Maria-Teresa Sebastià

Soil reflects ecosystem processes and is influenced by gradual biospheric changes, which can affect its biotic components. In fairy rings, soil morphology, physicochemical properties, and biota are interconnected within a shared environmental space. In La Bertolina grasslands, while fungal and bacterial genomics have been investigated, the micromorphological soil effects of these rings have not. This study micromorphologically analyzed thin sections of three fairy rings at four zones: the ring center, the zone of peak growth in 2013 (R13), the predicted growth zone for 2019 (R19), and outside the ring. From each zone, two thin soil sections were prepared, totaling 24 samples. Fungal structures were exhaustively described according to morphological criteria following reference by multiple authors. The soil was a calcareous, loamy Regosol, and showed moderately developed crumb or laminar microstructures. Nine types of fungal structures were identified, consistent with genomic findings in the zone. Although fungal abundance did not vary across zones, mesofauna droppings were more frequent in R13 and R19, which was related to higher nutrient or water availability due to the fungal activity. Regarding the groundmass of the topsoil, neither the composition nor the microstructure of the surface horizons varied according to the moment of appearance of the ring at the sampled points.

Summary: Grassland ecosystems are key biodiversity reservoirs, especially for the soil fauna. Among them, nematodes have been widely used as bioindicators to assess soil conditions and health in grasslands. However, their interactions with plant functional types, particularly in hot and dry climates like the Mediterranean region, remain poorly understood. The development of management strategies to optimise the resilience and productivity of grassland requires a better understanding of these biotic relationships. To address this, nematode communities were examined in grassland monocultures of three plant functional groups (grasses, legumes, forbs) in the Mediterranean. In France, Slovenia, Spain, Jordan and Lebanon, grasslands differing in soil properties and climate were investigated in autumn and spring in consecutive seasons. The genus composition of nematode assemblages was assessed, and their ecological indices and metabolic footprints were related to site characteristics and plant functional type. The Renkonen and Sörensen Index revealed that nematode communities differed greatly among regions. The western Mediterranean grasslands were dominated by bacterial feeders like Rhabditis and Eucephalobus , resulting in a low Channel Index and high Enrichment Index. The eastern Mediterranean sites showed higher proportions of fungal and plant feeders like Aphelenchus and Geocenamus and a higher Channel Index but lower Enrichment Index. Due to prolonged drought, the Spanish soils were infested by plant feeders such as Scutylenchus and Pratylenchus . The forage plant had only a minor impact, with legumes enhancing certain bacterial-feeding genera and grasses fostering some plant-parasitic nematodes. Overall, regional factors shape the nematode community more than forage plant cover. The dry conditions in the eastern Mediterranean, as well as the drought in Spain, increased herbivores and decreased top-down control, which is relevant in the light of future climate change.

Dehesas are mosaics of open grassland and standalone trees that are diversity reservoirs. However, they have recently faced abandonment and intensification, being replaced by plantations of fast-growing trees or subject to encroachment. Following a change in dehesa communities and structure, a change in soil microbial diversity and functionality in dehesas is expected, but dehesas’ microbial diversity is still a big unknown. In this work, we bring to light the soil prokaryotic taxonomic diversity in dehesa ecosystems and present a first approach to assessing their metabolic diversity through metabarcoding data. For this, we compared three dehesas dominated by different tree species: (i) one dehesa dominated by Quercus ilex; (ii) one dominated by Pinus pinea; and (iii) one dominated by a mixture of Q. ilex and Q. suber. At each dehesa, samples were taken under the canopy and in the open grassland, as well as through two seasons of peak vegetation productivity (autumn and spring). Our results show the following findings: (1) seasonality plays an important role in prokaryotic richness, showing higher values in autumn, and higher evenness in spring; (2) the effect of seasonality on the soil’s prokaryotic diversity is often modulated by the effect of tree species and canopy; (3) taxonomic diversity is driven mainly by the site effects, i.e., the opposite of the metabolic diversity that seemed to be driven by complex interactions among seasons, tree species, and canopies.

The element loss from forested ecosystems is largely reflected by the so‐called dissolved phase (20–60 nm, and >60 nm) were identified using field flow fractionation coupled with inductively coupled plasma mass spectrometry and an organic carbon detector for the size‐resolved detection of Al, Si, P, Ca, Mg, Mn, Fe, Zn, and organic C. Colloids accounted for up to 90%–100% of total element concentrations, emphasizing the importance of colloids for element cycling. Element concentrations showed seasonal patterns in stream waters. However, these patterns differed between the catchments with distinct soil groups: the catchments with dystrophic terrestrial soils exhibited peak flows in colloidal element concentrations during spring, whereas those with eutrophic terrestrial soils did during summer. By contrast, catchments with Gleysols showed season‐unrelated single peaks of high element export. Peak concentrations were also influenced by the climatic regime, which in turn impacted colloid abundance.

