# Organic Selenium Quality and Quantity in Soils Are Related to Organic Matter Composition and Driven by Land Use

## TL;DR

This study shows how land use and soil organic matter affect selenium forms in soils, influencing its availability to plants.

## Contribution

The study reveals novel connections between selenium speciation, organic matter composition, and land use in soils.

## Key findings

- Cropland soils contain more Se oxyanions and small hydrophilic organic Se.
- Forest and grassland soils have more large, aromatic organic Se compounds.
- Higher soil pH reduces selenium extractability by NaOH.

## Abstract

The
micronutrient
selenium (Se) exhibits a narrow range between
essentiality and toxicity. In soils, Se speciation influences its
mobility and plant availability, with implications for addressing
unsafe Se levels in plant-based nutrition. We investigated how Se
speciation varies with the molecular composition of organic matter
(OM) in 92 Swiss topsoils spanning different land uses (i.e., croplands,
grasslands, and forests). OM composition was characterized using pyrolysis-gas
chromatography/mass spectrometry (Py-GC/MS), while Se speciation was
determined in NaOH extracts using size exclusion chromatography coupled
to UV and elemental mass spectrometry (SEC-UV-ICP-MS/MS). We found
that Se speciation strongly relates to OM decomposition status and
pH, and drastically differs between soil land uses. Cropland soils
exhibited higher proportions of Se oxyanions and small hydrophilic
organic Se, whereas forest and grassland soils contained more larger,
aromatic organic Se compounds. Overall, these larger Se forms correlated
with fresh and/or poorly decomposed, plant-derived OM, while oxyanions
and small hydrophilic Se were linked to decomposed OM. Additionally,
Se extractability by NaOH decreased with increasing soil pH, which
may be due to stronger SOM stabilization, microbial processes, or
higher Ca contents at higher pH. These results have important implications
for Se plant availability considering land use changes and SOM degradation.