The availability of metallic nutrients in dryland soils, many of which are essential for the metabolism of soil organisms and vascular plants, may be altered due to climate changedriven increases in aridity. Biocrusts, soil surface communities dominated by lichens, bryophytes and cyanobacteria, are ecosystem engineers known to exert critical functions in dryland ecosystems. However, their role in regulating metallic nutrient availability under climate change is uncertain. Here, we evaluated whether well-developed biocrusts modulate metallic nutrient availability in response to 7 years of experimental warming and rainfall reduction in a Mediterranean dryland located in southeastern Spain. We found increases in the availability of K, Mg, Zn and Na under warming and rainfall exclusion. However, the presence of a well-developed biocrust cover buffered these effects, most likely because its constituents can uptake significant quantities of available metallic nutrients. Our findings suggest that biocrusts, a biotic community prevalent in drylands, exert an important role in preserving and protecting metallic nutrients in dryland soils from leaching and erosion. Therefore, we highlight the need to protect them to mitigate undesired effects of soil degradation driven by climate change in this globally expanding biome.

Current intensive agricultural practices, although highly successful in terms of production, have been found to be environmentally unsustainable. One of the crucial approaches to increase agricultural sustainability is the recycling of organic wastes, since these materials often contain many beneficial nutrients for soil and agriculture. Recently, pyrolytic conversion of biodegradable waste into charred material has gained global attention as an amendment to recycle nutrients while improving soil health. Increasing interest in the beneficial applications of pyrolyzed biomass has expanded multidisciplinary areas for science and engineering. The fertilizers used in this study were prepared by pyrolyzing mixtures of two abundant residues in Mediterranean areas: grape seeds and sewage sludge, in different proportions (100% GS, 75% GS-25% SS, 50% GS-50% SS, 25% GS-75% SS, 100% SS). In addition, fresh sludge was mixed with pyrolyzed grape seeds and included as an additional treatment. In this study, the positives and negatives of the application of biochars on agronomic potential and environmental risk have been addressed, taking into account P, Zn, Cu, Fe, As and Cd. In order to choose the best mixture, it is necessary to find a compromise between maximizing the beneficial elements that are translocated to the plant crop, as well as reducing the elements that are leached. Results of a 6-week greenhouse study indicated that the unpyrolyzed sludge mixture contained the largest amount of available nutrients Fe, Cu and P. In agreement, this treatment mixed with a Chromic Luvisol soil (40 ton/ha) increased the uptake of these elements in corn (Zea mays L.—LG ambitious). The yield also increased by 60% in this treatment. However, this mixture also contained more contaminants (As, Cd) which were extracted with Ammonium Bicarbonate-DTPA. According to our results, the treatments where grape seeds and sewage sludge were mixed at 50% and then pyrolyzed exhibited the optimal compromise between efficiency (nutrients uptake) and tolerable levels of potentially toxic elements in leachates.

Dryland soils provide different societal and environmental services, such as food supply and biodiversity support. In Europe, most of the dryland areas are devoted to agriculture. In the next decades, both European and worldwide drylands are expected to suffer with increased intensity due to the expected climate change‐derived rise in aridity. Many studies have focussed on aridity‐induced changes in major nutrients in drylands, but little is known of the impact of environmental and biogeochemical factors on micronutrients with critical roles in life, and as inorganic contaminants with ecotoxicological implications. We analysed and explored drivers of total and available concentrations of micronutrients (Co, Cu, Fe, Mo, Mn, Ni and Zn) and contaminants (As, Cd and Pb) in 148 soil samples collected from European drylands covering a wide range of aridity and of other geochemical parameters. The availability of micronutrients increased with their total content, decreased with pH and was enhanced by organic C content. Aridity decreased the availability of Fe, a key element in human diet. Our findings also highlight the scarcity of this micronutrient in European drylands, as well as of some other important micronutrients like Zn and Mo in agricultural soils. Total content was the main driver of the availability of Cd and Pb, and organic matter exerted synergistic effects on contaminant release. Our data show the need for precise management practices to be incentivised by agricultural and environmental policies in order to ensure micronutrient supply and avoid contamination, thus maintaining adequate levels of agricultural productivity and simultaneously preserving dryland ecosystems. Highlights Drylands are important for food production in Europe and sensitive to climate change. The occurrence of metals in European Union dry soils and the drivers influencing them were studied. Some micronutrients (Fe, Mo and Zn) were scarce while contaminants were abundant. SOC, pH and clays were the main drivers of element availability; aridity reduced Fe. Agricultural practises are needed to ensure nutrient supply and prevent contamination.

The contamination of soils and waters with organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), affect a large number of sites worldwide that need remediation. In this context soils amendments can be used to immobilise PAHs while maintaining soil functioning, with biochar being a promising amendment. In this experiment, phenantrene (Phe) was used as a frequent PAH contaminating soils and we studied the effect of three biochars at 1% applications to three different substrates, two agricultural topsoils and pure sand. We evaluated the changes in soil properties, sorption-desorption of Phe, and mineralisation of Phe in all treatments. Phe in pure sand was effectively sorbed to olive pruning (OBC) and rice husk (RBC) biochars, but pine biochar (PBC) was not as effective. In the soils, OBC and RBC only increased sorption of Phe in the silty soil. Desorption was affected by biochar application, RBC and OBC decreased water soluble Phe independently of the soil, which may be useful in preventing leaching of Phe into natural waters. Contrastingly, OBC and RBC slightly decreased the mineralisation of Phe in the soils, thus indicating lower bioavailability of the contaminant. Overall, biochar effects in the two tested soils were low, most likely due to the rather high soil organic C (SOC) contents of 2.2 and 2.8% with Koc values in the same range as those of the biochars. However, OBC and RBC additions can substantially increase adsorption of Phe in soils poor in SOC.

The contamination of soils and waters with organic pollutants, such as polycyclic aromatic hydrocarbons (PAHs), affect a large number of sites worldwide that need remediation. In this context soils amendments can be used to immobilise PAHs while maintaining soil functioning, with biochar being a promising amendment. In this experiment phenantrene (Phe) was used as a frequent PAH contaminating soils and we studied the effect of three biochars at 1% applications to three different substrates, two agricultural topsoils and pure sand. We evaluated the changes in soil properties, sorption-desorption of Phe, and mineralisation of Phe in all treatments. Phe in pure sand was effectively sorbed to olive pruning (OBC) and rice husk (RBC) biochars, but pine biochar (PBC) was not as effective. In the soils, OBC and RBC only increased sorption of Phe in the silty soil. Desorption was affected by biochar application, RBC and OBC decreased water soluble Phe independently of the soil, which may be useful in preventing leaching of Phe into natural waters. Contrastingly, OBC and RBC slightly decreased the mineralisation of Phe in the soils, thus indicating lower bioavailability of the contaminant. Overall, biochar effects in the two tested soils were low, most likely due to the rather high soil organic C (SOC) contents of 2.2 and 2.8% with Koc values in the same range as those of the biochars. However, OBC and RBC additions can substantially increase adsorption of Phe in soils poor in SOC.