A carbon-budget approach shows that reduced decomposition causes the nitrogen-induced increase in soil carbon in a boreal forest

Abstract

Nitrogen (N) addition causes rapid accumulation of carbon (C) in the soils of boreal forests. The C accumulation has been attributed to an increase in C supply to the soil, to a decrease in mineralization of organic C to CO2, or some combination of the two. We sought to quantify the proportional causes in a case study in a boreal Scots pine forest with or without annual N addition (at 50–100 kg N ha−1 yr−1). We continuously measured soil-surface CO2 exchange with large chambers (20-m2 surface area) over three growing seasons and derived flux rates from snowpack CO2 profiles during the winter. Models were used to disaggregate the CO2-exchange data into autotrophic and heterotrophic components. We also measured litterfall and inferred total belowground carbon flux (TBCF). We observed annual soil C accumulation to be higher by 104 g C m−2 yr−1 at the fertilized (F) plot compared to the unfertilized reference (R) stand. Total annual C supply to soil (Aboveground litterfall + TBCF) was not increased by the N addition despite a substantial increase in litterfall (+64 g C m−2 yr−1). Instead the sum decreased slightly, by 52 g C m−2 yr−1, because of a larger reduction in TBCF. This reduction in soil C supply led us to conclude that the soil C accumulation was entirely due to inhibited substrate mineralization. We speculate that the nitrogen increased soil C by inhibiting heterotrophy, specifically that portion of heterotrophy that is primed by autotrophic carbon.