Abstract
Key Findings: Combining physical fractionation and pyrolysis–gas chromatography/mass spectrometry (py-GC/MS) technique can help better understand the dynamics of soil organic matter (SOM). Background and Objectives: SOM plays a critical role in the global carbon (C) cycle. However, its complexity remains a challenge in characterizing chemical molecular composition within SOM and 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 organic matter (MAOM) to fine POM under N treatment was found. Only C content in fine POM was sensitive to N addition. Principal component analyses (PCA) showed that the coarse POM had the largest plant-derived markers (lignins, phenols, long-chain n-alkanes, and n-alkenes). Short-chain n-alkanes and n-alkenes, benzofurans, aromatics and polycyclic aromatic hydrocarbons mainly from black carbon prevailed in the fine POM. N compounds and polysaccharides from microbial products dominated in the MAOM. 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 MAOM fraction 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 SOM pools and help to understand the influence of N deposition on SOM transformation.
Highlights
As the largest carbon (C) pool in terrestrial ecosystems [1], soil organic matter (SOM) stores over three times the amount of C in either the atmosphere or biosphere [2], and functions as a net sink or source for atmosphere CO2 in various ecosystems [3]
Nitrogen addition resulted in a significant increase in the percentage of fine particulate organic matter (POM) and a decline in mineral-associated organic matter (MAOM) proportion compared with control plots (Figure 1)
This study aims to determine the quantity and chemical signatures of SOM particulate fractions, and how they are affected by N inputs
Summary
As the largest carbon (C) pool in terrestrial ecosystems [1], soil organic matter (SOM) stores over three times the amount of C in either the atmosphere or biosphere [2], and functions as a net sink or source for atmosphere CO2 in various ecosystems [3]. Since the accessibility of SOM to organisms is the first requisite for decomposition, physical fractionation method is based on the association of soil particle sizes and their spatial distribution [5]. The turnover rates normally decelerate as the particle size decreases [10,11], as a result of a combination of SOM stabilization mechanisms, including the change in chemical composition, increase in spatial inaccessibility and adsorption with mineral surfaces [12,13]. Lützow et al [4] concluded that smaller particles with a higher allocation of soil organic carbon (SOC) may not conform to longer turnover time It remains to be elucidated which manner (i.e., ‘truly’ stabilized, or potentially still ‘labile’ but just not accessible) could account for mineral-associated C with a longer mean turnover time [14]. Factor analysis revealed that the degradation extent of three fractions was largely distinct
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