The urban heat island (UHI) effect, the systematic elevation of temperatures in cities relative to surrounding rural areas, is one of the most documented consequences of urbanization, with profound implications for local climate, ecosystem functioning, and biogeochemical cycles (Oke, 1973; Luber and McGeehin, 2008; Tautenhahn et al., 2026). Among these cycles, soil respiration (SR) represents a major pathway of CO₂ flux from terrestrial ecosystems to the atmosphere (Davidson et al., 2002; Bond-Lamberty and Thomson, 2010; Decina et al., 2016), yet its dynamics under urban thermal and moisture regimes remain poorly understood. While numerous studies have investigated the environmental controls of SR in natural and agricultural systems (Chen et al., 2014; Jian et al., 2018, 2021), showing that seasonal CO₂ emissions are mainly driven by soil temperature (SST) (Hibbard et al., 2005), the extent to which the UHI modifies these relationships in urban soils has received comparatively little attention. The Q10 function, an exponential model describing SR as a function of temperature, remains the most widely used approach for SR prediction (Hashimoto et al., 2015; Lloyd and Taylor, 1994; Todd-Brown et al., 2013), but its applicability under the altered thermal and hydrological conditions of urban environments has rarely been tested.

However, assuming a constant temperature sensitivity of SR across all temperature ranges lacks ecological realism, as previously shown in mesic (Curiel Yuste et al., 2004; Lloyd and Taylor, 1994) and especially in water-limited ecosystems (Almagro et al., 2009; González-Ubierna et al., 2014; León et al., 2014). In such drought-prone systems, SST and soil moisture (SSM) often interact to regulate SR, with the process responding primarily to the most limiting factor (Correia et al., 2012; Almagro et al., 2009; Oyonarte et al., 2012).

Urban soils represent a particular case that is gaining increasing attention. Despite their importance, only a few studies have quantified soil CO₂ efflux in urban environments, mostly in temperate regions (Goncharova et al., 2020). Urban areas typically exhibit higher and more variable surface temperatures due to the urban heat island (UHI) effect compared with nearby rural or natural areas (Shi et al., 2012). In Mediterranean climates, however, the UHI effect is partially reversed, moderating cold winter temperatures more than summer heat, while rural zones may still experience extreme summer heat (Vardoulakis et al., 2013; Donateo et al., 2023). Under such conditions, SSM is expected to play a decisive role in SR dynamics. Although SR is generally constrained by low SSM during summer droughts, rewetting events often trigger abrupt and substantial CO₂ pulses (Jarvis et al., 2007; Unger et al., 2012; Matteucci et al., 2015).

These pulses are largely driven by soil biotic parameters (Fierer and Schimel, 2002). The relative influence of climatic, edaphic, and biotic factors (e.g., vegetation type, productivity, litterfall) on SR therefore remains an open question (Tang et al., 2020a; Yan et al., 2015). Vegetation can affect SR both directly, through its influence on soil carbon input and quality (Luo and Zhou, 2006; Yan et al., 2015), and indirectly, by modifying the soil microclimate, reducing temperature and evaporation and intercepting precipitation (Reichstein et al., 2002).

Some studies have compared SR across ecosystems (Tang et al., 2020b), but there is still a lack of research focused on microscale interactions among SR, climate, and biotic variables, especially in urban environments. In such human-modified systems, soils, microclimate, and vegetation structure are strongly altered by anthropogenic activities (Sæbø et al., 2003; Alía et al., 2025). These alterations affect soil quality (through contaminant deposition and recreational land use) and physical characteristics (e.g., compaction, artifacts) (Rebele, 1994; Sukopp and Werner, 1983), with significant implications for soil biogeochemical cycles and CO₂ emissions.

Despite these differences, SR in urban environments is frequently assumed to behave similarly to that in natural or rural systems (Karvinen et al., 2024). As a result, the influence of human-induced changes on soil CO₂ fluxes remains poorly understood. Microbial respiration, a major contributor to these fluxes, is known to be strongly affected by SST (Kaye et al., 2004; Lloyd and Taylor, 1994; Tomar and Baishya, 2020), yet this relationship has been scarcely examined in urban soils (Vasenev et al., 2021). While a few studies have explored microbial respiration in urban soils (Ghosh et al., 2016; Ivashchenko et al., 2019), spatially explicit datasets remain limited, hindering our understanding of how urban climate, particularly the UHI, affects microbial CO₂ emissions, especially in Mediterranean contexts (Vasenev et al., 2021). Moreover, there is a growing need to assess the temporal dynamics of SR responses to environmental drivers within stands dominated by specific plant communities (Chen et al., 2013).

