David J. Van Horn

Uptake of nutrients and organic C in streams in New York City drinking-water-supply watersheds

Abstract Uptake of nutrients and organic C was measured once annually between 2000 and 2002 in each of 10 streams within the water-supply source areas for New York City. Nutrients (PO43– and NH4+) and organic C (glucose and arabinose) were injected into the streams for 1 to 2 h, and uptake lengths were estimated from the longitudinal declines in downstream concentration relative to that of a conservative tracer. Uptake lengths increased with stream size and were converted to uptake velocities, Vf, to remove scaling effects. Vf s of PO43– and NH4+ varied inversely with the ambient concentration of total dissolved P (TDP) and total dissolved N (TDN), respectively, and were described by a model based on Michaelis–Menten kinetics. However, Vf s of glucose and arabinose were unrelated to the concentrations of any solute. Vf s of PO43–, NH4+, arabinose, and (with less certainty) glucose varied positively with measures of ecosystem metabolism (24-h community respiration and gross primary productivity). Uptake flux (U) of NH4+ also varied positively with ecosystem metabolism, but Us of PO43–, glucose, and arabinose did not. The Vf s of PO43– and NH4+ were positively related to invertebrate species richness and % forest cover, and negatively related to molecular tracer concentrations (polyaromatic hydrocarbons and fecal steroids [PO43–-Vf], fragrance materials [NH4+-Vf]) and population density. Spiraling, as a measure of ecosystem function, was sensitive to human impacts, most clearly through responses to nutrient loadings, but very probably through responses to other impacts as well.

Factors Controlling Soil Microbial Biomass and Bacterial Diversity and Community Composition in a Cold Desert Ecosystem: Role of Geographic Scale

Understanding controls over the distribution of soil bacteria is a fundamental step toward describing soil ecosystems, understanding their functional capabilities, and predicting their responses to environmental change. This study investigated the controls on the biomass, species richness, and community structure and composition of soil bacterial communities in the McMurdo Dry Valleys, Antarctica, at local and regional scales. The goals of the study were to describe the relationships between abiotic characteristics and soil bacteria in this unique, microbially dominated environment, and to test the scale dependence of these relationships in a low complexity ecosystem. Samples were collected from dry mineral soils associated with snow patches, which are a significant source of water in this desert environment, at six sites located in the major basins of the Taylor and Wright Valleys. Samples were analyzed for a suite of characteristics including soil moisture, pH, electrical conductivity, soil organic matter, major nutrients and ions, microbial biomass, 16 S rRNA gene richness, and bacterial community structure and composition. Snow patches created local biogeochemical gradients while inter-basin comparisons encompassed landscape scale gradients enabling comparisons of microbial controls at two distinct spatial scales. At the organic carbon rich, mesic, low elevation sites Acidobacteria and Actinobacteria were prevalent, while Firmicutes and Proteobacteria were dominant at the high elevation, low moisture and biomass sites. Microbial parameters were significantly related with soil water content and edaphic characteristics including soil pH, organic matter, and sulfate. However, the magnitude and even the direction of these relationships varied across basins and the application of mixed effects models revealed evidence of significant contextual effects at local and regional scales. The results highlight the importance of the geographic scale of sampling when determining the controls on soil microbial community characteristics.

Organic matter transport in New York City drinking-water-supply watersheds

Abstract Organic matter (OM) in streams that provide drinking water is a potential energy source for bacterial regrowth in distribution systems and a precursor for disinfection byproducts. Baseflow concentrations of OM were measured over a 3-y period in 60 streams divided evenly between water-supply regions east and west of the Hudson River (EOH or WOH) in New York State. A baseline of OM concentrations in the 2 regions was generated, and land use/cover variables were related to those baseline concentrations. Dissolved organic C (DOC), biodegradable DOC (BDOC), and particulate OM (POM) reflected regional differences in land use and point-source discharges. Three-year mean concentrations for these variables and for total organic C (TOC) were significantly lower in the WOH than in the EOH by factors of 1.5 to 2.3. Size fractionation of POM showed similarities between regions with >70% of particles in the 0.5- to 10-μm size class. DOC made up most of the TOC in both regions, and DOC:POC ratios ranged from 1.7 to 54.4. Stepwise multiple linear regressions revealed that agriculture and forest land uses explained most of the variation in OM concentrations in the WOH, whereas wetlands and point-source discharges, primarily associated with wastewater treatment plants, explained most of the variation in OM concentrations in the EOH. Despite the potential problems from OM for drinking water quality, OM is a natural and important component of stream ecosystems, so its total elimination from watersheds is neither advisable nor possible. Our data from watersheds in the WOH region with high percentages (>97%) of forested land use and from small to mid-sized watersheds in the EOH with no point-source discharges provide lower limits for OM concentrations and targets for best management practices.

Soil microbial responses to increased moisture and organic resources along a salinity gradient in a polar desert.

