Application of spent mushroom compost enhances wheat yield but reduces mycorrhizal associations and grain nutrient concentration.

Developing management practices that enhance crop yield while maintaining soil health is the foremost objective of the regenerative agriculture movement. One avenue to achieving this goal is using biofertilizers and alternative soil amendments to supplement or replace agrochemicals. Here we report the results of a pairwise field trial of spring wheat (Triticum aestivum) wherein we investigated individual and combined impacts of inoculation with arbuscular mycorrhizal fungi (AMF) and a spent mushroom compost amendment (herein mushroom compost). The symbiotic relationship between AMF and plants has been demonstrated to benefit the yield and nutritional quality of many crops by enhancing access to mineral nutrients and water. Mushroom compost, consisting of the devitalized residual substrate following harvest of edible mushrooms, is a byproduct of the mushroom industry and is comprised of a variety of nutrient-rich organic material inputs. Therefore, the objective of this study was to (1) determine the effect to which AMF and mushroom compost individually impact wheat yield and nutritional quality, and (2) examine if these effects are synergistic or antagonistic when both amendments are applied together. We found that mushroom compost addition, regardless of AMF inoculation, enhanced grain yield by ~ 40%, but reduced AMF root colonization level by ~ 25-40%. Additionally, despite yield increases, mushroom compost addition reduced grain phosphorus (P), potassium (K), and magnesium (Mg) concentrations by ~ 10% and boron concentration by ~ 45%. In fact, grain P, K, and Mg concentrations were all correlated with mycorrhizal colonization level. These results suggest that while spent mushroom compost additions enhanced grain yield, this may have led to a mineral nutrient 'dilution effect' exacerbated by negative impacts on AMF colonization and community composition.