Chapter 1: Background

About This Thesis

This thesis is divided into four chapters, not including the references. In Chapter 1, I provide important background information and a comprehensive literature review of the importance of organic matter, soil aggregates, restoration, and the “100 Sites for 100 Years” project. Chapter 2 is an invited book chapter in Progress in Soil Science Series that was published in 2014. This chapter includes analysis of data that demonstrate the role of soil aggregates in carbon sequestration in restored prairies. Chapter 3 is a more in- depth analysis of my study sites which makes up the body of this thesis. This chapter will be converted into a journal article and submitted to Applied Soil Ecology. In Chapter 4, I provide a brief synthesis of Chapters 1–3.

Organic matter and soil aggregates

More than two-thirds of the organic carbon (C) stored in terrestrial ecosystems is contained in soil organic matter (SOM) (Cheng et al., 2011; Miltner et al., 2012; Schlesinger, 1997; see also Figure 1.1). Soil organic matter consists of plant material and microbial biomass (Kögel-Knabner, 2001; Miltner et al., 2012). Primary productivity and decomposition are both characterized by SOM and respond differently to land management practices (Burke et al., 1989). The U.S. Central Plains Grasslands have lost up to 50 percent of their soil organic carbon (SOC), depending on management practices and location (Burke et al., 1989). Because SOM is a C pool, soil is considered to be a C sink, which can help decrease the atmospheric carbon dioxide (CO2) concentration and reduce the greenhouse effect (Cheng et al., 2011; Lützow et al., 2006). Fixation of CO2 by plants helps regulate atmospheric CO2 levels (Miltner et al., 2012). Restoration practices may, therefore, be important for reducing CO2 levels. 

Figure 1.1. Global Carbon Cycle. Storage measured in GIC (gigaton of carbon). Adapted from Global Greenhouse Warming. (n.d.). The global carbon cycle. In The Global Carbon Cycle. Retrieved from http://www.global-greenhouse-warming.com/global-carbon-cycle.html

Soil C levels are determined by organic matter (OM) inputs, primarily as plant residues, roots, root exudates, fungi, and OM losses that result from microbial decomposition (Liao et al., 2006; see also Figure 1.2). Organic matter inputs are important since they are binding agents in soil. Organic binding agents are composed of three different groups: persistent, temporary, and transient. Persistent binding agents account for 52 to 98 percent of total SOM, creating an organo-mineral fraction of the soil (Tisdall and Oades, 1982). Persistent binding agents consist of aromatic compounds associated with polyvalent metal cation complexes (Six et al., 2004; Tisdall and Oades, 1982). Roots, hyphae, saprophytic fungi, and vesicular-arbuscular mycorrhizal fungi all can serve as temporary binding agents. Transient binding agents consist primarily of polysaccharides, and are decomposed by microorganisms quickly (Tisdall and Oades, 1982). Organic binding agents play an important role in soil C levels and in the formation of soil aggregates.

Figure 1.2. Breakdown of OM in soil. Adapted from Grunwald, S. (n.d.). In Soil organic matter (SOM). Retrieved from http://soils.ifas.ufl.edu/faculty/grunwald/teaching/eSoilScience/organic.shtml

Soil aggregates play central roles in most ecosystems, both as storage complexes of OM and mediators of belowground C transport (Verchot et al., 2011). Soil aggregates are formed and stabilized by OM (An et al., 2010; McCarthy et al., 2008; Tang et al., 2011), primarily from roots and arbuscular mycorrhizal hyphae (Miltner et al., 2012; Tisdall and Oades, 1982). Traditional cultivation practices result in a decline in SOM, which then reduces aggregation. When large aggregates break down, microbes decompose the SOM, thereby decreasing aggregation in the soil (Six et al., 2000). Erosion and degradation in soil may, therefore, be determined by the stability of soil aggregates (An et al., 2010). Soil aggregate size and composition can be used as indicators of soil quality because they help decrease erosion and degradation, which also stabilizes C and prevents runoff into streams and rivers (An et al., 2010). Measurements of SOM and aggregate size at restoration sites should be considered during restoration practices.

Aggregate size is important because of the size-based variation in composition and function. The three primary soil aggregate sizes are (1) primary particles, (2) microaggregates, and (3) macroaggregates (Tisdall and Oades, 1982; Figure 1.3). Primary particles have diameters less than 50 μm, microaggregate diameters range from 50–250 μm, and macroaggregates have diameters greater than 250 μm (McCarthy et al., 2008; Figure 1.3). Microaggregates are primarily held together by microbial polysaccharides (Liao et al., 2006) and humic matter (Tang et al., 2011). Because microaggregates are physically protected by SOM, they are also able to store C longer than in macroaggregates (McCarthy et al., 2008). Macroaggregates are composed of multiple microaggregates that are held together by fungal hyphae and plant roots (Brady and Weil, 2010). Typically macroaggregates contain higher concentrations of OM than microaggregates (Jastrow, 1996). Macroaggregates also have a significantly faster turnover rate than microaggregates (Liao et al., 2006; McCarthy et al., 2008; Tisdall and Oades, 1982). That may be explained by chemical recalcitrance, organometallic complexes, or physical protection from SOM in microaggregates (Jastrow, 1996). For this study, only macro- and microaggregates were used.