R. David Evans

The role of ammonia volatilization in controlling the natural 15N abundance of a grazed grassland

Although the variation in natural 15N abundance in plants and soils is well characterized, mechanisms controlling N isotopic composition of organic matter are still poorly understood. The primary goal of this study was to examine the role of NH3 volatilization from ungulate urine patches in determining 15N abundance in grassland plants and soil in Yellowstone National Park. We additionally used isotopic measurements to explore the pathways that plants in urine patches take up N. Plant, soil, and volatilized NH3δ15N were measured on grassland plots for 10 days following the addition of simulated urine. Simulated urine increased 15N of roots and soil and reduced 15N of shoots. Soil enrichment was due to the volatilization of isotopically light NH3. Acid-trapped NH3δ15N increased from −28‰ (day 1) to −0.3‰ (day 10), and was lighter than the original urea-N added (1.2‰). A mass balance analysis of urea-derived N assimilated by plants indicated that most of the N taken up by plants was in the form of ammonium through roots. However, isotope data also showed that shoots directly absorbed 15N – depleted NH3-N that was volatilized from simulated urine patches. These results indicate that NH3 volatilization from urine patches enriches grassland soil with 15N and shoots are a sink for volatilized NH3, which likely leads to accelerated cycling of excreted N back to herbivores.

Common bacterial responses in six ecosystems exposed to 10 years of elevated atmospheric carbon dioxide

Six terrestrial ecosystems in the USA were exposed to elevated atmospheric CO(2) in single or multifactorial experiments for more than a decade to assess potential impacts. We retrospectively assessed soil bacterial community responses in all six-field experiments and found ecosystem-specific and common patterns of soil bacterial community response to elevated CO(2) . Soil bacterial composition differed greatly across the six ecosystems. No common effect of elevated atmospheric CO(2) on bacterial biomass, richness and community composition across all of the ecosystems was identified, although significant responses were detected in individual ecosystems. The most striking common trend across the sites was a decrease of up to 3.5-fold in the relative abundance of Acidobacteria Group 1 bacteria in soils exposed to elevated CO(2) or other climate factors. The Acidobacteria Group 1 response observed in exploratory 16S rRNA gene clone library surveys was validated in one ecosystem by 100-fold deeper sequencing and semi-quantitative PCR assays. Collectively, the 16S rRNA gene sequencing approach revealed influences of elevated CO(2) on multiple ecosystems. Although few common trends across the ecosystems were detected in the small surveys, the trends may be harbingers of more substantive changes in less abundant, more sensitive taxa that can only be detected by deeper surveys. Representative bacterial 16S rRNA gene clone sequences were deposited in GenBank with Accession No. JQ366086–JQ387568.

Responses of soil cellulolytic fungal communities to elevated atmospheric CO2 are complex and variable across five ecosystems

Elevated atmospheric CO(2) generally increases plant productivity and subsequently increases the availability of cellulose in soil to microbial decomposers. As key cellulose degraders, soil fungi are likely to be one of the most impacted and responsive microbial groups to elevated atmospheric CO(2). To investigate the impacts of ecosystem type and elevated atmospheric CO(2) on cellulolytic fungal communities, we sequenced 10,677 cbhI gene fragments encoding the catalytic subunit of cellobiohydrolase I, across five distinct terrestrial ecosystem experiments after a decade of exposure to elevated CO(2). The cbhI composition of each ecosystem was distinct, as supported by weighted Unifrac analyses (all P-values; < 0.001), with few operational taxonomic units (OTUs) being shared across ecosystems. Using a 114-member cbhI sequence database compiled from known fungi, less than 1% of the environmental sequences could be classified at the family level indicating that cellulolytic fungi in situ are likely dominated by novel fungi or known fungi that are not yet recognized as cellulose degraders. Shifts in fungal cbhI composition and richness that were correlated with elevated CO(2) exposure varied across the ecosystems. In aspen plantation and desert creosote bush soils, cbhI gene richness was significantly higher after exposure to elevated CO(2) (550 µmol mol(-1)) than under ambient CO(2) (360 µmol mol(-1) CO(2)). In contrast, while the richness was not altered, the relative abundance of dominant OTUs in desert soil crusts was significantly shifted. This suggests that responses are complex, vary across different ecosystems and, in at least one case, are OTU-specific. Collectively, our results document the complexity of cellulolytic fungal communities in multiple terrestrial ecosystems and the variability of their responses to long-term exposure to elevated atmospheric CO(2).

