Estimating carbon fluxes simultaneously in two soil fractions using 13C changes after vegetation replacements

GERVASIO PIÑEIRO; MAZZILLI, SEBASTIAN; ECLESIA, PAOLA; ESTEBAN JOBBÁGY; RIZZOTO, MARCOS; JACKSON, R.B.

Congreso; 8th International Conference on Applications of Stable Isotopes Techniques to cological Studies; 2012

For more than two decades soil researchers have estimated the proportion of C3 and C4 derived carbon in the soil organic matter based on 13C changes occurred after a vegetation shift or after 13C labelling. Carbon outputs from the soil and soil organic carbon (SOC) decomposition rates (k) have been also estimated based on this approach. Although, this method applies to a single pool model with known 13C/12C inputs, it has been also erroneously applied to models with multiple pools or data collected for multiple soil fractions. Here we present a new approach to estimate C inputs from vegetation to the soil and C fluxes from a labile to a recalcitrant soil fraction. Our method is easy to apply and is based on two simple equations based on mixing models. We use data collected in Uruguay grasslands (mostly C4 grasses) that have been replaced by Pine and Eucalyptus plantations. We collected soil a litter samples at five independent sites and fractionated soil samples using a common physical fractionation protocol that separates the more labile particulate organic matter fraction (POM) and the recalcitrant mineral associated organic matter fraction (MAOM). Our method gave estimates of decomposition rates of the POM and MAOM fraction and also maximum and minimum estimates of the amount of POM carbon that was transferred to the MOAM fraction after vegetation replacement. Our data suggest that in the top 5 cm of the soil and after approximately 25 years nearly all the carbon in the POM fraction is derived from trees and more than 60% of the MOAM fraction. In deeper soil layers these proportions decrease, but most of the POM is still derived from trees while the MAOM fraction has only 5 to 10% of C derived from plantations.