Peter D. Stahl

Soil microbiological properties 20 years after surface mine reclamation: spatial analysis of reclaimed and undisturbed sites

Spatial characteristics of soil microbial community structure and selected soil chemical factors were analyzed in soil surrounding Agropyron smithii (Western wheatgrass) and Artemisia tridentata (Wyoming big sagebrush) plants in sites reclaimed after surface mining and adjacent undisturbed sites in Wyoming. Microbial biomass C (MBC) and fatty acid methyl ester (FAME) biomarkers for total biomass, bacteria, and fungi were used as indicators of soil microbial community abundance and structure. In soil 20 years after reclamation FAME total microbial biomass, bacterial and fungal biomarkers, MBC and soil organic matter (SOM) averaged only 20, 16, 28, 44 and 36% of values found in undisturbed soils. In contrast to undisturbed soils, FAME biomarkers and MBC of reclaimed soils exhibited spatial correlation up to 42 cm. Reclaimed soils also exhibited localized enrichment of bacterial, fungal, and total microbial biomass, as well as depletion of inorganic N concentrations, around plant bases (, 10 cm), suggesting relatively poor soil exploration by roots and microorganisms compared to the undisturbed ecosystem. Strong spatial stratification of undisturbed SOM and soil NH4 pools was found with highest concentrations on the leeward side of shrubs, likely due to localized changes in microclimate and plant litter deposition. This indicates that shrub cover plays a central role in the establishment of site heterogeneity and regulation of ecological processes, such as C and N mineralization and immobilization, which has important implications for reclamation. q 2002 Elsevier Science Ltd. All rights reserved.

Microbial biomarkers as an indicator of ecosystem recovery following surface mine reclamation

Increased disturbance of terrestrial ecosystems in recent years for purposes of mineral extraction has created interest in development and optimization of reclamation methodologies for these lands. Currently, criteria for judging surface mine reclamation success, or progress toward reclamation goals, predominantly rely on aboveground indicators that fail to account for the abundance and composition of soil microbiota, an essential aspect of soil health. To test the utility of fatty acid methyl ester (FAME) biomarkers as indicators of reclamation progress, FAME bacterial, fungal, and total biomass biomarkers extracted from soil of surface mine reclamation sites of different ages and an adjacent undisturbed site were compared with other indicators of reclamation progress and ecosystem stability. Our results indicate that FAME microbial biomarkers and soil organic matter (SOM) contents were greatly impacted by disturbance. Discriminant analysis of FAME bacterial, fungal and total microbial biomass biomarkers, although clearly able to discriminate between disturbed and undisturbed ecosystems, indicated a trend towards the undisturbed condition with reclamation age. The ratio of FAME bacterial to fungal biomarkers reflected changes in other indicators of soil health (SOM, inorganic N concentration), suggesting that this ratio is a useful indicator of reclamation progress.

Soil microbial properties under permanent grass, conventional tillage, and no-till management in South Dakota

The objective of this study was to determine how no-till and conventional tillage affected microbial properties of soil quality, using permanent grass fields as a standard of local high-quality soils. Agricultural fields in South Dakota were matched within soil type and location so that each set contained one field managed in permanent grass cover, one managed for annual crops using conventional tillage, and one managed for annual crops using no-till. Microbial biomass and activities, particularly those related to nitrogen and phosphorus availability were examined in 0–15 cm soil samples. Most measures of microbial activity were similar in the two annually cropped management systems but less than under permanent grass. Soils managed under permanent grass contained approximately 160% greater labile carbon and 50% greater microbial biomass, and supported approximately 150% greater dehydrogenase enzyme activity, 50% greater respiration, and 180% greater acid phosphatase activity, than no-till and conventionally tilled soils, which were similar in these measures. Grassed soils also mineralized 460% greater N under anaerobic conditions and 80% greater N under aerobic conditions, while potential nitrification was 25% greater on average than cropped soils. Mycorrhizal fungi colonized 76% of observed root segments under grass, 63% under no-till, and 55% under conventional tillage. Among the suite of tests in this study, only alkaline phosphatase activity and C mineralized in 12 days were significantly different under no-till and conventional tillage, both greater under no-till. Crops grown in a no-till system may be expected to resist low available P as well as other stresses better than crops in conventionally tilled soils due to greater activity of alkaline phosphatase and marginally increased mycorrhizal colonization of roots. No-till management in central South Dakota has allowed enhancement of readily mineralizable C and alkaline phosphatase activity.

