The study area is the Saladas of Sástago-Bujaraloz, a complex of nearly 150 closed depressions hosting temporary saline wetlands — the saladas — scattered across three municipalities in the Central Ebro Basin (CEB). These playa-lakes belong to the European Natura 2000 network of protected areas and to the Ramsar list of wetlands of international importance. Some of their bottoms hold water ten times saltier than seawater, fed by groundwater reaching an extraordinary 175 dS m−1. The wetlands support a unique vegetation, unparalleled in Europe, sustained by alternating inundation and drying episodes under one of the driest climates in the continent. Yet this ecological treasure sits at the heart of an agricultural landscape undergoing radical transformation, and the researchers wanted to know precisely where the salt lies — in the wetland bottoms, in the fringes of natural vegetation, and in the fields where machinery now plows ever closer to the protected enclaves.

The technical challenge was considerable. The team combined two complementary survey strategies. First, they sampled soils along transects crossing the wetland-crop interface in three representative saladas — Guallar, Rollico, and Gramenosa — capturing the gradient from hypersaline bare bottoms through fringes of salt-tolerant halophytes into dry-farmed cereal plots. Second, in the salada Agustín, they surveyed three agricultural plots officially declared as arable land in the Land Parcel Identification System (LPIS), the EU-wide registry that serves as a key control mechanism under the CAP. Soil samples were extracted with a hand auger at 20 or 25 cm intervals, reaching depths from 50 to 260 cm, yielding 319 samples in total. Electrical conductivity was determined in 1:5 and 1:10 soil:water extracts in the laboratory, and the EC of the saturation extract (ECe) — the worldwide standard for expressing soil salinity — was measured or estimated for every sample. The relationship between the two fixed-ratio extracts proved remarkably robust, with determination coefficients of 0.98 and 0.97, confirming that the 1:5 extract can reliably substitute for the more laborious saturation extract even in gypsum-rich soils.

The spatial pattern that emerged is consistent and geomorphologically coherent. The highest salinity occurs in the bare soil bottoms of the playa-lakes, with mean EC1:5 values of about 15 dS m−1; intermediate values characterize the fringes of natural vegetation, ranging from 6.5 to 10.2 dS m−1; and the lowest values correspond to crop soils, from 3 to 4.6 dS m−1. The transects along Guallar and Rollico collected the widest salinity range, up to 27.7 dS m−1, while the vegetated saladas Gramenosa and Agustín showed narrower ranges of about 9 dS m−1. Even the cereal area contains every salinity phase except non-saline: 5 percent of crop samples were slightly saline, 14 percent moderately, 23 percent strongly, and 58 percent very strongly saline. In other words, there are no soils free of salinity in the agricultural zone — a fact that the CAP’s current subsidy framework, which treats these parcels as ordinary arable land, simply does not register.

The vertical distribution of salt adds another layer of complexity. In the natural areas, the salinity range is maximum at the surface horizons and decreases with depth, a pattern driven by the high evapotranspiration of the semiarid climate: capillary rise carries saline groundwater upward through the bare soil, and evaporation concentrates the salts in the upper layers, generating hypersaline crusts. In the agricultural plots of Agustín, by contrast, the profiles are more heterogeneous and the salinity range similar at surface and subsurface horizons — very likely a homogenizing effect of continuous tillage, which CAP compliance requires even under long fallow practices of 16 to 18 months. Deep plowing that breaks and removes limestone and gypsum strata gives crop roots access to saline materials that were previously buried. When the researchers compared integrated salinity over the top 50 cm (ECe50) and the top 100 cm (ECe100), they found that ECe100 was generally higher, and that the ECe50 of the playa-lake bottoms could be twelve times that of the crops in Guallar and more than twenty times in Rollico.

Groundwater adds its own signature. The water table was reached at depths from 31 to 195 cm along the transects, with one site flooded in the Rollico bottom. Once stabilized, the shallow groundwater showed mean electrical conductivities of 82.6 dS m−1 in Gramenosa, 133 dS m−1 in Guallar, and 144.3 dS m−1 in Rollico. Its chemistry is dominated by chloride (54 g L−1) followed by sulfate (27 g L−1), with sodium (21 g L−1) and magnesium (8 g L−1) as the main cations. The researchers also identified unexpected local hotspots of high salinity outside the wetland bottoms, in the leeward low-lying areas of aeolian deposits — spots where the water table was never reached during sampling or during visits over the following three months. Previous work has documented that geological materials and groundwater characteristics can change at distances of just a few meters in this area, and that tectonic joints and preferential flows contribute to salinity differences over similarly tiny distances.

