Profile and nano-scale distribution of soil organic carbon for upland and paddy soils from an alluvial plain in South China

Understanding the heterogeneous distribution of soil organic carbon (C) (SOC) in different soil environments is essential for predicting SOC preservation. However, SOC distribution differs in profile scales and molecular levels in various farmland soils, making it challenging to elucidate the environmental behaviors of SOC. In the present study, upland and paddy soil samples were both collected at different depths (0–100 cm) from an alluvial plain in the Pearl River Delta of South China. Wet chemistry experiments coupled with 13C-nuclear magnetic resonance (NMR) and spherical aberration correction scanning transmission electron microscopy (Cs-STEM) were used to determine the vertical distribution, chemical composition, and nano-scale sequestration mechanisms of SOC in upland and paddy soils. SOC content for upland soil, which was highest at the depth of 0–20 cm (6.98 ± 0.34 g·kg−1), decreased with increasing soil depth, while that of paddy soil had the highest content at a depth of 80–100 cm (86.66 ± 5.26 g·kg−1). Statistical analysis revealed that mineral-associated organic C (MAOC) in the upland soil and particulate organic C (POC) in the paddy soil were the major C fractions that were related to SOC content. Further analysis of NMR spectra showed that the molecular composition of upland SOC differs vertically, while the molecular composition and proportion of organic C in paddy soil varied with soil depth. Cs-STEM results revealed that the relative percentage of carboxyl C decreased from 0 to 20 cm (42.1%) to 80–100 cm (28.8%) with an increase in aromatic C from 7.7 to 27.2% for the upland soil, while the four C species showed similar abundance for paddy soil at different depths, consistent with the NMR results. Overall, the upland SOC was not as easily mineralized as that of paddy soil due to a lower percentage of easily oxidized carbon, higher percentage of MAOC, and higher decomposition degree. The results of this study would enrich our quantitative understanding of the profile distribution and chemical diversity of SOC from macro to micro scales in the alluvial plain from South China, and provide insights into C preservation in natural soils.