Influence of Orchard Floor Management and Compost Application Timing on Nitrogen Partitioning in Apple Trees

Abstract

Synchronizing the supply of plant-available nitrogen (N) from organic materials with the N needs of apple trees is essential to cost-effective organic apple production. Tree growth and organic matter mineralization are affected by orchard floor management. This study examines the effects of three orchard floor management systems, cultivation, wood chip mulch, and a legume cover crop, on the accumulation and partitioning of compost-derived N in young apple (Malus domestica Borkh.) trees at different compost application dates across two growing seasons. Compost enriched with 15N was applied to apple trees in April, May, and June of 2006 and 2007, and trees were excavated in Sept. 2007 to determine the fate of labeled compost N. Trees with wood chip mulch had significantly greater dry weight and N accumulation in vegetative tree components than trees with cultivation or legume cover. Fruit yields were similar between cultivation and wood chip treatments despite less vegetative growth under cultivation, as these trees partitioned more dry weight into fruit (44%) than wood chip mulch trees (31%). Nitrogen-use efficiency by trees was lower with a living legume cover crop than in other treatments due to competition for resources. In the cover crop aboveground biomass, 20% to 100% of the N was derived from compost. In comparison, only 5% to 40% of N in the decomposing wood chip mulch originated from compost. Tree reserves were an important source of N for spring fruit and leaf growth in all treatments, but significantly more so for trees in the cultivation treatment. Fruit and leaves were strong sinks for compost N early in the season, with trees allocating 72% of spring N uptake into leaves and fruit. In the summer, N uptake increased improving compost N-use efficiency. Summer N was preferentially allocated to perennial tissues (71%), bolstering N reserves. Trees with wood chip mulch performed well and had greater capacity to build N reserves, making wood chips ideal for establishing young organic apple orchards. However, as the orchard matures, it may be beneficial to switch to a groundcover that reduces tree vegetative growth.