Carbon transfer from ¹⁴C-labelled needles to mineral soil, and ¹⁴C-CO₂ production, in a young Pinus radiata Don stand

In forest ecosystems, few data are available on transfer rates of litterfall carbon (C) to mineral soil. Such data are, nevertheless, needed for predicting the effects of forest land-use change and management on soil C stocks. We here report the transfer of C from Pinus radiata litter to mineral soil over 117 weeks, using ¹⁴C-labelled needles mixed with unlabelled needles in lysimeters placed on the forest floor of a 5-year-old P. radiata stand. Mean annual temperature was about 12.9°C and mean annual rainfall about 995 mm; the soil was a well-drained silt loam. Measurements were made at 39, 78 and 117 weeks of ¹⁴C in residual litter and in 10-mm increments of mineral soil to 50 mm depth and 25-mm increments from 50 to 100 mm depth. Measurements were also made of ¹⁴C-CO₂ production, at frequent intervals over the first 5 weeks and then less frequently. Recovery of ¹⁴C in litter and mineral soil ranged from 51 to 66% at the different exposure times. Cumulative respired ¹⁴C-CO₂, expressed as a percentage of the initially added ¹⁴C, increased from 16 to 19% after exposure times of 39 and 117 weeks, respectively. Total recovery of the added ¹⁴C did not differ significantly (P > 0.12) with time over the three measurement periods, with means ranging from 68 to 84%. Apparent losses of ¹⁴C were associated with large replicate variability, with coefficients of variation ranging from 20 to 31%, and with some disturbance of litter in the lysimeters by birds. We estimate that 1.0-2.2% of the litter ¹⁴C was transferred annually to 10-100-mm depth of mineral soil. On the basis of microbial biomass-¹⁴C measurements, this transferred C contained proportionately more microbial (labile) C than did the older total C in mineral soil. Results therefore are consistent with other studies on forests with well-developed organic horizons indicating that C in humified organic matter, rather than in newly fallen litter, is the main above-ground source for transfer of C to the mineral soil.