ABSTRACT Microbial communities in extreme environments often have low diversity and specialized physiologies suggesting a limited resistance to change. The McMurdo Dry Valleys (MDV) are a microbially dominated, extreme ecosystem currently undergoing climate change-induced disturbances, including the melting of massive buried ice, cutting through of permafrost by streams, and warming events. These processes are increasing moisture across the landscape, altering conditions for soil communities by mobilizing nutrients and salts and stimulating autotrophic carbon inputs to soils. The goal of this study was to determine the effects of resource addition (water/organic matter) on the composition and function of microbial communities in the MDV along a natural salinity gradient representing an additional gradient of stress in an already extreme environment. Soil respiration and the activity of carbon-acquiring extracellular enzymes increased significantly (P < 0.05) with the addition of resources at the low- and moderate-salinity sites but not the high-salinity site. The bacterial community composition was altered, with an increase in Proteobacteria and Firmicutes with water and organic matter additions at the low- and moderate-salinity sites and a near dominance of Firmicutes at the high-salinity site. Principal coordinate analyses of all samples using a phylogenetically informed distance matrix (UniFrac) demonstrated discrete clustering among sites (analysis of similarity [ANOSIM], P < 0.05 and R > 0.40) and among most treatments within sites. The results from this experimental work suggest that microbial communities in this environment will undergo rapid change in response to the altered resources resulting from climate change impacts occurring in this region.

Ecoenzymatic Stoichiometry in Relation to Productivity for Freshwater Biofilm and Plankton Communities

The degradation of detrital organic matter and assimilation of carbon (C), nitrogen (N), and phosphorus (P) by heterotrophic microbial communities is mediated by enzymes released into the environment (ecoenzymes). For the attached microbial communities of soils and freshwater sediments, the activities of β-glucosidase, β-N-acetylglucosaminidase, leucine aminopeptidase, and phosphatase show consistent stoichiometric patterns. To determine whether similar constraints apply to planktonic communities, we assembled data from nine studies that include measurements of these enzyme activities along with microbial productivity. By normalizing enzyme activity to productivity, we directly compared the ecoenzymatic stoichiometry of aquatic biofilm and bacterioplankton communities. The relationships between β-glucosidase and α-glucosidase and β-glucosidase and β-N-acetylglucosaminidase were statistically indistinguishable for the two community types, while the relationships between β-glucosidase and phosphatase and β-glucosidase and leucine aminopeptidase significantly differed. For β-glucosidase vs. phosphatase, the differences in slope (biofilm 0.65, plankton 1.05) corresponded with differences in the mean elemental C:P ratio of microbial biomass (60 and 106, respectively). For β-glucosidase vs. leucine aminopeptidase, differences in slope (0.80 and 1.02) did not correspond to differences in the mean elemental C:N of biomass (8.6 and 6.6). β-N-Acetylglucosaminidase activity in biofilms was significantly greater than that of plankton, suggesting that aminosaccharides were a relatively more important N source for biofilms, perhaps because fungi are more abundant. The slopes of β-glucosidase vs. (β-N-acetylglucosaminidase + leucine aminopeptidase) regressions (biofilm 1.07, plankton 0.94) corresponded more closely to the estimated difference in mean biomass C:N. Despite major differences in physical structure and trophic organization, biofilm and plankton communities have similar ecoenzymatic stoichiometry in relation to productivity and biomass composition. These relationships can be integrated into the stoichiometric and metabolic theories of ecology and used to analyze community metabolism in relation to resource constraints.

Environmental controls over bacterial communities in polar desert soils

Productivity-diversity theory has proven informative to many investigations seeking to understand drivers of spatial patterns in biotic communities and relationships between resource availability and community structure documented for a wide variety of taxa. For soil bacteria, availability of organic matter is one such resource known to influence diversity and community structure. Here we describe the influence of environmental gradients on soil bacterial communities of the McMurdo Dry Valleys, Antarctica, a model ecosystem that hosts simple, microbially-dominated foodwebs believed to be primarily structured by abiotic drivers such as water, organic matter, pH, and electrical conductivity. We sampled 48 locations exhibiting orders of magnitude ranges in primary production and soil geochemistry (pH and electrical conductivity) over local and regional scales. Our findings show that environmental gradients imposed by cryptogam productivity and regional variation in geochemistry influence the diversity and structure of soil bacterial communities. Responses of soil bacterial richness to carbon content illustrate a productivity-diversity relationship, while bacterial community structure primarily responds to soil pH and electrical conductivity. This diversity response to resource availability and a community structure response to environmental severity suggests a need for careful consideration of how microbial communities and associated functions may respond to shifting environmental conditions resulting from human activity and climate variability.

