Soil Microbial Mediation of Mineral Uptake By Apple Rootstocks

Apple (Malus xdomestica) rootstocks confer beneficial traits, including mature tree size, precocity, and pest and disease resistance. The soil microbiome likely mediates some of these traits. Evidence suggests that carbon-based soil amendments, such as composts and mulches, increase crop growth by reducing pathogenic organisms and enhancing microbial activity. In previous work, we found that the soil microbiome can be manipulated by weed management and fertilizer inputs used for organic and integrated fruit production systems. Additionally, several studies have shown that soil microbiota influences apple replant disease incidence. In 2013, a pot-in-pot study was implemented to determine the impact rootstock genotype and fertilizer formulation have on apple tree growth and nutrient status, soil health, and microbial activity. An equivalent nitrogen rate was applied as five different fertilizer treatments: chicken litter compost, yardwaste compost, calcium nitrate, chicken litter compost + calcium nitrate, and yardwaste compost + calcium nitrate. There was also a non-fertilized control. Each fertilizer treatment was applied to five apple rootstock genotypes (‘Budagovsky 9’, ‘Malling 9’, ‘Geneva 41’, ‘Geneva 214’, and ‘Geneva 935’) grafted with ‘Gala cv. Brookfield’ in a completely randomized design with four replicates. After three years, total tree biomass and root biomass was greatest for ‘G.214’, and all the compost and compost + calcium nitrate treatments increased in tree biomass compared to the control. Tree size was greater than the control by 38% for the chicken litter treatment, 32% for the yardwaste + calcium nitrate treatment, 26% for the chicken litter + calcium nitrate treatment, and 22% for the yardwaste treatment. In 2015, leaf nitrogen concentration was greatest in ‘M.9’, ‘G.41,’ and ‘B.9’ rootstocks. Trees that received chicken litter, chicken litter + calcium nitrate, yardwaste + calcium nitrate, and calcium nitrate treatments had greater leaf nitrogen concentrations than the control. Rootstock genotype and fertilizer treatment also affected leaf phosphorous, potassium, magnesium, and boron concentrations. We also tested microbial community composition and function under these treatments. Overall, carbon-based fertilizers increased microbial biomass and activity and the amount of potentially mineralizable nitrogen. Molecular techniques were used to “fingerprint” the microbial community, as well as to identify the richness, diversity, and specific taxa associated with the roots and soil of each treatment. Soil DNA was matched to known organisms, and provided an interpretation of how rootstock genotype and fertilizers affected the soil microbiome and thus plant growth and fruit production.