A COMPARISON OF MYCORRHIZAL COLONIZATION IN CORN IN CONTINUOUS AND ORGANIC SYSTEMS

Caroline Brock, Dr. Walter Goldstein, Dr. Joshua Posner, Dr. Martha Rosemeyer

INTRODUCTION

Mycorrhizal fungi are a symbiotic organism that commonly occurs with many crop plants.  Most agricultural crops are colonized by an endomycorrhizae, vesicular arbuscular mycorrhizal fungi (VAM), whose presence is characterized by distinct fungal structures formed within the root cortex of the plant host.  The benefits that mycorrhizal fungi give to host plants include increased nutrient uptake, improved root health and expanded effective root surface area, as well as creating a desirable environment for other symbionts such as rhizobium.  However, mycorrhizal fungi also consume valuable carbohydrates from their host plants and may in certain situations cost the plant more in resources than the benefits associated with their presence (Johnson et al, 1997).  Mycorrhizal fungi seem to be especially stimulated in cropping systems which incorporate crop rotations, green manures, reduced tillage, minimize pesticides and chemical fertilizers while utilizing organic amendments (Douds et al., 1997). 

Continuous, organic cash grain and forage systems were selected for study as a result of the potentially important role that mycorrhizal fungi play in cropping systems.  A continuous corn system (CS1) and a three year organic forage/dairy system (CS5; corn-oats/field pea/alfalfa - hay) are both systems that occur on farms in the Upper Midwest.  An alternative three year organic cash grain (CS3; corn-soybean-wheat/red clover) was included in the comparison.  In addition, organic and biodynamic compost was added to the subplots in the two organic systems.  The study was focused on testing the following hypotheses.

Hypothesis # 1: Mycorrhizal colonization will be higher in the organic dairy and grain systems than in the continuous corn system as a result of beneficial management practices.  

Hypothesis # 2: The addition of compost will result in greater mycorrhizal colonization. 

METHODS

The research subplots were 4.57 m X 7.62 m (6 rows X 25 feet) on the edge of the larger WICST plots which were 0.3 hectares. Organic forage plots were only sampled in 2001 so there were four sampled treatments in 2000 and all seven were sampled in 2001 (Table 1).  The continuous (CS1) system received starter fertilizer and nitrogen fertilizer.  The organic cash grain (CS3) and organic forage (CS5) subplots each had three treatments consisting of control (receiving no dairy manure compost) (chk), a subplot treatment of dairy manure compost (og), and a subplot treatment of biodynamic dairy manure compost (bd).  (The organic systems were managed without chemicals but were not certified by a third party agency).  Compost was applied at a rate of 180 kg/hectare available nitrogen equivalent which is similar to the rate of chemical fertilizer that was applied on continuous corn (CS1).  Special botanical preparations were added to biodynamic subplot treatments that are thought to improve soil and crop quality in the farming system.  Weed and pest management was primarily chemical in the continuous system (CS1) compared to mechanical management in the organic systems (CS3 and CS5).  In 2000, corn in the organic cash grain (CS3) system was replanted with an early maturing variety over a month after the original planting date due to the lack of germination probably as a result of excess moisture and fungal infection. 

Three soil blocks (38.1 cm X 17.78 cm X 20.32 cm) were taken from representative corn plants in each of the subplots (refer to Table 2 for dates).  Roots were separated based on nodal system (refer to Figure 1). In 2000, nodes 1 through 3 were analyzed and in 2001, a weighted average of nodes seminal-five was analyzed.

Roots were stained according to a slight variation of the Phillips and Hayman method (Phillips and Hayman, 1970).  Stained roots were analyzed for mycorrhizal fungal root colonization based on the gridline intersection method described by Giovannetti & Mosse (1980) where root fragments are spread on a petri dish and a minimum of 750 intersections on a gridline were assessed for mycorrhizal colonization.  Each intersection was recorded as either being colonized or not colonized.

RESULTS

In contrast to hypothesis #1, mycorrhizal colonization levels did not differ by farming system in any sampling month (July, August, September) in 2000 (Figure 2; Table 3).  There was a trend however, in all months for higher levels of mycorrhizal colonization in continuous (CS1) corn than the organic cash grain (CS3) (Figure 2; Table 3).  In addition, mycorrhizal colonization was not statistically different with compost application (Figure 2; Table 3). 

Mycorrhizal colonization levels differed by cropping system in June 2001 (Figure 3; Table 4 column a).  In June, continuous corn had higher colonization levels than an average of organic check corn from the grain and forage rotation (Table 5; Contrast #1).  Check plots in cash grain systems had higher levels of mycorrhizal colonization than forage systems (Table 5; Contrast #2) and continuous grain had higher levels of mycorrhizal colonization than check organic cash grain (Table 5; Contrast # 3).  Organic cash grain had marginally higher levels of mycorrhizal colonization than organic managed forage systems (Table 5; Contrast #4 p<.10).  Compost did not significantly increase mycorrhizal colonization in the organic systems (Table 5; Contrast #5 & #6).  An average of mycorrhizal colonization levels in organic systems with organic compost were similar to organic plots treated with biodynamic compost (Table 5; Contrast #7). 

By August, the organic check plots showed higher rates of colonization and were nearly equivalent to that of continuous corn. Surprisingly, there was little increase in mycorrhizal colonization levels in subplots amended with compost (Figure 3; Table 4 column b).  Mycorrhizal colonization was not effected by biodynamic preparations (Figure 3; Table 4 column b).

