Structure and function of bacterial communities in ageing soils: Insights from the Mendocino ecological staircase

The ecological staircase of Mendocino (California, USA) is characterized by a succession of uplifted marine terraces that are derived from the same mineralogical parent material but have different ages, levels of fertility, and types of vegetation, from grassland in the youngest and most fertile terrace to a pygmy forest in the older terraces. Such conditions present a unique opportunity to determine how the structure, abundance, and function of bacterial communities vary with soil fertility along this natural chronosequence. Pyrosequencing analysis of 16S rRNA gene amplicons revealed that Acidobacteria, Proteobacteria, Actinobacteria, and Bacteroidetes were the most abundantly represented phyla. Bacteroidetes, Firmicutes, Verrucomicrobia were significantly enriched in the grasslands, while the less fertile forested terraces showed higher abundance of Acidobacteria Gp2 and Alphaproteobacteria. The pygmy forest soil harboured significantly more Actinobacteria and OP10 than the non-pygmy forest. Between samples from different terraces, the structure of the bacterial community clearly correlated with soil characteristics. Notably, the number of operational taxonomic units was greater in the fertile terrace, as was the density of culturable bacterial populations. Functional characterization of the soil culturable bacteria from the pygmy and non-pygmy forest terraces revealed that the soil bacteria from the non-pygmy terrace were significantly more effective in solubilizing minerals and more abundant than in the pygmy terrace. Our results provide new information on bacterial community structure as a function of soil age, land cover and fertility, which improve our understanding of soil evolution.     

Humus macro-morphology and soil microbial community changes along a 130-yr-old Fagus sylvatica chronosequence

We aimed to characterize humus macro-morphology and the associated soil microbial community within the unmodified litter (OL), the fragmented and humified layers (FH) and the organo-mineral (A) layer along a beech (Fagus sylvatica L.) forest chronosequence with four stand age-classes (15-, 65-, 95-, 130-yr-old) in Normandy, France. Humus macro-morphology was described with 36 quantitative and semi-quantitative variables. We measured microbial biomass N (N_mic), microbial N quotient (N_mic-to-N_t), fungal ergosterol, bacterial and fungal DNA using 16S and 18S rDNA real-time qPCR and evaluated the potential metabolic profile of heterotrophic bacteria within each soil layer and stand age-class. The log-transform ergosterol/fungal DNA ratio (EFR index) was used as an indicator related to active fungal biomass and the fungal/bacterial (F/B) ratio was calculated from qPCR results. There was a shift from mull (mainly dysmull) to moder humus forms along the chronosequence. While the N_mic did not change significantly, the N_mic-to-N_t decreased along the chronosequence in the OL layer. Ergosterol content increased in FH and A layers and the F/B ratio increased in the FH layer with increasing beech forest age. The EFR index was significantly higher in the OL and A layers of the oldest stands, whereas the highest EFR index in the FH layer occurred in the 15-yr-old stands. The functional diversity of heterotrophic bacteria was greater within OL and FH layers of 130-yr-old stands, but highest in the A layer of 15-yr-old stands while the Average Well Color Development remained stable for all soil layers. We found significant correlations between macro-morphology and microbial variables, especially between FH-based morphology and fungal biomass. Our main results are that beech forest maturation is accompanied by (1) an increase in fungal biomass in the FH layers and, (2) an increase in heterotrophic bacteria functional diversity in the organic layers. We have identified key macro-morphology variables that are good predictors of the structural and functional profile of the soil microbial community during beech forest development.     

红壤稻田土壤理化及生物学性状的动态变化特征

Rice production plays a crucial role in the food supply of China and a better understanding of the changes in paddy soil fertility and the management effects is of practical importance for increasing rice productivity. In this study, field sampling in a typical red soil region of subtropical China, Jiangxi Province, was used to observe changes in the soil physical, chemical, and biological properties in a cultivation chronosequence of paddy fields. After cultivation, clay (< 0.002 mm) content in the soil, which was 39% in the original uncultivated wasteland, decreased, to 17% in the 80-year paddy field, while silt (0.02--0.002 mm) content increased. Additionally, macroporosity increased and pore shapes became more homogeneous. Soil Ph generally increased. Soil organic C and total N content of the 0-10 cm layer increased from 4.58 and 0.39 g kg-1 to 19.6 and 1.62 g kg-1, respectively in the paddy fields after 30-year cultivation and then remained stable. Soil total P content increased from 0.5 to 1.3 g kg-1 after 3 years of rice cultivation, indicating that application of phosphate fertilizer could accelerate phosphorous accumulation in the soil. Total K content in the 0--10 cm soil layer for the 80-year paddy fields decreased by 28% compared to that of the uncultivated wasteland land. Total Fe and free Fe contents declined with years of cultivation. The bacterial population increased and urease activity noticeably intensified after years of cultivation. In this chronosequence it appeared that it took 30 years to increase soil fertility to a relatively constant value that was seen after 80 years of cultivation.     

土壤时间序列的构建及其在土壤发生研究中的意义

土壤时间序列在研究土壤演化速率与方向、建立土壤发生演化模型上具有重要的价值,并且能为土壤发生学理论的验证提供宝贵的信息。为了获得可靠的结果,时间序列的构建必须包括剖面内与剖面间母质均一性或母质不连续性的判定以及土壤的绝对或相对年龄的判定这两个重要过程。本文综述了土壤时间序列的类型及其构建方式,详细论述了在时间序列建立过程中母质不连续性的判定方式与土壤定年方法,并阐明了土壤时间序列方法在土壤发生研究中的重要意义,旨在为正确建立土壤时间序列、深刻理解土壤发生中的时间因素提供较为详尽的参考。     

Microbial biomass and soil organic carbon accumulation on a former lakebed near Novosibirsk,Russia

Shrinking of Lake Chany in W Siberia exposed 10,000 km² of former lakebed. On this new land we studied a chronosequence of young soils. The fraction of microbial C in total organic C had stabilized at 3% after 30 to 40 y. Concentration of organic C increased within the first 50 to 100 y to 4–5% in the first 10 cm, and to 2–3% at 10–20 cm depth. The amount of sequestered C is equivalent to about 2.5 d of current anthropogenic CO₂ emissions.     

海螺沟冰川退化迹地土壤序列的发育速率

土壤形态特征构建的土壤发育指数用来评价贡嘎山海螺沟冰川退化迹地土壤时间序列的发育状况.结果表明,描述性指标经过定量后能够较好地表征土壤发育的时间趋势.大部分土壤性质及剖面发育指数（PDI）和加权平均剖面发育指数（WPDI）随时间呈对数格局增加.表明土壤发育速率随时间而减缓.土壤性质发育速率大小顺序为：pH下降〉总质地〉结持性〉结构性〉红化率.剖面发育指数（PDI）和加权平均剖面发育指数（WPDI）作为剖面累积发育指标,可以用于不同土壤序列的发育速率比较.由于减少了剖面厚度的影响.加权平均剖面发育指数更能反映土壤发育在时间上的系统变化,也更真实地反映海螺沟土壤序列的成土速率.