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

Abstract

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 15 N tracer additions: in late April (post‐snowmelt, pre‐leaf‐out), late July (mid‐growing‐season), and late October (end of leaf‐fall). We expected that plant 15 N 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 15 N retention. Similar to most other 15 N tracer studies, we found that litter and soils dominated ecosystem retention of added 15 N. However, 15 N recovery in detrital pools varied tremendously by season, with >90% retention in spring and autumn and sharply reduced 15 N retention in late summer. During spring, over half of the 15 N retained in soil occurred within one day in the heavy (mineral‐associated) soil fraction. During summer, a large decrease in 15 N retention one week after addition coincided with increased losses of 15 NO 3 − to soil leachate and seasonal increases in soil and stream NO 3 − concentrations, although leaching accounted for only a small fraction of the lost 15 N ( 15 N into roots did not vary by season and accounted for 15 N 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.

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