DISCUSSION

In contrast to the initial hypothesis of higher levels of mycorrhizal colonization in organic systems amended with compost, lower mycorrhizal colonization levels were observed in the organic systems in this experiment and compost either did not effect or repressed mycorrhizal colonization in 2000 and 2001.  Also in contrast to the initial hypothesis of higher colonization in forage-based systems, there were significantly lower colonization levels than the cash grain systems in June 2001 and the levels were equivalent by August 2001. 

Lower mycorrhizal colonization levels may have been observed because corn is a more ideal host crop than the alternative crops in the longer organic rotations.  The effect of a crop rotation on mycorrhizal colonization is very specific to the species involved as crops vary in their suitability as hosts for mycorrhizal fungi (Thompson, 1997).  Organic systems used more tillage which could also have disrupted the hyphal structure (refer to Appendix II). Tillage may play a key role in explaining the higher levels of colonization in the continuous grain system as tillage is known to repress the development of mycorrhizal populations (McGonigle & Miller, 1993; Hamel, 1996; Johnson & Pfleger, 1992). 

Since mycorrhizal colonization did not consistently increase yield, stalk phosphorus  and grain oil content, it is possible that mycorrhizal fungi were playing a non-beneficial and/or parasitic role at this study site due to high levels of soil phosphorus.  In fact, percent mycorrhizal colonization in August 2000 & 2001 was negatively correlated with grain yield (R²=.14, p<.05) and (R²=.23, p<.005).   Also, there was not a consistent relationship between mycorrhizal colonization, and plant tissue phosphorus levels.  Although, plant phosphorus concentrations were positively correlated with mycorrhizal colonization in July 2000 (R²=.11), other sampling months in 2000 and August 2001 showed no correlation.  This also suggests that colonization did not play a role in plant phosphorus accumulation.

The complexity of nature, especially that part of nature which lies underground, makes the study of the effect of farming systems on root biology a challenging proposition.  The current knowledge of soil biology can be illustrated by the fact that soil ecologists can not explain the feeding strategies of more than 90% of soil biota (Wall and Virginia, 2000).  This study found that mycorrhizal colonization was not suppressed but actually enhanced in the continuous corn system.  These results were contrary to the initial expectations.  Further research is needed to understand the role of mycorrhizal fungi in corn production on rich prairie soils of the Upper Midwest. 

SUMMARY

Mycorrhizal colonization levels were compared in continuous corn and organic rotations (with and without compost).  Mycorrhizae are generally viewed as a symbiotic association between plants and fungi that provides increased access to immobile nutrients in exchange for plant carbohydrates.  In contrast to the initial hypothesis of higher levels of mycorrhizal colonization in organic systems amended with compost, lower mycorrhizal colonization levels were observed in the organic managed systems in this experiment and compost either did not effect or repressed mycorrhizal colonization in 2000 and 2001.  Lower mycorrhizal colonization levels may have been observed because corn is a more ideal host crop than the alternative crops in the organic systems.  Organic systems used more tillage which could also have disrupted the hyphal structure.  Since mycorrhizal colonization did not consistently increase yield, stalk phosphorus and grain oil content, it is possible that mycorrhizal fungi were playing a non-beneficial and/or parasitic role at this study site due to high levels of soil phosphorus.  Although mycorrizal levels were higher in continuous corn, further research should explore the role these fungi are playing in high phosphorus soils in the Upper Midwest. 

LITERATURE CITED

Douds Jr., D.D.,  Galvez, L, Franke-Synder, M., Redier, C. and Drinkwater, L.E.  1997. Effect of compost addition and crop rotation point upon VAM Fungi. Agriculture, Ecosystems and Environment 65. 257-266.

Galvez, L. Douds, D.D Jr, Wagoner, P., Longnecker L.R., Drinkwater L.E., and R.R. Janke. 1995. An overwintering cover crop increases inoculum of VAM fungi in agricultural soil. American Journal of Agriculture. Vol. 10 (4)

Hamel, C. 1996. Prospects and problems pertaining to the management of arbuscular mycorrhizae in agriculture. Agriculture Ecosystems & Environment. pp. 197-210.

Johnson, N. and F.L. Pfleger. 1992. Vesicular Mycorrhizae and Cultural Stresses. p. 71-101. In G. J. Bethlenfalvay and Linderman, R.G. (ed.) Mycorrhizae in Sustainable Agriculture. American Society of Agronomy, Madison WI.

Johnson, N.C. and J. H. Graham and F.A. Smith. 1997. Functioning of mycorrhizal associations along the mutalism-parasitism continuum. New Phytology. 135. 575-585.

Kesselbach.  1980.

McGonigle, T.P. M.H. Miller. 1993. Mycorrhizal development and phosphorus absorption in maize under conventional and reduced tillage. Soil Science Society of America. 57: 4 p.1002-1006.

Thompson, J. & R. Bowman, N. Seymour, D. Peck & T. Peck. 1997. Farming System Institute Crop Link Research Focus. Department of Primary Industries (DPI) Queensland, Australia.

Wall, D.H.,  and R.A. Virginia. 2000. The world beneath our feet: soil biodiversity and ecosystem functioning. Pages 225-241 in P.R. Raven and T. Williams, editors. Nature and human society: the quest for a sustainable world. National Academy of Sciences and National Research Council, Washington, DC.

Table 1. Treatments sampled in 2000 and 2001

¹Corn phase was sampled

Table 2. Root sampling dates

¹VAM = vesicular arbuscular mycorrhizal fungi

Table 3. Mean levels of mycorrhizal colonization in 2000

¹nodes 1-3

Table 4.  Mean levels of % mycorrhizal colonization in 2001

¹weighted average of seminal-five nodes

Table 5. Contrasts on the effect of farming system on % mycorrhizal colonization in June 2001

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