Soil microbes play a vital role in nutrient cycling, especially in carbon and nitrogen cycling. Microbes are influenced and regulated by many factors, such as anthropogenic disturbances, environmental conditions and soil properties. Little has been reported on soil microbial response to long term cultivation after reclamation of estuary wetland. One of possible reasons may be the heterogeneity of soil background and complicated land use history. The Chongming Island, located in the estuary of Yangtze River, was originated mainly from periodical reclamation on tidal flat. Because of an excellent homogeneous soil background and a relatively short and clear land use history on the island, the chronosequence of soils reflects mainly microbial response to soil development and cultivation. Phospholipid Fatty Acids (PLFA) are principle components of microbial cell membranes. Its pattern can be used as a fingerprint of soil microbes to provide insight information of soil bacterial and fungal communities because different microbes show different PLFA profile patterns.
This research focused on a chronosequence of 6 soil ages under two different cultivation (paddy rice and upland cropping) systems, including 0 (unclaimed wetland soil), 16, 40, 75, 120 and 300 years old soils. PLFA analysis results showed that after reclamation of coastal wetland, soil total PLFA, bacterial PLFA, Gram-positive (G⁺) PLFA and Gram-negative (G^-) PLFA all reduced significantly in both paddy rice and upland cropping soils. Microbial PLFA abundance then increased gradually over several decades in both paddy rice and upland cropping soils. After a long-term human cultivation, no significant difference in G⁺ PLFA was observed in 120 or 300 years old soils, either in paddy rice or upland cropping soils. No significant difference in total PLFA, bacterial PLFA or G^- PLFA was observed among 75, 120 or 300 years old soils. Principle component analysis (PCA) showed that two principle components, PC1 and PC2, could explain 51.12% and 18.57% of the total PLFA variation, respectively. PCA also revealed that PLFA profiles of 75, 120 and 300 years old soil were clearly different from those of 0, 16 and 40 years old soil. Cultivations of paddy rice and upland crops have significantly affected soil microbial structure. Total PLFA and G⁺ PLFA were significantly higher in 16 and 40 years old paddy soils than those of upland cropping soils. Bacterial and G^- PLFA were significantly higher in 40 years old paddy rice soils than those of upland cropping soils. Soil total PLFA and bacterial PLFA were significantly correlated to soil total nitrogen and clay content, suggesting strong relations between soil microbes and soil nutrient status. The PLFA results indicated that human cultivations have led to environmental stress on soil microbes and this stress was gradually relieved along with long-term cultivations, because of improved soil nutrient conditions due to inputs of carbon, nitrogen and phosphorus fertilizers.
This study provided deep insights into soil microbial structures of different soil ages based on PLFA analysis under paddy and upland cultivation systems, which are two major cropping systems in China. It was helpful for us to better understand the development of soil microbial communities along with soil succession in arable soils.