A long-term experiment set up in 1980 compared the effects of applying manures and chemical fertilizers on a paddy soil in the Taihu Lake region, China. Of the fourteen randomly distributed treatments consisting of different combinations of organic manure, inorganic nitrogen （N）, phosphorus （P）, and potassium （K）, and rice straw, eight were selected for the present study in 2007. Application of organic manure plus straw significantly increased soil organic carbon （SOC） content of the topsoil （0-10 cm） compared to that of chemical fertilizers alone. The content of SOC was relatively stable in the 10-30 cm layer in the chemical fertilizer treatments and in the 20-40 cm layer in the manure treatments. The stable carbon isotope ratio （513C） ranged from -24% to -28% and increased gradually with depth. The content of SOC was significantly （P 〈 0.05） negatively correlated with 513C. In the 0-20 cm layer, the 513C value significantly decreased in the treatments of manure alone （M）, manure and chemical N and P fertilizers （MNP）, manure and chemical N, P, and K fertilizers （MNPK）, manure, rice straw, and chemical N fertilizer （MRN）, and chemical N fertilizer and rice straw （CNR）, as compared with the no-fertilizer control. In the 30-50 cm layer, however, the ratio significantly increased in all the treatments except Treatment CNR. Mineralization of organic C peaked in the first 2-4 d of incubation and gradually leveled off thereafter over the first 3 weeks, being faster in the manure treatments than the chemical fertilizer treatments. The average rate of mineralization varied from 55.36 to 75.46 mL CO2 kg-1 d-1 and that of stable mineralization from 10 to 20 mL CO2 kg-1 d-1. In eight weeks of incubation, cumulative mineralization was always higher in the manure treatments than the chemical fertilizer treatments, being the highest in Treatment MRN. Combined humus in the soil was mainly （over 50%） composed of tightly combined fraction. The loosely combined humus and its r
MA LiYANG Lin-ZhangXIA Li-ZhongSHEN Ming-XingYIN Shi-XueLI Yun-Dong
An ancient irrigated paddy soil from the Neolithic age was excavated at Chuodunshan Site in the Yangtze River Delta, close to Suzhou, China. The soil organic matter (SOM) content in the prehistoric rice soil is comparable to the average SOM content of present rice soils in this region, but it is about 5 times higher than that in the parent materials. As possible biomarkers to indicate the presence of the prehistoric paddy soil, the bacterial communities were investigated using the techniques of aerobic and anaerobic oligotrophic bacteria enumeration, Biolog analysis, and polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE). The results showed that in the buried soil layers, the prehistoric paddy soil had the largest number of aerobic and anaerobic oligotrophic bacteria, up to 6.12 and 5.86 log cfu g-1 dry soil, respectively. The prehistoric paddy soil displayed better carbon utilization potential and higher functional diversity compared to the parent materials and a prehistoric loess layer. The Shannon index and richness based on DGGE profiles of bacterial 16S rRNA genes were higher in prehistoric paddy soil than those in the prehistoric loess soil. It might be concluded that the prehistoric irrigated rice cultivation accumulated the SOM in plowed soil layer, and thus increased soil bacterial populations, metabolic activity, functional diversity and genetic diversity. Bacterial communities might be considered as the sensitive indicators of the presence of the prehistoric paddy soil in China's Yangtze River Delta.
