Distribution and stability of n-alkyl biomolecules across particle-size fractions of different terrestrial ecosystems

Soil lipids constitute an important fraction of soil organic matter (OM) and are thought to be resistant against degradation from post-depositional processes. However, difference in vegetation composition, grain size distribution, climatic conditions and the mineralogical association can significantly impact the distribution and stabilization of lipid pools. To understand the impact of particle size fractions (PSF), in this study, soil OM and plants thriving on the same soil were collected from grassland, forest and mixed (comprising shrubs and grasses) regions of the lower Gangetic plain, India. Comparison of n-alkyl lipid distribution between the whole soil and PSF (250-125, 125-63, 63-32, 32-20 and <20) suggests that the distribution of n-alkanes and n-alkanoic acids in bulk soil mostly represents contributions from fresh and partially decomposed plant organic carbon, similar to the signatures observed in the coarser PSF. The SEM images in coarser PSF shows the presence of macro-aggregates and thin layers of clay coating over different minerals, which facilitated the accumulation and storage of freshly derived OM. Higher micro-aggregates constituting the finer PSF facilitate increased storage of n-alkyl lipids, but the OM is significantly modified, as evidenced by higher microbial lipid contribution in the finer micro-environment rich in bioavailable substrates. The grassland and mixed systems had higher microbial community proliferation and OM degradation in the finer fraction than the forest soils, which could be attributed to the water and air infiltration through the higher density of fine-roots. Difference in interaction of n-alkanoic acids and n-alkanes with clay mineral surfaces also resulted compartmentalization and variation in preservation of n-alkyl lipids across the grain size of soil. Thus, the micro-environmental conditions across the PSF are a critical determining factor that controls the residence and preservation of n-alkyl compounds, influencing the biochemical cycling of carbon at the micro-morphic level.