Niche and metabolic principles explain patterns of diversity and distribution: theory and a case study with soil bacterial communities

The causes of biodiversity patterns are controversial and elusive due to complex environmental variation, covarying changes in communities, and lack of baseline and null theories to differentiate straightforward causes from more complex mechanisms. To address these limitations, we developed general diversity theory integrating metabolic principles with niche-based community assembly. We evaluated this theory by investigating patterns in the diversity and distribution of soil bacteria taxa across four orders of magnitude variation in spatial scale on an Antarctic mountainside in low complexity, highly oligotrophic soils. Our theory predicts that lower temperatures should reduce taxon niche widths along environmental gradients due to decreasing growth rates, and the changing niche widths should lead to contrasting α- and β-diversity patterns. In accord with the predictions, α-diversity, niche widths and occupancies decreased while β-diversity increased with increasing elevation and decreasing temperature. The theory also successfully predicts a hump-shaped relationship between α-diversity and pH and a negative relationship between α-diversity and salinity. Thus, a few simple principles explained systematic microbial diversity variation along multiple gradients. Such general theory can be used to disentangle baseline effects from more complex effects of temperature and other variables on biodiversity patterns in a variety of ecosystems and organisms.

Characterization of Growing Bacterial Populations in McMurdo Dry Valley Soils through Stable Isotope Probing with 18O-water

Soil microbial communities of the McMurdo Dry Valleys, Antarctica (MDV) contain representatives from at least fourteen bacterial phyla. However, given low rates of microbial activity, it is unclear whether this richness represents functioning rather than dormant members of the community. We used stable isotope probing (SIP) with (18) O-water to determine if microbial populations grow in MDV soils. Changes in the microbial community were characterized in soils amended with H2 (18) O and H2 (18) O-organic matter. Sequencing the 16S rRNA genes of the heavy and light fractions of the bacterial community DNA shows that DNA of microbial populations was labeled with (18) O-water, indicating these micro-organisms grew in the MDV soils. Significant differences existed in the community composition of the heavy and light fractions of the H2 (18) O and H2 (18) O-organic matter amended samples (Anosim P < 0.05 of weighted Unifrac distance). Control samples and the light DNA fraction of the H2 (18) O amended samples were dominated by representatives of the phyla Deinococcus-Thermus, Proteobacteria, Planctomyces, Gemmatimonadetes, Actinobacteria and Acidobacteria, whereas Proteobacteria were more prevalent in the heavy DNA fractions from the H2 (18) O-water and the H2 (18) O-water-organic matter treatments. Our results indicate that SIP with H2 (18) O can be used to distinguish active bacterial populations even in this low organic matter environment.

Bacterial community composition of divergent soil habitats in a polar desert.

Edaphic factors such as pH, organic matter, and salinity are often the most significant drivers of diversity patterns in soil bacterial communities. Desert ecosystems in particular are model locations for examining such relationships as food web complexity is low and the soil environment is biogeochemically heterogeneous. Here, we present the findings from a 16S rRNA gene sequencing approach used to observe the differences in diversity and community composition among three divergent soil habitats of the McMurdo Dry Valleys, Antarctica. Results show that alpha diversity is significantly lowered in high pH soils, which contain higher proportions of the phyla Acidobacteria and Actinobacteria, while mesic soils with higher soil organic carbon (and ammonium) content contain high proportions of Nitrospira, a nitrite-oxidizing bacteria. Taxonomic community resolution also had a significant impact on our conclusions, as pH was the primary predictor of phylum-level diversity, while moisture was the most significant predictor of diversity at the genus level. Predictive power also increased with increasing taxonomic resolution, suggesting a potential increase in niche-based drivers of bacterial community composition at such levels.

Advancing the Food-Energy–Water Nexus: Closing Nutrient Loops in Arid River Corridors

Closing nutrient loops in terrestrial and aquatic ecosystems is integral to achieve resource security in the food-energy-water (FEW) nexus. We performed multiyear (2005-2008), monthly sampling of instream dissolved inorganic nutrient concentrations (NH4-N, NO3-N, soluble reactive phosphorus-SRP) along a ∼ 300-km arid-land river (Rio Grande, NM) and generated nutrient budgets to investigate how the net source/sink behavior of wastewater and irrigated agriculture can be holistically managed to improve water quality and close nutrient loops. Treated wastewater on average contributed over 90% of the instream dissolved inorganic nutrients (101 kg/day NH4-N, 1097 kg/day NO3-N, 656 kg/day SRP). During growing seasons, the irrigation network downstream of wastewater outfalls retained on average 37% of NO3-N and 45% of SRP inputs, with maximum retention exceeding 60% and 80% of NO3-N and SRP inputs, respectively. Accurate quantification of NH4-N retention was hindered by low loading and high variability. Nutrient retention in the irrigation network and instream processes together limited downstream export during growing seasons, with total retention of 33-99% of NO3-N inputs and 45-99% of SRP inputs. From our synoptic analysis, we identify trade-offs associated with wastewater reuse for agriculture within the scope of the FEW nexus and propose strategies for closing nutrient loops in arid-land rivers.