不同土地利用方式下土壤微生物群落水平生理图谱(CLP)与土壤碳、氮有效性之间的相互关系

The goal of this work was to assess soil microbial respiration, determined by the assay of community-level physiological profiling in an oxygen-sensitive microplate (O₂-CLPP), in response to endogenous C and several individual C substrates in the soils with different organic C contents (as a function of soil type and management practice). We also used the O₂-CLPP to determine the respiratory response of these soils to endogenous C and amended C substrates with N addition. A respiratory quotient (RQ) was calculated based on the ratio of the response to endogenous soil C vs. each C-only substrate, and was related to total organic carbon (TOC). For assessing N availability for microbial activity, the effect of N supplementation on soil respiration, expressed as N_ratio, was calculated based on the response of several substrates to N addition relative to the response without N. Soils clustered in 4 groups after a principal component analysis (PCA), based on TOC and their respiratory responses to substrates and endogenous C. These groups reflected differences among soils in their geographic origin, land use and C content. Calculated RQ values were significantly lower in natural forest soils than in managed soils for most C-only substrates. TOC was negatively correlated with RQ (r = - 0.65), indicating that the soils with higher organic matter content increased respiratory efficiency. The N addition in the assay in the absence of C amendment (i.e., only endogenous soil C present) had no effect on microbial respiration in any soil, indicating that these soils were not intrinsically N-limited, but substrate-dependent variation in N_ratio within soil groups was observed.     

Variability of communities and physiological characteristics between free-living bacteria and attached bacteria during the PAH biodegradation in a soil/water system

The biodegradation of polycyclic aromatic hydrocarbons (PAHs) via free-living and attached micro-organisms in soil/water systems was observed in order to examine the variability in the community dynamics and physiological profiles of the micro-organisms. As determined by fluorescence in situ hybridization (FISH), the Domain Bacteria, consisting of three phyla α-, β- and γ-Proteobacteria, reached 41.27–56.05% of all organisms in the soil/water system for PAH biodegradation. Among the free-living species, Proteobacteria, including Brevundimonas (Pseudomonas) diminuta, Caulobacter spp., Mycoplana bullata, Acidovorax spp. and Pseudomonas aeruginosa were found to be dominant—making up 93.51–99.80% of the population—and therefore seem to be associated with PAH biodegradation. Total plate count numbers and the count of Pseudomonas sp. present in the free-living population increased to between 10³ and 10⁶ CFU ml⁻¹ when clay with very low organic matter content was used as the matrix for PAH degradation. However, total plate count microbial numbers increased to only 10¹–10² CFU ml⁻¹ using natural soil from Taichung containing 1.883% organic matter. The soil organic content (SOM) seemed to affect the mass transfer of PAH in soil, leading to the difference in PAH biodegradation. Two different approaches, which included community-level physiological profiling (CLPP) and ectoenzymatic activities, were used to explain the functional diversity between free-living and attached bacteria. The free-living and attached bacterial communities from the clay system showed proportionately greater differences using CLPP. Relatively high levels of esterases, aminopeptidases and some specific glycolysis-gluconeogenesis enzymes gave an identifiable correlation with PAH biodegradation. The differences in bacterial composition, numbers and physiological characteristics show that free-living and attached micro-organisms may play different biochemical roles in PAH degradation in soil.     

Variations of soluble organic nitrogen and microbial nitrogen in deciduous forest soils along an urban-rural gradient

Soluble organic N and microbial N pools in terrestrial ecosystems have been less studied than those of inorganic N. Therefore, cross-system variation in their pool sizes and seasonal dynamics, both absolute and relative to inorganic N pools, needs to be quantified so that their ecological importance in different ecosystems can be evaluated. We measured extractable soil organic N (ESON), microbial biomass N (MBN), and the net production rates of ESON in red oak-dominated remnant forests, along an urban-rural gradient in the New York City metropolitan area. We were interested in (1) determining the seasonal dynamics of ESON and MBN, and (2) examining whether the contrasts in land use (urban, suburban, rural) surrounding these forest remnants were associated with different amounts of ESON and MBN. This field-based study was conducted continuously for 16 months. Yearly average ESON concentrations ranged from 60 to 140 mg kg⁻¹ soil organic matter (SOM), 3-4 times those of inorganic N, and average MBN ranged from 600 to 1100 mg kg⁻¹ SOM. There was a considerable MBN increase in spring in all plots across the gradient. The average increase expressed on an areal basis (to a depth of 7.5 cm) ranged from 1.75 to 4.19 g N m⁻². The N incorporated into the microbial biomass in spring was gradually released later in the growing season (the mean MBN decrease ranged from 1.11 to 3.82 g N m⁻²). The spring MBN increase could be an important retention mechanism for conserving soil inorganic N when plant N uptake may be low. The amplitude in the seasonal dynamics of ESON was far less than that of inorganic N, as was that of net ESON production rates when compared to net N mineralization. These suggest a closer coupling of plant N uptake with inorganic N, much more than with ESON. Both ESON and MBN were significantly higher in rural soils than in urban soils, and both concentrations were positively correlated with SOM content. The variation in ESON, MBN, or SOM associated with this urbanization gradient suggests that the form of N exported, the plant N budget and soil N retention mechanisms may be differentially affected by urban, suburban and rural land uses.