Exploring the Signature of Lipid Biomolecules Influenced by Soil Aggregates Across Terrestrial Ecosystems

Soils across terrestrial ecosystems comprise aggregates with varying biological and physicochemical properties that impact plant lipid distribution. This study examines the role of aggregates characteristics in the distribution of n-alkyl lipids (n-alkanes and n-alkanoic acids) in forest, grassland, and mixed (shrubs and grasses) ecosystems across five different particle size fractions (PSF) in the lower Ganga region (India). Fresh plant-derived n-alkyl lipid signature, similar to the bulk soil, mostly associates sand PSF (>63 μm), which constitutes larger aggregates formed by extensive biological (fungal hyphae and roots) and physical (clay coating) components. Fragmented soil aggregates comprise mostly the silt PSF, which allowed restricted storage of plant n-alkyl lipids and increased microbial contribution. Stable clay-rich microaggregates in finer PSF (<20 μm) vary across ecosystems, influencing the storage and modification of plant-derived n-alkyl lipid signatures. Principle component analysis showed that the n-alkyl lipid signature across PSF in forest soil is distinct and more variable than other two ecosystems. In grassland and mixed PSF, extensive belowground root processes fragments and reduces soil aggregates, which limits the modification of plant n-alkyl lipids when compared to physically stable aggregates found in forest ecosystems. The susceptibility of n-alkanoic acids to microbial decomposition resulted in consistent replacement and a stable profile across soil fractions, while n-alkanes exhibit greater variability due to differences in aggregate protection. Such difference in biochemical response between n-alkyl lipids highlights the crucial role of aggregate characteristics in mineral protection and/or microbial decomposition of OM, which contributes to microscale carbon dynamics across ecosystems.