Richard A. Gill

Nonlinear grassland responses to past and future atmospheric CO2

Carbon sequestration in soil organic matter may moderate increases in atmospheric CO2 concentrations (Ca) as Ca increases to more than 500 µmol mol-1 this century from interglacial levels of less than 200 µmol mol-1 (refs 1–6). However, such carbon storage depends on feedbacks between plant responses to Ca and nutrient availability. Here we present evidence that soil carbon storage and nitrogen cycling in a grassland ecosystem are much more responsive to increases in past Ca than to those forecast for the coming century. Along a continuous gradient of 200 to 550 µmol mol-1 (refs 9, 10), increased Ca promoted higher photosynthetic rates and altered plant tissue chemistry. Soil carbon was lost at subambient Ca, but was unchanged at elevated Ca where losses of old soil carbon offset increases in new carbon. Along the experimental gradient in Ca there was a nonlinear, threefold decrease in nitrogen availability. The differences in sensitivity of carbon storage to historical and future Ca and increased nutrient limitation suggest that the passive sequestration of carbon in soils may have been important historically, but the ability of soils to continue as sinks is limited.

Relationship Between Root Biomass and Soil Organic Matter Pools in the Shortgrass Steppe of Eastern Colorado

ABSTRACT We examined the distribution of soil organic carbon (SOC) fractions and roots with depth to improve our understanding of belowground carbon dynamics in the shortgrass steppe of northern Colorado. Weaver and others (1935) found that the surface 15 cm of soil contained over 70% of the total roots found in a tallgrass prairie soil profile, while only accounting for 40% of the profile soil organic matter. We asked whether the relationship between roots and SOC that Weaver and others (1935) found in the tallgrass prairie was also found in the shortgrass steppe. Weaver and others (1935) suggested that the dissimilarity between belowground biomass and SOC with depth is the result of variability in decomposition rates. In an effort to determine whether patterns of SOC are the result of short-term plant input patterns or decomposition, we measured the 14C content of potentially mineralizable C and particulate organic matter (POM) C ten years after pulse labeling shortgrass steppe vegetation. We also estimated the mass specific decomposition rate constant (kPOM) for POM C through a shortgrass steppe soil profile. We found that the distribution of roots and SOM in the shortgrass steppe were similar to those observed in tallgrass prairie (Weaver and others 1935), with a higher proportion of total root biomass in the surface soils than total soil organic matter. Fifty-seven percent of root biomass was found in the surface 15-cm, while this same soil layer contained 23 percent of profile soil organic C. We measured the highest accumulation of 14C at the soil surface (12.0 ng 14C·m-2·cm-1 depth), with the least accumulation from 75-100 cm (0.724 ng 14C·m-2·cm-1 depth). The highest values of potentially mineralizable C were at the soil surface, with no significant differences in total mineralizable C among the 10-100 cm soil depths. The contribution of POM C to total C reached a profile minimum at the 15-20 cm depth increment, with profile maxima in the surface 5 cm and from 75-100 cm. We estimated that the proportion of particulate organic matter lost annually (kPOM) reached a profile maximum of 0.097 yr-1 within the 10-15 cm depth increment. The 75-100 cm depth increment had the lowest kPOM value at 0.058 yr-1. Thus, within the same physical fraction of SOC, decomposition rates vary with depth by nearly twofold. This pattern of high decomposition rates from 10-15 cm with lower decomposition rates at the soil surface and deeper in the soil profile may be the result of higher water availability in sub-surface soils in the shortgrass steppe.

POTENTIAL NITROGEN CONSTRAINTS ON SOIL CARBON SEQUESTRATION UNDER LOW AND ELEVATED ATMOSPHERIC CO2

The interaction between nitrogen cycling and carbon sequestration is critical in predicting the consequences of anthropogenic increases in atmospheric CO2 (hereafter, Ca). The progressive N limitation (PNL) theory predicts that carbon sequestration in plants and soils with rising Ca may be constrained by the availability of nitrogen in many ecosystems. Here we report on the interaction between C and N dynamics during a four-year field experiment in which an intact C3/C4 grassland was exposed to a gradient in Ca from 200 to 560 micromol/mol. There were strong species effects on decomposition dynamics, with C loss positively correlated and N mineralization negatively correlated with Ca for litter of the C3 forb Solanum dimidiatum, whereas decomposition of litter from the C4 grass Bothriochloa ischaemum was unresponsive to Ca. Both soil microbial biomass and soil respiration rates exhibited a nonlinear response to Ca, reaching a maximum at approximately 440 micromol/mol Ca. We found a general movement of N out of soil organic matter and into aboveground plant biomass with increased Ca. Within soils we found evidence of C loss from recalcitrant soil C fractions with narrow C:N ratios to more labile soil fractions with broader C:N ratios, potentially due to decreases in N availability. The observed reallocation of N from soil to plants over the last three years of the experiment supports the PNL theory that reductions in N availability with rising Ca could initially be overcome by a transfer of N from low C:N ratio fractions to those with higher C:N ratios. Although the transfer of N allowed plant production to increase with increasing Ca, there was no net soil C sequestration at elevated Ca, presumably because relatively stable C is being decomposed to meet microbial and plant N requirements. Ultimately, if the C gained by increased plant production is rapidly lost through decomposition, the shift in N from older soil organic matter to rapidly decomposing plant tissue may limit net C sequestration with increased plant production.