The second pillar of the study is the electromagnetic sensor (EMS), a portable instrument whose advent since the late twentieth century has made field assessment and mapping of soil salinity affordable in cost and time. The team used a hand-held Geonics EM38 along the wetland-crop transects, taking readings in horizontal (EMH) and vertical (EMV) coil positions nine days after a 32.2 mm rain, in trafficable soil conditions. In Agustín, a DualEm 1S sensor mounted on a plastic sled towed by a tractor acquired georeferenced readings automatically, connected to a GPS and a field computer where the data were recorded in real time. A preliminary map of readings was created in the field using kriging in ArcGIS to decide the soil sampling points — a strategy that dramatically reduces the number of samples needed. The calibration between sensor readings and laboratory EC varied between saladas, between horizontal and vertical readings, and between integrated soil depths. Rollico showed no difference between EMH and EMV, with maximum determination coefficients of 94 and 96 percent at 100 cm depth; Guallar’s fit was acceptable down to 80 cm before dropping sharply; and Gramenosa yielded very low values, likely because the dense, nitrophilous vegetation cover made it impossible to precisely distinguish soil cover changes when selecting sites.

The best overall relationship was found with a soil depth of 0–100 cm and with horizontal EMS readings, and the resulting calibrated salinity maps provided estimates at unsampled sites across the Agustín plots. Almost the complete fringe of Agustín depicted ECe above 4 dS m−1, with salinity decreasing toward the crops at higher topographic positions where the water table deepens. A concentric distribution was observed, with ECe above 10 dS m−1 at the center and the rest of the area between 2 and 10 dS m−1. The highest values occurred at the leeward depression of an aeolian deposit currently cultivated but with limited production. The identified salinity patterns, the authors conclude, are consistent and applicable across the whole area of about 150 saladas, and can be extrapolated with the help of digital mapping techniques and soil salinity proxies.

Why does this matter for policy? Spain contains the largest surface of saline land in the European Union, and the CAP currently ignores salinity when distributing monetary aid to farmers — a gap the researchers describe as generating economic suffocation in rural areas of the region. Their proposal is concrete: a new agri-environment-climate measure could classify plots with ECe above 10 dS m−1 in more than 50 percent of their surface as unsuitable for cultivation. The threshold draws on established yield-response science: previous studies established that an EC50 of 11.3 dS m−1 reduces durum wheat and barley yields by half, while barley alone tolerates much higher levels, with an EC50 of 18 dS m−1. Plots of limited agricultural potential could form part of a new category of eligible areas within the LPIS as non-productive areas prone to natural variable salinity, ensuring farmers do not lose their subsidies while protecting the sensitive wetland-crop interface. The European Court of Auditors has recommended that targeted on-the-spot data may be required to accurately identify this type of land as eligible — a requirement well suited to soil salinity, given its intrinsic temporal variability.

The stakes are rising. Over recent decades, Monegros has undergone radical transformations: irrigation schemes in operation and in progress, intensive swine farms legally spreading slurry that causes an excess of 441 tonnes of nitrogen in the basin, and upcoming solar and wind installations whose effects on near-surface wind speed, ground temperature, and evaporation remain poorly known. Farmers have demanded conversion to irrigation for decades, and several studies indicate this would raise the shallow phreatic level due to perched groundwater and low-permeability geological materials, decreasing arable land through flooding and salinization. The detailed geomorphological maps, based on aerial photographs from 1956–1957, captured the landscape before homogenization by land consolidation — a baseline for interpreting salinity distribution and predicting where new irrigation will mobilize salt. The saladas of Monegros, the authors write, are only an example of the co-occurrence of inland saline environments sharing conservation and production criteria, a challenge repeated in semiarid regions worldwide. Their approach — manual sampling along topographic transects combined with calibrated EMS readings across flat areas — offers a replicable model for delimiting non-productive saline land within Natura 2000 sites, turning a hidden edaphic variable into a visible, quantifiable criterion for the future of European agriculture.

Subject of Research: Soil Salinity, Agriculture, and Nature Conservation in Monegros, NE Spain

Article Title: Soil Salinity, Agriculture, and Nature Conservation in Monegros, NE Spain

Article References: Tierra, M., Castañeda, C., Gracia, F. J., & Medina, E. T. (2026). Soil Salinity, Agriculture, and Nature Conservation in Monegros, NE Spain. Spanish Journal of Soil Science, 15, Article 15359. https://doi.org/10.3389/sjss.2025.15359

Image Credits: AI Generated

DOI: 10.3389/sjss.2025.15359

Keywords: soil salinity, Common Agricultural Policy, saline wetlands, Natura 2000, electromagnetic sensor, Monegros, Spain, agriculture, Soil, Salinity, Nature, Conservation