Molecular Ratio of PAH as a Tool to Reveal Ancient Farming Practice from Paleo-Paddy Soils in the Yangtze Delta of China.
Yuanhua Dong, Zhihong Cao, Jiuhai Li, Hui Wang, Qiong An, zhengyi Hu, Linzhang Yang, Xinggui Lin, and Rui Yin. Institute of Soil Science, Chinese Academy of Sciences, 71 East Beijing Road, Nanjing, 210008, China
China is a country of long rice cultivation history which can be dated back to the Neolithic Age. About 200 sites with fossil rice grains have been discovered, and mainly distributed in the Yangtze River Valley. A total of 46 pieces of prehistoric irrigated rice fields with size of 0.4-16m2 was excavated at Chuo-dun-shan site in Suzhou in 2003. The results of carbonized rice, phytolith, rice pollen, and corresponding farming tools indicated the fields were used for rice cultivation, and the 14C showed that those fields can be dated back to 6280 BP, which belongs to Ma-jia-bang cultural period. Soil samples from a paddy soil profile (P01) and an adjacent control profile of dry land (P03) at the site were collected in 2003 and analyzed by HPLC for 15 Polycyclic Aromatic Hydrocarbons (PAHs). The sum of 15 PAHs were 202.9 and 207.7 µg/kg in the surface soils for P01 and P03 respectively, which were mainly deposited from atmosphere. In P01, the total PAHs concentration sharply decreased to 56.0 µg/kg with the depth of profile, but was still higher than those in ancient dry land soil and bottom soil which were with a total of 32.0-36.9 µg/kg. In P01, the concentrations of 2- and 3-ring PAHs took a larger portion of 63 percents to the total PAHs, and naphthalene and phenanthrene were the most abundant compounds. The results from the principal component and cluster analysis indicated that Chrysene, Benzo(k)fluoranthene, Benzo(a)anthracene, Indeno(1,2,3-cd)pyrene, Benzo(b)fluoranthene, Pyrene, Benzo(a)pyrene, Dibenzo(ah)anthracen and Fluoranthene were originated mainly from anthropogenic sources, Fluorene and Phenathrene were formed in biological process under reducing conditions, Naphthalene, Benzo(ghi)perlene and Anthracene may come from both the anthropogenic and biogenic processes. The molecular ratios of phenanthrene/ anthracene and benzo(a)anthracene/chrysene and 13C-NMR spectrum of soil organic matter showed that PAHs in paleo-paddy soil were derived mainly from rice straw burning, which could imaged the fact that a farming practice of ²ploughing by burning and planting by water² was adopted in the prehistoric paddy fields