Climate change is expected to exacerbate the UHI effect, as increasing global temperatures enhance heat accumulation in urban areas (Oke, 1973; Luber and McGeehin, 2008). This intensified warming alters soil thermal regimes and moisture dynamics, potentially influencing SR – particularly in densely built environments and megacities (Argüeso et al., 2015; IPCC, 2018). Understanding these interactions is therefore essential to accurately assess CO₂ fluxes in urban landscapes.

We hypothesized that SR in Mediterranean urban systems is significantly influenced by both SST and SSM, and that biotic factors such as plant community composition modulate these relationships. To test this hypothesis, we continuously monitored SR in the four most abundant frequent plant communities within Mediterranean urban green spaces in Madrid, measuring soil CO₂ efflux, SST, and SSM quarterly over one year across three urban sites.

Soils were notably homogeneous in texture across all sites and plant communities, with sand content ranging from 71 % to 77 %. The most notable differences between communities were observed in pH, which was consistently higher in AH-D soils (8.22 ± 0.01) and lower in PG-D soils (6.33 ± 0.11), and in WHC, which was markedly higher in PG-D communities (53.25 ± 9.09 %), reflecting the greater root biomass and organic matter accumulation typically associated with perennial grasslands.

The soil physicochemical characterization revealed a notable textural homogeneity across all study sites and plant communities, with all soils classified as Sandy Loam (sand content 71 %–77 %). This uniformity supports the comparability of measurements across sites and suggests that differences in soil CO₂ efflux among plant communities are more likely attributable to vegetation cover and its influence on microclimate and organic matter inputs than to contrasting soil physical properties. Nevertheless, marked differences in WHC were observed between communities, with PG-D soils showing the highest values (53.25 ± 9.09 %), which may partly reflect the greater root biomass and organic matter accumulation typically associated with perennial grasslands. Given that soil moisture emerged as the dominant driver of CO₂ efflux in this study, the higher water retention capacity of PG-D soils may contribute to explaining the respiration dynamics observed in this community. Additionally, the consistently lower pH recorded in PG-D soils (6.33 ± 0.11) compared to the remaining communities (7.32–8.22) may favour higher microbial activity, as near-neutral pH values are generally considered optimal for soil microbial communities (Rousk et al., 2010)

The climatic variables (both atmospheric and soil-related) followed a typical Mediterranean pattern, characterized by cold – wet winters and warm – dry summers that generate highly transient soil moisture dynamics (Seager et al., 2019). The climatic evolution drove CO₂ emissions in a consistent, parallel pattern. In our study site, soil moisture was markedly higher in winter and sharply declined through spring and summer compared to atmospheric moisture, consistent with the strong seasonal decoupling between precipitation and temperature that controls soil water availability in Mediterranean ecosystems (Viola et al., 2008). No differences were found between plant communities with the exception of CO₂ emissions, with PH-M producing higher soil CO₂ emission rates, and showing higher variability, than the other communities.

Soil respiration under Mediterranean climates clearly does not follow the general assumptions derived from other climatic regions (González-Ubierna and Lai, 2019). SST is not the clear driver of SR, and at the global scale there is no simple positive correlation between them because their relationship often follows a Gaussian-like response, with an optimum temperature beyond which SR declines (Lellei-Kovács et al., 2011; Anjileli et al., 2019). This non-linear behaviour has been reported in Mediterranean agricultural soils (González-Ubierna and Lai, 2019), natural ecosystems (Anjileli et al., 2019) and is now also observed under urban conditions. The Q10 index is therefore not useful in highly variable climates, such as Mediterranean ones (Chang et al., 2014; Meyer et al., 2018), and is not at all recommended under urban conditions (Wu et al., 2021 highlighted the doubts about Q10 index in a changing climate). Consistent with this, our own model comparison confirmed that the Q10 formulation performed markedly worse than both the Gaussian and the proposed mechanistic model. The UHI effect does not seem to substantially modify the shape of the SR–SST relationship, whereas the influence of soil water availability and rewetting pulses, expressed as a RW_i, appears to be largely suppressed under urban conditions even in the absence of irrigation, likely due to reduced effective infiltration, soil sealing and altered rainfall partitioning (Scalenghe and Marsan, 2009; Gillefalk et al., 2021). Some authors have highlighted the impact of the UHI effect on drying intensity and the consequent reduction or elimination of rewetting pulses on CO₂ emissions (Tautenhahn et al., 2026). Soil moisture and RW_i showed a strong positive influence on soil CO₂ emissions, without a clear maximum, which contradicts the patterns reported for Mediterranean soils under natural and semi-natural conditions. In contrast to the typical Mediterranean pattern, where drying–rewetting events generate discrete CO₂ pulses superimposed on a temperature-driven seasonal cycle (Talmon et al., 2011; León et al., 2014; Bautista et al., 2021), SR in our urban sites increased monotonically with SSM and RW_i within the observed range, without a marked optimum. In relation with RW_i, the relation was consistent below 20, since with higher values, and due to poor water management in general (Tautenhahn et al., 2026), produced a weaker influence.

