Keitt, T.H., Addis, C., Mitchell, D., Salas, A. & Hawkes, C.V. Climate change, microbes, and soil carbon cycling.
Climate Change and Microbial Ecology: Current and Future Trends (2016).
Publisher's VersionAbstractMicrobial responses to climate change will partly control the balance of soil carbon storage and loss under future temperature and precipitation conditions. We propose four classes of response mechanisms that can allow for a more general understanding of microbial climate responses. We further explore how a subset of these mechanisms results in microbial responses to climate change using simulation modeling. Specifically, we incorporate soil moisture sensitivity into two current enzyme-driven models of soil carbon cycling and find that moisture has large effects on predictions for soil carbon and microbial pools. Empirical efforts to distinguish among response mechanisms will facilitate our ability to further develop models with improved accuracy.
Lovell, J.T., et al. Promises and challenges of eco-physiological genomics in the field: tests of drought responses in switchgrass.
Plant Physiology pp. 00545.2016 (2016).
Publisher's VersionAbstractIdentifying the physiological and genetic basis of stress tolerance in plants has proven to be critical to understanding adaptation in both agricultural and natural systems. However, many discoveries were initially made in the controlled conditions of greenhouses or laboratories, not in the field. To test the comparability of drought responses across field and greenhouse environments, we undertook three independent experiments using the switchgrass reference genotype Alamo AP13. We analyzed physiological and gene-expression variation across four locations, two sampling times and three years. Relatively similar physiological responses and expression coefficients of variation across experiments masked highly dissimilar gene expression responses to drought. Critically, a drought experiment utilizing small pots in the greenhouse elicited nearly identical physiological changes as an experiment conducted in the field, but an order of magnitude more differentially expressed genes. However, we were able to define a suite of several hundred genes that were differentially expressed in each experiment. This list was strongly enriched in photosynthesis, water status and reactive oxygen species responsive genes. The strong across-experiment correlations between physiological plasticity-but not differential gene expression-highlight the complex and diverse genetic mechanisms that can produce phenotypically similar responses to various soil water deficits.
Kim, S., Williams, A., Kiniry, J.R. & Hawkes, C.V. Simulating diverse native C 4 perennial grasses with varying rainfall.
Journal of Arid Environments 134, 97-103 (2016).
Publisher's VersionAbstractRainfall is recognized as a major factor affecting the rate of plant growth development. The impact of changes in amount and variability of rainfall on growth and production of different forage grasses needs to be quantified to determine how climate change can impact rangelands. Comparative studies to evaluate the growth of several perennial forage species at different rainfall rates will provide useful information by identifying forage management strategies under various rainfall scenarios. In this study, the combination of rainfall changes and soil types on the plant growth of 10 perennial forage species was investigated with both the experimental methods, using rainout shelters, and with the numerical methods using the plant growth simulation model, ALMANAC. Overall, most species significantly increased basal diameter and height as rainfall increased. Like measured volume, simulated yields for all species generally increased as rainfall increased. But, large volume and yield increases were only observed between 350 and 850 mm/yr. Simulating all species growing together competing agrees relatively well with observed plant volumes at low rainfall treatment, while simulating all species growing separately was slightly biased towards overestimation on low rainfall effect. Both simulations agree relatively well with observed plant volume at high rainfall treatment.
Kivlin, S.N. & Hawkes, C.V. Temporal and spatial variation of soil bacteria richness, composition, and function in a Neotropical rainforest.
PloS ONE 11, 7, e0159131 (2016).
Publisher's VersionAbstractThe high diversity of tree species has traditionally been considered an important controller of belowground processes in tropical rainforests. However, soil water availability and resources are also primary regulators of soil bacteria in many ecosystems. Separating the effects of these biotic and abiotic factors in the tropics is challenging because of their high spatial and temporal heterogeneity. To determine the drivers of tropical soil bacteria, we examined tree species effects using experimental tree monocultures and secondary forests at La Selva Biological Station in Costa Rica. A randomized block design captured spatial variation and we sampled at four dates across two years to assess temporal variation. We measured bacteria richness, phylogenetic diversity, community composition, biomass, and functional potential. All bacteria parameters varied significantly across dates. In addition, bacteria richness and phylogenetic diversity were affected by the interaction of vegetation type and date, whereas bacteria community composition was affected by the interaction of vegetation type and block. Shifts in bacteria community richness and composition were unrelated to shifts in enzyme function, suggesting physiological overlap among taxa. Based on the observed temporal and spatial heterogeneity, our understanding of tropical soil bacteria will benefit from additional work to determine the optimal temporal and spatial scales for sampling. Understanding spatial and temporal variation will facilitate prediction of how tropical soil microbes will respond to future environmental change.
Averill, C. & Hawkes, C.V. Ectomycorrhizal fungi slow soil carbon cycling.
Ecology Letters 19, 8, 937-947 (2016).
Publisher's VersionAbstractRespiration of soil organic carbon is one of the largest fluxes of CO2 on earth. Understanding the processes that regulate soil respiration is critical for predicting future climate. Recent work has suggested that soil carbon respiration may be reduced by competition for nitrogen between symbiotic ectomycorrhizal fungi that associate with plant roots and free-living microbial decomposers, which is consistent with increased soil carbon storage in ectomycorrhizal ecosystems globally. However, experimental tests of the mycorrhizal competition hypothesis are lacking. Here we show that ectomycorrhizal roots and hyphae decrease soil carbon respiration rates by up to 67% under field conditions in two separate field exclusion experiments, and this likely occurs via competition for soil nitrogen, an effect larger than 2 °C soil warming. These findings support mycorrhizal competition for nitrogen as an independent driver of soil carbon balance and demonstrate the need to understand microbial community interactions to predict ecosystem feedbacks to global climate.
