Objectives We explored the changes in distribution among aggregate fractions and in chemical composition of soil organic matter of topsoil across different soil landscapes within a small watershed in a rural hilly region. This is to provide a new insight for the dynamics of soil organic carbon at different degradation levels in this region.
Methods Undisturbed topsoil (0–15 cm) cores were sampled in protected forest land (FL), orchard (OR), dry cropland (DL) and paddy field (PF) within a small rural watershed in Lishui District, Nanjing Municipality, Jiangsu Province, China. Soil organic carbon (SOC) content was measured for bulk sample, macroaggregates (2000–250 μm), microaggregates (250–53 μm), silt-clay fraction (<53 μm). Subsequently, bulk topsoil was subject to extraction sequentially by total solvent (TSE), base hydrolysis (BHY) and cupric oxidation (CUO) to obtain free lipids, bound lipids and lignin phenols, which were analyzed for biomarker molecules abundances with GC/MS detection and Shannon diversity index.
Results Compared to FL, bulk topsoil SOC content in OR, DL and PF decreased by 70%, 57%, and 51%, respectively; macroaggregate SOC decreased by 85%, 81% and 71%, respectively; microaggregate SOC decreased by 74%, 79%, and 67%, respectively; silt-clay fraction SOC decreased by 48%,18%, and 3% , respectively. The range of molecular abundance of free lipids, bound lipids and lignin phenols were 2.24–6.74, 4.81–14.87 and 3.51–6.16 mg/g SOC, respectively. Across the studied soil landscapes, the total abundance of biomarker molecules in these extractions was generally in an order: FL>PF>OR>DL. Although total abundance of biomarker molecules of lignin phenols was similar between FL and PF. Furthermore, relative to FL, the molecular abundance of fatty acids, alkanols, steroids and terpenoids reduced in OR, DL and PF, while those of alkanes, contribution of microbial derived organic C and alkanols significantly increased. Of the bound lipids, there was a higher abundance of hydroxy acids in FL and OR while abundance of alkanoic acids peaked in DL and PF. In addition, the Shannon index of molecular diversity of free and bound lipids were higher at FL and PF than in OR and DL while that of lignin phenols was highest in PF, followed by DL, lowest in OR and FL.
Conclusions Topsoil SOC varied in a wide range not only in amount and aggregate fraction distribution but also in molecular composition assessed by biomarkers detection. Compared to protected forestland, we observed a significant reduction of total SOC, aggregates-associated SOC, molecular diversity of extracted biomarkers, while microbial derived organic C was relatively increased. However, SOC and molecular diversity were higher in paddy fields than in dry croplands. Suitable soil management especially the input of organic materials is an important way to improve the soil health of cropland.
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