Spatial Stratification of Soil Bacterial Populations in Aggregates of Diverse Soils

Most soil microbial community studies to date have focused on homogenized bulk soil samples. However, it is likely that many important microbial processes occur in spatially segregated microenvironments in the soil leading to a microscale biogeography. This study attempts to localize specific microbial populations to different fractions or compartments within the soil matrix. Microbial populations associated with macroaggregates and inner- versus total-microaggregates of three diverse soils were characterized using culture-independent, molecular methods. Despite their relative paucity in most surveys of soil diversity, representatives of Gemmatimonadetes and Actinobacteria subdivision Rubrobacteridae were found to be highly abundant in inner-microaggregates of most soils analyzed. By contrast, clones affiliated with Acidobacteria were found to be relatively enriched in libraries derived from macroaggregate fractions of nearly all soils, but poorly represented in inner-microaggregate fractions. Based upon analysis of 16S rRNA, active community members within microaggregates of a Georgian Ultisol were comprised largely of Gemmatimonadetes and Rubrobacteridae, while within microaggregates of a Nebraska Mollisol, Rubrobacteridae and Alphaproteobacteria were the predominant active bacterial lineages. This work suggests that microaggregates represent a unique microenvironment that selects for specific microbial lineages across disparate soils.

EFFECTS OF DIFFERENT GEOGRAPHIC ISOLATES OF GLOMUS ON THE WATER RELATIONS OF AGROPYRON SMITHII

The influence of vesicular-arbuscular mycorrhizae on the water relations of the rangeland grass Agropyron smithii was compared for two different geographic isolates of both Glomus macrocarpum var. macrocarpum and G. microcarpum. Spores of these fungi were collected from two sites in Wyoming that differed in total annual precipitation and were used to inoculate greenhouse-grown plants. Leaf resistances to water vapor loss (R^) were about 11% lower in all mycorrhizal vs. nonmycorrhizal plants at high soil and plant water potentials (>-0.2 MPa) and up to 47% lower at soil and plant water potentials near -6 MPa. Plants infected with G. microcarpum had R^ values that were up to 26% lower (1520 vs. 1120 m s_1) than G. macrocarpum-infected plants at the lowest xylem pressure potentials. However, changes in xylem pressure potentials over a soil water potential range from 0 to -6 MPa were not statistically different between plants infected with the two fungal congeners. Plants infected with G. microcarpum from the dry Wyoming site had statistically different responses to drying soil than did plants infected with G. microcarpum collected from the more mesic site with an approximate 9% difference in R^ (1120 m s~l vs. 1220 m s~*) at a xylem pressure potential near - 6 MPa. As was the case for the fungal congeners, xylem pressure potentials of the host A. smithii did not differ between the two G. microcarpum isolates over the range of soil drying.

Analysis of soil whole- and inner-microaggregate bacterial communities.

Although soil structure largely determines energy flows and the distribution and composition of soil microhabitats, little is known about how microbial community composition is influenced by soil structural characteristics and organic matter compartmentalization dynamics. A UV irradiation-based procedure was developed to specifically isolate inner-microaggregate microbial communities, thus providing the means to analyze these communities in relation to their environment. Whole- and inner-microaggregate fractions of undisturbed soil and soils reclaimed after disturbance by surface coal mining were analyzed using 16S rDNA terminal restriction fragment polymorphism (T-RFLP) and sequence analyses to determine salient bacterial community structural characteristics. We hypothesized that inner-microaggregate environments select for definable microbial communities and that, due to their sequestered environment, inner-microaggregate communities would not be significantly impacted by disturbance. However, T-RFLP analysis indicated distinct differences between bacterial populations of inner-microaggregates of undisturbed and reclaimed soils. While both undisturbed and reclaimed inner-microaggregate bacterial communities were found dominated by Actinobacteria, undisturbed soils contained only Actinobacteridae, while in inner-microaggregates of reclaimed soils Rubrobacteridae predominate. Spatial stratification of division-level lineages within microaggregates was also evidenced, with Proteobacteria clones being prevalent in libraries derived from whole microaggregates. The fractionation methods employed in this study therefore represent a valuable tool for defining relationships between biodiversity and soil structure.

