Effects of Soil Aggregation and Tillage Practices on Soil Quality, Structure and Nutrient Cycling
Soil structure and aggregation are important to plant growth and production. Various microbes play an important role in the formation and maintenance of soil structure (Lynch and Bragg, 1985). Microflora, in particular fungi, contribute directly to the formation and stabilization of soil aggregates through hyphal entanglement of soil particles and deposition of extracellular polysaccharides that bind soil particles together (Tisdall, 1991). In general, studies of the role of fungi in soil structure have been few, mainly because of the perceived difficulty of analysis. However, new immunoassay techniques are now available that can identify microbes at the order, genus, and even species level (Caesar-TonThat et al., 2001).
Soil OM (soil organic C and N concentrations) is a key indicator of soil quality and productivity because of its favorable effects on the physical, chemical, and biological properties (Bauer and Black, 1994; Doran and Parkin, 1994; Low, 1972; Tisdall and Oades, 1982; Dormarr, 1983; Gupta and Germida, 1988; Elliot, 1986). It plays critical roles on nutrient cycling, water retention, root growth (Sainju and Kalisz, 1990; Sainju and Good, 1993), erosion control, plant productivity, and environmental quality. Consequently, plant growth will be diminished when improper cultivation practices decrease pore space, reduce size distributions of water stable aggregates (WSA), minimize soil/root hair contact and increase soil bulk densities (Wienhold and Halvorson, 1998). Mollisol soils (mostly Typic Argiborolls), common to the MonDak region, are easily compacted at medium to high moisture contents, which further degrades soil structure. However, increasing soil OM will help reduce the deleterious effects of global warming by sequestering atmospheric CO2 and other greenhouse gasses (Lal and Kimble, 1997; Paustian et al., 1997).
Tillage practices can negatively impact both the physical structure and microbial components of the soil, and reduces soil organic C and N by increasing residue degradation, disrupting soil aggregation, and increasing aeration (Dalal and Mayer, 1986; Balesdent et al., 1990; Cambardella and Elliott, 1993). Similarly, fallowing reduces organic C and N by not replacing organic matter lost by mineralization through crop residue addition (Grant, 1997). In contrast, practices that reduce residue incorporation and aggregate degradation, such as no-till or strip tillage, may conserve and/or maintain soil organic C and N (Doran, 1987; Havlin et al., 1990; Franzluebbers et al., 1995b) and increase microbial biomass (Linn and Doran, 1984; Gupta and Germida, 1988; Havlin et al., 1990; Drury et al., 1991, Caesar-TonThat et al. 2001). Higher cropping intensity also minimizes OM loss by sustaining annual residue returns to the soil (Sherrod et al., 2003). Holland and Coleman (1987) reported that aggregation resulting from fungal hyphae was more easily established in soils that were minimally disturbed, and that fungi generally dominate in soils under reduced tillage.
Recovery of desirable soil structure (indicated by size distributions of WSA) under no-till may take as long as 50 years or more (Tisdall and Oades, 1980; Dormarr and Smoliak, 1985). Changes in soil organic C and N as a result of management practices occur slowly because of their large pool size and inherent spatial variability (Franzluebbers et al., 1995a; Salinas-Garcia et al., 1997). In contrast, active fractions of soil organic C and N, including potential C and N mineralization (PCM and PNM) and microbial biomass C and N (MBC and MBN) are indicators of microbial activity, N mineralization potentials, and inorganic N. However, these vary seasonally due to changes in plant residue amounts, management practices (Franzluebbers et al., 1995a; Salinas-Garcia et al., 1997), rhizodeposition of organic materials from roots (Buyanovsky et al., 1986), and seasonal changes in soil moisture and temperature (Kaiser and Heinemeyer, 1993). Similarly, particular organic C and N (POC and PON) fractions are regarded as intermediate pools of soil organic C and N (Beare et al., 1994; Franzluebbers et al., 1999). These fractions have been identified as early indicators of changes in soil organic C and N levels that influence soil aggregation and nutrient dynamics in the soil (Franzluebbers et al., 1995a; 1995b; Salinas-Garcia et al., 1997; Six et al., 1999).