The plant community showed a modest effect on SR, with a single significant difference between PH-M and the other three communities (AH-D, AG-H, and PG-D). Because soil and atmospheric temperature did not differ significantly among plant communities, and because the visiting order of parks and sampling points was systematically rotated and randomized across a wide daily window (08:00–19:00), the higher emissions observed in PH-M cannot be attributed to a systematic measurement-time temperature bias, but rather point to an intrinsic community-level effect, likely related to soil disturbance and litter quality. PH-M exhibited the highest interannual variability and the greatest net CO₂ emission rates, whereas PG-D showed the most stable temporal pattern over the year. No differences in climatic conditions were observed among communities, indicating that the effect is linked to plant community composition rather than to climate, or than the influence of plants on climate. Wang et al. (2014) highlighted the impact of plant communities on modifying SST sensitivity of SR. The effect of litter inputs is widely related to SR (Stanek and Stefanowicz, 2019; Woś et al., 2023; Zhang and Wang, 2015), due to their impact on microbial communities. In urban conditions, however, litter is not only coming from vegetation, and human wastes significantly alter soil carbon cycle and dynamics. PH-M is indicative of soil perturbation (Molina et al., 2023), which could explain the differences found. This was translated into a higher variability on the SR and RW_i relationship in PH-M communities, due to the disruption of macronutrient cycles and its impact on microbial activities (Torija et al., 2026). In contrast, AH-D displayed the most stable interannual SR–RW_i relationship, which can be attributed to more balanced nutrient cycles and to lower enzymatic and microbial activities relative to the more disturbed PH-M communities, in agreement with studies linking higher functional stability to less disturbed, structurally more complex ecosystems (Gough et al., 2021).

The Gaussian model describing the relationship between SR and SST exhibited a maximum at 18.4 °C, slightly lower than the optimum temperature reported for peri-urban agricultural soils in the same area (González-Ubierna et al., 2014). In semi-arid and Mediterranean climates, this Gaussian behaviour has been attributed to the combined effects of SST and SSM, which jointly constrain soil CO₂ efflux under warm and dry, or cold and wet conditions (Lellei-Kovács et al., 2011; Chang et al., 2014). These studies have also shown that the SR optimum shifts towards higher temperatures when SSM is higher, whereas low SSM can inhibit microbial activity and thus lower the temperature at which soil respiration peaks. In our case, compared with the 2014 study, the data indicated predominantly high-moisture conditions (according to the original SSM classes), above 0.20 m³ m⁻³, previously identified as a threshold that constrains soil respiration and masks its dependence on soil temperature (Rey et al., 2002; Tedeschi et al., 2006). Thus, SR did not exhibit a moisture optimum but instead increased exponentially with SSM, which is consistent with the absence of very low SSM values and likely prevented moisture limitation. Under these conditions, the interaction between relatively warm SST and consistently high SSM may promote an exponential rather than a Gaussian SR–SSM relationship (Lellei-Kovács et al., 2016). In Mediterranean ecosystems, once soil water content exceeds the limiting threshold, the typical Arrhenius-type exponential increase of soil respiration with soil temperature re-emerges and respiration tends to increase monotonically with soil moisture, rather than showing bell-shaped moisture responses. Under these conditions, the interaction between relatively warm SST and consistently high SSM may therefore promote an exponential, rather than Gaussian, SR–SSM relationship.

