Interactive effects of winter warming and nitrogen deposition on temperate ecosystem soil nitrogen dynamics

Background/Question/Methods The nitrogen dynamics of temperate ecosystems are regulated in part by the ability of these systems to retain nitrogen over winter, at a time when plant nitrogen uptake is low.  Although climate warming may extend the length of plant growing seasons, changes to soil freezing dynamics over winter, such as an increase in the frequency of freeze-thaw cycles, may ultimately lead to increased nitrogen leaching and trace gas losses from these systems.  We estimated soil nitrogen mineralization and leaching losses in a field experiment designed to examine the interactive effects of climate warming and atmospheric nitrogen deposition in a temperate old field.  The two warming treatments (year-round and winter-only) made use of overhead infrared heaters to provide up to three degrees of warming at the soil surface.  Nitrogen was applied as ammonium nitrate at a rate of six grams per square meter per year.  We quantified net nitrogen mineralization using buried PVC tubes with mixed-bed resin disks at the bottom to capture inorganic nitrogen in leachate, and used lysimeters buried at 50 cm depth to measure nitrogen concentrations in soil solution below the rooting zone.
Results/Conclusions Warming treatments commenced in November 2006 and significantly increased ammonium concentrations in lysimeter samples collected from warmed plots in late fall but had no significant effect on concentrations of nitrate and soluble organic nitrogen at this time.  Spring melt in 2007 was characterized by higher concentrations of soluble nitrogen in lysimeter samples than in the fall.  Nitrate concentrations in lysimeter samples decreased significantly in warmed plots.  Net nitrogen mineralization over the winter of 2007 was significantly higher in ambient plots than in winter-warmed plots.  However, overall the effects of warming on soil nitrogen dynamics were subtle over the first year of the experiment.