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Negative correlation between soil salinity and soil organic carbon variability

Foto: Amir Hosseini

We often overlook the soil beneath our feet. Still, it is a vibrant and complex ecosystem that plays a critical role in supporting life on Earth. 

Imagine soil as a community of microorganisms, nutrients, and organic matter. They all work together to sustain plant life and regulate our planet’s climate.

Soil salinity

Soil salinity refers to the concentration of soluble salts in the soil. They can have both natural and human-induced origins.

In regions with arid or semi-arid climates, salts accumulate over time due to factors like mineral weathering and irrigation practices. Soil salinity is typically measured using electrical conductivity of the soil solution. It is represented by the unit of measure called “decisiemens per meter” (dS m-1). Higher values indicate higher salinity levels. Excessive soil salinity can negatively impact soil fertility, inhibit plant growth, and disrupt the soil ecosystem balance.

Salts near soil surface, Iran’s Bushehr Province. Image credit: Hassani et al.

Soil Organic Carbon

Soil organic carbon content (SOC) can be considered the lifeblood of the soil. It consists of a variety of organic materials derived from plant and animal residues, microbial biomass, and decomposed organic substances.

SOC is vital for maintaining soil health and promoting nutrient cycling. Think of it as the fuel that powers the soil ecosystem.  It provides essential nutrients and energy for microbial activity and plant growth.

It is known that soil salinity impacts SOC, yet the specific direction and magnitude of SOC variability of in relation to soil salinity remain poorly understood.

The relationship between soil salinity and SOC

In a recent study, an international team of scientists from Norway, UK, and Germany, has investigated the relationship between soil salinity and SOC.  (www.pnas.org/doi/10.1073/pnas.2317332121)

The analysis, led by NILU scientist Dr Amirhossein Hassani, revealed a general negative correlation between soil salinity and SOC. An increase in soil salinity was associated with a decline in SOC. Non-croplands experienced a sharper decline compared to croplands.

Besides soil salinity, other environmental factors were found to significantly explain the SOC variability. Among them were soil total nitrogen, land cover, sample depth, soil pH, and precipitation seasonality index.

Predictive models indicated that a one standard deviation increase in soil salinity (~2.75 dS m-1) correlates with a SOC decrease of approximately 4.4% (relative to ~17.9 g kg-1) and 9.26% (~36 g kg-1) in croplands and non-croplands, respectively.

The magnitude of the change varied depending on the land cover type.

Vegetation/cropland mosaics and evergreen needle-leaved trees showed the greatest declines in croplands and non-croplands, respectively.

Impact of one standard deviation increase in soil salinity (~2.75 dS m-1) on topsoil (0 – 7 cm) Soil Organic Carbon (SOC) content at the location of soil profiles/samples (Hassani et al, 2024).

Why do these findings matter?

It is recommended to reduce agricultural practices that cause soil salinization to preserve stocks of SOC. Soil salinization negatively affects SOC content, which is crucial for soil health, fertility, and climate regulation.

Practices such as improved irrigation management, drainage systems, crop rotation, reduced tillage, and optimized fertilization can help. These strategies prevent salt accumulation, maintain soil structure, and enhance the supply of organic matter.