It should be noted that the 18.4 °C breakpoint was estimated from a pooled dataset spanning a full annual cycle, and therefore predominantly reflects seasonal rather than diurnal variation in SST, given the defined daily sampling window and rotated visiting order described in Sect. 2.2. The narrow 95 % confidence interval of this estimate (17.17–19.79 °C) further supports its robustness as a meaningful threshold rather than a statistical artefact.

In cold conditions (SST < 18.4 °C), SST positively and approximately linearly influenced soil CO₂ emissions and was the main driver of SR variability, as typically observed in temperate climates (Richardson et al., 2012; Zhou et al., 2014). The strong relationship between SST and SR meant that SSM and RW_i showed no significant statistical relationship with SR, although SSM still tended to exhibit a positive association with soil CO₂ efflux. The relatively high SSM values (above 0.20 m³ m⁻³, previously identified as a threshold that constrains soil respiration and masks its dependence on soil temperature (Rey et al., 2002; Tedeschi et al., 2006), and which are represented by the disconnect between SSM and SST, likely avoided limitations to microbial and root activities and may therefore explain the weak influence of water-related variables on SR variation (Chang et al., 2014). This suggests that under urban Mediterranean conditions, there are no clear rewetting pulses at high SSM combined with low SST (Morillas et al., 2017). Within this part of the response surface, PH-M produced higher CO₂ emissions than all other communities except AG-H when controlling for soil water variables, and higher emissions than AH-D when the effect of SST was removed.

In warmer conditions (SST > 18.4 °C), soil water variables became the main drivers of SR, with both SSM and RW_i exerting a positive influence on soil CO₂ emissions, whereas SST had no detectable effect or even a slight negative effect once water-related variables were accounted for (Chang et al., 2014; Conant et al., 2004). This pattern is consistent with Mediterranean and semi-arid studies showing that soil moisture can overrule the temperature dependence of soil respiration under warm and dry conditions (Rey et al., 2002; Chang et al., 2014). The constriction of SSM at high SST (values frequently falling below 0.20 m³ m⁻³ during part of the year within this temperature range, with a negative correlation between the two variables) likely limited microbial and root activities, reducing CO₂ emissions and driving their variability, in line with reported moisture thresholds that constrain soil respiration and mask its dependence on soil temperature (Rey et al., 2002; Tedeschi et al., 2006; Chang et al., 2014). Within this portion of the response surface, the PH-M community exhibited higher SR than the other communities even after controlling for climatic variables (SST, SSM and RW_i), which agrees with previous findings that vegetation type and associated soil properties can generate consistent differences in soil CO₂ efflux under similar microclimatic conditions (Rey et al., 2011; Han et al., 2014).

The development of a specific model allows us to analyze these relationships deeper. The biological rationale for the negative temperature effect is grounded in the interaction between soil temperature and soil moisture (Rey et al., 2022; Lellei-Kovács et al., 2016; González-Ubierna and Lai, 2019). Consider a soil system in which microbial activity drives CO₂ emissions, in the absence of moisture, microbial communities would progressively decline, reducing respiration rates (Manzoni et al., 2012). Under these conditions, increasing temperature would accelerate desiccation and microbial mortality, further decreasing respiration. When moisture is reintroduced, however, temperature and moisture interact synergistically: higher moisture availability enables microbial activity, and the combination of warmth and moisture jointly stimulates CO₂ emissions (Moyano et al., 2013; Brangarí et al., 2020). The critical constraint is that, in Mediterranean environments, increasing temperature inevitably leads to decreasing soil moisture, the two variables are negatively correlated. This physical coupling means that temperature and moisture cannot increase simultaneously without bound, and it is precisely this constraint that generates the unimodal response of soil respiration to temperature: emissions rise as temperature increases while moisture remains sufficient, but decline once temperature-driven desiccation reduces moisture below the level needed to sustain microbial activity (Alster et al., 2023). In the same way, the proposed model first describes how CO₂ emissions would evolve with temperature alone, under the limiting assumption of zero moisture, and then introduces the moisture-temperature interaction to capture the full observed dynamics. The result is a biologically interpretable model that reproduces the unimodal structure of the data and is consistent with the known controls of microbial respiration in water-limited ecosystems (Peng et al., 2026).

