Natural Resources, School of

 

Date of this Version

2015

Citation

Ecology, 96(10), 2015, pp. 2653–2668

Comments

Copyright 2015 by the Ecological Society of America

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

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.

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