Soil processes drive seasonal variation in retention of 15N tracers in a deciduous forest catchment.

Seasonal patterns of stream nitrate concentration have long been interpreted as demonstrating the central role of plant uptake in regulating stream nitrogen loss from forested catchments. Soil processes are rarely considered as important drivers of these patterns. We examined seasonal variation in N retention in a deciduous forest using three whole-ecosystem N-15 tracer additions: in late April (post-snowmelt, pre-leaf-out), late July (mid-growingseason), and late October (end of leaf-fall). We expected that plant N-15 uptake would peak in late spring and midsummer, that immobilization in surface litter and soil would peak the following autumn leaf-fall, and that leaching losses would vary inversely with N-15 retention. Similar to most other N-15 tracer studies, we found that litter and soils dominated ecosystem retention of added N-15. However, N-15 recovery in detrital pools varied tremendously by season, with.90% retention in spring and autumn and sharply reduced N-15 retention in late summer. During spring, over half of the N-15 retained in soil occurred within one day in the heavy (mineral-associated) soil fraction. During summer, a large decrease in N-15 retention one week after addition coincided with increased losses of (NO3-)-N-15 to soil leachate and seasonal increases in soil and stream NO3- concentrations, although leaching accounted for only a small fraction of the lost N-15 (< 0.2%). Uptake of N-15 into roots did not vary by season and accounted for <4% of each tracer addition. Denitrification or other processes that lead to N gas loss may have consumed the rest. These measurements of N-15 movement provide strong evidence for the dominant role of soil processes in regulating seasonal N retention and losses in this catchment and perhaps others with similar soils.