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Big sagebrush (Artemisia tridentata) is a widespread and locally dominant shrub that is a key driver of water fluxes and storage in western North America. There are several recognized subspecies of big sagebrush that occupy different microsites across the landscape according to moisture availability, yet little is known about how these subspecies vary in drought tolerance or water management strategies. We measured diurnal and seasonal (i.e., early-, mid-, and late summer) variation in water status and transport efficiency, transpiration, and stomatal regulations in two subspecies of big sagebrush (A.t. wyomingensis and A.t. vaseyana) at the Reynolds Creek Critical Zone Observatory in southwestern Idaho. We hypothesized that water status, transport efficiency, and stomatal regulations would be greater in A.t. vaseyana compared to A.t. wyomingensis, because the first subspecies occupies wetter microsites. Predawn and midday water potentials were up to 2× more negative in A.t. wyomingensis compared to A.t. vaseyana, and transpiration was up to ~ 4× greater in A.t. vaseyana compared to A.t. wyomingensis. A.t. vaseyana was more isohydric with ~ 1.5× greater stomatal sensitivity to atmospheric vapor pressure deficit and a smaller hydroscape, compared to A.t. wyomingensis. Our data demonstrate that there are fundamental differences in plant–water relations among these subspecies that constitute the vast, but not homogeneous sagebrush landscapes. These important differences can have implications for modeling water fluxes in big sagebrush-dominated communities at shrub to ecosystem scales.

Sagebrush ecosystems consist of different communities of species and subspecies of sagebrush marked by distinct ecotones along elevation gradients, yet few studies have quantified how ecosystem-scale carbon dioxide (net ecosystem exchange, NEE) and water fluxes (evapotranspiration, ET), as well as their environmental drivers, vary among communities dominated by different sub/species of sagebrush at daily and seasonal time scales. To address this knowledge gap, we measured daytime (6 a.m.–6 p.m.) NEE and ET using a tent chamber and associated environmental drivers in three sagebrush communities spanning an elevation gradient of 1 425–2 111 m at the Reynolds Creek Critical Zone Observatory in southwestern Idaho. Daytime NEE and ET were greatest at the highest elevation (snow-dominated) site during the study period except NEE in June. By late summer, NEE declined by > 80% at the lower (rain-dominated) sites but only 50% at the highest site, compared with maximal values in June. In contrast, ET declined ∼95% in late summer compared with June at all three sites. Ecosystem-scale NEE and ET were mainly controlled by soil moisture and vapor pressure deficit at the rain-dominated sites and by deep soil moisture and air temperature at the snow-dominated site. Cumulative (June−August) modeled daytime NEE was greatest at the midelevation site, whereas cumulative daytime ET was greatest at the highest-elevation site. Ecosystem models often assume that sagebrush landscapes are homogeneous and do not differ in fluxes and controls, yet our data demonstrate that there are fundamental differences in CO2 and water fluxes and their controls among different shrub communities that should be accounted for in these models.

Dryland ecosystems play an important role in determining how precipitation anomalies affect terrestrial carbon fluxes at regional to global scales. Thus, to understand how climate change may affect the global carbon cycle, we must also be able to understand and model its effects on dryland vegetation. Dynamic Global Vegetation Models (DGVMs) are an important tool for modeling ecosystem dynamics, but they often struggle to reproduce seasonal patterns of plant productivity. Because the phenological niche of many plant species is linked to both total productivity and competitive interactions with other plants, errors in how process-based models represent phenology hinder our ability to predict climate change impacts. This may be particularly problematic in dryland ecosystems where many species have developed a complex phenology in response to seasonal variability in both moisture and temperature. Here, we examine how uncertainty in key parameters as well as the structure of existing phenology routines affect the ability of a DGVM to match seasonal patterns of leaf area index (LAI) and gross primary productivity (GPP) across a temperature and precipitation gradient. First, we optimized model parameters using a combination of site-level eddy covariance data and remotely-sensed LAI data. Second, we modified the model to include a semi-deciduous phenology type and added flexibility to the representation of grass phenology. While optimizing parameters reduced model bias, the largest gains in model performance were associated with the development of our new representation of phenology. This modified model was able to better capture seasonal patterns of both leaf area index (R2 = 0.75) and gross primary productivity (R2 = 0.84), though its ability to estimate total annual GPP depended on using eddy covariance data for optimization. The new model also resulted in a more realistic outcome of modeled competition between grass and shrubs. These findings demonstrate the importance of improving how DGVMs represent phenology in order to accurately forecast climate change impacts in dryland ecosystems.

