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Landscapes are frequently delineated by nested watersheds and river networks ranked via stream orders. Landscapes have only recently been delineated by their interfluves and ridge networks, and ordered based on their ridge connectivity. There are, however, few studies that have quantitatively investigated the connections between interfluve networks and landscape morphology and environmental processes. Here, we ordered hillsheds using methods complementary to traditional watersheds, via a hierarchical ordering of interfluves, and we defined hillsheds to be landscape surfaces from which soil is shed by soil creep or any type of hillslope transport. With this approach, we demonstrated that hillsheds are most useful for analyses of landscape structure and processes. We ordered interfluve networks at the Calhoun Critical Zone Observatory (CZO), a North American Piedmont landscape, and demonstrated how interfluve networks and associated hillsheds are related to landscape geomorphology and processes of land management and land-use history, accelerated agricultural gully erosion, and bedrock weathering depth (i.e., regolith depth). Interfluve networks were ordered with an approach directly analogous to that first proposed for ordering streams and rivers by Robert Horton in the GSA Bulletin in 1945. At the Calhoun CZO, low-order hillsheds are numerous and dominate most of the observatory’s ∼190 km2 area. Low-order hillsheds are relatively narrow with small individual areas, they have relatively steep slopes with high curvature, and they are relatively low in elevation. In contrast, high-order hillsheds are few, large in individual area, and relatively level at high elevation. Cultivation was historically abandoned by farmers on severely eroding low-order hillsheds, and in fact agriculture continues today only on high-order hillsheds. Low-order hillsheds have an order of magnitude greater intensity of gullying across the Calhoun CZO landscape than high-order hillsheds. In addition, although modeled regolith depth appears to be similar across hillshed orders on average, both maximum modeled regolith depth and spatial depth variability decrease as hillshed order increases. Land management, geomorphology, pedology, and studies of land-use change can benefit from this new approach pairing landscape structure and analyses. 

Long-term environmental research networks are one approach to advancing local, regional, and global environmental science and education. A remarkable number and wide variety of environmental research networks operate around the world today. These are diverse in funding, infrastructure, motivating questions, scientific strengths, and the sciences that birthed and maintain the networks. Some networks have individual sites that were selected because they had produced invaluable long-term data, while other networks have new sites selected to span ecological gradients. However, all long-term environmental networks share two challenges. Networks must keep pace with scientific advances and interact with both the scientific community and society at large. If networks fall short of successfully addressing these challenges, they risk becoming irrelevant. The objective of this paper is to assert that the biogeosciences offer environmental research networks a number of opportunities to expand scientific impact and public engagement. We explore some of these opportunities with four networks: the International Long-Term Ecological Research Network programs (ILTERs), critical zone observatories (CZOs), Earth and ecological observatory networks (EONs), and the FLUXNET program of eddy flux sites. While these networks were founded and expanded by interdisciplinary scientists, the preponderance of expertise and funding has gravitated activities of ILTERs and EONs toward ecology and biology, CZOs toward the Earth sciences and geology, and FLUXNET toward ecophysiology and micrometeorology. Our point is not to homogenize networks, nor to diminish disciplinary science. Rather, we argue that by more fully incorporating the integration of biology and geology in long-term environmental research networks, scientists can better leverage network assets, keep pace with the ever-changing science of the environment, and engage with larger scientific and public audiences. 

As bedrock weathers to regolith – defined here as weathered rock, saprolite, and soil – porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Co-located geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from saprolite to weathered bedrock is more gradational, over a depth range of 11–18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth’s surface.

 

Soil carbonates (i.e., soil inorganic carbon or SIC) represent more than a quarter of the terrestrial carbon pool and are often considered to be relatively stable, with fluxes significant only on geologic timescales. However, given the importance of climatic water balance on SIC accumulation, we tested the hypothesis that increased soil water storage and transport resulting from cultivation may enhance dissolution of SIC, altering their local stock at decadal timescales. We compared SIC storage to 7.3 m depth in eight sites, each having paired plots of native vegetation and rain‐fed croplands, and half the sites having additional irrigated cropland plots. Rain‐fed and irrigated croplands had 328 and 730 Mg C/ha less SIC storage, respectively, compared to their native vegetation (grassland or woodland) pairs, and irrigated croplands had 402 Mg C/ha less than their rain‐fed pairs (p < .0001). SIC contents were negatively correlated with estimated groundwater recharge, suggesting that dissolution and leaching may be responsible for SIC losses observed. Under croplands, the remaining SIC had more modern radiocarbon and a δ¹³C composition that was closer to crop inputs than under native vegetation, suggesting that cultivation has led to faster turnover and incorporation of recent crop carbon into the SIC pool (p < .0001). The losses occurred just 30–100 years after land‐use changes, indicating SIC stocks that were stable for millennia can rapidly adjust to increased soil water flows. Large SIC losses (194–242 Mg C/ha) also occurred below 4.9 m deep under irrigated croplands, with SIC losses lagging behind the downward‐advancing wetting front by ~30 years, suggesting that even deep SIC were affected. These observations suggest that the vertical distribution of SIC in dry ecosystems is dynamic on decadal timescales, highlighting its potential role as a carbon sink or source to be examined in the context of land use and climate change.

