The timing and magnitude of rainfall events are expected to change in future decades, resulting in longer drought periods and larger rainfall events. Although microbial community composition and ...function are both sensitive to changes in rainfall, it is unclear whether this is because taxa adopt strategies that maximise fitness under new regimes. We assessed whether bacteria exhibited phylogenetically conserved ecological strategies in response to drying‐rewetting, and whether these strategies were altered by historical exposure to experimentally intensified rainfall patterns. By clustering relative abundance patterns, we identified three discrete ecological strategies and found that tolerance to drying‐rewetting increased with exposure to intensified rainfall patterns. Changes in strategy were primarily due to changes in community composition, but also to strategy shifts within taxa. These moisture regime‐selected ecological strategies may be predictable from disturbance history, and are likely to be linked to traits that influence the functional potential of microbial communities.
Seasonally frozen ground (SFG) and permafrost underlay approximately half of the land surface in the Northern Hemisphere. It is anticipated that climate warming will degrade both types of frozen ...ground, altering groundwater discharge to streams. While the effects of permafrost degradation on groundwater discharge have been analyzed, quantification of how groundwater discharge in degrading permafrost differs from that in SFG is lacking. This study simulates coupled groundwater and heat transport under freeze‐thaw conditions for four representative hillslopes underlain by either continuous permafrost or SFG and compares groundwater discharge outputs under projected warming scenarios over decadal scales. Model results show that without warming there is more groundwater discharge in hillslopes with SFG than permafrost. After a century of warming, groundwater discharge increases for both kinds of frozen ground, but permafrost experiences a larger increase than SFG. These findings have implications for aquatic ecosystems and prioritizing water resource planning.
Key Points
A coupled groundwater and heat transport model is used to compare discharge in hillslopes with seasonally frozen ground and permafrost
Without warming groundwater discharge is greater in hillslopes with seasonally frozen ground than permafrost
Warming may increase groundwater discharge for both hillslopes with a larger increase for permafrost
Climate models project that precipitation patterns will likely intensify in the future, resulting in increased duration of droughts and increased frequency of large soil rewetting events, which are ...stressful to the microorganisms that drive soil biogeochemical cycling. Historical conditions can affect contemporary microbial responses to environmental factors through the persistence of abiotic changes or through the selection of a more tolerant microbial community. We examined how a history of intensified rainfall would alter microbial functional response to drying and rewetting events, whether this historical legacy was mediated through altered microbial community composition, and how long community and functional legacies persisted under similar conditions. We collected soils from a long-term field manipulation (Rainfall Manipulation Plot Study) in Kansas, USA, where rainfall variability was experimentally amplified. We measured respiration, microbial biomass, fungal:bacterial ratios and bacterial community composition after collecting soils from the field experiment, and after subjecting them to a series of drying–rewetting pulses in the lab. Although rainfall history affected respiration and microbial biomass, the differences between field treatments did not persist throughout our 115-day drying–rewetting incubation. However, soils accustomed to more extreme rainfall did change less in response to lab moisture pulses. In contrast, bacterial community composition did not differ between rainfall manipulation treatments, but became more dissimilar in response to drying–rewetting pulses depending on their previous field conditions. Our results suggest that environmental history can affect contemporary rates of biogeochemical processes both through changes in abiotic drivers and through changes in microbial community structure. However, the extremity of the disturbance and the mechanism through which historical legacies occur may influence how long they persist, which determines the importance of these effects for biogeochemical cycling.
Headwater catchments have a direct impact on the water resources of downstream lowland regions as they supply freshwater in the form of surface runoff and discharging groundwater. Often, these ...mountainous catchments contain expansive permafrost that may alter the natural topographically controlled groundwater flow system. As permafrost could degrade with climate change, it is imperative to understand the effect of permafrost on groundwater flow in headwater catchments. This study characterizes groundwater flow in mountainous headwater catchments and evaluates the effect of permafrost in the context of climate change on groundwater movement using a three‐dimensional, finite element, hydrogeologic model. The model is applied to a representative headwater catchment on the Qinghai‐Tibet Plateau, China. Results from the model simulations indicate that groundwater contributes significantly to streams in the form of baseflow and the majority of groundwater flow is from the shallow aquifer above the permafrost, disrupting the typical topographically controlled flow pattern observed in most permafrost‐free headwater catchments. Under a warming scenario where mean annual surface temperature is increased by 2°C, reducing the areal extent of permafrost in the catchment, groundwater contribution to streamflow may increase three‐fold. These findings suggest that, in headwater catchments, permafrost has a large influence on groundwater flow and stream discharge. Increased annual air temperatures may increase groundwater discharge to streams, which has implications for ecosystem health and the long‐term availability of water resources to downstream regions.