Differences in water and carbon fluxes along a climate/elevation gradient within a sagebrush ecosystem are quantified, and inferences are made about climate warming using a network of eddy covariance systems. Sites are located within the Reynolds Creek Critical Zone Observatory in southwestern Idaho, USA, with elevations ranging from 1425 to 2111 m, annual precipitation ranging from 290 to 795 mm and annual temperature ranging from 9.1 to 5.4 °C. Annual gross ecosystem production (GEP) for the sites averaged (± uncertainty) 385 ± 6, 549 ± 19, 684 ± 25, and 818 ± 26 gC m−2 from lowest to highest elevation. Annual net ecosystem production indicated that the sites are carbon sinks with annual uptake typically ranging from 100 ± 10 to 200 ± 30 gC m−2. Exceptions to this are: the lowest elevation site which was carbon neutral (1 ± 16 gC m−2) during a year with a summer rainfall respiration pulse, and the highest elevation site, where carbon uptake dropped to 42 ± 20 gC m−2 during a heavy snow year. Carbon flux and evapotranspiration (ET) peaked about a month earlier at the lower elevation sites, but with limited precipitation, these sites encountered water stress for much of the growing season. Model simulations suggest that climate warming will likely have a negligible impact on annual ET and GEP at lower elevations, but rather shift ET and GEP earlier in the season and prolong the period of water stress. ET and GEP may increase with climate warming at higher elevations where precipitation is above a threshold of about 450 mm.

Pedotransfer functions (PTFs) have been developed to estimate soil fine fraction bulk density (BDFF) using the relationships with soil organic carbon content (SOC) and particle size distribution. Current PTF's overlook the influence of lithology on BDFF estimations. In this study, we examine this influence by developing two PTF's for total bulk density (BDT), which includes both fine (FF) and coarse fragments (CF), using measured percent SOC in soils derived from felsic and mafic lithologies (90 felsic and 52 mafic, 142 total). Our results show that SOC is highly correlated with BDT in soils derived from felsic (R2 value of 0.79, p 2 mm) to determine estimated BDFF. We use an independently collected validation subset of 70 samples to compare our model against 23 published PTFs. When accounting for lithological differences in BDT and adjusting for site-specific CF content, which is highly variable vertically and horizontally within the watershed, we observe substantial improvements (average of 10.05 ± 4.89%) in BDFF estimation and associated errors compared to other PTFs. Findings from our study demonstrate that incorporating both lithology and CF content into site-specific empirical relationships can provide reliable BDFF and consequently soil carbon stock estimates.

There is increasing evidence of environmental change impacts on freshwater ecosystem services especially through land use and climate change. However, little is known about how land conservation could help mitigate adverse water-sustainability impacts. In this paper, we utilized the InVEST tool and the Residual Trends method to assess the joint effects and relative contributions of climate change and land conservation on freshwater ecosystem services in the Portneuf River catchment in Idaho, USA. We developed five hypothesized scenarios regarding gain and loss in the enrollment of Conservation Reserve Program (CRP), the largest agricultural land-retirement program in the U.S., plus riparian buffer and assessed their interactions with climate change. Results suggest that the realized water yield in the Portneuf River catchment would possibly be 56% less due to climate change and 24% less due to the decline of CRP enrollment. On the contrary, if CRP enrollment is promoted by ~30% and riparian buffer protection is implemented, the water supply reduction in the year 2050 could be changed from 56% to 26%, the total phosphorus (TP) and total nitrogen (TN) export would be reduced by 10% and 11%, and the total suspended sediment (TSS) reduced by 17%. This study suggests that increasing implementation of the CRP would likely preserve key freshwater ecosystem services and assist proactive mitigation, especially for semiarid regions vulnerable to changing climate conditions.

