Control of Nitrification by Tree Species in a Common-Garden Experiment

We studied the effect of tree species on nitrification in five young plantations and an old native beech coppice forest at the Breuil experimental site in central France. The potential net nitrification (PNN) of soil was high in beech, Corsican pine, and Douglas fir plantations (high nitrifying stands denoted H) and low in spruce and Nordmann fir plantations as well as in native forest stands (low nitrifying stands denoted L). We hypothesized that tree species would stimulate or inhibit nitrification in transplanted soil cores within a few years after the cores were transplanted between stands. We first initiated a transplant experiment where soil cores were exchanged between all stands. The PNN remained high in soil cores from H transferred to H and low in soil cores from L transferred to L. The PNN increased considerably after 16 months in soil cores transferred from L to H, whereas the transfer of soil cores from H to L decreased the PNN only slightly after 28 months. In a second transplant experiment, forest floor material was exchanged between the Douglas fir (H) and the native forest (L) stand. Six months later, the forest floor from the native forest had increased the PNN of the Douglas fir soil considerably, whereas the forest floor from Douglas fir did not affect the PNN of the soil in the native forest stand. It was concluded that beech, Corsican pine, and Douglas fir rapidly stimulate soil nitrification by either activation of suppressed nitrifier communities and/or colonization by new nitrifier communities. Conversely, the slow and irregular reduction of nitrification in spruce, Nordmann fir, and native forest was probably due to the low and heterogeneously distributed flux of inhibiting substances per volume of soil. Our experiments suggest that the inhibition of nitrification is not tightly connected to forest floor leachates, but that the forest floor both reflects and maintains the major ongoing processes. In the long term, humus build up and the production of inhibiting substances may completely block the nitrification activity.