Long-term anthropogenic nitrogen deposition enhances carbon sequestration in boreal forest soils
It is proposed that carbon (C) sequestration in response to reactive nitrogen (Nr) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO2 emissions. While previous studies have helped clarify the magnitude by which Nr deposition enhances C sequestration by forest vegetation, there remains a paucity of long-term experimental studies evaluating how soil C pools respond. We conducted a long-term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50 kg N ha-1 yr-1) in the middle boreal zone of northern Sweden in order to understand how atmospheric Nr deposition affects soil C accumulation, soil microbial communities, and total soil respiration (i.e. autotrophic and heterotrophic combined). We hypothesized that soil C sequestration will increase, and soil microbial biomass and total soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition.
Our data showed that the low N addition treatment caused a non-significant increase in the organic horizon C pool of ~15%, and a significant increase of ~30% in response to the high N treatment relative to the control. The relationship between C sequestration and N addition in the organic horizon was linear, with a slope of 10 kg C kg-1 N. We also found a concomitant decrease in total microbial and fungal biomasses, and a ~11% reduction in total soil respiration in response to the high N treatment. Our data complement previous data from the same study system describing aboveground C sequestration, indicating a total ecosystem sequestration rate of 26 kg C kg-1 N. These estimates are far lower than suggested by some previous modelling studies, and thus will help improve and validate current modelling efforts aimed at separating the effect of multiple global change factors on the C balance of the boreal region.