John Lloyd recently completed his Ph.D. degree
in entomology at The Ohio State University,
Ohio Agricultural Research and Development Center,
in Wooster, and is currently an assistant professor
in the Department of Plant, Soil, and
Entomological Sciences at the University of Idaho in
Mulches with high carbon-to-nitrogen (C:N) ratios are thought by some to induce nitrogen deficiencies in plants by stimulating microbial growth, which depletes underlying soils of available nitrogen. On the other hand, mulches with low C:N ratios may increase soil fertility and plant growth as nutrients are released from mulch as it decomposes. We quantified the effects of mulching with composted yard waste (C:N ratio less than 20:1) and recycled wood pallets (C:N ratio greater than 100:1), as well as fertilization, on soil organic matter, microbial activity, nutrient availability, and the growth of river birch and rhododendron.
Both mulches increased soil organic matter and microbial biomass, which is consistent with the hypothesis that growth of soil microbes is limited by available soil carbon. Effects on nitrogen availability and plant growth, however, were highly dependent on the C:N ratio of the mulch. Mulching with low C:N composted yard waste dramatically increased total soil nitrogen, while mulching with ground wood had little effect. The rate at which nitrogen was released from decomposing mulch was much greater in plots mulched with composted yard waste, which substantially increased nitrogen availability and plant growth, as well as flower production of rhododendron.
Fertilization increased the growth of plants in the bare soil and wood pallet treatments, but had no effect on the growth of plants mulched with composted yard waste, which demonstrates that composted yard waste serves as high-quality organic fertilizer as it decomposes. On the other hand, the high degree of microbial immobilization of nitrogen in plots mulched with wood pallets induced nutrient deficiencies and greatly decreased plant growth. The high carbon content of the ground wood stimulated the growth of soil microbes, which out-competed plants for the limited nutrient supply. Fertilization relaxed competition between plants and microbes, but only to a degree.
This study demonstrates conclusively that organic mulches can have major effects on soil fertility and plant growth that are dependent on their C:N ratio. Understanding the dominant influence of soil microbes on nitrogen availability is key to understanding dynamics of soil fertility.