Post Flood Syndrome and Vesicular-Arbuscular Mycorrhizal Fungi

In the past few years large areas of the Midwest and Great Plains have been inundated with water and plant P deficiency is often a problem after such events. Soil and plant samples were collected in 1994 in Missouri and Iowa from sites flooded and fallowed in 1993. Plants were P deficient during early vegetative growth in spite of soil tests indicating adequate P availability and typical levels of starter fertilizer. The flood and fallow problem was associated with the loss of root colonizing vesicular-arbuscular mycorrhizal (VAM) fungi that benefit the plant by increasing uptake of P. The VAM fungi are dependent upon the plant for growth and reproduction and loss of host significantly reduces VAM fungal populations in soil. The VAM fungal populations increased with the presence of a crop in the year following the flood. A greenhouse study on the effect of flooding on VAM fungi indicates the number of VAM hyphae or spores producing colonization were not affected by an extended flooding period and therefore are related to the lack of host for an extended period of time in the same manner as “long fallow disorder” in Australia. The research conducted indicates that the Post Flood Syndrome was associated with the loss of VAM fungi. Use of starter fertilizer was beneficial in correcting Post Flood Syndrome but broadcast fertilizer application appeared to have an insignificant effect on reducing the P deficiency. The application of 80 lb P/acre as starter fertilizer produced plants that were not P deficient. Research QuestionAfter a flood, Flood Syndrome often develops in crops planted in the subsequent year. Phosphorus deficiency during the early stages of plant growth is the primary characteristic of Flood Syndrome. The objectives of this study were (i) to determine whether vesicular-arbuscular mycorrhizal (VAM) fungal colonization was decreased after flooding and fallow in fields affected by the 1993 flood in Iowa and Missouri, (ii) whether the decrease in VAM fungi ability to colonize roots was associated with plant P deficiency, and (iii) whether the decrease in potential for VAM fungi to colonize the roots of host plants was associated with lack of host or saturated soil associated with flooding. Vesicular arbuscular-mycorrhizal fungi are beneficial soil fungi that colonize roots of almost all agriculturally important plants. They require a host plant for nutrition and completion of their life cycle and fungal survival decreases when plants are not present. Vesicles are specialized structures produced by some fungi, which are used for lipid storage. Arbuscules are a specialized mycorrhizal structure produced when VAM fungi are active and the term mycorrhiza means root fungus. Literature SummaryFloods that destroy crops or prevent planting often produce a condition called Flood Syndrome the next cropping year. Crops after flooding are often P deficient, although soil-testing procedures often find more available P in soils after flooding than in nonflooded soils. The Flood Syndrome and “long fallow disorder” are both characterized by P deficient plants. In the case of long fallow disorder, Zn deficiency is also a problem. Absence of a mycorrhizal host plant during the fallow period decreases mycorrhizal colonization potential for the succeeding crop and results in P deficiency symptoms in plants that are mycorrhizal dependent, such as corn, soybean, sunflower, and cotton. Corn plants are stunted and show severe P deficiency when mycorrhizal hyphae or spores producing colonization are greatly decreased in soil. Similar problems occur with soybean and wheat plants after fallow in the USA. Study DescriptionThis research was conducted on fields flooded in 1993 in Holt County in Missouri and Polk County in Iowa. The soil types, at the Polk County site, were Colo loam and Kato loam and at the Holt County site, a Dupo silt loam. The sites were flooded from 2 to 6 wk. The flooding either killed the crop, which was were not replanted, or prevented crops from being planted. Each flooded site had an adjacent field or part of the field that suffered no flood damage that served as controls. At planting, samples were taken from sites for VAM fungal bioassay and soil chemical analysis. Fertilizer treatments included starter P of 25 to 80 lb/acre. Shoot biomass, mycorrhizal colonization of roots, P, Ca, Mg, K, and Zn content of shoots were measured during the growing season and final corn grain yield was measured at maturity. Soil in greenhouse pots was also flooded from 1 to 3 wk to determine the effect of water logging on VAM fungi survival. Applied QuestionsDid flooding and fallow affect arbuscular-mycorrhizal fungal survival and was this associated with decreased P uptake by plants? There was a significant decrease in VAM fungal hyphae or spores producing colonization in soils flooded the previous year. Both VAM fungal colonization potential and activity were decreased, based on mycorrhizal root colonization and arbuscule formation. In the bioassay of field soils, the ability of VAM fungi to colonize roots was decreased in flooded and fallowed soils. The loss of VAM fungi in the roots of plants could affect plant P and Zn uptake. As the season progressed, VAM fungal root colonization of plants grown in flooded and fallowed soil increased to a level similar to that in plants from soils that were not flooded and plant P content was not deficient The VAM fungi in roots from flooded soil would have less effect on P uptake of plants grown in flooded and fallowed soil than plants grown in nonflooded because of the higher intensity (total fungal hyphae in roots) of root colonization. The decrease in shoot weight and grain yield from plants grown in flooded fields was probably associated with decreased VAM fungal root colonization during early stages of plant development, which resulted in lower plant P concentrations. The data from this study support the concept that Hood Syndrome is very similar to the long fallow disorder discovered in Australia in that the effect on plants was associated with a marked decrease in VAM fungal hyphae or spores producing colonization. In the greenhouse study on flooded soils, root colonization by VAM fungi and their activity, as measured by arbuscule production, was not affected. The results of this experiment would support the hypothesis that lack of host plants during an extended fallow was the reason for decreasing VAM fungal hyphae or spores producing colonization and not extended water saturation of soil. During the year following flooding and fallow, the growing plants were a host for VAM fungi and the population was increased. However, extreme loss of VAM fungi may require more time to rebuild the population. In some cases, a plant which is less dependent upon VAM fungi can be planted. Soybean is more tolerant to decreased VAM fungal colonization and sorghum has even greater tolerance. In general, the shorter and coarser the root system, the greater the host plant dependence on mycorrhiza. Is P fertilizer treatment adequate to overcome the loss of the arbuscular-mycorrhizal fungus from soil? Although there was no treatment without starter fertilizer applied, plants with the lowest rate of 25 lb P/acre had severe P deficiency, which resulted in 32 bu/acre yield decrease in the flooded field, compared with the nonflooded field. Broadcasting P fertilizer did not have a significant impact on P uptake by plants. When 60 to 80 lb P/acre as starter fertilizer was applied to plots, plants did not show any P deficiency but flooded fields had a 7 to 16 bu/acre decrease in grain yield. An alternative to P fertilization, when soil P is adequate, could be to grow a fall cover host crop to rebuild the VAM fungal population.