Our results reinforce the idea that SST and SSM interact to control soil CO₂ emissions under Mediterranean conditions (including now urban ones), with SR responding to whichever is the most limiting factor (Almagro et al., 2009; Chang et al., 2014; Bautista et al., 2021). These findings point to a more complex and nuanced role of the UHI in regulating urban soil CO₂ fluxes than is commonly assumed. Rather than acting primarily as a thermal stimulator of microbial activity, as would be predicted by temperature-driven models such as Q10, the UHI in Mediterranean urban settings appears to operate predominantly as a drying agent, particularly during the summer period when its thermal effect is most intense and soil moisture reaches its seasonal minimum. This interpretation is consistent with the emerging concept of the Urban Dry Island (UDI), whereby the replacement of vegetated surfaces with impervious materials reduces evapotranspiration and infiltration, thereby lowering soil moisture availability even in unsealed green spaces embedded within the urban matrix (Hao et al., 2023; Scalenghe and Marsan, 2009). Under such conditions, the thermal energy that the UHI adds to the system is largely dissipated as sensible rather than latent heat, further accelerating soil desiccation and limiting the microbial and root activity that drives SR (Manoli et al., 2019). It should be noted, however, that the study period registered relatively high soil moisture conditions for much of the year, consistent with above-average precipitation during the monitoring period. Under these conditions, moisture limitation was only partially expressed, which may explain why SR responded exponentially to SSM rather than showing the bell-shaped optimum typical of more severely water-limited systems. This suggests that the drying effect of the UHI, while structurally present, was partially buffered by the interannual climatic variability of the monitoring period, and that its full suppressive impact on SR may be more pronounced in drier years. Consequently, the net effect of the UHI on soil CO₂ emissions in Mediterranean cities is paradoxical: rather than enhancing SR as observed in temperate urban systems, it tends to suppress average emission rates while simultaneously increasing their temporal variability – driven by episodic rewetting pulses that briefly overcome moisture limitation (Tautenhahn et al., 2026; Gillefalk et al., 2021). This drying-dominated UHI effect also helps explain the suppression of rewetting pulses observed in our sites compared to non-urban Mediterranean ecosystems, likely attributable to reduced effective infiltration and altered rainfall partitioning in the urban environment (Scalenghe and Marsan, 2009; Gillefalk et al., 2021).

Plant communities differed in their modelled respiration parameters, revealing ecologically interpretable patterns. The inverse relationship between basal respiration and maximum respiration capacity, with communities with lower basal rates tending to reach higher maxima, suggests a functional trade-off between constitutive microbial activity and the capacity to respond to optimal temperature and moisture conditions. PH-M consistently showed the highest SR both annually and at the temperature optimum, reflecting the greater soil perturbation and labile carbon availability associated with this community (Molina et al., 2023; Torija et al., 2026). In contrast, PG-D exhibited the highest basal SR and the lowest temperature optimum, consistent with the more stable and recalcitrant carbon inputs characteristic of perennial grassland communities (Stanek and Stefanowicz, 2019). AH-D displayed intermediate behaviour across all parameters.

The most ecologically informative pattern was the positive correlation between the sensitivity of SR to soil moisture and the temperature optimum for SR: AG-H and PH-M, the communities with the strongest moisture influence on respiration, also showed the highest temperature optima (Schipper et al., 2019; Zhou et al., 2014). For AG-H, dominated by the annual grass Hordeum leporinum, this pattern is consistent with its phenological strategy. In Mediterranean urban environments, AG-H develops during autumn–winter and reaches peak biomass and root activity in spring, precisely when urban soils in Madrid retain moisture while soil temperatures rise rapidly (Molina et al., 2023; Xu and Baldocchi, 2004). This creates a narrow but intense seasonal window during which labile rhizospheric carbon inputs are highest and microbial activity is tightly coupled to both temperature and moisture simultaneously (Placella et al., 2012; Bahn et al., 2008), conditions that are expected to enhance the apparent sensitivity of soil respiration to these two drivers. As the growing season ends and soils dry, control shifts almost entirely to moisture availability (Chang et al., 2014; Almagro et al., 2009), explaining why AG-H shows the lowest basal respiration values despite its high maximum capacity.