Numerous studies have examined bacterial communities in biological soil crusts (BSCs) associated with warm arid to semiarid ecosystems. Few, however, have examined bacterial communities in BSCs associated with cold steppe ecosystems, which often span a wide range of climate conditions and are sensitive to trends predicted by relevant climate models. Here, we utilized Illumina sequencing to examine BSC bacterial communities with respect to climatic gradients (elevation), land management practices (grazing vs. non-grazing), and shrub/intershrub patches in a cold sagebrush steppe ecosystem in southwestern Idaho, United States. Particular attention was paid to shifts in bacterial community structure and composition. BSC bacterial communities, including keystone N-fixing taxa, shifted dramatically with both elevation and shrub-canopy microclimates within elevational zones. BSC cover and BSC cyanobacteria abundance were much higher at lower elevation (warmer and drier) sites and in intershrub areas. Shrub-understory BSCs were significantly associated with several non-cyanobacteria diazotrophic genera, including Mesorhizobium and Allorhizobium-Neorhizobium-Pararhizobium-Rhizobium. High elevation (wetter and colder) sites had distinct, highly diverse, but low-cover BSC communities that were significantly indicated by non-cyanobacterial diazotrophic taxa including families in the order Rhizobiales and the family Frankiaceae. Abiotic soil characteristics, especially pH and ammonium, varied with both elevation and shrub/intershrub level, and were strongly associated with BSC community composition. Functional inference using the PICRUSt pipeline identified shifts in putative N-fixing taxa with respect to both the elevational gradient and the presence/absence of shrub canopy cover. These results add to current understanding of biocrust microbial ecology in cold steppe, serving as a baseline for future mechanistic research.

Despite a multitude of small catchment studies, we lack a deep understanding of how variations in critical zone architecture lead to variations in hydrologic states and fluxes. This study characterizes hydrologic dynamics of 15 catchments of the U.S. Critical Zone Observatory (CZO) network where we hypothesized that our understanding of subsurface structure would illuminate patterns of hydrologic partitioning. The CZOs collect data sets that characterize the physical, chemical, and biological architecture of the subsurface, while also monitoring hydrologic fluxes such as streamflow, precipitation, and evapotranspiration. For the first time, we collate time series of hydrologic variables across the CZO network and begin the process of examining hydrologic signatures across sites. We find that catchments with low baseflow indices and high runoff sensitivity to storage receive most of their precipitation as rain and contain clay‐rich regolith profiles, prominent argillic horizons, and/or anthropogenic modifications. In contrast, sites with high baseflow indices and low runoff sensitivity to storage receive the majority of precipitation as snow and have more permeable regolith profiles. The seasonal variability of water balance components is a key control on the dynamic range of hydraulically connected water in the critical zone. These findings lead us to posit that water balance partitioning and streamflow hydraulics are linked through the coevolution of critical zone architecture but that much work remains to parse these controls out quantitatively.

