PARENT SESSION
Oral Session #98: Carbon and Nitrogen Cycling: N-addition, N-flux.
Presiding: C. Hoover
Friday, August 9. 9:00 AM to 12:00 PM. Greenlee Meeting Room, TCC.

Nitrogen additions change soil carbon structure and turnover times.

Townsend, Alan^*,1, Neff, Jason², Bowman, William¹, Lehman, Scott¹, ¹ University of Colorado, Boulder, CO² USGS, Lakewood, CO

ABSTRACT- Soil organic matter (SOM) decomposition is one of the most basic and fundamental of ecosystem processes. However, while we know fairly well how it responds to changes in temperature and moisture, considerable uncertainty remains over how nitrogen (N) regulates SOM turnover. We used ¹⁴C and compound-specific ¹³C analyses of SOM from long-term N fertilization plots at the Niwot Ridge LTER site to show that N additions change both the turnover and chemical structure of SOM pools. For example, we separated SOM into light and heavy fractions using a density-dependent fractionation technique, and found that radiocarbon values for the light SOM fraction from nitrogen fertilized plots averaged 52 parts per mil, whereas the control plot average was 120 parts per mil. Thus, N additions appear to stimulate decomposition rates for a large fraction of SOM dominated by material with higher C:N ratios and turnover times of roughly 10-40 years. In contrast, added N appears to slow the turnover of more recalcitrant but N rich SOM pools, and overall SOM turnover slowed slightly in response to N fertilization. These data imply potentially important feedbacks in soil carbon dynamics in response to a rapidly changing global nitrogen cycle. Moreover, the complex nature of the SOM responses suggests that such feedbacks could easily be missed using only data on SOM pool sizes and soil respiration rates.

KEY WORDS: soil carbon, nitrogen , decomposition, isotopes