There is considerable evidence supporting the involvement of polysaccharides in soil aggregation (Clapp and Emerson, 1965; Cheshire et al. 1983; Martin, 1945; Robert and Chenu 1992; Clapp et al. 1962; Tisdal and Oades, 1982; Angers and Mehuys, 1999; Haynes and Francis, 1993). Wright and Upadhyaya (1998) isolated a glycoprotein, produced by Glomales (phylum Zygomyceta [vascular arbuscular mycorrhizal fungi]), which is highly correlated with soil aggregate stability. However, these obligate fungi cannot be cultured in the laboratory because their survival depends on the photosynthetically derived carbon provided by their specific host plants. Thus, it is also important to investigate the very large populations of non-obligate fungi (easily propagated in the laboratory) for their role in soil formation and aggregation and to help understand the mechanisms by which these fungi help bind or aggregate soil particles.
Polysaccharides that contain uronic acid groups (primary carboxyl [COOH]) have ion-exchange properties that, when in the presence of di- or trivalent cations, play an important role in binding clay particles into aggregates (Martin, 1971; Chenu, 1993). Extracellular polyuronic acids are produced by plants (Vermeer and McCully, 1982; Watt et al. 1993), bacteria (Clapp et al. 1962; Griffiths and Burns 1972; Chenu 1989; Molope et al. 1987; Robertson et al. 1991; Robertson and Firestone, 1992; Skvortsov and Ignatov, 1998), and fungi (Tisdall, 1991; Singleton et al. 1990; Caesar- TonThat, 2002.). Using electron microscopy and staining of soil fabrics, Foster (1981) found that polysaccharides coat clay platelets and occur in crevices of submicron size within mineral aggregates, which helps explain how microbial polysaccharides stabilize clay aggregates.
Limited studies have shown that long-term tillage reduces soil carbohydrate content and impacts soil structure (Cheshire et al. 1984; Robertson et al. 1991; Puget et al. 1994; Dormaar, 1984; Hu et al. 1995; Beare et al. 1997; Murayama, 1984; Schlecht-Pietsch et al. 1994). There are few reports on the effects of different agricultural management practices on uronic acids used in soil aggregation. Lately, Kiem and Kogel-Knabner (2003) found that soil aggregates (200-250 um) from two different sites contained less galacturonic acids under conventional management with fertilizers than management without fertilizers. We believe that many of the complex biological, chemical, and physical processes involved in soil aggregation can be deduced by understanding the dynamics of the acidic polysaccharides. Furthermore, uronic acids can be easily measured, and should provide a reliable indicator of soil health once we understand the relationships between uronic acids and soil aggregating microorganisms (Caesar-TonThat et al. 2001), glomalin (Wright and Upadhyaya, 1998) and soil OM content.
Biotic and abiotic N dynamics are influenced by residue quality (e.g., C/N ratios), residue placement depth (e.g., surface vs buried), and various climatic and soil environmental factors (i.e. temperature, clay content and pH) (Barrett et al. 2002). Plant and microbial enzymes also play a key role in soil nutrient cycling (Kiss et. al. 1975; Ladd, 1978; Bahl and Agrawal, 1972; Tabatabai, 1994). Enzymes, primarily produced by microbes, accumulate in the soil as they are released from living cells, disintegrated cells and enzymes bound to cellular constituents (Kiss et al., 1975).
Several studies have been published on the potential use of enzyme activity as an index of soil productivity or microbial activity (Weaver et al. 1994; Dick et al. 1996) and as indicators of soil condition (Monreal and Bergstrom, 2000; Kandeler et al., 1999; Tscherko and Kandeler, 1999, Gupta et al., 1988; Klein and Koths, 1980). Additional research is needed thoughto understand the complex interactions between enzymes and the surrounding plant and soil material in order to develop meaningful indicators of microbial status and soil physico-chemical condition (AKA: soil quality).