Stream drying and wildfire are projected to increase with climate change in the western United States, and both are likely to impact stream chemistry patterns and processes. To investigate drying and wildfire effects on stream chemistry (carbon, nutrients, anions, cations, and isotopes), we examined seasonal drying in two intermittent streams in southwestern Idaho, one stream that was unburned and one that burned 8 months prior to our study period. During the seasonal recession following snowmelt, we hypothesized that spatiotemporal patterns of stream chemistry would change due to increased evaporation, groundwater dominance, and autochthonous carbon production. With increased nutrients and reduced canopy cover, we expected greater shifts in the burned stream. To capture spatial chemistry patterns, we sampled surface water for a suite of analytes along the length of each stream with a high spatial scope (50-m sampling along ~2,500 m). To capture temporal variation, we sampled each stream in April (higher flow), May, and June (lower flow) in 2016. Seasonal patterns and processes influencing stream chemistry were generally similar in both streams, but some were amplified in the burned stream. Mean dissolved inorganic carbon (DIC) concentrations increased with drying by 22% in the unburned and by 300% in the burned stream. In contrast, mean total nitrogen (TN) concentrations decreased in both streams, with a 16% TN decrease in the unburned stream and a 500% TN decrease (mostly nitrate) in the burned stream. Contrary to expectations, dissolved organic carbon (DOC) concentrations varied more in space than in time. In addition, we found the streams did not become more evaporative relative to the Local Meteoric Water Line (LMWL) and we found weak evidence for evapoconcentration with drying. However, consistent with our expectations, strontium-DIC ratios indicated stream water shifted toward groundwater-dominance, especially in the burned stream. Fluorescence and absorbance measurements showed considerable spatial variation in DOC sourcing each month in both streams, and mean values suggested a temporal shift from allochthonous toward autochthonous carbon sources in the burned stream. Our findings suggest that the effects of fire may magnify some chemistry patterns but not the biophysical controls that we tested with stream drying.

Ephemeral and intermittent streams are increasing with climate and land use changes, and alteration in stream water presence or flow duration will likely affect litter decomposition and nutrient dynamics in channel and riparian zones more than uplands. To investigate the influence of varying climate and stream flow regimes on rates of decomposition and associated nutrient dynamics, we used a space-for-time substitution design in which we deployed a common leaf litter across a range of ephemeral to seasonally- intermittent stream reaches (10) and landscape positions (channel, riparian, upland) in Arizona, USA over an 18-month period. We also measured soil physio-chemical properties and nutrient dynamics associated with these reaches and positions. Consistent with expectations, rates of litter decomposition (k) decreased significantly in the channels as cumulative percentage (%) of water presence decreased below 40%. Indeed, differences in cumulative duration of water presence as well as channel bed material silt content explained 80% of the variation in k across flow regimes. In contrast, decay rates of the common litter across sites were surprisingly similar in upland and riparian positions despite large differences in climate, specifically precipitation (160-516 mm). Relatively similar litter nitrogen immobilization and soil moisture in upland and riparian environments helped to explain the lack of difference in k and soil nutrient dynamics in these environments. Collectively, our findings indicate that stream water presence may be a more important indicator of ephemeral and intermittent stream function than stream flow alone and that riparian zones in these dryland regions may be less responsive to changes in climate and associated subsidies of stream flow.

The lasting influence humans have on Earth’s critical zone—and how geologic forces have mediated those influences—is revealed in studies of soil and carbon migration.

Soil thickness is a fundamental variable in many earth science disciplines due to its critical role in many hydrological and ecological processes, but it is difficult to predict. Here we show a strong linear relationship (r2 = 0.87, RMSE = 0.19 m) between soil thickness and hillslope curvature across both convergent and divergent parts of the landscape at a field site in Idaho. We find similar linear relationships across diverse landscapes (n = 6) with the slopes of these relationships varying as a function of the standard deviation in catchment curvatures. This soil thickness-curvature approach is significantly more efficient and just as accurate as kriging-based methods, but requires only high-resolution elevation data and as few as one soil profile. Efficiently attained, spatially continuous soil thickness datasets enable improved models for soil carbon, hydrology, weathering, and landscape evolution., Soil thickness is a key parameter in earth system models, yet how it varies spatially at catchment scales is largely unknown due to measurement challenges. Here, the authors show that a continuous field of thicknesses can be predicted using high-resolution topography and a few soil thickness measurements.

Urbanization has a massive impact on ecological function and impedes the provisioning of environmental services. The interaction between urbanization and the environment has been researched extensively. However, the existing research focused on the urbanization characteristics of large metropolitan areas. In this paper, we investigate the urban fragmentation patterns of small and mid-sized cities in Idaho, USA. To capture the urban morphology for each site within the study we use both concentric ring and transect analysis as well as several spatial/landscape metrics. Our results show that the characteristics of urbanization in small and mid-sized cities (and those in second-tier regions that attract relatively little scholarly attention) are consistent with growth phase theory of urbanization, urbanization along a gradient, and urbanization patterns of metropolises.

