioning but is part of a complex organism integrated by plants, mineral substrates and organisms inhabiting the soil matrix, the rhizosphere and the phyllosphere. All these elements working together provide our food and the fibers we use, and regulate the hydrological and atmospheric cycles, including the carbon cycle. The vast majority of nature-based solutions (NbS) use the soil-plant organism as a tool or alter it in one or another form. Therefore, the environmental services provided by soil should be included in NbS monitoring plans from one or both of the following perspectives: either soil restoration is a Key Performance Indicator of the impact of the action or conserving and improving soil health makes part of the environmental co-benefits required for an NbS to be admitted as such. The importance of soil for the C cycle is better appreciated when considering that terrestrial ecosystems hold about 500 Pg C in plant biomass, and 1500-2000 Pg C in the form of soil organic matter, and that soils and the leaf litter lying on the ground store two to three times more organic C than the atmosphere. Furthermore, as soil carbon has not been comprehensively addressed through the soil profile, data most likely underestimate soil organic carbon (SOC) stocks in many parts of the world. SOC stocks can be dramatically affected by NbS when the programed actions include changes in land use. For example, in mineral soils reforesting croplands with native vegetation or planting trees on grasslands can increase SOC stocks by up to 25-40% in the upper horizons. Changes in soil management can significantly influence the CO 2 sequestration/emission balance. In this sense it is known that maintaining soil continuously covered by vegetation in woody crops can result in a positive annual increase of 0.32 Mg ha -1 yr -1 in the upper soil cm, then contributing to compensate for greenhouse gas emissions. Increasing or preserving SOC stocks triggers a number of col-lateral environmental benefits such as increased nutrient storage and water retention capacity, greater resistance to erosion, improved fertility for food production, etc., although unfortunately trade-offs between these services often hamper the assessment of NbS impacts. An important consequence of disturbing soil is affecting soil biodiversity. Soil is the most biodiverse compartment of terrestrial ecosystems. From 1.01 x 10 11 species described on Earth, 1.04 x 10 10 inhabit soil and interact with plants to provide fundamental services. In fact, most services attributed to plants are totally or partially produced by soil organisms. Despite overwhelming evidence of the importance of maintaining and restoring soil health, it is seldom considered in NbS monitoring plans. In a recent in-deep review of indicators proposed for NbS monitoring in EC funded projects and reference reports on NbS, only 49 indicators (of 570) addressed soil. The first limitation when promoting soil introduction in monitoring plans is the lack of consensus on the meaning of “Soil Health” and, specifically, on the most adequate indicators to operationalize its assessment. Soil multifunctionality suggests constructing integrated indexes built on minimal sets of indicators covering physical, chemical, and biological properties, but this approach is often impeded by trade-off between soil services and dis-services. The close interdependence between soil properties and local climate, geological materials, topography, land use history, and landscape structure hinder stablishing universal reference or “optimal” values for each indicator or expected patterns of evolution over time. In practice, methodological problems arise from the cryptic character of soil. Despite increasing attempts to replace field sampling and subsequent laboratory analyses with satellite imagery and pedotransfer functions, very few of these methods are reliable for application on a small spatial scale. The lack of budget allocated to soil sampling and analysis in NbS projects is one of the major constraints to implement consistent monitoring. To add complexity, soil response to manipulation may be very slow, and the results of an intervention can take several years to be detectable so monitoring must be extended beyond the duration of the projects. In this communication, we will explain how we deal with all these limitations, based on our experience in writing the monitoring plans of some EC projects addressing NbS, specifically the PHUSICOS project (https://www.phusicos.eu/), already concluded, and the ongoing CARDIMED project (https://www.cardimed-project.eu/). This will cover a wide range of situations, from urban and industrial cases to agricultural transformations and NbS addressing risk reduction. We will move from assessing the impact of simple interventions to complex cases that require soil to be included in WEFE NEXUS approaches.