Spring ephemeral herbs and nitrogen cycling in a northern hardwood forest:an experimental test of the vernal dam hypothesis

In the late 1970s R.N. Muller and EH. Bormann posited their "vernal dam" hy pothesis, stating that spring-ephemeral herbs in deciduous forests serve as a temporary sink for N when overstory trees are dormant, and then release this N later, in the summer, when the trees are active. This hypothesis has gained wide acceptance. yet two of its critical assumptions have never bee n experimentally tested: (1) that N taken up by spring ephemerals would oth erwise be lost from the ecosystem, and (2) that N from senesced ephemeral t issues contributes to increased rates of summertime N mineralization. To te st these assumptions, I quantified patterns of N cycling and loss from a se t of paired plots, half of which served as controls and from half of which all spring-ephemeral plants were removed. There were no significant differe nces in NO3- leaching between plots with and without spring ephemeral veget ation. These results are consistent with the relatively low rates of N upta ke by the dominant spring ephemeral, Allium tricoccum, and its apparent pre ference for NH4+, which is far less mobile in soil than NO3-. In addition, based on sequential sampling, I found that soil microorganisms took up 8 ti mes as much N during the spring than did spring-ephemeral herbs (microbial uptake=3.19 vs, plant uptake=0.41 g N m(-2)), suggesting that microbial imm obilization of N is the dominant sink for N during this season. Removal of spring ephemeral vegetation also had no effect on summertime rates of net N mineralization. Furthermore 2, the addition of spring ephemeral litter to soil+forest floor microcosms did not significantly increase rates of N mine ralization in a laboratory incubation. Instead, this experiment demonstrate d the overwhelming influence of forest floor litter in controlling the rele ase of mineral N from these soils. Overall, neither assumption of the verna l dam hypothesis holds true in this ecosystem, where patterns of N cycling and loss appear to be dominated by microbial decomposition of forest floor material and soil organic matter.