Artificial soil sealing in urban areas has attracted increasing attention due to its potential hazard to urban ecosystems. It has negative impacts on soil function and the urban environment, since the impervious surface can hamper the exchange of material and energy between the soil and other environmental compartments. However, information about the effects of artificial soil sealing in urban area on soil quality and properties, especially the microbiological components, is still limited. Ten plots which differed in land use were selected from Nanjing City, China, to investigate the effects of impervious surfaces on microbiological characteristics in urban soil. Plot types were paved road, residential paved square, residential paved alley, and grassed area. Soil microbial biomass carbon (Cmic) and nitrogen (Nmic), and activities were analysed, and the microbial functional diversity of fine earth (<2mm material) was characterised by the Biolog EcoPlate technique. Mean concentrations of soil organic carbon (SOC), Cmic, and Nmic in fine earth from the impervious areas (0–20cm) were, respectively, 6.5gkg-1, 55.8mgkg-1, and 12.2mgkg-1, which were significantly lower than concentrations from grass areas. Urban sealing also resulted in decreases in soil microbial activity and functional diversity, but the influences on soil microbial diversity varied among land uses, with road pavement having the most negative effect. Substrate use patterns showed that microorganisms in urban sealed soils had higher utilisation of polymers (P<0.05) but lower use of carbohydrates and amines/amides (P<0.05). These findings demonstrate that SOC in the sealed soils was more stable than in open soils, and the installation of impervious surfaces such as asphalt and concrete, which are very common in urban areas, can result in decreases in SOC content, soil microbial activity, and diversity in urban soil.

Agroforestry systems are widely practiced in tropical forests to recover degraded and deforested areas and also to balance the global carbon budget. However, our understanding of difference in soil respiration rates between agroforestry and natural forest systems is very limited. This study compared the seasonal variations in soil respiration rates in relation to fine root biomass, microbial biomass, and soil organic carbon between a secondary forest and two agroforestry systems dominated by Gmelina arborea and Dipterocarps in the Philippines during the dry and the wet seasons. The secondary forest had significantly higher (p < 0.05) soil respiration rate, fine root biomass and soil organic matter than the agroforestry systems in the dry season. However, in the wet season, soil respiration and soil organic matter in the G. arborea dominated agroforestry system were as high as in the secondary forest. Whereas soil respiration was generally higher in the wet than in the dry season, there were no differences in fine root biomass, microbial biomass and soil organic matter between the two seasons. Soil respiration rate correlated positively and significantly with fine root biomass, microbial biomass, and soil organic C in all three sites. The results of this study indicate, to some degree, that different land use management practices have different effects on fine root biomass, microbial biomass and soil organic C which may affect soil respiration as well. Therefore, when introducing agroforestry system, a proper choice of species and management techniques which are similar to natural forest is recommended.

AIM: To estimate the concentrations, stoichiometry and storage of soil microbial biomass carbon (C), nitrogen (N) and phosphorus (P) at biome and global scales. LOCATION: Global. METHOD: We collected 3422 data points to summarize the concentrations and stoichiometry of C, N and P in soils, soil microbial biomass at global and biome levels, and to estimate the global storage of soil microbial biomass C and N. RESULTS: The results show that concentrations of C, N and P in soils and soil microbial biomass vary substantially across biomes; the fractions of soil elements C, N and P in soil microbial biomass are 1.2, 2.6 and 8.0%, respectively. The best estimates of C:N:P stoichiometry for soil elements and soil microbial biomass are 287:17:1 and 42:6:1, respectively, at global scale, and they vary in a wide range among biomes. The vertical distribution of soil microbial biomass follows the distribution of roots up to 1 m depth. MAIN CONCLUSIONS: The global storage of soil microbial biomass C and N were estimated to be 16.7 Pg C and 2.6 Pg N in the 0–30 cm soil profiles, and 23.2 Pg C and 3.7 Pg N in the 0–100 cm soil profiles. We did not estimate P in soil microbial biomass due to insufficient data and insignificant correlation between soil total P and climate variables used for spatial extrapolation. The spatial patterns of soil microbial biomass C and N were consistent with those of soil organic C and total N, i.e. high density in northern high latitude, and low density in low latitudes and the Southern Hemisphere.