Dryland biological soil crust cyanobacteria show unexpected decreases in abundance under long‐term elevated CO2

Biological soil crusts (biocrusts) cover soil surfaces in many drylands globally. The impacts of 10 years of elevated atmospheric CO2 on the cyanobacteria in biocrusts of an arid shrubland were examined at a large manipulated experiment in Nevada, USA. Cyanobacteria-specific quantitative PCR surveys of cyanobacteria small-subunit (SSU) rRNA genes suggested a reduction in biocrust cyanobacterial biomass in the elevated CO2 treatment relative to the ambient controls. Additionally, SSU rRNA gene libraries and shotgun metagenomes showed reduced representation of cyanobacteria in the total microbial community. Taxonomic composition of the cyanobacteria was similar under ambient and elevated CO2 conditions, indicating the decline was manifest across multiple cyanobacterial lineages. Recruitment of cyanobacteria sequences from replicate shotgun metagenomes to cyanobacterial genomes representing major biocrust orders also suggested decreased abundance of cyanobacteria sequences across the majority of genomes tested. Functional assignment of cyanobacteria-related shotgun metagenome sequences indicated that four subsystem categories, three related to oxidative stress, were differentially abundant in relation to the elevated CO2 treatment. Taken together, these results suggest that elevated CO2 affected a generalized decrease in cyanobacteria in the biocrusts and may have favoured cyanobacteria with altered gene inventories for coping with oxidative stress.

BioEarth: Envisioning and developing a new regional earth system model to inform natural and agricultural resource management

As managers of agricultural and natural resources are confronted with uncertainties in global change impacts, the complexities associated with the interconnected cycling of nitrogen, carbon, and water present daunting management challenges. Existing models provide detailed information on specific sub-systems (e.g., land, air, water, and economics). An increasing awareness of the unintended consequences of management decisions resulting from interconnectedness of these sub-systems, however, necessitates coupled regional earth system models (EaSMs). Decision makers’ needs and priorities can be integrated into the model design and development processes to enhance decision-making relevance and “usability” of EaSMs. BioEarth is a research initiative currently under development with a focus on the U.S. Pacific Northwest region that explores the coupling of multiple stand-alone EaSMs to generate usable information for resource decision-making. Direct engagement between model developers and non-academic stakeholders involved in resource and environmental management decisions throughout the model development process is a critical component of this effort. BioEarth utilizes a bottom-up approach for its land surface model that preserves fine spatial-scale sensitivities and lateral hydrologic connectivity, which makes it unique among many regional EaSMs. This paper describes the BioEarth initiative and highlights opportunities and challenges associated with coupling multiple stand-alone models to generate usable information for agricultural and natural resource decision-making.

Linking community and ecosystem development on Mount St. Helens

In the two decades following the 1980 eruption of Mount St. Helens in Washington State, the N2-fixing colonizer Lupinus lepidus is associated with striking heterogeneity in plant community and soil development. We report on differences in nutrient availability and plant tissue chemistry between older, dense patches (core) of L. lepidus and more recently established low density patches (edge). In addition, we conducted a factorial nitrogen and phosphorus fertilization experiment in core patches to examine the degree of N and P limitation in early primary succession. We found that there were no significant differences in N or P availability between core and edge L. lepidus patches during the dry summer months, although nutrient availability is very low across the landscape. In the high density patches we found lower tissue N content and higher fiber content in L. lepidus tissue than in the younger edge patches. The addition of nutrients substantially altered plant community composition, with N addition causing an increase in other forb biomass and a corresponding competition-induced decline in L. lepidus biomass. The majority of the positive biomass response came from Hypochaeris radicata. In the second year of the fertilization experiment, the addition of N significantly increased total community biomass while L. lepidus biomass declined by more than 50%. The response of every species other than L. lepidus to N additions suggests that N may be the macronutrient most limiting plant production on Mount St. Helens but that the gains in productivity were somewhat offset by a decline of the dominant species. By the third year of the experiment, L. lepidus began to increase in abundance with P addition. This result suggests co-limitation of the community by N and P.

