We determined the effects of mulching with composted yard waste and recycled ground wood pallets, with
and without fertilization, on soil organic matter, microbial activity, nitrogen cycling, and growth of river
birch, Betula nigra. Both mulches increased soil organic matter content and more than doubled microbial
respiration, which is consistent with the hypothesis that soil microbes are generally carbon-limited. Effects
on nitrogen availability, however, were highly dependent on the C:N ratio of the mulch. Mulching with
composted yard waste, with a low C:N ratio of 17:1, dramatically increased total soil N as well as N
mineralization rate. This increased plant available N, as well as foliar nitrogen concentration and growth of
river birch. Fertilization had no effect on the growth of trees mulched with composted yard waste, which
indicates that composted yard waste serves as a high quality organic fertilizer that can meet fully the
nutrient requirements of both microbes and plants as it decomposes. On the other hand, mulching with
high C:N (125:1) wood pallets stimulated microbial growth while adding little nitrogen to the soil.
Microbes immobilized a high proportion of the limited nitrogen pool, thereby slowing the growth of river
birch. These results are consistent with the hypothesis that soil microbes out-compete plants for nitrogen,
and that addition of organic matter with high C:N ratios can induce nutrient deficiencies in plants by
stimulating microbial growth. Fertilization relaxed this competition, increasing the growth of river birch
that had been mulched with ground wood. 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 the dynamics of
mulch effects on soil fertility.