Retention of surface nitrate additions in a temperate old field: implications for atmospheric nitrogen deposition over winter and plant nitrogen availability

In many temperate ecosystems, rates of atmospheric nitrogen deposition remain high over winter despite decreased agricultural activity over this season. The extent to which this nitrogen is accessible for plant growth over the following growing season may depend strongly on uptake by plants and soil microorganisms from late fall through early spring, when the majority of aboveground plant tissue has senesced. We added Ca((NO3)-N-15)(2) (5 atom %N-15) at a rate of 2 g m(-2) of N (corresponding to 100 mg N-15 m(-2)) to the surface of plots in a temperate old field during either late fall, winter, spring melt or early spring. We quantified the recovery of excess N-15 in the soil microbial biomass and soil extracts following spring melt and in aboveground plant tissue at the peak of the plant growing season. Nitrate additions had no significant effect on total aboveground plant biomass, relative species abundance or percent tissue nitrogen. However, mean excess N-15 in aboveground plant tissue varied significantly among treatments, with values of 8.1, 2.6, 0.3 and 7.3 mg m(-2) for late fall, winter, spring melt and early spring addition plots, respectively. Corresponding values of excess N-15 were 3.1, 1.4 and 0.2 mg m(-2) in microbial biomass, and 0.17, 0.07 and 0.03 mg m(-2) in soil extracts, for late fall, winter and spring melt addition plots, respectively. Overall, these results indicate that nitrogen retention from late fall through early spring may depend highly on plant uptake in this system, and that only a small fraction of the nitrogen that accumulates in the winter snow pack may be available to plants.