Climate-driven changes in carbon (C) cycling of forested ecosystems have the potential to alter long-term C sequestration and the global C balance. Prior studies have shown that C uptake and partitioning in response to hydrologic variation are system specific, suggesting that a comprehensive assessment is required for distinct ecosystems. Many sub-humid montane forest ecosystems in the US are projected to experience increased water limitation over the next decades and existing water-limited forests can be used as a model for how changes in the hydrologic cycle will impact such ecosystems more broadly. Toward that goal we monitored precipitation, net ecosystem exchange and lateral soil and stream C fluxes in three semi-arid to sub-humid montane forest catchments for several years (WY 2009–2013) to investigate how the amount and timing of water delivery affect C stores and fluxes. The key control on aqueous and gaseous C fluxes was the distribution of water between winter and summer precipitation, affecting ecosystem C uptake versus heterotrophic respiration. We furthermore assessed C stores in soil and above- and below-ground biomass to assess how spatial patterns in water availability influence C stores. Topographically-driven patterns in catchment wetness correlated with modeled soil C stores, reflecting both long-term trends in local C uptake as well as lateral redistribution of C leached from upslope organic soil horizons to convergent landscape positions. The results suggest that changes in the seasonality of precipitation from winter snow to summer rain will influence both the amount and the spatial distribution of soil C stores.

The impact of land management actions such as prescribed fire remains a key uncertainty in understanding the spatiotemporal patterns of carbon cycling in the Western USA. We therefore quantified carbon exchange and aboveground carbon stocks following a prescribed fire in a mountain big sagebrush ecosystem located in the northern Great Basin, USA. Specifically, we examined the changes in plant functional type, leaf area index, standing aboveground carbon stocks, net ecosystem production (NEP), gross ecosystem production (GEP), and ecosystem-level respiration (Reco) for 2 years before and 7 of 9 years after a prescribed fire. Post-burn GEP and Reco exceeded pre-burn GEP and Reco within 2 years and remained elevated. The variation in GEP and Reco provided no evidence of a large and prolonged net efflux of carbon in the 9 years after the fire. Rather, NEP indicated the site was a sink before and after the fire, with little change in sink strength associated with the burn. Re-sprouting and recruitment of grasses and forbs drove the post-burn increase in GEP. Woody shrub growth was the dominant control on aboveground biomass accumulation after fire, with shrub aboveground biomass reaching ~ 11% of pre-burn biomass after 5 years. The rapid recovery of GEP and the growth of mid-successional shrubs suggest ecosystem-level carbon fluxes and stocks can recover rapidly after fire in mesic mountain big sagebrush ecosystems.

The storage and flux of carbon from soils, the planet's third largest carbon pool, strongly influence the global carbon cycle and are essential, but poorly constrained, parameters for global climate models. An estimated 40% of all soil carbon is stored as inorganic carbonate minerals. Despite a recognition of the importance of soil inorganic carbon (SIC) in soil carbon storage, few studies have quantified pedon-scale variability in SIC storage. We examine different stages of carbonate development and accumulation rates between gravelly and non-gravelly soils. Studies often ignore carbonate coatings on gravels in measurements of soil inorganic carbon (SIC). By quantifying and differentiating the fine ( 2 mm) fractions of SIC in the Reynolds Creek Experimental Watershed in southwestern Idaho, we show that gravel coatings contain up to 44% of total SIC at a given site. Among the 26 soil sites examined throughout the watershed, an average of 13% of the total SIC is stored as carbonate coatings within in the gravel fraction. We measured a high level of pedon-scale field variability (up to 220%) among the three sampled faces of 1 m 3 soil pits. Analytical error associated with the modified pressure calcimeter (0.001–0.014%) is considerably less than naturally occurring heterogeneities in SIC within the soil profile. This work highlights and quantifies two sources of uncertainty in studies of SIC needed to inform future research. First, in gravelly sites, the > 2 mm portion of soils may store a large percentage of SIC. Second, SIC varies considerably at the pedon-scale, so studies attempting to quantify carbon storage over landscape scales need to consider this variability.