Microbial biomass is an important source of soil organic matter, which plays crucial roles in the maintenance of soil fertility and food security. However, the mineralization and transformation of microbial biomass by the dominant soil macrofauna earthworms are still unclear. We performed feeding trials with the geophagous earthworm Metaphire guillelmi using 14C-labelled bacteria (Escherichia coli and Bacillus megaterium) cells, fungal (Penicillium chrysogenum) cells, protein, peptidoglycan, and chitin. The mineralization rate of the microbial cells and cell components was significantly 1.2–4.0-fold higher in soil with the presence of M. guillelmi for seven days than in earthworm-free soil and 1–11-fold higher than in fresh earthworm cast material. When the earthworms were removed from the soil, the mineralization of the residual carbon of the microbial biomass was significantly lower than that in the earthworm-free soil, indicating that M. guillelmi affects the mineralization of the biomass in soil in two aspects: first stimulation and then reduction, which were attributed to the passage of the microbial biomass through the earthworm gut, and that the microorganisms in the cast could play only minor roles in the stimulated mineralization and residual stabilization of microbial biomass. Large amounts (8–29%) of radiolabel of the tested microbial biomass were assimilated in the earthworm tissue. Accumulation of fungal cells (11%) and cell wall component chitin (29%) in the tissue was significantly higher than that of bacterial cells (8%) and cell wall component peptidoglycan (15%). Feeding trails with 14C-lablled microbial cells and cell components provided direct evidence that microbial biomass is a food source for geophagous earthworm and fungal biomass is likely a more important food source for earthworms than bacterial biomass. Findings of this study have important implications for the roles of geophagous earthworms in the fate of microbial biomass in soil.

Bioenergy feedstock production systems face many challenges, among which is the lack of guidelines on sustainable biomass harvest thresholds and tillage cropping systems that maintain soil quality and productivity. We used the ALMANAC crop model to evaluate four biomass removal rates, 0%, 50%, 75% and 100%, and four tillage cropping systems, continuous No Till (NT), and Conventional Till (CT), and periodically plowed or subsoiled NT lands at Shorter, AL, for a Lynchburg loamy sand soil, over 51 yr of actual weather data: 1960–2010. Farmers periodically plow or subsoil NT lands to alleviate problems of drainage, pests, and soil compaction. Given the importance of soil organic carbon (SOC) as a soil quality indicator, we premised sustainability upon the maintenance of SOC at or above the initial SOC levels. As expected, NT had the highest SOC and lowest bulk density (BD) across the four biomass removal rates and gained the highest percent SOC over the 51-yr simulation period. For this study, the 75% biomass removal rate was applied sustainably on NT energy sorghum production systems, giving an annual harvestable biomass yield of 18.0±0.9, residue biomass, 6.2±0.3, and a root biomass of 7.2±0.4Mgha−1. However, the 75% removal rate also significantly increased soil bulk density, a critical indicator of soil compaction, by 30%. Compared to conventional tillage, subsoil tillage maintained SOC and better alleviated soil compaction in NT systems, but at the reduced biomass removal threshold of 50%. Long-term biomass removal resulted in reduced total biomass yields over time due to nutrient depletion as reflected by increased N stress days on subsequent crops. We attributed the N stress to N immobilization by the decomposing residues, reduced mineralization and N losses. Additional inputs will be needed to avoid increased N uptake from the soil which could result in soil mining.

Studying the influence of plants on soil biological variables in an arid zone is important to the understanding of soil processes and relationships between above and below ground. The objective of this study was to quantify the pattern and degree of soil heterogeneity for soil moisture and its relationship with microbial biomass carbon and soil respiration using geostatistical techniques at stand scale of an arid scrubland. The experiment was conducted in a scrubland landscape using a 2 × 2 m grid within a 16 × 14 m plot in the lower reach of Sangong River watershed in Xinjiang, northwest China. The results revealed the following: (1) Soil moisture and soil microbial biomass carbon had moderate spatial variation, but soil respiration had strong variation. Spatial variability of soil moisture in the study plot decreased when soil moisture changed from wet in April to dry in June. In addition, correlations of soil moisture with microbial biomass carbon and soil respiration were positive and significant (p < 0.005). (2) Variation of soil microbial biomass carbon and soil moisture had a strong spatial autocorrelation in the study plot, mainly induced by structural factors, and the spatial autocorrelation of microbial biomass carbon and soil respiration was mainly determined by soil moisture. (3) The location of the high-value positions of soil moisture, soil microbial biomass carbon and soil respiration were clearly around the positions of scrubs in the study plot. Such information provided some insights to explain the spatial heterogeneity of soil properties at stand scale in an arid region.

