Choice of organic amendments in tomato transplants has lasting effects on bacterial rhizosphere communities and crop performance in the field

Vegetable transplant media used in certified organic crop production must both comply with regulations and meet seedling nutrient demand. Transplant media amendments allowable in organic production often contain complex microbial communities, however little is known about their effects on transplant rhizosphere microorganisms and if these effects carry over to mature plants in the field. To address this, we compared (i) plant-based (sesame meal, alfalfa meal) amendments, (ii) composted manure-based (vermicompost, thermogenic compost, industry standard) amendments and (iii) a non-amended peat and vermiculite base mix for organic tomato (Lycopersicon esculentum) production. Organic transplant media amendments affected germination rates, transplant growth in the greenhouse, crop growth in the field and final yields. Transplant biomass and early yield were highest for vermicompost and plant-based amendments. Total yield was highest for 20% alfalfa and sesame meal amendments in the first season, however this high rate of amendment negatively impacted germination. No significant differences in yield were found among amended treatments in the second season where plant-based amendment rates were reduced to1 and 2.5% for sesame meal and 5% for alfalfa meal. Amendments also influenced bacterial community structure in both the transplant media and in the rhizosphere of the tomato plants. Terminal restriction fragment length polymorphism (T-RFLP) analysis showed significant differences in bacterial communities between all amendments and these differences persisted for at least one month after seedlings were transplanted to the field. Amendment-associated differences in bacterial communities diminished over the course of the field season. By harvest only vermicompost and the base media had unique T-RFLP profiles. Comparing thermogenic compost and vermicompost made from the same starting material showed that the composting process influenced the bacterial community in the transplant material as well as subsequent communities in the crop rhizosphere. Overall, our results show that the type and rate of organic transplant media amendment can strongly influence transplant quality and subsequent crop performance in the field as well as rhizosphere bacterial communities long after seedlings are transplanted to field soil.

Research highlights
► Choosing effective organic transplant media amendments is important for growers.
► Organic amendments influence the bacterial community composition of transplant media.
► Amendment-driven changes in rhizosphere bacterial communities persist in field soil.