Key Points:
A headwater catchment with permafrost is simulated using a groundwater model
Permafrost exerts a large influence on groundwater flow and stream discharge
With climatic warming there will be increased groundwater discharge to streams
Free-living nitrogen fixation (FLNF) in the rhizosphere, or N fixation by heterotrophic bacteria living on/near root surfaces, is ubiquitous and a significant source of N in some terrestrial systems. ...FLNF is also of interest in crop production as an alternative to chemical fertilizer, potentially reducing production costs and ameliorating negative environmental impacts of fertilizer N additions. Despite this interest, a mechanistic understanding of controls (e.g., carbon, oxygen, nitrogen, and nutrient availability) on FLNF in the rhizosphere is lacking but necessary. FLNF is distinct from and occurs under more diverse and dynamic conditions than symbiotic N fixation; therefore, predicting FLNF rates and understanding controls on FLNF has proven difficult. This has led to large gaps in our understanding of FLNF, and studies aimed at identifying controls on FLNF are needed. Here, we provide a mechanistic overview of FLNF, including how various controls may influence FLNF in the rhizosphere in comparison with symbiotic N fixation occurring in plant nodules where environmental conditions are moderated by the plant. We apply this knowledge to a real-world example, the bioenergy crop switchgrass (
), to provide context of how FLNF may function in a managed system. We also highlight future challenges to assessing FLNF and understanding how FLNF functions in the environment and significantly contributes to plant N availability and productivity.
Soil moisture is a major driver of microbial activity and thus, of the release of carbon (C) into the Earth's atmosphere. Yet, there is no consensus on the relationship between soil moisture and ...microbial respiration, and as a result, moisture response functions are a poorly constrained aspect of C models. In addition, models assume that the response of microbial respiration to moisture is the same for all ecosystems, regardless of climate history, an assumption that many empirical studies have challenged. These gaps in understanding of the microbial respiration response to moisture contribute to uncertainty in model predictions.
We review our understanding of what drives microbial moisture response, highlighting evidence that historical precipitation can influence both responses to moisture and sensitivity to drought. We present two hypotheses, the ‘climate history hypothesis’, where we predict that baseline moisture response functions change as a function of precipitation history, and the ‘drought legacy hypothesis’, in which we suggest that the intensity and frequency of historical drought have shaped microbial communities in ways that will control moisture responses to contemporary drought. Underlying mechanisms include biological selection and filtering of the microbial community by rainfall regimes, which result in microbial traits and trade‐offs that shape function.
We present an integrated modelling and empirical approach for understanding microbial moisture responses and improving models. Standardized measures of moisture response (respiration rate across a range of moistures) and accompanying microbial properties are needed across sites. These data can be incorporated into trait‐based models to produce generalized moisture response functions, which can then be validated and incorporated into conventional and microbially explicit ecosystem models of soil C cycling. Future studies should strive to analyse realistic moisture conditions and consider the role of environmental factors and soil structure in microbial response.
Microbes are the engines that drive C storage and are sensitive to changes in rainfall. A greater understanding of the factors that govern this sensitivity could be a key part of improving predictions of soil C dynamics, climate change and C‐climate feedbacks.
Read the free Plain Language Summary for this article on the Journal blog.
Read the free Plain Language Summary for this article on the Journal blog.
Warming in the Arctic is occurring at twice the rate of the global average, resulting in permafrost thaw and a restructuring of the Arctic hydrologic cycle as indicated by increased stream discharge ...during low‐flow periods. In these cold regions, permafrost thaw is postulated to increase low‐flow discharge, or baseflow, through either: (a) localized increases in groundwater storage and discharge to streams due to increased aquifer transmissivity from thickening of the freeze–thaw layer above permafrost known as the active layer or (b) long‐term increases in regional groundwater circulation via enhancement of groundwater–surface water interactions due to extensive permafrost loss over decades. While increasing baseflow has been observed throughout northern Eurasia, the precise mechanistic causes remain elusive. In this study, we differentiate between where these two subsurface physical mechanisms of baseflow increase are occurring by performing a baseflow recession analysis using daily streamflow records from 1913 to 2003 for 139 stations in northern Eurasia underlain by varying permafrost areal extents. Results indicate that from 1913 to 2003, the majority of catchments underlain by continuous permafrost have an increasing trend in their recession flow intercepts, a proxy for increasing active layer thickness. Alternatively, the majority of catchments underlain by permafrost types that are less spatially extensive (e.g., discontinuous, sporadic, isolated, or no permafrost) have decreasing trends in their recession flow intercepts, indicating that a potential increase in active layer thickness is not the driving factor of baseflow variations in these catchments. This may indicate that in catchments underlain by continuous permafrost, active layer thickening correlates with increases in baseflow, whereas, in other catchments with less extensive permafrost, increases in baseflow may be caused by wholesale permafrost loss and vertical talik expansion that enhances regional groundwater circulation. The results of this work may inform our understanding of the subsurface mechanisms responsible for the changing Arctic hydrologic cycle.
