EJP-Soil First call: SoilsalAdapt Preadapting soil biology for increased tolerance to elevated salinities due to climate change

Salinization is a major cause of agricultural land degradation in arid areas, as well as in regions threatened by climate-driven sea level rise, where reduced crop yields are a common consequence. If soil as an ecosystem can "learn" to tolerate saline irrigation through the gradual adaptation of soil organisms, this means that farmers can prepare their soil to remain fertile, even when they occasionally need to use saline water for irrigation. Farmers would then be able to manage the increasingly widespread problem of soil salinization. However, little is known about how soil organisms react to increased salinity. Research shows that soil organisms from saline soils are more tolerant to salt than those from non-saline soils. Based on this, the project aims to test how the adaptive abilities of soil organisms function when exposed to different saltwater concentrations in the soil, as well as to measure and predict the effects on various scales. Three soil types (sand, silt, and clay) have been irrigated with diluted saltwater and regular tap water (0, 3, and 6 dS/m) for 3 months, before all are subjected to a shock irrigation with 9 dS/m later in the growing season. Seawater typically has 52 dS/m. These experiments were carried out in pot experiment, in four replicates, by our partners at Lincoln University in England. Various crops have been cultivated. In 2024, we successfully tested the effects of the different saltwater regimes on soil microbial development of tolerance to NaCl in soil samples from this experiment. There is some variation in response depending on soil type, but generally, we see little difference in the development of NaCl tolerance between the saltwater treatments up until the shock irrigation of 9 dS/m, regardless of soil type. In soil collected after the shock irrigation, however, we see that in cases where irrigation with 3 and 6 dS/m was used, tolerance to 9 dS/m is higher than where 0 dS/m was used beforehand. This may be important when considering whether fresh- or brackish water can be used in fields during freshwater shortages. Going forward, we will obtain a new set of samples to investigate whether this difference has been leveled out after two years of regular maintenance irrigation with normal rainwater. We will also conduct pot experiments to examine the interaction between plant roots and soil microbes, as well as the ability of the microbes to drive denitrification under the same saline irrigation scenarios.

Saltwater contamination is a major contributor to soil degradation in arid areas. In temperate coastal areas dry periods force freshwater level closer to the saline water. The ground water will become increasingly brackish. Growers are faced with either: 1) irrigating with much more expensive treated tap water; or 2) not irrigating which significantly increases the chance of crop failure. Growers are increasingly forced to use the part-saline ground water for irrigation in summer, but the short- and long-term effects are poorly understood. Soil organisms are key to soil fertility, but very little is known about how they respond to increased salinity. Previous work show that saline irrigation affects soil communities, and that soil organisms from saline soils are more tolerant to salt than those from non-saline soils. We will tests whether Darwinian selection processes can ‘push’ soil biological communities to become increasingly tolerant of saline conditions by adaptation. We will test a range of saline concentrations, measure and predict the various impacts of this approach at three scales. At the micro-scale the NO partner will focus on the effects on the rhizosphere. The UK partner will look at medium-scale pot to whole field-scale changes in total soil biodiversity and function by R&DNA sequencing and compare the fertility of soils of various history of saline concentrations in the UK and Portugal. We will also measure any cost of saline irrigation and adaption and how long any effects last. Lastly, the landscape-scale impacts of increased salinity will be modelled by the PT partner, to predict the European and global extent of the issue, and identify the management practices to counter salinization and conserve the soil functions. If soils can ‘learn’ to tolerate saline irrigation through more gradual biological adaptation, growers may condition soils to remain fertile despite the use of saline irrigation and manage this increasingly pressing issue.