Distribution of Perylenequinone Pigments in Several Soils in the United States.

Understanding the stability of soil organic matter (SOM) is important for accurate modeling of carbon cycle and for predicting climate change. However, since SOM consists of a wide variety of organic compounds, from easily biodegradable to decomposition-resistant components, SOM stability will alter depending on soil properties as well as their chemical structure. Therefore, data for stability of organic carbon in soils measured from specific organic compounds is more pertinent than those obtained from bulk SOM. Previously, we proposed that perylenequinone pigments (PQs), a chromophore of green fraction of humic acid produced by soil fungi, could be used as an indicator for predicting SOM stability. This is because PQs occurs naturally in soils in various soil orders. Also, the ratio of PQs concentration to SOM concentration (PQs/SOM) from buried horizons aged nearly 7.5 kyr was found significantly higher than surface soils, indicating high stability of PQs relative to other SOM components. In this study, we investigated PQs content in several soil samples collected from various parts of the United States (US) to understand their distribution and stability.

We observed PQs in surface soils collected from various parts of the US used in this study (0.56 to 139 mg/kg). The PQs content in Mollisols from New Mexico were relatively higher when compared with other soils orders (e.g. Gelisols, Andisols, Oxisols, Ultisols, Alfisoils, Inceptisols and Entisols) used in this study. PQs content in US soil was relatively lower than that of Japanese soils such as Spodozols, Andisols and Inceptisols (26-918 mg/kg). Additionally, the PQs/SOM ratio from buried horizons in this study was significantly higher than surface soil, similar to what was reported in the study with Japanese soils. Results from this study strongly indicate that PQs is more stable than other SOM components, and widely distributed in various soil type around the world.