- Fruits: berries (blueberries)
- Crop Production: biological inoculants, nutrient cycling, organic fertilizers, tissue analysis
- Natural Resources/Environment: indicators
- Production Systems: agroecosystems, organic agriculture, transitioning to organic
- Soil Management: nutrient mineralization, organic matter, soil analysis, soil chemistry, soil microbiology, soil quality/health
We conducted an observational study to determine the effect of organic and conventional management on plant health and soil biology in blueberry fields in Michigan. Organically grown fruit had a higher incidence of anthracnose rot while conventionally grown fruit had a higher incidence of Alternaria rot. Mycorrhizal colonization levels were significantly higher in organic blueberries. Organic and conventional blueberry production practices differed in their effects on soil processes. Lower midseason levels of inorganic N but higher levels of potentially mineralizable N in organically managed soils provides evidence for greater reliance on soil processes in meeting crop N needs in organically managed blueberry fields. Short- and long-term soil incubations and potential soil enzyme activity assays demonstrated that organic management practices enhance biological activity in soil while conventionally managed soils tend to accumulates in slow-cycling soil organic matter fractions. Microbial N and P acquisition via secretion of N- and P- degrading enzymes diverged according to management, with a shift towards N acquisition in soils on organic farms and P acquisition in soils on conventional farms.
A greenhouse experiment demonstrated that inoculation with mycorrhizal fungi increased shoot growth of plants fertilized with feather meal, while compost enhanced the survival of mycorrhizal fungi. 165 days after compost and feather meal application, the N-supplying capacity of compost was nearly exhausted while feather meal continued to release N. This finding suggests protein fertilizer is more likely than compost to elevate late-season N levels in blueberry fields.
Highbush blueberries (Vaccinium corymbosum L.) are adapted to moist, coarse-textured acidic soils with high organic matter content, and not easily cultivated in soils lacking these characteristics (Ballinger, 1966; Haynes and Swift, 1986). If basic cultural requirements are met, blueberry fields in Michigan remain productive for 40 years or longer (Hanson and Mandujano, 1997). This is noteworthy because there is evidence that long-term additions of fertilizer N in cropping systems may deplete soil organic matter (SOM) and reduce crop yield potential over time (Khan et al., 2007). Recent studies have shown that mineral N inputs intensify the activity of enzymes linked to mineralization of the most labile soil C pools, resulting in declines in biologically active SOM (Sinsabaugh et al., 2002; Keeler et al., 2009). The remarkable longevity of intensively managed blueberry fields in Michigan indicates that unique characteristics of the highbush blueberry and its culture may create feedbacks between management, soil, and plants such that conclusions drawn from studies conducted in annual cropping or unmanaged ecosystems may not be applicable to blueberry fields in Michigan.
The highbush blueberry hosts a unique symbiosis between roots and soil fungi known as ericoid mycorrhizae (ERM). ERM are thought to perform a similar function to root hairs, which facilitate water and nutrient uptake from soil in most plants but are absent from blueberries and other Ericaceae-family plants (Vander Kloet, 1980). Ericaceous hair roots have a diameter of <100 µm, a C:N ratio of 20:1, and median turnover time of 120 days (Valenzuela-Estrada et al., 2008). In the Ericaceae, hair roots constitute 40 to 90% of the total root biomass, and it is common to find 90% of the outer layer of cells of hair roots colonized by ERM fungi in natural ecosystems (Read, 1996; Scagel, 2003). ERM fungi can utilize nitrogen (N) in virtually all forms that are found in soil, including inorganic (nitrate and ammonium) and organic (amino acids, proteins, chitin, and tannin-precipitated protein) compounds (Read et al., 2004). By synthesizing a wide array of enzymes, ERM are able to attack structural components (e.g. cellulose and lignin) of soil organic matter and mobilize biochemically protected nutrients including N and P (Read, 1996). Studies using isotope-labeled compounds demonstrated that organic N uptake by ERM-colonized plants was significantly higher than by non-ERM plants grown in aseptic culture (Sribley and Read, 1974) and unsterilized soil (Stribley and Read, 1980). ERM play a major role in niche partitioning in natural ecosystems by allowing ericaceous plants to utilize organic forms of N in soil and detritus that is less accessible by ectomycorrhizal plants and not utilized by arbuscular or non-mycorrhizal plants (Michelsen et al., 1996). N contained in more recalcitrant organic matter may be accessed by ERM fungi via secretion of peroxide, which reacts with Fe+2 in acidic soils to generate hydroxyl radicals which catalyze non-specific combustion of lignin molecules (Burke and Cairney, 1998). Ericaceous plants are highly dependent on ERM for N acquisition in their native state, where soil conditions typically limit the breakdown of organic matter and accumulation of inorganic N (Read and Perez-Moreno, 2003). Native Vaccinium spp. in a forest understory obtained 86% of N via ERM hyphae (Hobbie and Hobbie, 2006). The symbiosis is not without cost to the plant, as ERM are a sink for up to 20% of net photosynthetic C (Hobbie and Hobbie, 2006).
