Variability of the CO2 vertical soil profile production and its isotopic composition in a beech forest

Abstract:

The use of stable isotopes has become a useful research tool to understand the complexity of processes involved in the variability of the vertical profile of CO₂ production (P).

In this context, an approach considering diffusion as the only gas transport, the Flux-Gradient Approach (FGA) is used to relate the temporal variation in soil P and its isotopic composition (δ¹³P). P of the different soil layers ([1] 0 cm – -10 cm / [2] -10 cm – -20 cm / [3] -20 cm – -40 cm) and their isotopic signature can be computed from continuous measurements of the vertical soil CO₂ concentration profile and its isotopic signature and combined to simultaneous measurements of the soil efflux, trunk efflux, their respective isotopic signature and eddy flux measurements. The field campaign was conducted at the beech forest of Hesse (France) in 2011 from April to September.

The results show significant temporal variations in P and δ¹³P. For P, large intra and inter day fluctuations are observed and are explained by a dependence on temperature specific to each layer. For δ¹³P, the horizon 2 show significant daily fluctuations which are related to lag-timed soil moisture and water use efficiency. These fluctuations dependency appears to be consistent with a response of P sources to environment and to the impact of photosynthetic δ¹³C fluctuations on δ¹³P.

Comparison of isoprod, defined as the product of CO₂ production and its isotopic composition, and soil and trunk isofluxes shows that horizon 2, the largest contributor to soil P (50 % to 77 % of total soil P depending on the period) is mainly affected by root respiration. The fluctuations of horizon 1 isoprod are influenced both by autotrophic and heterotrophic sources. Moreover, some fluctuations can be generated by other physical processes inducing P and δ¹³P variability, highlighting methodological issues that need to be taken into account to improve FGA.