Applications of Stable Isotope Measurements for Early‐Warning Detection of Ecological Change

Publisher Summary This chapter describes the rationale and framework for stable isotope monitoring to assess ecological condition and change at ecosystem to global scales. The isotope ratios of compounds in aquatic, terrestrial, and atmospheric environments are very sensitive to changes in ecological processes. As such, isotope measurements may serve as an early-warning signal of ecological changes related to ecosystem functions. This unique application and role for stable isotope monitoring can greatly assist management efforts and inform environmental policy. This chapter goes on to propose a specific framework for developing an isotope-monitoring network and the spatial modeling necessary to detect and understand ecological change at a continental scale. The framework is based on isotopic measurements of atmospheric inputs, ecosystem outputs, changes between inputs and outputs as elements are cycled within ecosystems, and sentinel organisms as integrators and indicators of ecological change. Such a framework has the capacity to provide unique insight into how climate and land-use changes and associated biotic and abiotic disturbances impact ecological functioning and connectivity across large regions to continents.

Analysis of low-concentration gas samples with continuous-flow isotope ratio mass spectrometry: eliminating sources of contamination to achieve high precision.

Developments in continuous-flow isotope ratio mass spectrometry have made possible the rapid analysis of delta13C in CO2 of small-volume gas samples with precisions of < or = 0.1 per thousand. Prior research has validated the integrity of septum-capped vials for collection and short-term storage of gas samples. However, there has been little investigation into the sources of contamination during the preparation and analysis of low-concentration gas samples. In this study we determined (1) sources of contamination on a Gasbench II, (2) developed an analytical procedure to reduce contamination, and (3) identified an efficient, precise method for introducing sample gas into vials. We investigated three vial-filling procedures: (1) automated flush-fill (AFF), (2) vacuum back-fill (VBF), and (3) hand-fill (HF). Treatments were evaluated based on the time required for preparation, observed contamination, and multi-vial precision. The worst-case observed contamination was 4.5% of sample volume. Our empirical estimate showed that this level of contamination results in an error of 1.7 per thousand for samples with near-ambient CO2 concentrations and isotopic values that followed a high-concentration carbonate reference with an isotope ratio of -47 per thousand (IAEA-CO-9). This carry-over contamination on the Gasbench can be reduced by placing a helium-filled vial between the standard and the succeeding sample or by ignoring the first two of five sample peaks generated by each analysis. High-precision (SD < or = 0.1 per thousand) results with no detectable room-air contamination were observed for AFF and VBF treatments. In contrast, the precision of HF treatments was lower (SD > or = 0.2 per thousand). VBF was optimal for the preparation of gas samples, as it yielded faster throughput at similar precision to AFF.

Relationships between the El Niño–Southern Oscillation, precipitation, and nitrogen wet deposition rates in the contiguous United States

Human activities have significantly increased reactive nitrogen (N) in the environment, leading to adverse effects on various ecosystems. We used 1979-2012 seasonal inorganic N wet deposition data from the National Atmospheric Deposition Program to evaluate the relationship between the El Nino Southern Oscillation (ENSO) and N wet deposition in the contiguous U.S. The correlations between precipitation and inorganic N wet deposition were the strongest and most spatially extensive during winter; up to 62% and 53% of the 2- to 6-year variations of precipitation and N wet deposition rates, respectively, in the Rocky Mountains, along the coast of the Gulf of Mexico, and near the Great Lakes can be explained by variation in the NINO3.4 climate index, which was used as a measure of ENSO activity. During El Nino winters, precipitation and N wet deposition rates were higher than normal in the southern U.S., while La Nina events brought higher precipitation and N wet deposition to the Rocky Mountains and Great Lakes regions. Wintertime N wet deposition correlations held through springtime in the Great Lakes and the Northeast; however, correlations between NINO3.4 and precipitation were not significant at most sites, suggesting factors besides precipitation amount contributed to the 2- to 6-year variation of N wet deposition in these regions. As the frequency, strength, and types of ENSO change in the future, inter-annual variability of N wet deposition will be affected, indirectly affecting spatial distribution of dry N deposition and potentially changing the overall spatial patterns of N deposition.