In an early study of the ERM colonization in response to varying inorganic N concentration, Stribley and Read (1976) showed that ERM colonization was reduced at high concentrations of inorganic N despite prolific superficial growth of fungal mycelium over roots. This suggested that initial establishment of the symbiosis may be regulated to some extent by the plant host. However, studies conducted under field conditions have shown inconsistent responses of ERM colonization levels to soil N enrichment (Johannson, 2000; Yang et al., 2002), which is likely due in part to the complex nature of the soil environment. In a meta-analysis of studies that included N fertilization or enrichment and measures of mycorrhizal colonization, the percentage of root length colonized by mycorrhizae was reduced by an average of 5.8% (Treseder, 2004)
Blueberries are a major crop in Michigan, and the number of organic blueberry producers in the region is increasing steadily (pers. comm., Michigan Department of Agriculture accredited organic certifiers, list available at http://www.michigan.gov/mda/0,1607,7-125-1569_25516-55175--,00.html). However, organic blueberry culture has not been researched as extensively as conventional blueberry production systems in which use of synthetic fertilizers and pesticides is allowed. According to a survey taken at the 2007 Great Lakes Fruit and Vegetable Expo, regional blueberry growers have considered organic production but are reluctant to transition over a major portion of their production area because of anticipated yield declines during the 3-year transition period and uncertainty associated with the conversion to a somewhat unknown production system. In addition, organic fruit and vegetable growers with diversified crops are interested in producing blueberries on a small scale but are limited by a lack of cultural information on organic blueberry production. In 2008, about 60 acres of blueberries were certified organic in Michigan, which amounts to only 0.3% of the total blueberry production area. A few blueberry farms in the region have been managed with organic practices for more than 30 years, but the majority of certified organic blueberry fields in Michigan began the organic transition after 2005. These certified organic blueberry farms provide an opportunity to compare two distinct management systems in terms of soil microorganism communities, soil biological activity, mycorrhizal colonization, and disease incidence. We hypothesize that: 1) blueberries on organic farms are more N-limited and allocate more C below-ground to increase nutrient uptake, resulting in higher ERM colonization, while ERM will be less abundant on conventional farms due to heightened levels of soil N; 2) turnover rates of labile soil C will be higher on conventional farms due to synthetic N inputs, leading to diminished labile soil C compared to organic farms; 3) the turnover rate of lignin as measured by phenol oxidase and peroxidase activity, will be unaffected by management but correlated to ERM infection due to the lignin-degrading ability of ERM fungi, and 4) the N-supplying capacity of soil from conventional farms will be lower than organic farms due to synthetic N-mediated depletion of labile C pools and resultant lower biological activity and nutrient turnover; and 5) the prevalence of diseases will vary between management systems due to contrasts in pesticide efficacy and other factors.
Nitrogen fertilizers in use on organic farms are limited to materials derived from natural sources (http://www.ams.usda.gov/AMSv1.0/nop) such as compost and protein meals. Conventional growers satisfy crop N needs with inorganic nitrogen or urea because synthetic N is generally less costly than organic sources of N. Ericaceous plants such as highbush blueberry (Vaccinium corymbosum L.) are better able to utilize organic nitrogen when colonized by ERM fungi (Stribley and Read, 1980; Bawja and Read, 1986; Sokolovski et al., 2002). Scagel (2005) reported that nursery-grown blueberries supplied with organic fertilizers grew more vigorously when inoculated with ERM fungi, but the growth of plants fertilized with synthetic nutrients was superior to those grown with organic fertilizer and less affected by ERM inoculation. However, Scagel (2005) applied synthetic and organic fertilizers at the same rate of total N and did not consider the possibility for different nitrogen release patterns of organic and synthetic fertilizers over time. High levels of inorganic nitrogen may inhibit ERM colonization of ericaceous plants (Stribley and Read, 1976). The question of whether high availability of organic nitrogen inhibits ERM colonization has not been tested. Organic forms of nitrogen such as amino acids stimulate fungal growth when added to soil (Lucas et al., 2007). In Michigan, soil inorganic nitrogen levels are higher in conventional compared to organic blueberry fields when plant nitrogen demand is highest, but leaf tissue nitrogen content does not differ significantly by management type (unpublished data). In addition, ERM colonization in Michigan is, on average, higher in fields under organic management (unpublished data). Taken together, these data suggest ERM may be particularly important in plant uptake of nutrients from organic fertilizers. The objective of this study was to determine the effect of nitrogen fertilizer type on growth and ERM colonization of container-grown blueberries inoculated with three species of ERM fungi.
Our primary objective is to understand the relationship between mycorrhizal colonization, plant health, and soil biology in organic and conventional blueberry fields in Michigan.