Longevity and turnover of roots in the shortgrass steppe: influence of diameter and depth

We used minirhizotrons to determine patterns of root longevity andturnover for the perennial bunchgrass Bouteloua gracilisinthe shortgrass steppe of eastern Colorado, USA. We hypothesized that rootlongevity would be partially controlled by root diameter, following previouslyobserved patterns in woody plants. In addition, we hypothesized that rootturnover would be greatest in surface soil horizons and decrease with depth dueto variation in soil moisture availability and temperature. Root longevity wascorrelated with root diameter. Median life span of roots > 0.4mm was approximately 320 days, while roots < 0.2mmhad a median life span of 180 days. There was approximately a 6%decreasein the likelihood of mortality with a 0.10-mm increase inroot diameter, controlling for the effect of depth in the soil profile. Rootlength production and mortality were highest in the upper20 cm of the soil profile and decreased with depth.However,because root length density also decreased with depth, there were nosignificantdifferences in turnover rate of root length among sampling intervals. Turnoverwas approximately 0.86 yr−1 based on root length production,while turnover was 0.35 yr−1 using root length mortality as ameasurement of flux. The imbalance between turnover estimates may be aconsequence of the time the minirhizotrons were in place prior to imaging or mayresult from our lack of over-winter measures of mortality. Our worksuggests that Bouteloua gracilis roots have complex lifehistory strategies, similar to woody species. Some portion of the root systemishighly ephemeral, while slightly larger roots persist much longer. Thesedifferences have implications for belowground carbon and nitrogen cycles in theshortgrass steppe.

N-P co-limitation of primary production and response of arthropods to N and P in early primary succession on Mount St. Helens volcano.

Background The effect of low nutrient availability on plant-consumer interactions during early succession is poorly understood. The low productivity and complexity of primary successional communities are expected to limit diversity and abundance of arthropods, but few studies have examined arthropod responses to enhanced nutrient supply in this context. We investigated the effects of nitrogen (N) and phosphorus (P) addition on plant productivity and arthropod abundance on 24-yr-old soils at Mount St. Helens volcano. Methodology/Principal Findings We measured the relative abundance of eight arthropod orders and five families in plots that received N, P, or no nutrients for 3–5 years. We also measured plant % cover, leaf %N, and plant diversity. Vegetation responded rapidly to N addition but showed a lagged response to P that, combined with evidence of increased N fixation, suggested P-limitation to N availability. After 3 yrs of fertilization, orthopterans (primarily Anabrus simplex (Tettigoniidae) and Melanoplus spp (Acrididae)) showed a striking attraction to P addition plots, while no other taxa responded to fertilization. After 5 yrs of fertilization, orthopteran density in the same plots increased 80%–130% with P addition and 40% with N. Using structural equation modeling, we show that in year 3 orthopteran abundance was associated with a P-mediated increase in plant cover (or correlated increases in resource quality), whereas in year 5 orthopteran density was not related to cover, diversity or plant %N, but rather to unmeasured effects of P, such as its influence on other aspects of resource quality. Conclusions/Significance The marked surprising response to P by orthopterans, combined with a previous observation of P-limitation in lepidopteran herbivores at these sites, suggests that P-mediated effects of food quantity or quality are critical to insect herbivores in this N-P co-limited primary successional system. Our results also support a previous suggestion that the availability of N in these soils is P-limited.

Atmospheric CO2 and soil extracellular enzyme activity: a meta-analysis and CO2 gradient experiment

Rising atmospheric CO2 concentrations can alter carbon and nutrient cycling and microbial processes in terrestrial ecosystems. One of the primary ways microbes interact with soil organic matter is through the production of extracellular enzymes, which break down complex organic molecules and release nutrients into the soil. We conducted a meta-analysis of 34 studies that examined responses in microbial enzyme activity to elevated CO2. We also conducted a field study of soil enzyme activity in a tallgrass-prairie ecosystem growing in sandy loam (lower organic matter content) and clayey soils (higher organic matter content) exposed to a continuous gradient of 250 to 500 ppm CO2. Of the 10 enzyme groups examined in the meta-analysis, including those degrading starch, β-glucan, cellulose, xylan/hemicellulose, lignin, organic P, and organic N, only the activity of one enzyme that degrades the C- and N-containing building blocks of chitin (N-acetyl-glucosaminidase) increased consistently at elevated CO2 by an average of 12.6% (p < 0.05), especially in field studies and in woody ecosystems. In our field study, increasing CO2 from subambient to elevated concentrations reduced the activity of leucine aminopeptidase by 32% in the black clay soil during the peak of the growing season, while β −1,4-N-acetyl-glucosaminidase increased by 44% near the end of the season, indicating increased N limitation with increasing CO2. In the sandy loam soil, alkaline phosphatase activity increased by 42% with CO2 enrichment at the end of the growing season, suggesting CO2-induced phosphorus limitation in these soils. Additionally, a 53–83% decrease in the carbon cycling enzymes cellobiohydrolase, α-glucosidase, and xylosidase activity with increased CO2 was found in July. Our field study shows that soil type can strongly influence how microbial functioning may change with rising CO2 concentrations and that microbial responses associated with C-, N-, and P-cycling are likely to change—and may already have changed—with increasing CO2 under some soil types and conditions. Our meta-analysis revealed that, despite variable enzyme activities with CO2, chitinase activity increased consistently with CO2 across ecosystems.

