Impact of soil biology on nitrogen cycling and weed suppression under newly established organic orchard floor management systems

Sustainable methods of weed control and nitrogen fertility are a significant challenge in organic orchard production systems. These studies were conducted to determine whether soil tillage, living cover mulch establishment, or amendment of soil with Brassicaceae seed meal (BSM), clove oil herbicide, or wood chip mulch, could be used to suppress weeds and positively impact soil quality to result in enhanced nitrogen (N) supply and tree health. Orchard floor management treatments were established in a newly planted apple orchard and tree health was assessed based upon increase in tree circumference and leaf nutrient status. Soil cores were periodically collected to evaluate soil quality and N supply using biological and chemical indicator analyses. Compost amendment was enriched with 15N fertilizer to track N partitioning among soil, living cover and wood chip mulch residues, and tree leaf components. In greenhouse experiments, weed emergence, stimulation of Pythium spp. populations and root infection by these oomycetes, were monitored in orchard soils amended with BSM's that varied in glucosinolate content. None of the orchard floor treatments produced an ideal combination of weed control, maximum tree growth, sufficient leaf nutrient content, and improved soil quality. Rather, soil quality improvements tended to be achieved at the expense of tree performance. Living cover mulch and BSM amendment resulted in soil quality improvement, but tree health was negatively impacted. Wood chip mulch and clove oil herbicide did not positively impact soil quality and resulted in lower tree health. Soil tillage negatively impacted soil quality but tree health was greatest in this treatment. Weed suppression by BSM amendment was correlated with soil enrichment and root infection by resident pathogenic Pythium spp and not glucosinolate content or composition. These studies indicate that in the short-term, the standard practice of soil tillage is the most effective way to control weeds and meet tree N needs, but it may not be desirable in the long-term. Brassicaceae seed meal amendment can be used to meet tree N needs and selectively enhance resident pathogenic Pythium spp. for the purpose of weed control, but further research is necessary to determine ideal timing and application rates.