1a. Objectives (from AD-416):
One objective of this research project is to explore the use of arbuscular mycorrhizal [AM] fungi in crop production. The sources of AM fungi examined will be both indigenous fungi present in the soil and inoculum produced using on-farm technology developed as part of the parent CRIS project. The role of indigenous AM fungi in crop growth will be explored via creative use of tillage operations and an exclusion technique. The use of inoculum will be tested for crops in which seedlings are first grown in the greenhouse for later out planting to the field. Preliminary experiments will be conducted for the latter to optimize greenhouse culture regimes to produce seedlings that are well colonized by AM fungi and of satisfactory size and vigor via organic methods. A second objective of this project is to pursue modifications to the technology for the on-farm production of AM fungi to produce resultant inocula in a form amenable to mechanical application to the field. A third objective is to study the impact of the transition to no-till agriculture upon carbon sequestration and AM fungus populations in row crop agriculture.
1b. Approach (from AD-416):
The proper utilization of inoculum of AM fungi to produce vegetable seedlings for out planting to the field requires modification of routine greenhouse fertilization regimes. Too much fertilizer will inhibit colonization of plants, wasting the investment in inoculum. Conventional growers have a variety of chemical fertilizer preparations at their disposal. Organic growers are limited in this respect, and for the most part cannot manipulate the levels of individual mineral nutrients in fertility regimes. Since AM fungi are particularly useful in agricultural systems shunning chemical fertilizers and pesticides, it is important that organic growers have a greenhouse regime amenable to produce well colonized, healthy seedlings. Greenhouse potting media amendments such as compost, and high N low P fertilizers such as fish emulsion, will be studied for their effect upon AM fungus colonization of roots and overall plant growth. The role and use of AM fungi in crop production will be studied several ways. First, prior observations indicate that inoculation with AM fungi produces a significant growth response in the field when the soil has experienced disturbance, such as roto-tilling to control weeds prior to planting. This will be studied using a variety of repetitions of roto-tilling, at varying times before planting mycorrhizal and non-mycorrhizal leek (Allium porrum) seedlings. Leeks will be used because their mycorrhizal dependency makes them good test organisms. A second study of the use of AM fungi in crop production will utilize sweet corn. A recently adopted method for the production of sweet corn entails the production of seedling sin the greenhouse which are them later transplanted to the field. The use of AM fungi has not been examined in this production system. Uninoculated and pre-inoculated seedlings will be transplanted to the field and the effect of the inoculation upon plant growth and final yield will be quantified. The use of AM fungus inoculum produced on-the-farm is primarily limited to the production of seedlings in the greenhouse. Here, the inoculum can be efficiently and economically mixed into horticultural potting media. Application of inoculum to the field is labor intensive and limited to special, small-scale operations. The on-farm system of inoculum production produces AM fungi in a compost and vermiculite mixture. Other amendments to the system, such as porous biochar, will be tested for their use as carrier materials amenable to mechanical application to planting sites in the field. Work at the Rodale Institute’s Farming Systems Trial has shown greater carbon sequestration in organically than conventionally farmed soils. This result came from farming systems that incorporated conventional tillage at all points of the crop rotation. No-till has recently been incorporated as a treatment in the Farming Systems Trial, offering the opportunity to study the potential added benefits to carbon sequestration via reduced tillage.
3. Progress Report:
Arbuscular mycorrhizal (AM) fungi are naturally-occurring beneficial soil fungi that form a symbiosis with most crop plants. The plants benefit through enhanced: uptake of mineral nutrients, water stress resistance, and disease resistance. Vegetable seedlings grown first in the greenhouse prior to transplanting to the field, can benefit by being colonized by AM fungi during the greenhouse growth phase. The unique nutrient sources utilized in organic agriculture make it more difficult to obtain high AM fungus colonization of vegetable crop roots during the greenhouse growth phase. The use of compost in organic potting media can inhibit colonization of roots by AM fungi due to the resulting phosphorus (P) content of the potting medium. Plants growing in soil high in available P restrict the colonization of their roots by AM fungi. Peppers and tomatoes are moderately responsive to AM fungus colonization and our previous work showed that AM fungus colonization of roots of these plants exhibits strong sensitivity to P levels. Therefore, greenhouse nutrient regimes must be properly formulated to ensure P concentrations are low enough to allow colonization, yet supply sufficient levels of other nutrients, notably nitrogen, to support plant growth. Outplanting uncolonized seedlings due to over-fertilization in the greenhouse would confer no advantage, and result in money wasted on commercially-purchased inoculum. Research at The Rodale Institute with USDA-ARS seeks to find an organic greenhouse production regime that will produce significant levels of AM fungus colonization while increasing plant health. Two cultivars of peppers, two cultivars of tomato, and one of leek were grown in a commercially-available organic potting media and another media mixed on site containing 20% compost by volume. Supplemental nitrogen was supplied as blood meal or fish hydrolysate to subsets of each group. Both media were amended with AM fungus inoculum produced on the farm in seven gallon plastic “grow-bags” with bahiagrass as plant host, or left unamended for controls. Results showed better overall growth in the potting mix containing 20% compost than in the commercial mix, and better colonization when N was supplied via fish hydrolysate than as blood meal. A subsample of these plants has been transplanted to the field to determine the impact of their greenhouse growth conditions and AM fungus colonization upon eventual yield of peppers and tomatoes.