Denitrifier community response to seven years of ground cover and nutrient management in an organic fruit tree orchard soil

- Soil nirK was impacted by ground cover management in an organic apple orchard.
- Compost increased soil nirK richness, diversity, and dissolved organic carbon.
- nirK richness and clustering increased from 2007 to 2013 with compost and wood chips.

Mechanisms through which microorganisms are impacted by surface application of ground cover and nutrient source in organic orchard management are not fully understood. Twelve ground cover (compost, wood chips, paper mulch, or mow-and-blow) by fertilizer (poultry litter, organic commercial, or a no-fertilizer control) treatment combinations were applied annually in April beginning in 2006, with soil collected in March 2007 and 2013 to determine the 1-year and 7-year effects of treatments on soil chemical and biological properties. Organic matter (OM) increased through time regardless of ground cover treatment averaged across fertilizers, with the greatest increase occurring with compost addition. Soil water content, microbial biomass N, ammonium-N, and nitrate-N were greater in 2013 than in 2007 at the 0-10 cm depth. Denaturing gradient gel electrophoresis analysis of denitrifying (nirK) communities revealed that fertilizers did not have a significant main effect or interaction with ground covers. Greater nirK richness and diversity coincided with clustering of the nirK community composition within compost treatments, and greater dissolved organic carbon concentrations with compost compared to wood chip or mow-and-blow treatments in 2007. By 2013 nirK communities in wood chip and compost ground covers tended to cluster together. Furthermore, soil nirK richness increased significantly from 2007 to 2013 in compost and wood chip treatments and was greatest in 2013 under those two ground covers. Diversity with wood chip addition increased from among the least in 2007 to among the greatest in 2013. Greater richness and diversity and community composition in the nirK community within a year of compost additions and gradually over time with wood chip addition indicate potential for enhanced denitrification in low OM, organic apple orchard soil receiving compost and wood chips mulches.