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 ¹⁵N 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 ¹⁵N 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 ¹⁵N retention. Similar to most other ¹⁵N tracer studies, we found that litter and soils dominated ecosystem retention of added ¹⁵N. However, ¹⁵N recovery in detrital pools varied tremendously by season, with .90% retention in spring and autumn and sharply reduced ¹⁵N retention in late summer. During spring, over half of the ¹⁵N retained in soil occurred within one day in the heavy (mineral-associated) soil fraction. During summer, a large decrease in ¹⁵N retention one week after addition coincided with increased losses of ¹⁵NO₃^- to soil leachate and seasonal increases in soil and stream NO₃^- concentrations, although leaching accounted for only a small fraction of the lost ¹⁵N (,0.2%). Uptake of ¹⁵N 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 ¹⁵N 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.