Different molecular characterization of soil particulate fractions under N deposition in a subtropical forest

Abstract

Key Findings: Combining physical fractionation and pyrolysis-gas chromatography/mass spectrometry (py-GC/MS) technique can help better understand the dynamics of soil organicmatter (SOM). Background and Objectives: SOMplays a critical role in the global carbon (C) cycle. However, its complexity remains a challenge in characterizing chemical molecular composition within SOMand under nitrogen (N) deposition. Materials and Methods: Three particulate organic matter (POM) fractions within SOM and under N treatments were studied from perspectives of distributions, C contents and chemical signatures in a subtropical forest. N addition experiment was conducted with two inorganic N forms (NH4Cl and NaNO3) applied at three rates of 0, 40, 120 kg N ha-1 yr-1. Three particle-size fractions ( > 250 μm, 53-250 μm and < 53 μm) were separated by a wet-sieving method. Py-GC/MS technique was used to differentiate between chemical composition. Results: A progressive proportion transfer of mineral-associated organicmatter (MAOM) to fine POMunderNtreatmentwas found. Only C content in fine POMwas sensitive to N addition. Principal component analyses (PCA) showed that the coarse POM had the largest plant-derivedmarkers (lignins, phenols, long-chain n-alkanes, and n-alkenes). Short-chain n-alkanes and n-alkenes, benzofurans, aromatics and polycyclic aromatic hydrocarbonsmainly fromblack carbon prevailed in the fine POM.Ncompounds and polysaccharides frommicrobial products dominated in theMAOM. Factor analysis revealed that the degradation extent of three fractions was largely distinct. The difference in chemical structure among three particulate fractions within SOM was larger than treatments between control and N addition. In terms of N treatment impact, the MAOMfraction had fewer benzofurans compounds and was enriched in polysaccharides, indicating comparatively weaker mineralization and stronger stabilization of these substances. Conclusions: Our findings highlight the importance of chemical structure in SOMpools and help to understand the influence of N deposition on SOM transformation.