Thirty one years of spatially distributed air temperature, relative humidity, dew point temperature, precipitation amount, and precipitation phase data are presented for the Reynolds Creek Experimental Watershed, which is part of the Critical Zone Observatory network. The air temperature, relative humidity, and precipitation amount data are spatially distributed over a 10 m Lidar-derived digital elevation model at an hourly time step using a detrended kriging algorithm. This dataset covers a wide range of weather extremes in a mesoscale basin (237 km2) that encompasses the rain-snow transition zone and should find widespread application in earth science modeling communities. Spatial data allows for a more holistic analysis of basin means and elevation gradients, compared to weather station data measured at specific locations. Files are stored in the NetCDF file format, which allows for easy spatiotemporal averaging and/or subsetting. Data are made publicly available through an OPeNDAP-enabled THREDDS server hosted by Boise State University Libraries in support of the Reynolds Creek Critical Zone Observatory (https://doi.org/10.18122/B2B59V).

In mountains with seasonal snow cover, the effects of climate change on snowpack will be constrained by landscape-vegetation interactions with the atmosphere. Airborne lidar surveys used to estimate snow depth, topography, and vegetation were coupled with reanalysis climate products to quantify these interactions and to highlight potential snowpack sensitivities to climate and vegetation change across the western U.S. at Rocky Mountain (RM), Northern Basin and Range (NBR), and Sierra Nevada (SNV) sites. In forest and shrub areas, elevation captured the greatest amount of variability in snow depth (16–79%) but aspect explained more variability (11–40%) in alpine areas. Aspect was most important at RM sites where incoming shortwave to incoming net radiation (SW:NetR↓) was highest (∼0.5), capturing 17–37% of snow depth variability in forests and 32–37% in shrub areas. Forest vegetation height exhibited negative relationships with snow depth and explained 3–6% of its variability at sites with greater longwave inputs (NBR and SNV). Variability in the importance of physiography suggests differential sensitivities of snowpack to climate and vegetation change. The high SW:NetR↓ and importance of aspect suggests RM sites may be more responsive to decreases in SW:NetR↓ driven by warming or increases in humidity or cloud cover. Reduced canopy-cover could increase snow depths at SNV sites, and NBR and SNV sites are currently more sensitive to shifts from snow to rain. The consistent importance of aspect and elevation indicates that changes in SW:NetR↓ and the elevation of the rain/snow transition zone could have widespread and varied effects on western U.S. snowpacks.

In drylands worldwide, biological soil crusts (BSC) form a thin photosynthetic cover across landscapes, and provide vital benefits in terms of stabilizing soil and fixing nitrogen (N) and carbon (C). Numerous studies have examined the effects of climate and disturbance on BSC functions; however, few have characterized these responses in rolling BSCs typical of northern ecosystems in the Intermountain West, US. With temperature increases and shifts in precipitation projected, it is unclear how BSCs in this region will respond to climate change, and how the response could affect their capacity to perform key ecosystem functions, such as providing ‘new’ N through biological N2 fixation. To address this important knowledge gap, we examined nitrogenase activity (NA) associated with rolling BSCs along a climatic gradient in southwestern Idaho, US, and quantified how acetylene reduction rates changed as a function of climate, grazing (using exclosures), and shrub-canopy association. Results show that warmer, drier climates at lower elevations hosted greater cover of late successional BSC communities (e.g., mosses and lichens), and higher NA compared with colder, wetter climates at higher elevations. Highest NA (0.5–29.3 µmol C2H4 m−2 h−1) occurred during the early summer/spring, when water was more available than in late summer/autumn. Activity was strongly associated with soil characteristics including pH and ammonium concentrations suggesting these characteristics as potentially strong controls on NA in BSCs. The relationship between grazing and NA varied with elevation. Specifically, lower elevation sites had lower NA at grazed locations, whereas higher elevation sites had higher NA with grazing. At both low and high ends of the elevation gradient, shrub-canopy associated BSCs maintained two to three times higher NA compared to BSCs in the interspace among shrubs. Taken together, our findings indicate that the controls and rates of NA in BSCs vary seasonally and strongly with climate in the Intermountain West, and that drier springs are likely to influence rates of NA more than warmer summers.

