CHAPTER 2: THE EFFECT OF SOME SOIL PROPERTIES AND EXTERNAL
2.2 Soil factors
2.2.7 Fungi and bacteria
Among soil organisms, fungi have been found to be very important in the formation and stabilization of soil aggregates via both direct and indirect contributions. The direct effect is through the hyphae network that binds the soil particles and forces them together or aligns soil particles along the expanding hyphae (Siddiky et al., 2012; Tisdall et al., 2012). Indirectly, the arbuscular mycorrhizal fungi (AMF) secrete glomalin-related soil protein (GRSP) or polysaccharides that may glue and bind soil particles together (Rillig et al., 2005; Kohler et al., 2010; Siddiky et al., 2012). The improvement of soil aggregation also provides a conducive and protected environment for soil microorganisms and facilitates root oxygenation (Denef et al., 2001). Arbuscular mycorrhizal fungi also alter the community structure of microorganisms, both in their own surroundings and in the host plant rhizosphere (Rillig et al., 2005; Siddiky et al., 2012). An increase in AMF in the soil may increase the population of other soils organisms that feed on them. A combination of AMF and Collembola positively increased the proportion of water stable aggregates in an Albic Luvisol collected from the experimental farm of the Freie Universität Berlin (Figure 2.3) (Siddiky et al., 2012).
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Figure 2.3: The effects of Collembola (C), arbuscular mycorrhizal fungi (M) and their interaction (CM) on the proportion of water stable aggregates (WSA) in four aggregate fractions of an Albic Luvisol under sorghum and Daucus (wild carrot) compared to the control treatment (Con) at the experimental farm of the Freie Universität Berlin (Siddiky et al., 2012).
Collembola are one of the most abundant groups of soil arthropods that feed on AMF.
Collembola also improve the soil structure through their feeding behavior as they incorporate considerable amounts of SOM into faecal pellets, which increase the soil surface area and accessibility for bacterial and fungal utilization and thus increase decomposition (Rillig et al., 2005; Siddiky et al., 2012). A positive and significant relationship between soil microorganisms (total bacteria, anaerobes and fungi) and soil AS was reported (Andrade et al., 1998; Figure 2.4), although fungi showed a stronger effect (i.e. higher correlation coefficient) on AS than bacteria or anaerobes.
Fungi are known to be more effective soil aggregating and stabilizing microorganisms than other soil microflora according to Beare et al. (1997). Rillig et al. (2005) reported that fungi can increase microbial communities in their surroundings that are possibly involved in soil aggregation processes by exuding photosynthesis-derived carbon into the mycorrhizosphere which serves as food for them. The effect of total bacteria and anaerobes on AS accounted for approximately 40% and 43.5% only, respectively, though it was significant according to Andrade et al. (1998) (Figure 2.4). Since the correlation between AS and fungi was about 70%,
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it could be deduced that the contribution of the microorganisms to AS ranged from 40 to 70%
and further soil aggregation was possibly due to other factors such as clay type and content, OC and other microorganisms that were not measured in the study. A negative correlation between fungi and AS at the beginning and the positive correlation observed at the later stage of the research conducted by Kihara et al. (2012) indicated that fungi only play a significant role in soil aggregation beyond a certain threshold (0.7 Simpson’s index in their study). This relationship suggested that fungus species that are effective in the formation of macroaggregates thrive in low density and can be replaced by less effective fungus species as the diversity increases.
Kohler et al. (2010) conducted research in a saline soil and found no relationship between hyphae and AS and a negative relationship between GRSP and AS which was associated with the increase in Na concentration in the soil. González-Chávez et al. (2004) stated that the GRSP produced by fungi is very efficient in sequestering different toxic elements including Na that have negative effects on AS. Another research study by Caesar-TonThat (2002) reported that polysaccharides and GRSP rich soil treated with sodium tetraborate showed that Na can destroy long-chain polysaccharides and so disrupt soil aggregates.
The role of microorganisms on AS is also influenced by the environment and the amount of food available where they are found. Graham and Haynes (2006) measured AS and microbial biomass populations from the inter-rows and intra-rows of sugarcane and found higher AS and microbial biomass in the intra-rows. The intra-rows are usually moister and have more organic matter than the inter-rows making the environment more favourable for microorganisms and production of mucilage that cements the soil particles and microaggregates to increase AS.
Microorganisms might have been adversely affected by environmental stress such as a sparsity of labile carbon (food for microorganisms) or water stress in the inter-rows in comparison with the intra-rows.
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Figure 2.4: The relationship between soil microorganisms and soil aggregation measured under split-root sorghum plants grown in multi-compartment containers. The soils in the individual compartments were permeated by (M) arbuscular mycorrhizal roots and arbuscular mycorrhizal hyphae; (H) arbuscular mycorrhizal hyphae only; (R) non-arbuscular mycorrhizal roots; or (S) free of roots and arbuscular mycorrhizal hyphae as bulk soil. Data points indicate the number of colony-forming units of the groups of organisms assayed. The organisms found in the different compartments are represented by different symbols (Andrade et al., 1998).
2.3 External factors