No abstract

Core Ideas Straw returned twice each year (WC‐MC and WH‐MC) significantly enhanced SOC and DOC. Return of both wheat and maize chopped straw had highest β‐1,4‐xylosidase, invertase, and enhanced Alphaproteobacteria and Gammaproteobacteria. Retention of high wheat stubble and return of chopped straw had highest β‐1,4‐glucosidase, cellobiohydrolase, and enhanced Acidobacteria and Planctomycetes. Amount and complexity of straw added led to differences in C, enzyme activity, and microbial composition. High wheat stubble and crushed straw improved yields and bacterial richness, but had less C sequestration. Little is known about the responses of organic carbon fractions, enzymes activity, and soil bacterial communities and diversity to different straw return modes in intensive maize‐wheat double‐cropping systems. An 8‐yr field experiment was conducted to evaluate the effects of different straw return modes. Four treatments were compared: (i) return of both wheat and maize chopped straw (WC‐MC); (ii) retention of high wheat stubble and return of chopped maize straw (WH‐MC); (iii) retention of high wheat stubble and no return of maize straw (WH‐MN); and (iv) no return of straw of both wheat and maize (WN‐MN). Soil organic carbon, dissolved organic carbon, and particulate organic carbon content increased significantly in the WC‐MC and WH‐MC. Beta‐1‐4‐xylosidase and invertase were higher in the WC‐MC treatment than in WH‐MC, while the highest β‐1,4‐glucosidase activities were observed in WH‐MC. However, the activity of polyphenol oxidase was low in all three straw return treatments. Alphaproteobacteria and Gammaproteobacteria dominated in WC‐MC, while the Acidobacteria and Planctomycetes phyla dominated WH‐MC. The two high wheat stubble retention modes caused significant increases in bacterial richness. Redundancy analysis showed that C input amount and organic carbon content were the most influential factors determining bacterial community structure. The observed effects were due to straw amount and the complex composition of WH‐MC added during the wheat growing season compared to WC‐MC. In conclusion, the straw return mode that employs high wheat stubble retention and chopped maize straw return is sufficient to maintain organic carbon levels and improve the soil microenvironment

The Cenozoic Xihu Sag in the East China Sea Shelf Basin contains large reserves of coals together with liquid petroleum derived from coal‐associated sediments. However, the origin of the petroleum is not well understood. In this study, biomarker assemblages in a suite of recently discovered light oils and condensates from the Paleogene succession in the western margin of in the Xihu Sag were investigated using gas chromatography – mass spectrometry. The objectives were to investigate the samples' thermal maturity and the depositional environment of the precursor source rocks which generated the oils. The light oils are believed to have been derived from coaly source rocks in the Eocene Pinghu Formation. Assessment of thermal maturity based on CPI, pristane/n‐C17 ratio and isomerisation ratios of C29 steranes and C31 homohopanes suggest that the hydrocarbons have a relatively low maturity in the early to mid oil generation window. The distribution of isoprenoids relative to n‐alkanes, the high pristane/phytane ratios (5.1–10.7), the almost complete absence of gammacerane and C33+ homohopanes, and the low dibenzothiophene/phenanthrene ratios indicate that the source rocks of the hydrocarbons were deposited in a relatively oxic and sulphate‐poor fluvio‐deltaic environment which was favourable for coal measure development. Abnormally abundant gymnosperm‐derived diterpanes including labdane, 19‐norisopimarane, fichtelite, rimuane, pimarane, isopimarane, 17‐nortetracyclic diterpene, phyllocladanes and abietane were detected in the samples analysed. 16a(H)‐Phyllocladane was identified unambiguously and kauranes were confirmed to be absent. In addition, three 19‐norisopimarane isomers, 13β(H)‐atisane, and 20‐normethylatisane were tentatively identified in the studied samples. The distributions of n‐alkanes, isoprenoids and regular steranes, the presence of 4β(H)‐eudesmane and oleanane, high Pr/Ph ratios and the abundant diterpanes together suggest that the hydrocarbons were derived from a coaly source rock. Gymnosperms of the conifer families Cupressaceae (especially the former Taxodiaceae) and Pinaceae are interpreted to be the major source of the diterpanes and to have made a significant contribution to the coaly source rock. However, the low abundance of oleanane relative to diterpanes may underestimate the contribution from angiosperms relative to gymnosperms. This could be due to differential preservation and alteration of the di‐ and triterpenoid precursors during diagenesis and the occurrence of non‐specific precursors in higher land plants.