Trophic cascades in ornamental landscapes mediated by soil organic matter: Effects of mulch and fertilization on microbial activity, nutrient cycling, plant carbon allocation, and insect herbivores

Nutrient cycling theory postulates that soil microbes generally increase in response to increased soil organic matter, and that competition between plants and microbes for nutrients is mediated by the carbon:nitrogen (C:N) ratio of the organic matter. When low C:N organic matter is decomposed, nitrogen is mineralized in excess of microbial demands, increasing nitrogen availability for plants. Conversely, decomposition of high C:N organic matter increases nitrogen immobilization, which decreases nitrogen availability and plant growth. Plant defense theory predicts that faster growing plants in nutrient-rich environments are less resistant to herbivores than slower growing plants in nutrient-limited environments because of a physiological tradeoff between growth and chemical defense. When integrated, these theories predict atrophic cascade from soil microbes through plants to insect herbivores that is mediated by effects of soil organic matter on nutrient availability and host plant quality. The objective of this research was to test this prediction in ornamental landscape microcosms by quantifying effects of mulching with composted yard waste (C:N ratio ≤ 25:1) and recycled ground wood pallets (C:N ratio ≥ 88:1), with and without fertilization, on soil organic matter, microbial activity, nitrogen cycling, as well as plant growth, secondary metabolism, and insect resistance. Results were consistent with the proposed trophic cascade. Both mulches increased soil organic matter and microbial biomass, but effects of yard waste were more substantial. However, effects on nitrogen cycling, plant growth, and insect herbivores were dependent on the C:N ratio of the mulch. Low C:N yard waste increased, and high C:N ground wood decreased, nitrogen mineralization rates, nitrogen availability, foliar nitrogen, and growth of rhododendron, taxus, and river birch, as well as flower production of rhododendron. Fertilization of plots mulched with ground wood relaxed competition between plants and microbes for nutrients and increased the growth of both. Mulching with ground wood increased, and fertilization decreased foliar phenolic concentrations and insect resistance, consistent with the proposed tradeoff between plant growth and herbivore defense. Results of this study contribute to development of soil management regimes for ornamental landscapes that diminish reliance on pesticides and fertilizers, while increasing market demand for recycled organic wastes.