Multifractal fingerprints of rain events on soil moisture and respiration in a Mediterranean grassland

Mediterranean grasslands operate near the edge of water limitation and are strongly driven by short, discrete rainfall events. Yet, we still know little about how the event-scale dynamics of soil moisture (SWC), soil temperature (ST) and soil respiration (reorganize between wet and dry years. Here we use multifractal detrended fluctuation analysis (MFDFA) on time series in El Escorial (central Spain) to characterise post-rain dynamics in two contrasting years: a relatively wet year (2022) and a dry year (2024). We focus on April (spring) and September (), and six series and interactions of the soil–plant–atmosphere system: SWC, ST, CO₂ and the pairs SWC–ST, SWC–CO₂, ST–CO₂. For each post-rain window (several days after individual events) we quantify for these six series, and compare their behaviour across seasons and years.

In April 2022, Δα is moderate and H2 shows a stable, moisture-dominated backbone: SWC–SWC and SWC–ST are highly persistent, while CO₂–CO₂ and ST–CO₂ are often antipersistent while still moderately multifractal, indicating that CO₂ acts mainly as a reactive signal to water and temperature. In April 2024, Δα increases markedly for CO₂–CO₂ and SWC–CO₂, and their H2 shifts towards stronger persistence, while ST–CO₂ becomes more antipersistent. This points to a reorganisation whereby, under early-season water stress, carbon–moisture couplings become the main carriers of complexity and memory, and ST becomes a more reactive pathway. In September 2022, multifractality remains moderate but a strongly negative asymmetry in SWC–SWC and SWC–CO₂ reveals sharp rewetting and respiration pulses driven by soil moisture. In September 2024, Δα becomes very high for SWC–SWC, SWC–ST and CO₂–CO₂, with H2 ≈ 0.9–1.0 for SWC–SWC, SWC–ST and CO₂–CO₂, while asymmetry shifts: extremes move from moisture-dominated (negative in SWC–CO₂) to carbon-dominated (positive in CO₂–CO₂) and ST–CO₂ becomes strongly antipersistent.

In conclusion, these results show that using SWC, ST and and their interactions it is possible to identify distinct post-rain “modes” of ecosystem functioning: (1) a wet-year regime with a persistent SWC–ST backbone and moisture-driven pulses, and (2) a dry-year regime where long-range memory strengthens in SWC–ST–CO₂ but extremes and intermittency shift into the carbon subsystem, indicating loss of hydrological buffering and increased carbon–thermal stress after rainfall events. Such event-scale indicators could be used to inform adaptive grassland and land management strategies in Mediterranean regions, by identifying when ecosystems are approaching critical thresholds of water and carbon stress.

Acknowledgement: This paper is part of the project Clasificación de Pastizales Mediante Métodos Supervisados—SANTO, from Universidad Politécnica de Madrid (project number: RP220220C024). And funded by the European Union. Views and opinions expressed are however those of the author(s) and do not necesarily reflect those of the European Union or European Research Executive Agency (REA). Neither the European Union nor the granting authority can be held responsible for them.