These findings have direct implications for the management of urban green spaces in Mediterranean cities. Since soil moisture, and not temperature, is the dominant driver of CO₂ efflux, management strategies that preserve or enhance soil water availability are likely to have a greater influence on the urban soil carbon balance than those focused on thermal regulation alone. Practical measures include reducing impervious surface cover adjacent to green spaces to improve lateral water inputs, enhancing soil infiltration capacity through reduced compaction and the incorporation of organic amendments, and implementing moisture-sensitive irrigation regimes adapted to the pronounced seasonal drought characteristic of Mediterranean cities. The Gaussian temperature response identified here further suggests that conventional temperature-based models used in urban carbon accounting tools will systematically overestimate soil CO₂ emissions during summer months in water-limited Mediterranean cities, as they do not account for the simultaneous decline in soil moisture. Incorporating moisture-temperature coupling into urban green space management planning and carbon monitoring protocols would improve the accuracy of urban carbon budgets and support more effective climate adaptation strategies.

Some limitations of this study should be acknowledged. First, the monitoring period (April 2023–April 2024) was characterised by above-average precipitation for the Madrid region, which may have partially buffered the moisture limitation typically observed in Mediterranean urban soils during dry years. The dominance of soil moisture as a driver of SR and the absence of a clear moisture optimum may therefore reflect the specific hydroclimatic conditions of the monitoring period, and the relationships identified here should be validated across contrasting hydrological years. Second, total soil CO₂ efflux was measured without partitioning autotrophic and heterotrophic components, as vegetation was removed prior to each measurement to avoid above-ground interference. While this approach is standard in field-based SR studies and minimises confounding effects of above-ground plant respiration, it precludes direct attribution of CO₂ fluxes to microbial versus root-derived sources. Finally, the study was conducted across three urban green spaces in Madrid, and the generalisability of the findings to other Mediterranean cities or urban contexts with different soil management histories, vegetation structures, or degrees of imperviousness should be tested in future research.

Soil respiration in the studied Mediterranean urban green spaces does not follow the conventional Q10 exponential model. Instead, a Gaussian (unimodal) relationship between SR and soil temperature was identified, with a positive correlation below a breakpoint of 18.4 °C and a negative effect above this threshold, consistent across all plant communities studied. This finding challenges the universal applicability of temperature-based respiration models in water-limited urban environments and suggests that Q10-based approaches may misrepresent soil CO₂ dynamics in Mediterranean cities.

Soil moisture emerged as the dominant driver of SR in the studied urban green spaces, exhibiting a positive exponential relationship with CO₂ efflux across all plant communities. The rewetting index showed an additional positive logarithmic effect on SR for values below 20, reflecting the importance of episodic precipitation pulses in triggering CO₂ emissions in these water-limited systems. Above this threshold, the relationship weakened, likely due to reduced effective infiltration under urban conditions.

The apparent negative effect of temperature on SR above 18.4 °C is not a direct thermal suppression of microbial activity, but rather an indirect consequence of the strong negative coupling between soil temperature and soil moisture in the Mediterranean climate, and should be considered when developing models of urban soil carbon fluxes in comparable Mediterranean urban settings.

A mechanistic model was developed describing SR as the joint outcome of temperature-driven moisture loss and moisture-stimulated CO₂ emissions. The model successfully reproduced the unimodal structure of the observed data and provided biologically interpretable parameters for each plant community, offering a more ecologically realistic alternative to monotonic temperature-based formulations for water-limited urban ecosystems.

The urban heat island effect did not act primarily as a thermal stimulator of SR, as is commonly assumed for temperate urban systems. Instead, it appears to operate predominantly as a drying agent, suppressing average SR rates while increasing their temporal variability through episodic rewetting pulses. This drying-dominated role of the UHI, while consistent with the observed data, was partially buffered by above-average precipitation during the monitoring period, and its full suppressive impact on SR may be more pronounced in drier years.

Plant community identity had a limited overall effect on SR dynamics. The only significant exception was the perennial herb community dominated by Malva spp., which consistently produced higher CO₂ emission rates and greater temporal variability than the other communities, likely reflecting greater soil perturbation and labile carbon availability associated with this ruderal community. Among the remaining communities, the annual grassland dominated by Hordeum leporinum showed the strongest sensitivity of SR to soil moisture and the highest temperature optimum, consistent with its phenological strategy of concentrating root activity and labile carbon inputs during the moist and warming spring period.

These findings collectively highlight the need for moisture-integrated approaches to accurately model soil CO₂ fluxes in Mediterranean urban green spaces such as those studied here, and underscore the importance of accounting for the drying role of the urban heat island when projecting urban carbon budgets, pending validation across a broader range of cities and hydrological years.