Beyond indicator frequency: a systematic review towards integrated impact assessment of soil-based solutions to mitigate land desertification processes.

Desertification, defined by the United Nations Convention to Combat Desertification (UNCCD) as land degradation in drylands driven by the interaction between climate variability and human activities, represents an escalating global threat, particularly in drought-prone regions. In Europe, large areas are already classified as high to very highly vulnerable to degradation, a situation expected to intensify under projected climate change scenarios and continued land-use pressures.

Recent estimates suggest that between 60 and 70% of soils in Europe are considered unhealthy, highlighting the urgent need for effective soil protection. Soil degradation compromises key ecosystem functions, including food production, water retention, nutrient cycling, carbon storage, and biodiversity conservation. If critical thresholds are exceeded, the resulting environmental and socio-economic consequences may become irreversible.

The EU Horizon project TERRASAFE aims to empower local communities in southern Europe and northern Africa to address the growing threat of desertification by promoting a suite of innovations, including nature-based solutions (biochar, compost, technosols, and hydrogels), sensor-based monitoring tools, and social approaches. The present work focuses exclusively on the four nature-based solutions, as these innovations directly modify soil properties and processes and are therefore most suitable for systematic evaluation through biophysical indicators. Although these solutions have been already tested across a range of dryland environments, their reported impacts remain unevenly investigated and fragmented across disciplines, indicators, and experimental scales. In this context, a robust understanding of the current state of knowledge is essential.

To address this need, the present study conducts a systematic review of the scientific literature assessing the effects of these four solutions under arid, semi-arid, and Mediterranean climatic conditions, with a specific focus on the indicators used to evaluate desertification-related processes. A wide range of soil, plant, and ecosystem indicators was examined and synthesized to determine whether they are being investigated evenly or whether critical indicators remain underrepresented in the current state of the art.

Preliminary screening reveals a marked dominance of physical, chemical, and productivity-related indicators, while biological, ecotoxicological, and eco-physiological indicators appear comparatively neglected. Identifying these knowledge gaps is pivotal to avoid partial interpretations of solution performance and to support broader, integrated impact assessments that adequately capture key desertification mechanisms.

In a subsequent phase, building on this representativeness analysis, the study will advance beyond indicator frequency, to identify which indicators most effectively explain desertification processes. This will provide a foundation to more targeted, meaningful, and decision-relevant monitoring strategies in regions with high vulnerability to desertification.