Determination of the relationship between soil nutrients, mycorrhizae, and plant health in organic blueberry production
We completed a second field season of a survey of plant health and soil biology in eight pairs of organic and conventional blueberry fields matched by USDA-NRCS soil series. Considered together, the observed management-specific effects on ericoid mycorrhizal colonization and soil C, N, and P cycling suggest a shift in the functions of soil microbial communities results from the transition from conventional to organic management practices.
In a six-month experiment conducted first in a growth chamber and then a greenhouse, inoculation with ericoid mycorrhizal fungi increased the shoot growth of plants fertilized with feather meal but not compost. After 165 days, mineral nitrogen concentrations in the container substrate were 30 ppm in the feather meal treatment and less than 1 ppm in the compost treatment, indicating more gradual but sustained release of N from feather meal compared to compost. At the end of the experiment, only plants fertilized with compost were colonized by mycorrhizae, which may be related to non-symbiotic, saprotrophic survival of ericoid mycorrhizal fungi in organic matter supplied with compost but lacking in the feather meal treatment. In the latter 90 days of the experiment, plants were subjected to environmental stresses (insect feeding and high temperature) in the greenhouse. Compost added to the container substrate may have facilitated survival of ericoid mycorrhizal fungi and subsequent recolonization of blueberry hair root cells when plant shoot growth resumed after plants were transferred to a lower temperature greenhouse and insect pests were eradicated.
Our primary objective is to understand the relationship between mycorrhizal colonization, plant health, and soil biology in organic and conventional blueberry fields in Michigan.
In the field survey, samples were collected on three dates to assess seasonal variation in mycorrhizal colonization, soil C and N cycling, and soil enzyme activity. The specific laboratory protocols that we followed will be described in the final report. Correlation analysis of mycorrhizal colonization and soil variables was carried out on both sand and muck soil types together (n=16), and on sands and mucks separately (n=12 and n=4, respectively) due to extreme values in soil parameters on muck soils.
We investigated growth and mycorrhizal colonization of container-grown blueberry plants fertilized with feather meal or compost as nitrogen sources and inoculated, separately or in combination, with three species of mycorrhizal fungi in a 180-day experiment. Plants were first grown in a growth chamber and moved to a greenhouse in the latter 90 days of the experiment.
Ericoid mycorrhizal colonization levels were higher in organic blueberries and did not vary by sampling date. Inorganic soil nitrogen was 1.5 times higher in conventional fields in July, and slightly elevated in organic fields in October. Potentially mineralizable soil nitrogen was higher in organic fields in July. Soils under organic management had higher labile soil C content, B-glucosidase and N-acetylglucosaminidase activity, and a wider ratio of N:P enzyme activity than conventionally managed soils, while conventionally managed soils had more slow-cycling soil C.
Of the 30 field variables assessed, ericoid mycorrhizal colonization was significantly correlated with soil pH and hair-root diameter (positive correlations), and field age, soil ammonium, and B-glucosidase, cellobiosidase, and phosphatase activity (negative correlations), and marginally correlated with total soil N content and cellobiosidase activity (negative correlations).
Compost increased shoot length over feather meal on all dates except the final measurement. Among plants fertilized with feather meal, those inoculated with the ERM fungus Rhizoscyphus ericae (UAMH 9270) had more shoot growth compared to plants inoculated with Oidiodendron maius (UAMH 9263), an unidentified fungus associated with ericaceous roots (UAMH 9264), or a combined inoculum of all three fungi, while uninoculated plants had reduced shoot growth compared to plants inoculated with any fungus. Inoculation with ericoid mycorrhizal fungi did not affect the shoot growth of plants fertilized with compost. At 165 days after fertilizer addition, the concentration of inorganic N in leachate collected from the feather meal fertilizer treatment container substrate were nearly 30 ppm and less than 1 ppm in the compost fertilizer treatment container substrate. Plants fertilized with compost had higher mycorrhizal colonization than those fertilized with feather meal 180 days after inoculation, the only time that mycorrhizal colonization was quantitatively assessed.
Impacts and Contributions/Outcomes
Prior to this study, the effect of organic and conventional management practices on soils in blueberry fields had not been described. Our results strongly suggest that biological attributes of blueberry soils are differentially impacted by organic and conventional management. Our findings are useful for understanding soil biology in cultivated highbush blueberries and may contribute to the development of science-based recommendations for organic blueberry production in Michigan and other regions. A greenhouse study demonstrated that feather meal and compost fertilizer differ in nitrogen release over time, and in their effect on mycorrhizal colonization, and that inoculation with mycorrhizal fungi is more beneficial to the growth of container-grown plants fertilized with feather meal than compost.
We presented the survey of organic and conventional farms at the 2009 MSU Organic Research Symposium (poster), the 2009 MSU Plant Science Graduate Symposium (poster), a regional blueberry grower meeting in December 2009 attended by approximately 50 growers (presentation), the 2010 Organic Farming Conference in La Crosse, WI (poster), and the 2010 Michigan Organic Conference in East Lansing, MI (poster).
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