Calculating the long-term stable nitrogen sink in northern European forests

Nitrogen accumulation rates in a boreal Scots pine monoculture in central Sweden and three groups of mixed Scots pine/Norway spruce/silver birch forests in northern Sweden are estimated by using measured and calculated litter fall data, calculated limit values for litter decomposition, and the nitrogen concentration at the limit value (N-limit). Humus has been accumulating in these forests for 120-3000 years. The N-limit value is determined by extrapolating a linear relationship between accumulated litter mass loss and the increasing litter N concentration. Values for foliar litter fall and N, it are used to calculate the quantity of an hypothesized stable N fraction remaining after accumulated mass loss has reached an asymptotic plateau (the limit value). This stable remaining amount of nitrogen is compared to measured year and amounts of N in the humus layers of the forests. Measured rates of N accumulation in humus range from 0.147 to 0.255 g N m(-2) year(-1) differ from estimated rates of N accumulation in humus by 1.8-30.4%, the higher value possibly due to a slow succession from N-poor pine to N-rich birch. not accounted for in the model. Relating the nitrogen accumulated in these forests at maturity to a nitrogen budget based on estimated rates of nitrogen input, N, fixation, denitrification and leaching to the mineral soil gives a good agreement of about 0.30-0.35 g N m(-2) years stored in humus and vegetation at forest maturity. In these forests, this low rate of N accumulation and storage can apparently continue for thousands of years until the system is "reset" by forest fire. We conclude that in undisturbed boreal forests, once N is bound in recalcitrant fractions it is stable, and that the humus is thus a long-term nitrogen sink. (C) 2004 Elsevier SAS. All rights reserved.