Mediterranean wood pastures are the result of traditional silvo-pastoral uses that shaped these ecosystems into a mosaic of trees and open grassland. This ecosystem structure is generally associated with increased soil fertility under tree canopies. However, the response of herbaceous plant functional types (PFTs)—grasses, legumes, and non-legume forbs—to these heterogeneous microenvironments (under the canopy vs. open grassland) remains largely unknown, particularly regarding carbon (C) and nitrogen (N) acquisition and use. Even less is known about how different tree species and environmental conditions influence these responses. In this study, we aim to assess how tree canopies influence carbon and nitrogen cycling by comparing the effects of traditional oak stands and pine plantations on herbaceous PFTs and soil dynamics. For that we use C and N content and natural isotopic abundances (δ¹³C and δ¹⁵N) as proxies for biogeochemical cycling. Our results show that ecosystem C and N patterns depend not only on herbaceous PFTs and the presence or absence of tree canopies but also on tree species identity and environmental conditions, including climate. In particular, pine-dominated plantations exhibited lower nitrogen availability compared to those dominated by oak, suggesting that oak stands may contribute more effectively to enhance soil fertility in Mediterranean wood pastures. Furthermore, the canopy effect was more pronounced under harsher environmental conditions, highlighting the role of trees in buffering environmental stress, particularly in arid regions. This suggests that changes in tree cover and tree species may drive complex changes in ecosystem C and N storage and cycling.

Fairy ring fungi are considered keystone species in grasslands due to their strong impact on soil physicochemical properties, but their effect on the associated bacterial community is poorly understood. Here, we analyze shifts in soil bacterial diversity and community composition across fairy rings using Illumina metabarcoding. A total of 254,135 MiSeq reads and between 405 and 1444 operational taxonomic units (OTUs) per soil sample were observed in a montane grassland in the Eastern Pyrenees. We found a strong reduction in all bacterial diversity indices inside the ring-affected zones compared to the outside grassland, especially in the stimulation (current ring) zone. The exception were Firmicutes, the dominant taxa in the grassland, which increased their relative abundance further in fairy ring-affected zones. The recovery of bacterial populations after the fungal front passage highlights the strong resilience of the bacterial communities to this biotic disturbance.

A better understanding of the role of plant composition and N cycle on agroecosystems is necessary, as these will be affected by future developments in agriculture intensification. To explore the effect of plant diversity on yield and carbon (C) and nitrogen (N) balances in forage mixtures, identifying potential co-benefits between functions. We analyzed results from a field experiment where plants of three forage species (a grass, a legume, and a non-legume forb) were cultivated in monocultures and mixtures. Three years after sward establishment, dry matter yield, together with δ¹⁵N, δ¹³C, and C and N content in plant and soil material were measured. In addition, we analyzed a second scenario to investigate the effect of fertigation with pig slurry (δ¹⁵N = +8.4‰) on the C and N balances of forage species. Results support the hypothesis that C and N allocation is affected by plant diversity. Plant composition affected N source (% N derived from air, % N derived from soil, and % N transferred in mixtures). In addition, sown diversity increased yield and modulated C and N balances. The δ¹⁵N of samples was affected by both plant composition and fertigation. These results are consistent with previous work showing strong plant composition effects on N-balances, and the potential role that legumes play in enhancing nitrogen sources (derived from the atmosphere) into forage mixture systems. This study contributes to the prediction of suitable sown plant community composition and N management for the optimum agriculture with increased productivity and at the same time reduced environmental impact.

Legume-based livestock forage systems are widespread and socially relevant agroecosystems that are essential for delivering high-quality forage. Therefore, it is critical to determine how climate change and management might affect the functioning of forage agroecosystems. The objective of this study was to explore the potential of sainfoin swards under different management regimes by combining results from an experiment on species diversity effects on yield developed under extreme climatic conditions and a survey of sainfoin (Onobrychis viciifolia Scop.) swards and nutritive value in mountain regions in Catalonia, Western Mediterranean. Our results show that 12–59% of the variability in forage nutritive value parameters was accounted for by management. Young cattle-grazed swards showed higher nutrition quality that decreased faster than sheep-grazed or only mown swards. Mixing sainfoin with orchardgrass increased fiber content (e.g., +22% neutral detergent fiber) and decreased protein (−26%) and lignin (−11%) compared to sainfoin monocultures. However, under the extreme climatic conditions of the last three years, the only diversity effect found on yield was the identity of the species present. We propose that a combination of different sainfoin management regimes might be beneficial regionally for both quality forage production and overall biodiversity conservation, whereas traditional systems may be vulnerable to changing climatic conditions.