Influence of soil texture and mineralogy on organic matter content and composition in physically separated fractions soils of Thailand

There is a limited knowledge on the role of soil texture and clay mineralogy in governing the content and composition of organic matter (OM) associated with physically-separated fractions in major Thai soils. To extend the understanding in this context, we sampled a range of Thai soils representing four major mineralogy groups, i.e. siliceous, mixed mineralogy, kaolinitc, and smectitic soils. The clay fraction and four density fractions [two particulate OM, free-light (fPOM) and occluded (oPOM), <1.85gcm−3; and two heavy fractions, 1.85–2.60gcm−3 (HFOM1.85–2.60) and >2.60gcm−3 (HFOM>2.60)] of soils from the top- and sub-soil horizons with varying clay content (58–967gkg−1) and mineralogy were analyzed for organic carbon (OC) and total nitrogen (N) contents. Additionally, chemical composition of the OM in the clay fraction of soils from 0–20cm, 20–40cm and 40–70cm and the four density fractions from soils at 20–40cm depth was characterized using diffuse reflectance infrared Fourier transformed (DRIFT) spectroscopy. The OM associated with oPOM (OC only) and HFOM1.85–2.60 (OC and total N) fractions was significantly increased with increasing clay content among soil groups and the degree of association depended on the dominating clay mineral (albeit significantly at P<0.05 for some soil groups only). The relative proportion (%rA) of aliphatic-C in the clay and HFOM1.85–2.60 fractions significantly increased with increasing sand content, and significantly decreased with increasing clay content. Whereas, opposite relationships were observed for the aromatic-C and amide N (amide-II) with sand and clay contents, respectively. The organic-mineral fractions (clay or HFOM1.85–2.60) of the coarse-textured siliceous soils were enriched with aliphatic-C and depleted in amide N, whereas the organo-mineral fractions of the fine-textured soils were enriched with aromatic-C and amide N. Aliphatic-C represented the dominating organic structure in the siliceous soils, whereas aromatic-C and amide N represented typical forms of OM in the fine-textured soil groups. The present study provided insights into the role of soil texture and clay mineralogy in governing the content and composition of OM pools in Thai soils with relevance for sustaining soil fertility and mitigating C emissions.