Paddy management on different soil types does not promote lignin accumulation

Paddy soil management is generally thought to promote the accumulation of soil organic matter (SOM) and specifically lignin. Lignin is considered particularly susceptible to accumulation under these circumstances because of the recalcitrance of its aromatic structure to biodegradation under anaerobic conditions (i.e., during inundation of paddy fields). The present study investigates the effect of paddy soil management on SOM composition in comparison to nearby agricultural soils that are not used for rice production (non-paddy soils). Soil types typically used for rice cultivation were selected, including Alisol, Andosol and Vertisol sites in Indonesia (humid tropical climate of Java) and an Alisol site in China (humid subtropical climate, Jiangxi province). These soil types represent a range of soil properties to be expected in Asian paddy fields. All uppermost A horizons were analysed for their SOM composition by solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopy and for lignin-derived phenols by the CuO oxidation method. The SOM composition was similar for all of the above named parent soil types (non-paddy soils) and was also not affected by paddy soil management. A substantial proportion (up to 23%) of the total aryl-carbon in some paddy and non-paddy soils was found to originate from condensed aromatic-carbon (e.g., charcoal). This may be attributed to the burning of crop residues. On average, the proportion of lignin was low and made up 20% of the total SOM, and showed no differences between straw, particulate organic matter (POM), and the bulk soil material. The results from CuO oxidation are consistent with the data obtained from solid-state ¹³C NMR spectroscopy. The extraction of lignin-derived phenols revealed low VSC (vanillyl, syringyl, cinnamyl) values for all investigated soils in a range (4 to 12 g kg^-1 OC) that was typical for agricultural soils. In comparison to adjacent non-paddy soils, the data do not provide evidence for a substantial accumulation of phenolic lignin-derived structures in the paddy soils, even for those characterized by higher organic carbon (OC) contents (e.g., Andosol- and Alisol (China)-derived paddy soils). We conclude that the properties of the parent soil types are more important for the lignin content of the soils than the effect of paddy management itself.