A land surface 14C transfer model and numerical experiments on belowground 14C accumulation and its impact on vegetation 14C level

Abstract

A model simulating transport and exchange for 14C (or 14CO2) in a land surface ecosystem was developed and the belowground 14C accumulation and its impact on vegetation 14C accumulation at a hypothetical cultivated field were studied with the model through numerical experiments. The developed model involved physical 14CO2 transport in surface atmosphere and soil and physiological 14CO2 exchanges in leaves, and was incorporated into a dynamical model (SOLVEG-II) that calculates transport and exchange for heat, water and CO2. The model was tested through a simulation of an existing-experiment on an acute exposure of grape plants to 14CO2. The calculated 14C amount in leaves agreed with the observations within a factor of 1.7. A hypothetical scenario used for the numerical experiments considered an annual 14C input into surface soil layers via 14C-enriched foliage or root litter under a continually heightened atmospheric 14CO2 concentration. The specific activity of 14C in the surface soil layers increased with time and several decades after the start of accumulation it eventually converged to eight times the initial specific activity. At this equilibrium state, the increased belowground 14CO2 production enhanced the atmospheric 14CO2 level and, consequently, 14CO2 uptake by vegetation increased to 1.1 times the control calculated without belowground 14C accumulation. The model results also demonstrated that 14C accumulated in soil can maintain an enhanced vegetation 14C level for at least several decades even after the end of accumulation.