 

Intensive agricultural land use can have detrimental effects on landscape properties, greatly accelerating soil erosion, with consequent fertility loss and reduced agricultural potential. To quantify the effects of such erosional processes on hillslope morphology and gain insight into the underlying dynamics, we use a twofold approach. First, a statistical analysis of topographical features is conducted, with a focus on slope and gradient distributions. The accelerated soil erosion is shown to be fingerprinted in the distribution tails, which provide a clear statistical signature of this human‐induced land modification. Theoretical solutions are then derived for the hillslope morphology and the associated creep and runoff erosion fluxes, allowing us to distinguish between the main erosional mechanisms operating in disturbed and undisturbed areas. We focus our application on the landscape at the Calhoun Critical Zone Observatory in the US Southern Piedmont, where severe soil erosion followed intensive cotton cultivation, resulting in highly eroded and gullied hillslopes. The observed differences in hillslope morphologies in disturbed and undisturbed areas are shown to be related to the disruption of the natural balance between soil creep and runoff erosion. The relaxation time required for the disturbed hillslopes to reach a quasi‐equilibrium condition is also investigated. © 2019 John Wiley & Sons, Ltd.

 

ABSTRACT 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. IMPORTANCE 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. 

Abstract. The critical zone (CZ), the dynamic living skin of the Earth, extends from the top of the vegetative canopy through the soil and down to fresh bedrock and the bottom of the groundwater. All humans live in and depend on the CZ. This zone has three co-evolving surfaces: the top of the vegetative canopy, the ground surface, and a deep subsurface below which Earth's materials are unweathered. The network of nine CZ observatories supported by the US National Science Foundation has made advances in three broad areas of CZ research relating to the co-evolving surfaces. First, monitoring has revealed how natural and anthropogenic inputs at the vegetation canopy and ground surface cause subsurface responses in water, regolith structure, minerals, and biotic activity to considerable depths. This response, in turn, impacts aboveground biota and climate. Second, drilling and geophysical imaging now reveal how the deep subsurface of the CZ varies across landscapes, which in turn influences aboveground ecosystems. Third, several new mechanistic models now provide quantitative predictions of the spatial structure of the subsurface of the CZ.
Many countries fund critical zone observatories (CZOs) to measure the fluxes of solutes, water, energy, gases, and sediments in the CZ and some relate these observations to the histories of those fluxes recorded in landforms, biota, soils, sediments, and rocks. Each US observatory has succeeded in (i) synthesizing research across disciplines into convergent approaches; (ii) providing long-term measurements to compare across sites; (iii) testing and developing models; (iv) collecting and measuring baseline data for comparison to catastrophic events; (v) stimulating new process-based hypotheses; (vi) catalyzing development of new techniques and instrumentation; (vii) informing the public about the CZ; (viii) mentoring students and teaching about emerging multidisciplinary CZ science; and (ix) discovering new insights about the CZ. Many of these activities can only be accomplished with observatories. Here we review the CZO enterprise in the United States and identify how such observatories could operate in the future as a network designed to generate critical scientific insights. Specifically, we recognize the need for the network to study network-level questions, expand the environments under investigation, accommodate both hypothesis testing and monitoring, and involve more stakeholders. We propose a driving question for future CZ science and a hubs-and-campaigns model to address that question and target the CZ as one unit. Only with such integrative efforts will we learn to steward the life-sustaining critical zone now and into the future.

 

Abstract. Long-term environmental research networks are one approach toadvancing local, regional, and global environmental science and education. Aremarkable number and wide variety of environmental research networks operatearound the world today. These are diverse in funding, infrastructure,motivating questions, scientific strengths, and the sciences that birthed andmaintain the networks. Some networks have individual sites that wereselected because they had produced invaluable long-term data, while othernetworks have new sites selected to span ecological gradients. However, alllong-term environmental networks share two challenges. Networks must keeppace with scientific advances and interact with both the scientific communityand society at large. If networks fall short of successfully addressing thesechallenges, they risk becoming irrelevant. The objective of this paper is toassert that the biogeosciences offer environmental research networks a numberof opportunities to expand scientific impact and public engagement. Weexplore some of these opportunities with four networks: the InternationalLong-Term Ecological Research Network programs (ILTERs), critical zoneobservatories (CZOs), Earth and ecological observatory networks (EONs),and the FLUXNET program of eddy flux sites. While these networks were foundedand expanded by interdisciplinary scientists, the preponderance of expertise andfunding has gravitated activities of ILTERs and EONs toward ecology andbiology, CZOs toward the Earth sciences and geology, and FLUXNET towardecophysiology and micrometeorology. Our point is not to homogenize networks,nor to diminish disciplinary science. Rather, we argue that by more fullyincorporating the integration of biology and geology in long-termenvironmental research networks, scientists can better leverage networkassets, keep pace with the ever-changing science of the environment, andengage with larger scientific and public audiences.