Kivlin, S.N. & Hawkes, C.V. Tree species, spatial heterogeneity, and seasonality drive soil fungal abundance, richness, and composition in Neotropical rainforests.
Environmental Microbiology n/a - n/a (2016).
Publisher's VersionAbstractTropical ecosystems remain poorly understood and this is particularly true for belowground soil fungi. Soil fungi may respond to plant identity when, for example, plants differentially allocate resources belowground. However, spatial and temporal heterogeneity in factors such as plant inputs, moisture, or nutrients can also affect fungal communities and obscure our ability to detect plant effects in single time point studies or within diverse forests. To address this, we sampled replicated monocultures of four tree species and secondary forest controls sampled in the drier and wetter seasons over two years. Fungal community composition was primarily related to vegetation type and spatial heterogeneity in the effects of vegetation type, with increasing divergence partly reflecting greater differences in soil pH and soil moisture. Across wetter vs. drier dates, fungi were 7% less diverse, but up to four-fold more abundant. The combined effects of tree species and seasonality suggest that predicted losses of tropical tree diversity and intensification of drought have the potential to cascade belowground to affect both diversity and abundance of tropical soil fungi. This article is protected by copyright. All rights reserved.
Giauque, H. & Hawkes, C.V. Historical and current climate drive spatial and temporal patterns in fungal endophyte diversity.
Fungal Ecology 20, 108–114 (2016).
Publisher's VersionAbstractHorizontally-transmitted foliar endophytic fungi can moderate plant tolerance to abiotic and biotic stress. Previous studies have found correlations between climate and endophyte beta diversity, but were unable to clearly separate drivers related to long-term climate, annual weather, and host plants. To address this, we characterized endophyte communities in the perennial C4 grass, Panicum hallii, across a precipitation gradient in central Texas over 3 years. A total of 65 unique leaf endophytes were isolated and identified based on ITS and LSU regions of rDNA. Mean annual rainfall and temperature were the primary drivers of endophyte richness and community composition, followed by annual weather conditions. In contrast, little explanatory value was provided by plant host traits, vegetation structure, or spatial factors. The importance of historical climate and annual weather in endophyte distributions suggests that species sort by environment and are likely to be affected by future climate change.
Averill, C., Waring, B.G. & Hawkes, C.V. Historical precipitation predictably alters the shape and magnitude of microbial functional response to soil moisture.
Global Change Biology 22, 1957-1964 (2016).
Publisher's VersionAbstractSoil moisture constrains the activity of decomposer soil microorganisms, and in turn the rate at which soil carbon returns to the atmosphere. While increases in soil moisture are generally associated with increased microbial activity, historical climate may constrain current microbial responses to moisture. However, it is not known if variation in the shape and magnitude of microbial functional responses to soil moisture can be predicted from historical climate at regional scales. To address this problem, we measured soil enzyme activity at 12 sites across a broad climate gradient spanning 442–887 mm mean annual precipitation. Measurements were made eight times over 21 months to maximize sampling during different moisture conditions. We then fit saturating functions of enzyme activity to soil moisture and extracted half saturation and maximum activity parameter values from model fits. We found that 50% of the variation in maximum activity parameters across sites could be predicted by 30-year mean annual precipitation, an indicator of historical climate, and that the effect is independent of variation in temperature, soil texture, or soil carbon concentration. Based on this finding, we suggest that variation in the shape and magnitude of soil microbial response to soil moisture due to historical climate may be remarkably predictable at regional scales, and this approach may extend to other systems. If historical contingencies on microbial activities prove to be persistent in the face of environmental change, this approach also provides a framework for incorporating historical climate effects into biogeochemical models simulating future global change scenarios.
Sikes, B.A., Hawkes, C.V. & Fukami, T. Plant and root-endophyte assembly history: interactive effects on native and exotic plants.
Ecology 97, 484-493 (2016).
Publisher's VersionAbstractDifferences in the arrival timing of plants and soil biota may result in different plant communities through priority effects, potentially affecting the success of native vs. exotic plants, but experimental evidence is largely lacking. We conducted a greenhouse experiment to investigate whether the assembly history of plants and fungal root endophytes could interact to influence plant emergence and biomass. We introduced a grass species and eight fungal species from one of three land-use types (undisturbed, disturbed, or pasture sites in a Florida scrubland) in factorial combinations. We then introduced all plants and fungi from the other land-use types 2 weeks later. Plant emergence was monitored for 6 months, and final plant biomass and fungal species composition assessed. The emergence and growth of the exotic Melinis repens and the native Schizacharyium niveum were affected negatively when introduced early with their “home” fungi, but early introduction of a different plant species or fungi from a different site type eliminated these negative effects, providing evidence for interactive priority effects. Interactive effects of plant and fungal arrival history may be an overlooked determinant of plant community structure and may provide an effective management tool to inhibit biological invasion and aid ecosystem restoration.