Ephemeral streams are abundant in drylands, yet we know little about how their vegetation differs from surrounding terrestrial zones and about their projected response to regional warming and drying. We assessed plant communities at seven ephemeral streams (and terrestrial zones) distributed among three climatic settings in Arizona. Compared to terrestrial zones, riparian zones had similar herbaceous cover but greater woody vegetation volume. They supported more plant species, with several woody taxa restricted to the ephemeral zone (consistent with the idea that herbaceous plants are rain-dependent while riparian trees rely on runoff stored in stream sediments). Their herbaceous communities had high compositional overlap with terrestrial zones and may sustain regional diversity as droughts intensify. Presumably owing to periodic flood disturbance, riparian plant communities had greater evenness than terrestrial zones, many of which were dominated by Eragrostis lehmanniana . Patterns along the climatic gradient suggest that increasing aridity will reduce the number of herbaceous (and total) plant species within riparian zones (110 species per stream in semihumid settings, 88 in semiarid, 48 in arid) and drive compositional shifts from perennials grasses and forbs to annuals. Hotter and drier conditions will drive sharp declines in herbaceous cover, converting riparian savanna to xeroriparian scrubland.

Soil profiles are rarely homogeneous. Resource availability and microbial abundances typically decrease with soil depth, but microbes found in deeper horizons are still important components of terrestrial ecosystems. By studying 20 soil profiles across the United States, we documented consistent changes in soil bacterial and archaeal communities with depth. Deeper soils harbored communities distinct from those of the more commonly studied surface horizons. Most notably, we found that the candidate phylum Dormibacteraeota (formerly AD3) was often dominant in subsurface soils, and we used genomes from uncultivated members of this group to identify why these taxa are able to thrive in such resource-limited environments. Simply digging deeper into soil can reveal a surprising number of novel microbes with unique adaptations to oligotrophic subsurface conditions., While most bacterial and archaeal taxa living in surface soils remain undescribed, this problem is exacerbated in deeper soils, owing to the unique oligotrophic conditions found in the subsurface. Additionally, previous studies of soil microbiomes have focused almost exclusively on surface soils, even though the microbes living in deeper soils also play critical roles in a wide range of biogeochemical processes. We examined soils collected from 20 distinct profiles across the United States to characterize the bacterial and archaeal communities that live in subsurface soils and to determine whether there are consistent changes in soil microbial communities with depth across a wide range of soil and environmental conditions. We found that bacterial and archaeal diversity generally decreased with depth, as did the degree of similarity of microbial communities to those found in surface horizons. We observed five phyla that consistently increased in relative abundance with depth across our soil profiles: Chloroflexi, Nitrospirae, Euryarchaeota, and candidate phyla GAL15 and Dormibacteraeota (formerly AD3). Leveraging the unusually high abundance of Dormibacteraeota at depth, we assembled genomes representative of this candidate phylum and identified traits that are likely to be beneficial in low-nutrient environments, including the synthesis and storage of carbohydrates, the potential to use carbon monoxide (CO) as a supplemental energy source, and the ability to form spores. Together these attributes likely allow members of the candidate phylum Dormibacteraeota to flourish in deeper soils and provide insight into the survival and growth strategies employed by the microbes that thrive in oligotrophic soil environments.

