McSwiney, C.P., H.A. Collins, G.P. Robertson
Presented at the ASM at Snowbird (2000-08-02 to 2017-12-05 )
In conventional agricultural systems, nitrogen fertilizer can stimulate N2O fluxes whereas N inputs to forested systems often do not. At the Kellogg Biological Station we have examined N2O production in conifer and deciduous forests that have been fertilized with 0 or 30 kg N/ha/yr. From the outset of the experiment N2O production was greater in the conifer than in the deciduous stands. In the first year of the experiment N2O fluxes were similar in fertilized and control plots for each forest type. After six years of fertilization, fertilized forest plots in both stand types produced more N2O than did the unfertilized control plots. We hypothesize that the reason for the time lag in the N2O response to fertilization is the accumulation of organic matter in the forest floor that buffers the forested system from inputs of N. In initial tests of this hypothesis we transferred intact conifer forest floor into conventional agricultural systems. We monitored surface N2O fluxes from soil with and without forest floor additions. Forest floor material was also combined with agricultural soil in laboratory experiments to determine whether organic matter from the forested systems depresses N2O production when fertilizer is added to soils. Contrary to expectations, forest floor additions stimulated rather than reduced N2O production in agricultural soils both in the field and in the laboratory. In a separate experiment, when forest floor leachates were added both with and without fertilizer to agricultural soil, N2O production was stimulated only by conifer forest floor leachate with fertilizer. Experiments are now underway to define the mechanisms responsible for the stimulation of N2O fluxes when forest floor material is introduced to the agricultural systems.
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