Organic orchard soil management practices affect soil biology and organic matter.

Organic production practices may vary in their effects on soil biology. We investigated the effects on soil biological properties of four strategies for managing tree-row vegetation and soil fertility in organic apple orchards. The experimental orchard was cv. 'Ambrosia' on B.9 rootstock planted spring 2006 in a 4 m x 1 m spacing. There were 6 replicate 10-tree plots of each of four treatments applied to the 1.8 m wide tree-row: 1) 'compost/tillage' - compost applied each spring with shallow tillage for weed management; 2) 'alfalfa mulch' - alfalfa grown in alleys was mown and transferred onto the tree-row; 3) 'bark mulch' - compost applied in the first year was covered by mulch of wood chips and thereafter fertilized with soluble organic fish fertilizer; 4) 'plastic mulch' - compost applied in the first year was covered by weed fabric and thereafter fertilized as for treatment 3. Soil samples taken in October of 2007, 2009 and 2011 were analyzed for microbial biomass and phosphatase activity, soil nematodes, root growth, and infection of fine roots by root-lesion nematodes (Pratylenchus penetrans) and mycorrhizae. Mycorrhizae were assessed via PCR of fine roots with mycorrhizal primers, cloning and sequencing. Bark mulch and alfalfa mulch maintained the largest soil microbial biomasses, phosphatase activity and soil nematode populations. Indices of nematode community structure that reflect functional diversity of the soil food web and importance of fungal-based decomposition pathways were both greater under bark mulch than other treatments. Apple root biomass increased under bark mulch and plastic mulch relative to the compost/tillage and alfalfa mulch treatments. There were no significant effects on infection of roots by root-lesion nematodes or mycorrhizae.