Abstract Objectives The current study investigates the moderating effect of perceived social support on associations between child maltreatment severity and adult trauma symptoms. We extend the ...existing literature by examining the roles of severity of multiple maltreatment types (i.e., sexual, physical, and emotional abuse; physical and emotional neglect) and gender in this process. Methods The sample included 372 newlywed individuals recruited from marriage license records. Participants completed a number of self-report questionnaires measuring the nature and severity of child maltreatment history, perceived social support from friends and family, and trauma-related symptoms. These questionnaires were part of a larger study, investigating marital and intrapersonal functioning. We conducted separate, two-step hierarchical multiple regression models for perceived social support from family and perceived social support from friends. In each of these models, total trauma symptomatology was predicted from each child maltreatment severity variable, perceived social support, and the product of the two variables. In order to examine the role of gender, we conducted separate analyses for women and men. Results As hypothesized, increased severity of several maltreatment types (sexual abuse, emotional abuse, emotional neglect, and physical neglect) predicted greater trauma symptoms for both women and men, and increased physical abuse severity predicted greater trauma symptoms for women. Perceived social support from both family and friends predicted lower trauma symptoms across all levels of maltreatment for men. For women, greater perceived social support from friends, but not from family, predicted decreased trauma symptoms. Finally, among women, perceived social support from family interacted with child maltreatment such that, as the severity of maltreatment (physical and emotional abuse, emotional neglect) increased, the buffering effect of perceived social support from family on trauma symptoms diminished. Conclusions The results of the current study shed new light on the potential for social support to shield individuals against long-term trauma symptoms, and suggest the importance of strengthening perceptions of available social support when working with adult survivors of child maltreatment.
For any enzyme-catalyzed reaction to occur, the corresponding protein-encoding genes and transcripts are necessary prerequisites. Thus, a positive relationship between the abundance of gene or ...transcripts and corresponding process rates is often assumed. To test this assumption, we conducted a meta-analysis of the relationships between gene and/or transcript abundances and corresponding process rates. We identified 415 studies that quantified the abundance of genes or transcripts for enzymes involved in carbon or nitrogen cycling. However, in only 59 of these manuscripts did the authors report both gene or transcript abundance and rates of the appropriate process. We found that within studies there was a significant but weak positive relationship between gene abundance and the corresponding process. Correlations were not strengthened by accounting for habitat type, differences among genes or reaction products versus reactants, suggesting that other ecological and methodological factors may affect the strength of this relationship. Our findings highlight the need for fundamental research on the factors that control transcription, translation and enzyme function in natural systems to better link genomic and transcriptomic data to ecosystem processes.
Climate warming may alter the quantity and timing of groundwater discharge to streams in high alpine watersheds due to changes in the timing of the duration of seasonal freezing in the subsurface and ...snowmelt recharge. It is imperative to understand the effects of seasonal freezing and recharge on groundwater discharge to streams in warming alpine watersheds as streamflow originating from these watersheds is a critical water resource for downstream users. This study evaluates how climate warming may alter groundwater discharge due to changes in seasonally frozen ground and snowmelt using a 2‐D coupled flow and heat transport model with freeze and thaw capabilities for variably saturated media. The model is applied to a representative snowmelt‐dominated watershed in the Rocky Mountains of central Colorado, USA, with snowmelt time series reconstructed from a 12 year data set of hydrometeorological records and satellite‐derived snow covered area. Model analyses indicate that the duration of seasonal freezing in the subsurface controls groundwater discharge to streams, while snowmelt timing controls groundwater discharge to hillslope faces. Climate warming causes changes to subsurface ice content and duration, rerouting groundwater flow paths but not altering the total magnitude of future groundwater discharge outside of the bounds of hydrologic parameter uncertainties. These findings suggest that frozen soil routines play an important role for predicting the future location of groundwater discharge in watersheds underlain by seasonally frozen ground.
Key Points
A 2‐D coupled groundwater and heat transport model is used to evaluate groundwater discharge in an alpine watershed with frozen ground
The timing of subsurface freeze and thaw controls groundwater discharge to streams while snowmelt timing controls groundwater discharge to hillslopes
Warming simulations indicate that frozen soil routines play an important role in delineating future groundwater discharge locations
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