International audience | Background and aims Soil aggregate stability depends on plant community properties, such as functional group composition, diversity and biomass production. However, little is known about the relative importance of these drivers and the role of soil organisms in mediating plant community effects. Methods We studied soil aggregate stability in an experimental grassland plant diversity gradient and considered several explanatory variables to mechanistically explain effects of plant diversity and plant functional group composition. Three soil aggregate stability measures (slaking, mechanical breakdown and microcracking) were considered in path analyses. Results Soil aggregate stability increased significantly from monocultures to plant species mixtures and in the presence of grasses, while it decreased in the presence of legumes, though effects differed somewhat between soil aggregate stability measures. Using path analysis plant community effects could be explained by variations in root biomass, soil microbial biomass, soil organic carbon concentrations (all positive relationships), and earthworm biomass (negative relationship with mechanical breakdown). Conclusions The present study identified important drivers of plant community effects on soil aggregate stability. The effects of root biomass, soil microbial biomass, and soil organic carbon concentrations were largely consistent across plant diversity levels suggesting that the mechanisms identified are of general relevance.

Aims: To determine the effect of grassland degradation on the soil carbon pool in alpine grassland. Methods: In this study, we calculated the carbon pool in the above-and below-ground biomass, the soil microbial biomass carbon pool, the total organic carbon pool and the soil total carbon. Results: Grassland degradation has resulted in decreases in biomass and carbon content and has changed the ratio of roots to shoots. However, there was less influence of degradation on dead root biomass. There was most likely a lag effect of changes in dead root biomass following grassland degradation. In the alpine grassland ecosystem, the carbon pool in soil accounts for more than 92 % of the total carbon both in vegetation and soil. The carbon in alpine grassland is stored primarily in the form of total organic carbon below-ground. As organic carbon decreases, the ratio of the microbial biomass carbon pool to the total organic carbon pool increases and then declines with increasing degradation level. Along the grassland degradation gradient, the total vegetation biomass (above-and below-ground) and the soil carbon pool (microbial biomass C, total organic C and total C) all decreased. © 2012 Springer Science+Business Media Dordrecht.

Soil microbes contribute to native plant species successful resistance against invasive plant. Three native tree species, Heteropanax fragrans (HF), Cinnamomum burmanii (CB), and Macaranga tanarius (MT) were effective in controlling the notorious invasive vine Mikania micrantha (MM). Biomass production and allocation patterns (shoot/root biomass ratio (shoot/root)) are important indicators of MM climbing coverage and competitive light-capturing capacity. An investigation was conducted to test the role of soil microbes associated with the three native tree species to inhibit MM biomass production and shift MM shoot/root. Rhizosphere soils originating from preculture HF, CB, MT, and MM plots were collected separately for use as inocula. The inocula were mixed with sterilized river sand at a 1:9 (w/w) ratio to grow MM. The fungicide carbendazim (methyl benzimidazol-2-ylcarbamate) was applied to half the treatments to kill pathogenic soil fungi. Two nutrient levels were established based on the natural soil nutrient concentration from a field stand invaded by MM. MM were grown from seeds in a glasshouse, harvested 15 weeks after sowing, and separated into shoot and root portions. Results showed that under interaction of soil origin and nutrient levels, MM biomass production was unchanged, but biomass allocation patterns were significantly different. MM biomass production grown in the three native tree soils under two nutrient levels was similar or higher than MM biomass production in MM conspecific soil, indicating the absence of species-specific pathogens that inhibited MM biomass production in native tree soils. However, in both conspecific and tree soils, MM biomass production was significantly reduced in the presence of pathogenic soil fungi, i.e. MM experienced significant fungal inhibition, demonstrating the pathogenic soil fungi promoted native tree resistence to MM. MM exhibited decreased shoot biomass allocation when cultivated in native tree soil relative to MM conspecific soil under field stand nutrient level conditions. Reduced resource allocation to shoot biomass could result in diminished capacity to climb, cover, and subsequent smother to native trees, and reduced surface area exposed to available light. Following fungicide application, significant biomass allocation differences disappeared, suggesting the native tree soil fungi were responsible for decreasing MM shoot biomass. The overall results indicated tree soil fungi serve an integral role in controlling invasive MM through fungal inhibition on MM biomass production, and shifts in MM biomass allocation patterns.