Crop rotation and Management Practices Effects on Arbuscular Mycorrhizal Fungus (AMF)
James J. Hoorman, Assistant Professor, Ohio State University Extension Educator
Modern cropping rotations and management practices have an effect on microorganisms in the soil. In a typical corn and soybean rotation, live plants and active roots are present less than a third of the time during a calendar year (Magdoff and van Es, 2000). Restoring ecosystem functionality requires feeding the soil life and reducing tillage. Tillage destroys microbial habitat and the microbial food system. Almost 60% of soil organic matter (SOM) in temperate regions and 75% in the topics have been depleted by conventional tillage due to oxidation (Lal, 2004; Intergovernmental Panel on Climate Change, 1996).
A no-till (NT) system with cover crops (CC) provide more months of living roots and plants to restore C stores and improve soil ecology. Arbuscular mycorrhizal fungus (AMF) populations are associated with glomalin and water aggregate stability (WAS) aggregates. Glomalin is a glycoprotein produced by AMF (Rillig et al. 2001) and glomalin soil concentration is positively correlated with WAS (Wright and Upadhyaya, 1998), and improved soil structure. Nearly 90% of the SOM is located within soil aggregates (Jastrow et al., 2000). Current management practices (tillage, P fertilization and fallow cropping), NT, and CC affects on AMF colonization, glomalin production, and WAS are reviewed.
A two year Canadian study by Kabir et al. (1997) compared conventional tillage (CT; fall plowing plus spring disking), reduced tillage (RT; spring disking) and NT with corn receiving either inorganic (N or K) or organic (liquid dairy manure) fertilizer for AMF colonization. They found that densities of total and active AMF hyphae were significantly lower in CT than RT and NT. Fertilization did not affect AMF colonization in a sandy loam soil but were abundant in manured clay soils.
Galvez et al. (2000) compared a low input (organic) system to conventional agriculture and the low input systems NT and RT had higher AMF spore populations than CT. Corn plants grown in low input NT had the highest shoot P concentrations, highest P use efficiency, and enhanced AMF colonization.
Pikul et al. (2009) compared NT to chisel-tilled (CHT) for ten years, examining the quantity and quality of SOM. Pikul et al. (2009) concluded that NT resulted in better SOM quality than CHT because the decaying plant root systems remained undisturbed favoring AMF growth. These studies show the negative effects of tillage, inorganic P fertilization, and fallow cropping on GRSP, WAS, and SOM in relation to AMF colonization. Cover crops and glomalin also effect soil aggregation and stability.
Glomalin (GRSP) produced by AMF is highly correlated with WAS (Rillig and Steinberg, 2002). Driver et al. (2004) found that about 80% of the glomalin was found in hyphal walls and was released on decomposition rather than being secreted. Hyphal turnover (5-7 days half-life) is the main pathway for glomalin deposition, released to the soil from dead mycelia fragments. Rillig and Steinberg, (2002) and Driver et al., (2004) research shows how glomalin increases soil GSRP and WAS and improves soil structure.
Douds et al. (1993) found that CT yielded lower levels of AMF than low-input systems with cover crops planted between cash crops. Greenhouse bioassays showed 2.5-10 fold greater AMF colonization of plants growing in soil from low-input systems with cover crops than conventional systems. These studies show that changes in the soil ecology have a large impact on the soil quality.
INTERACTIONS WITH THE SOIL MICROBIAL COMMUNITY
Arbuscular mycorrhizal fungi modify their growth environment by producing a glycoprotein called glomain which is highly correlated with WAS. AMF under stress or less favorable growing conditions (compacted soils), poor soil structure) increased glomalin production despite smaller hyphal length (Rillig and Steinberg, 2002). Driver et al. (2004) found that about 80% of the glomalin was found in hyphal walls and was released on decomposition rather than being secreted. Hyphal turnover (5-7 days half-life) is the main pathway for glomalin deposition, released to the soil from dead mycelia fragments.
Purin and Rillig (2007) theorize that the primary role of glomalin is for fungal cell wall physiology and for defense (less palatable to grazing predators). A secondary role is in changing the soil environment associated with GRSP and WAS. Rillig and Mummey (2006) reviewed the contributions of AMF on soil structure from the soil fungus hyphae, to the individual root, and to whole plant communities. Fungal species and diversity promote soil aggregation to different degrees. The authors suggest that a multifunctional perspective be used to study AMF and feedback mechanism between soil structure and AMF arguing that the entire plant and microbial community interact to improve the soil environment.
Andrade et al. (1998a) found that roots and AMF may not associate with soil bacteria randomly, but rather in a hierarchical structure of mutual preferences. The mycorrhizal status of soils may selectively influence persistence of bacterial inoculants and affect native bacteria. Andrade et al. (1998b) showed that the plant roots and AMF enhanced WSA stability individually and additively in concert, and suggest that they affect microorganism numbers indirectly by providing habitable pore space in the WSA.
Rillig (2004) argues that research at the ecosystem level is less prominent but potentially more promising and states that too much emphasis has been put on individual plant hosts and not enough research emphasis on whole plant communities and soil ecology. Rillig notes that human-induced disturbances (global climate change and agro-ecosystem management) decrease AMF functions. He discusses four interacting routes via which AMF influence soil ecosystem processes on C cycling: 1) increasing plant species diversity and community C cycling, 2) indirect effects on soil microbial communities by changing bacteria communities in the rhizosphere, 3) individual host plant physiology (drought effects, plants as C sinks and associated ecosystem effects), and 4) direct effects of AMF and glomalin at the ecosystem level especially on soil structure and ultimately increases in C storage. Rillig hypothesizes that AMF increases infiltration, decreases runoff, promotes AMF colonization and increased C sequestration which are the same effects observed when NT and CC are used together in the soil.
Current human and agro-ecosystem management practices (tillage, P fertilization, fallow cropping) may have negative effects on AMF, and impact C cycling and storage in soil aggregates. AMF were lower in CT and RT compared to NT. No-till improves AMF colonization and glomalin production and is correlated with WAS and improved C sequestration. Glomalin in AMF is primarily a defense mechanism but has major secondary effects on WAS, soil structure, and the microbial community. Further increases in C sequestration will require agricultural practices that process greater quantities of SOM. A majority of plants have an association with AMF (Fitter et al., 2000). The review of the literature suggests that major factors that affect soil aggregation include soil fauna, microorganisms, roots, and inorganic and physical processes. AMF influence ecosystem processes directly through host physiology and AMF hyphae glomalin decomposition on WAS and indirectly changing plant and soil microbial communities.
Further research is needed on no-till and cover crop’s role in AMF colonization, glomalin production, and WAS. Continuous NT combined with CC may increase AMF populations and ultimately sequesters more C in the soil ecosystem. The CC supplies the carbon and the root exudates for AMF populations to produce glomalin or GRSP when the main grain crops are not being grown (in a corn-soybean rotation this may be greater than 68% of the calendar year). The AMF in the cover crops are generally of a different species than grain crops but provide many soil ecosystem services including improved soil structure, decreased bulk density, and improved water infiltration. Cover crops may provide a stable environment and a constant supply of C for AMF to process and incorporate into microaggregates and macroaggregates, where 90 percent of the C is stored. No-till and CC may mimic the natural environment, restore ecosystem functions, and restore balance to the soil ecosystem that past human activities have disrupted.
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