Population variation in the mycorrhizal fungus Glomus mosseae: breadth of environmental tolerance

Responses to a range of environmental conditions were examined and compared in three populations of Glomus mosseae from dissimilar habitats to assess phenotypic plasticity. The G. mosseae populations were observed (1) in three soils with different physical-chemical characteristics, (2) at three different levels of soil moisture and (3) under three temperature regimes. Using Melilotus officinalis as host, measurements were made of mycorrhiza formed, spore production and influence of the endophytes on host biomass production and shoot phosphorus concentration. The three G. mosseae populations were found to vary significantly in their responses under different environmental conditions and in their breadth of tolerance to environmental factors. Results were interpreted as indicating that phenotypic plasticity is variable among populations of G. mosseae but not sufficient to explain the wide distribution of this fungus.

Spatial and temporal variability of bacterial 16S rDNA-based T-RFLP patterns derived from soil of two Wyoming grassland ecosystems

Abstract Spatial and temporal variability of soil bacterial 16S rDNA terminal restriction fragment (TRF) size variation was evaluated in a homogeneous grassland (HG) dominated by the turf-forming grass Bouteloua gracilis and in a shrubland (SL) dominated by Artemisia tridentata (Wyoming big sagebrush). Temporal variability was also evaluated on the HG site over a growing season. No trends toward dissimilarity were detected with temporal (180 days) or spatial (up to 100 m) distance in the HG system. Terminal-restriction fragment length polymorphism (T-RFLP) profiles of the SL site exhibited pronounced small-scale spatial variability (<70 cm), although spatial analysis indicated weak spatial autocorrelation to distances greater than 36 cm. While shrub-induced nutrient localization was shown to significantly influence T-RFLP profiles, very little of the variability could be accounted for on the basis of spatial characteristics, suggesting that soil bacterial 16S rDNA composition of this site is predominantly controlled at scales other than those measured. Average dissimilarity values differed greatly between the two sites (0.27 and 0.59 for HG and SL sites, respectively). These results suggest that plant community structure strongly influences bacterial community composition in these semiarid ecosystems, highlighting the importance of considering spatial variability when designing field studies related to bacterial diversity in ecosystems having patchy or heterogeneous plant cover.

Fungal presence in paired cultivated and uncultivated soils in central Iowa, USA

Abstract Amounts of fungal biomass in adjacent cultivated and uncultivated soils in central Iowa were estimated and compared by quantifying soil ergosterol concentrations and lengths of fungal hyphae present. Both indices of fungal biomass, with one exception, indicated that there was at least twice as much fungal biomass in uncultivated soil as in cultivated soil. Levels of microbial biomass carbon in uncultivated soils were also determined to be at least twice that in cultivated soils. Data collected in this study indicate that fungi may be more significantly affected by agricultural soil management practices than other components of the soil microbial community. For two of the soils examined, calculated estimates denote that fungal biomass carbon represented approximately 20% of the total microbial biomass carbon in cultivated soil and about 33% of the microbial biomass carbon in uncultivated soil. Results of this study indicate that conventional agricultural practices result in a significant reduction of fungal biomass production in soil. Implications of differences in fungal biomass between the soils are discussed.

MYCORRHIZAL FUNGI ASSOCIATED WITH BOUTELOUA AND AGROPYRON IN WYOMING SAGEBRUSH-GRASSLANDS

The spore populations of vesicular-arbuscular (VA) mycorrhizal fungi at four sagebrushgrassland sites in Wyoming were examined, characterized, and quantified. The sampled soils contained 232-696 spores per 100 g dry soil and three to six species of VA mycorrhizal fungi (Glomus fasciculatus, G. macrocarpus, G. microcarpus, G. mosseae, Entrophospora infrequens, and an unidentified form). Community composition and relative prominence (frequency, density) of the six species were related both to geographic location of the site and abiotic factors and to identity of the plant symbiont. Environmental influence on spore populations is discussed.