Biotic resistance and disturbance: rodent consumers regulate post‐fire plant invasions and increase plant community diversity

Biotic resistance and disturbance are fundamental processes influencing plant invasion outcomes; however, the role of consumers in regulating the establishment and spread of plant invaders and how disturbance modifies biotic resistance by consumers is unclear. We document that fire in combination with experimental exclusion of rodent consumers shifted a native desert shrubland to a low-diversity, invasive annual grassland dominated by Bromus tectorum (cheatgrass). In contrast, burned plots with rodents present suppressed invasion by cheatgrass and developed into a more diverse forb community. Rodents created strong biotic resistance to the establishment of aggressive plant invaders likely through seed and seedling predation, which had cascading effects on plant competition and plant community diversity. Fire mediated its positive effects on plant invaders through native plant removal and by decreasing the abundance and diversity of the rodent community. The experimental disruption of plant and consumer-mediated biotic resistance of plant invaders using fire and rodent exclusion treatments provides strong evidence that native plants and rodents are important regulators of plant invasion dynamics and plant biodiversity in our study system. While rodents conferred strong resistance to invasion in our study system, fluctuations in rodent populations due to disturbance and climatic events may provide windows of opportunity for exotic plant species to escape biotic resistance by rodent consumers and initiate invasions.

HydroServer Lite as an open source solution for archiving and sharing environmental data for independent university labs

Abstract Managing, archiving, and sharing large amounts of data are essential tasks in ecological laboratories, and detailed data management plans are now required by major funding agencies. Many independent research labs may lack the technical or financial resources needed to support some of the more comprehensive data management solutions that have become available. In this paper we describe an open-source solution to data management, archiving, and sharing that can be implemented and customized by someone with limited computer programming experience using free software and standardized web services. This software, HydroServer Lite, is a light-weight database and data management web-based application that integrates with and makes data available on a large data sharing network developed by the Consortium of Universities for the Advancement of Hydrologic Sciences, Inc. (CUAHSI). The CUAHSI Hydrologic Information System facilitates data sharing through a network of local HydroServers that are registered with the central registry. Each HydroServer may contain a variety of ecological and climate data, stored in a standardized relational database model. Someone searching for data that are registered in the central registry can query the network by source, location, variable type, and dates. These data can be downloaded from the local HydroServer to a computer in an office or lab where they can be manipulated and analyzed without compromising the data in the archives. We offer this HydroServer Lite case study as a possible solution for independent research laboratories looking for a data management system that requires little technical expertise or initial cost to set up.

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.

Influencing highly religious undergraduate perceptions of evolution: Mormons as a case study

AbstractBackground Students frequently hold an incorrect view of evolution. There are several potential barriers that prevent religious students, specifically, from engaging evolutionary theory in the classroom. This study focuses on two hypothesized barriers on learning evolutionary theory in a highly religious model population, specifically members of The Church of Jesus Christ of Latter-day Saints (LDS or Mormon): (1) religious views stemming from incorrect or inadequate understanding of the Mormon church’s neutral stance on evolution and (2) misunderstanding of the theory of evolution. The LDS population at Brigham Young University provides the ideal setting for studying evolution education among religious individuals in a controlled environment. To ascertain the prevalence and effect of these barriers, we measured the relationship between acceptance of evolution and knowledge of evolution, religiosity, and understanding of religious doctrine on evolution in introductory non-majors biology courses. Additionally, we measured the effect of including a discussion on religious doctrine in the classroom. Students in all sections, except for one control section, were taught a unit on evolution that included a discussion on the neutral LDS doctrine on evolution. Data was gathered pre, post, and longitudinally.ResultsOur data demonstrate a positive relationship between knowledge and acceptance of evolution, a positive relationship between understanding of religious doctrine and acceptance of evolution, and a negative relationship between religiosity and acceptance of evolution. Additionally, when an in-class discussion was held addressing the LDS doctrine on evolution students became more accepting of the principles of evolution.ConclusionsThese data provide compelling evidence that an accurate understanding of their religious doctrines and knowledge of evolution can lead to greater acceptance of the basic concepts of evolution among highly religious students.