Compost and Geneva® series rootstocks increase young ‘Gala’ apple tree growth and change root-zone microbial communities

Abstract

Rootstocks are used in commercial apple (Malus ×domestica Borkh.) orchards to create dwarf trees that are highly precocious, but there are gaps in understanding how soil fertility management practices may affect soil health, tree nutrient status, plant associated soil microbial communities, and orchard productivity when different rootstock genotypes are used. For three consecutive years, we tested six fertilizer treatments, on five rootstock genotypes (each was scion grafted with ‘Brookfield Gala’). The fertilizer treatments included, (1) a no fertilizer control, 40 kg/ha plant available N from (2) calcium nitrate Ca(NO3)2 fertigation, (3) chicken litter compost, or (4) yard waste compost, and, two integrated compost-Ca(NO3)2 treatments, which were comprised of 20 kg/ha N from either (5) chicken litter or (6) yard waste compost and 20 kg/ha from Ca(NO3)2. The rootstock genotype treatments included, ‘Budagovsky 9’, ‘Geneva® 41’, ‘G.214’, ‘G.935’, and ‘Malling 9’. Trees were planted into 38 L pots containing local soil and Stalite®. After three years, total tree biomass was greater in the chicken litter, chicken litter-Ca(NO3)2, yardwaste, and yardwaste-Ca(NO3)2 treatments than the non-fertilized control, regardless of the rootstock genotype. In 2015, the Ca(NO3)2, chicken litter, and the two integrated compost-Ca(NO3)2 treatments had greater leaf tissue N than the control and yardwaste treatments. Leaf tissue P and B were consistently greater in the Geneva® rootstocks than B.9 or M.9. In the third year of the experiment, the compost and integrated treatments had greater soil organic matter, cation exchange capacity, potentially mineralizable soil nitrogen, and soil microbial respiration than the control. Additionally, bacterial and fungal microbial community compositions were affected by rootstock genotype, fertilizer treatment, and time and these effects were correlated with changes in tree growth and soil properties. These results suggest that compost and integrated compost-Ca(NO3)2 nutrient applications can be used to increase plant growth, leaf mineral content, soil fertility, and microbial activity in newly established apple orchards. Additionally, the Geneva® rootstocks that we tested appear to be more effective at acquiring soil minerals than other dwarfing genotypes and, therefore, may be well suited for use in fertility management plans that derive all or part of their nutrients from compost. Further research into the roles played by microbial communities in apple tree growth and soil fertility is supported.