Seasonal and Diurnal Variations in Atmospheric and Soil Air 14CO2 in a Boreal Scots Pine Forest

Abstract

We present a radiocarbon (C-14) dataset of tropospheric air CO2 forest soil air CO2, and soil CO2 emissions over the course of one growing season in a Scots pine forest in southern Finland. The CO2 collection for C-14 accelerator mass spectrometry (AMS) analysis was done with a portable, suitcase-sized system, using molecular sieve cartridges to selectively trap CO2 The piloting measurements aimed to quantify the spatial, seasonal and diurnal changes in the C-14 content of CO2 in a northern forest site. The atmospheric samples collected above the canopy showed a large seasonal variation and an 11 parts per thousand difference between day and nighttime profiles in August. The higher Delta C-14 values during night are partly explained by a higher contribution of C-14-elevated soil CO2, accumulating in the nocturnal boundary layer when vertical mixing is weak. We observed significant seasonal trends in Delta C-14-CO2 at different soil depths that reflected changes in the shares of autotrophic and heterotrophic respiration. Also the observed diurnal variation in the Delta C-14 values in soil CO2 highlighted the changes in the origin of CO2, with root activity decreasing more for the night than decomposition.