Effects of long-term nitrogen addition and atmospheric nitrogen deposition on carbon accumulation in Picea sitchensis plantations

This study aimed to assess the combined effects of
long-term nitrogen (N) supply and nitrogen deposition
(Ndep) on carbon (C) accumulation within
Sitka spruce [Picea sitchensis (Bong.) Carr.] plantations
in Scotland. Six study sites established from
1970 to 1982 were periodically N-fertilized, monitored
over time and commonly surveyed in 2010.
Soil, aboveground biomass, and ground vegetation
C stock changes were analyzed; aboveground C
stocks were correlated with total additional N
experienced at each site, that is, the sum of
experimental N supply (Nadd) and site-specific
accumulated Ndep from 1900 to 2010. Results
showed a positive N effect on aboveground tree C
stock and no decline in tree growth was observed
either during fertilization or after the latest N
addition. The amount of C in litter was significantly
higher in experimentally N-treated plots, whereas
the amount of C in understory vegetation was
higher in control plots. Pooling all the compartments
(that is, understory vegetation, litter, soil,
and tree biomass) the total ecosystem C content
was estimated for each site, and at most sites a
higher C stock was estimated for N-treated plots.
Differences in aboveground C accumulation rates
between treated and control plots were lower at
sites with high levels of accumulated Ndep. Our
results indicate that site-specific accumulated Ndep
should be considered to understand tree growth
responses to N fertilization.