Weathering of rock to regolith: The activity of deep roots in bedrock fractures
Graphical abstract
Introduction
Plants play a key role in weathering regolith in the critical zone, but this role varies as a function of water use, rooting depth and distribution, and associated mycorrhizal fungi (Reneau and Dietrich, 1991, Van Breemen et al., 2000, Balogh-Brunstad et al., 2008, Fimmen et al., 2008, Graham et al., 2010, Schulz et al., 2016). Of particular importance are the mineral weathering reactions that consume CO2 and organic acids produced by plant roots and soil microorganisms (Leake et al., 2008, Ahmed and Holmström, 2015). Such weathering processes exert important controls on global C cycling and climate change over geological timescales. Interactions between physical, chemical, and biological processes transform bedrock into soil and provide inorganic nutrients to terrestrial biota. When bedrock is physically and chemically weathered, it enhances rock porosity, which is crucial for changing biologically inert rock into materials from which plants and microorganisms can extract water and nutrients (Brantley, 2010, Wald et al., 2013). For example, as early as the 1800s, Jackson (1840) found that the expansion of biotite due to oxidation may further enhance fracture propagation and the degradation of rock to regolith. Plant roots can also promote these chemical and physical weathering processes and alter the morphology of the bedrock (Graham et al., 1994, Frazier and Graham, 2000, Schenk and Jackson, 2005, Graham et al., 2010).
The weathering potential of tree roots depends, in part, on rooting depth. Rooting depth is a direct function of climate, particularly annual precipitation and potential evapotranspiration (Schenk and Jackson, 2002a, Schenk and Jackson, 2002b), species (Gale and Grigal, 1987), soil thickness (Stone and Kalisz, 1991, Anderson et al., 1995, Sternberg et al., 1996, Hubbert et al., 2001a, Hubbert et al., 2001b, Witty et al., 2003, Bornyasz et al., 2005, Graham et al., 2010), inherent and dynamic soil properties (Kochenderfer, 1973, Nicoll et al., 2006), and bedrock properties (Witty et al., 2003). Plant roots are predominantly located in the upper portions of the soil profile, and Schenk and Jackson (2005) found that on a global scale around half of all roots are located in the top 30 cm of soil and 95% are in the top 2 m. Vertical rooting depth is generally assumed to be limited in shallow soils because root growth is restricted by the solid bedrock below, and thus most studies of root dynamics are confined to the uppermost soil horizons. Nevertheless, many landscapes are characterized by shallow soils that are underlain by actively weathering bedrock containing fractures that can allow soil, gases, water, and roots to move downward. Roots have been observed to penetrate many meters into bedrock along joints and fracture planes, particularly in upland areas (Hellmers et al., 1955, Scholl, 1976, Stone and Kalisz, 1991, Anderson et al., 1995, Canadell and Zedler, 1995, Jackson et al., 1999, Hubbert et al., 2001a, Hubbert et al., 2001b, Egerton-Warburton et al., 2003, Rose et al., 2003, Witty et al., 2003, Bornyasz et al., 2005, Graham et al., 2010, Estrada-Medina et al., 2013). Despite the common observance of roots in rock fractures, rarely has the rooting environment within fractures been explored, partially due to the difficulties and expense of excavating solid rock (Maeght et al., 2013).
Studies of the distribution of deep roots in rocks are largely restricted to arid and drought-prone environments where deep roots allow woody vegetation to access water from below the soil in weathered bedrock reserves (Lewis and Burgy, 1964, Zwieniecki and Newton, 1995, Hubbert et al., 2001a, Hubbert et al., 2001b, Egerton-Warburton et al., 2003, Rose et al., 2003, Witty et al., 2003, Bornyasz et al., 2005, Schenk, 2008, Duniway et al., 2010, Graham et al., 2010, Schwinning, 2010). The majority of these studies focus on the water-holding capacity of weathered rocks, but they rarely address the physical and biogeochemical dynamics of this environment. Moreover, in temperate regions with higher rainfall, trees do not experience the same water limitations as arid environments. Indeed, Gaines et al. (2015) found that the isotopic signature of stem water in a central Pennsylvania forest showed that trees mainly obtained their water from the upper soil horizons. Thus, the advantages of deep roots in humid environments are less clear. Additionally, studies of deeply rooted systems have investigated only a few lithologies including limestone (Hasselquist et al., 2010, Estrada-Medina et al., 2013) and granite (Hubbert et al., 2001a, Hubbert et al., 2001b, Witty et al., 2003, Bornyasz et al., 2005, Graham et al., 2010, Poot et al., 2012).
We tested environment of deep roots in rock fractures as well as the role of deep roots in weathering bedrock. In detail, we investigated the abundance and activity of roots in shale bedrock fractures, characterized the growing environment of the roots within the fractures by examining the adjacent materials and porefluid chemistry, and assessed the potential of roots in rock fractures to promote rock weathering along a catena in a forested catchment in the northern Appalachian Mountains (i.e., a catchment close to the Shale Hills experimental watershed in the Susquehanna Shale Hills Critical Zone Observatory; SSHCZO) where the climate is temperate and humid. Assessing the role of deep roots in rock will lead to a better understanding of controls on rooting depth and hillslope regolith development.
Section snippets
Site description
Our study area, Missed Grouse Gulch, is a temperate, forested watershed located in the Appalachian Valley and Ridge Province of central Pennsylvania. The site is just two valleys (~ 0.25 km) north of the Shale Hills experimental watershed in the SSHCZO (Fig. 1). We selected the Missed Grouse Gulch site to study deep root activity because it features lithology, soils, and vegetation similar to the well-studied Shale Hills catchment and is easily accessed by excavation equipment. We could not
Rock fracture distribution
Bedrock fractures in the vertical pit faces imaged by our photography were predominantly oriented along, or parallel to, bedding, but could be found in any orientation (Fig. 2A–C). In all three pits where we reached bedrock, we observed large vertical fractures that were generally oriented perpendicular to bedding (see asterisk in Fig. 2A for an example). These vertical fractures were often the widest fractures observed in the pits (typically > 0.5 cm) and often had high root densities and
Discussion
Most studies of the distribution of deep roots that penetrate into bedrock are restricted to arid and drought-prone environments where researchers have concluded that deep roots allow woody vegetation to access water from below the soil in weathered bedrock reserves. Thus, these studies typically focus on root activity as it relates to water stress rather than the biogeochemical dynamics of the fractured rock environment and the potential for rock weathering by roots. With our extensive
Conclusions
We have presented one of the most extensive physical and chemical datasets available for soils, shale bedrock, and fractures in a weathering system. We focused on pits excavated along a catena in the Missed Grouse Gulch catchment that is equivalent to the better known Shale Hills catchment in the SSHCZO. The pits show that plant roots are able to penetrate fractured bedrock to depths of ~ 180 cm (and likely go much deeper). Overall, our data suggest that the roots and fracture fill in shale
Acknowledgments
We thank Joseph Harding, Brosi Bradley, and numerous undergraduate assistants for data collection and technical assistance. We conducted this research at the Penn State Stone Valley Forest, which is funded by The Pennsylvania State University College of Agricultural Sciences, Department of Ecosystem Science and Management and managed by the staff of the Forestlands Management Office. We thank Marjorie Schulz and an anonymous reviewer who provided constructive comments that helped us improve the
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Current affiliation: Cary Institute of Ecosystem Studies, Millbrook, NY, USA & City University of New York, Advanced Science Research Center, New York, NY, USA.