The Effects of Rootstock and Fertilizer Selection on Apple Orchard Productivity and Soil Microbial Community Ecology

In apple (Malus domestica Borkh.) orchards, rootstock genotype and soil fertility management practices potentially impact soil health and nutrient status, plant associated soil microbial communities, and orchard productivity. Apple growers select specific rootstocks to confer beneficial traits, including size control, precocity, and pest and disease resistance. Rootstocks may also influence microbial communities, resulting in changes to microbially facilitated ecosystem services, such as nutrient availability, that affect tree growth and productivity. Many apple growers apply synthetic nitrogen (N) fertilizers to improve fruit yield and quality. In excess of tree requirements, nitrogen fertilizers have been shown to decrease crop yield and quality, and contaminate ground and surface waters. The addition of carbon-based fertilizers, such as compost, has been shown to reduce N loss, increase soil organic matter (OM), cation exchange capacity (CEC), water holding capacity and the availability of certain minerals. In 2013, a pot-in-pot study was implemented to determine the effects of yard waste and chicken litter composts, fertigation with calcium nitrate [Ca(NO₃)₂] fertilizer, and combinations of compost and Ca(NO₃)₂ fertigation on apple tree growth and nutrient status, soil health, and microbial activity. For this study, ‘Brookfield Gala’ scions were grafted onto five rootstocks, ‘Budagovsky 9’, ‘Malling 9’, ‘Geneva 41’, ‘G. 214’, and ‘G. 935’. Trees were planted in pots containing Poplimento Silt Loam series orchard soil. Four individual tree replicates of each rootstock were treated with 40 kg·ha^-1 N from either chicken litter compost, yard waste compost, Ca(NO₃)₂  or a combination of 20 kg·ha^-1 from compost and 20 kg·ha^-1 from Ca(NO₃)₂. Unfertilized containers served as a control. Applications occurred in 2013 and 2014. In 2013, relative to the untreated control, yard waste increased OM by 66% and CEC by 49%, and chicken litter increased OM by 24% and CEC by 40%. In 2014, relative to the untreated control, yard waste increased OM by 88% and CEC by 56%, and chicken litter increased OM by 69% and CEC by 65%. Leaf nitrogen concentration was not significantly different among rootstocks or fertilizer treatments. However, both rootstock selection and fertilizer treatment affected leaf phosphorous, potassium, magnesium, and boron concentrations. Neither rootstock selection nor fertilizer treatments affected microbial community composition, as measured by terminal restriction fragment length polymorphisms, during the 2013 growing season. Leaf nutrient status, microbial respiration, trunk cross sectional area, soil carbon to nitrogen ratio, microbial biomass, and microbial community composition will continue to be measured in 2015.