Introduction
Global energy production and activities preceding production are expected to expand into new landscapes in response to the increased demand for energy (McDonald et al. 2009) . Numerous negative consequences are associated with such activities including fragmentation (Trainor et al. 2016), habitat loss (Shaffer & Buhl 2016), alterations to soil structure (Stumpf et al. 2016), and the loss of productive landscapes (Allred et al. 2015). However, in many cases (i.e., surface mining) reclamation is required post-extraction to promote a return of productivity to the landscape (SMCRA 1977). Among the many ecosystems affected by the development of energy-based infrastructure are the grasslands of the Northern Great Plains (NGP) (Preston & Kim 2016), which is where multiple forms of non-renewable resources are being extracted, including lignite coal. Current traditional best reclamation practices on lignite coalmines result in reclaimed lands being successfully released from performance bonds after 10 years. Yet, ensuring soil structural recovery and sustaining a diverse plant community on reclaimed grasslands remains a challenge, especially as time since reclamation progresses (Bohrer et al. 2017a; Bohrer et al. 2017b).
Surface mining activities require large-scale excavation of earthen materials resulting in the complete deconstruction of ecosystems (Holl 2002; Pauletto et al. 2016). Excavation of all existing vegetation and deconstruction of soil profiles is among the first stages of surface mining. Such activities results in extreme alterations to soil structure, specifically larger soil aggregates ( Stumpf et al. 2016). Soil aggregates are further degraded by the vibrations during the course of transportation (McSweeney & Jansen 1984). These cumulative impacts on soil aggregates become problematic during reclamation as heavy load-bearing pressures from reclamation equipment (responsible for stabilizing and grading the newly constructed landscape) compress the degraded soil aggregates (McSweeney & Jansen 1984; Bohrer et al. 2017b). Compression of these altered aggregates creates compacted soil conditions, and such conditions have the potential to cause many obstacles when attempting to establish and sustain a desired plant community.
Connectivity of macropores within the soil matrix is essential for water infiltration, promotion nutrient of cycling, and providing plant roots accessibility to resources like nutrients, water, oxygen, and heat ( Stoessel et al., 2018). Soil compaction increases bulk density and penetration resistance (PR), reducing the distribution of macropores in the soil profile (Jabro et al. 2014), affecting growth of plant roots (Tardieu 1994; Unger & Kaspar 1994), the accessibility of water to plant roots (Haygarth & Ritz, 2009), and the overall movement of water (Kulli et al. 2003). Plants must exert more energy to obtain water and nutrients, and if water cannot be obtained the plants become stressed. Additionally, limited infiltration and pooling may also occur at either the surface or subsurface, which impacts the availability of water and/or oxygen to plant roots (Hamza & Anderson 2004; Stoessel et al. 2018) and increase the likelihood of soil erosion (Stoessel et al. 2018). Decreased macropores can also impede the ability of plant roots to maneuver within the soil profile and altering the growth patterns (Hernandez-Ramirez et al. 2014; Beckett et al. 2017). Finding a solution to improve root growth and water movement becomes vital during the reclamation process.
Alleviating soil compaction can be accomplished using a variety of anthropogenic methods, including mechanized disruption of soil or amending the soil with organic matter (Hamza & Anderson 2004). Tilling is one of the most common land management practices used to decrease soil compaction (Schneider et al. 2017). This technique breaks up the compressed layer of soil and increases the amount and distribution of macropores (Hangen et al. 2002). Tilling-like practices applied to reach subsoil depths is often referred to as ripping, or subsoiling (Schneider et al. 2017). An additional means of decreasing compaction is the integration of organic matter (OM), e.g. material such as straw, into the soil (Getahun et al. 2018). This management practice can aid in alleviating compaction in two ways. The capabilities of OM to absorb water improves the soil water-holding capacity enhancing the availability of water to plant roots (Zhao et al. 2014). Also, as organic materials decompose they aid in soil aggregation by adding organic carbon (Sheoran 2010). The application of these practices improves the pore space distribution which in turn promotes water movement, root exploration, and decreases the bulk density and penetration resistance. Ultimately, these actions have the potential to improve growing conditions and the establishment of desirable native grassland species. However, these conditions may also promote invasive species like Kentucky bluegrass (Poa pratensis ), a cool season invasive grass of special concern in the NGP.
Grasslands in the NGP are being disturbed to support energy production-based infrastructure (Preston & Kim 2016), but mandatory reclamation for surface-mining operations provides an opportunity for native grasslands to be replaced by new reconstructed grasslands. Unfortunately, conditions of older reclaimed grasslands, both above and belowground, are not presenting ecological qualities representative of functional grasslands (Bohrer et al. 2017a; Bohrer et al. 2017b), prompting a need to investigate alternative reclamation practices. The objective of this study was to compare how different combinations of alternative reclamation practices can influence community composition, reduce PR, and improve soil water movement. We expect to observe quantifiable differences between the different combinations when assessing the plant community composition, PR, and volumetric soil moisture.