Research examining the influence of parent material on soil and landscape development is limited in dryland environments. In volcanic soils, most studies examining soil development and succession have been conducted in humid environments, where weathering occurs rapidly. This study was conducted in a semi-arid environment to examine the influence of volcanic parent material morphology on soil development and ecohydrologic properties that may regulate primary succession. Four study sites were selected across young (~ 2.0 ka) and old (~ 12.1 ka) end members of two different parent materials (cinder cones and pahoehoe lava flows). We hypothesized that soils and associated hydraulic properties would evolve more rapidly on the cinder cones compared to the pahoehoe flows owing to greater surface area on the cinder. For each site, we quantified plant cover, soil organic matter, bulk density, texture, soil water retention, and hydraulic conductivity. The old cinder cone soils accumulated approximately 9% soil organic matter and increased in silt plus clay fraction by 23% relative to the young soil. This increased water retention and decreased hydraulic conductivity. Similarly, plant cover increased by 47%. The old pahoehoe site remained dominated by unweathered basalt, where silty eolian material accumulated preferentially in crevices. Soil organic matter and silt plus clay fraction increased by 18% and 49% respectively in old pahoehoe soils, resulting in increased water retention but little change in hydraulic conductivity. Plants were restricted to crevices and cover increased by only 34%. We observed the cinder cone sites had extensive soil and plant community development from the young to old sites. The pahoehoe soils and plant communities established earlier than on cinder cones, but showed only modest functional changes between young and old sites. The combined characterization of soil development, hydraulic properties, and plant community dynamics highlights potential ecohydrologic mechanisms controlling successional processes in young semi-arid landscapes.

Chemical stabilization of microbial-derived products such as extracellular enzymes (EE) onto mineral surfaces has gained attention as a possibly important mechanism leading to the persistence of soil organic carbon (SOC). While the controls on EE activities and their stabilization in the surface soil are reasonably well-understood, how these activities change with soil depth and possibly diverge from those at the soil surface due to distinct physical, chemical, and biotic conditions remains unclear. We assessed EE activity to a depth of 1 m (10 cm increments) in 19 soil profiles across the Critical Zone Observatory Network, which represents a wide range of climates, soil orders, and vegetation types. For all EEs, activities per mass of soil correlated positively with microbial biomass (MB) and SOC, and all three of these variables decreased logarithmically with depth (p < 0.05). Across all sites, over half of the potential EE activities per mass soil consistently occurred below 20 cm for all measured EEs. Activities per unit MB or SOC were substantially higher at depth (soils below 20 cm accounted for 80% of whole-profile EE activity), suggesting an accumulation of stabilized (i.e. mineral sorbed) EEs in subsoil horizons. The pronounced enzyme stabilization in subsurface horizons was corroborated by mixed-effects models that showed a significant, positive relationship between clay concentration and MB-normalized EE activities in the subsoil. Furthermore, the negative relationships between soil C, N, and P and C-, N-, and P-acquiring EEs found in the surface soil decoupled below 20 cm, which could have also been caused by EE stabilization. This finding suggests that EEs may not reflect soil nutrient availabilities deeper in the soil profile. Taken together, our results suggest that deeper soil horizons hold a significant reservoir of EEs, and that the controls of subsoil EEs differ from their surface soil counterparts.

The Reynolds Creek Experimental Watershed (RCEW) was established in 1960 as an "outdoor hydrological laboratory" to investigate hydrological processes of interest in the interior northwestern part of the United States. Initial emphasis was on installing and testing instrumentation and data collection and dissemination. The initial instrumentation network sampled the climatic gradient within the 239-km(2) watershed and focused on specific subwatersheds for intensive instrumentation. This network has expanded and supported ad hoc research and provides a stable platform for the development of long-term programs supporting research and model development in snow hydrology, climate change, water and energy balance, land management, carbon cycling, and critical zone hydrology. Recently, the challenge taken up at the RCEW is to integrate different processes over space for applications to larger areas outside the watershed. The presence of steep local environmental gradients associated with topography in addition to more gradual, elevational gradients requires high-resolution modeling. The snow hydrology program has demonstrated the potential for high-resolution, process-based modeling across large landscapes. The direct linkage of biogeochemical processes with hydrological processes ultimately requires a multidisciplinary approach that has been adopted at the RCEW since inclusion in the Critical Zone Observatory program. We think that coupling of these processes will lead to a better understanding and management of natural resources on the landscape. NSF RC CZO [EAR 1331872] This work was partially funded by NSF RC CZO Cooperative Agreement no. EAR 1331872. Public domain – authored by a U.S. government employee