High alternative oxidase activity in cold soils and its implication to the Dole Effect

Abstract

Variations in the Dole Effect, which have been used to infer past changes in biospheric productivity, are strongly affected by isotopic discrimination in soil respiration. Respiration through the alternative oxidase (AOX) pathway is associated with a higher discrimination than the one associated with the "normal" dark respiration pathway (the cytochrome pathway, COX). However, observations of O 2 discrimination and AOX activity in undisturbed natural environments are scarce. In the current study we measured the O 2 concentration and stable isotopes in the root zone of tundra, boreal forest and alpine forest soils. To estimate the discrimination from this data, we have performed O 2 diffusion experiments in gamma-sterilized soil columns, with varying soil clay content. The discrimination found in the diffusion experiments was independent of clay content, and the value found, 14 2‰, is the same as the one for binary diffusion of O 2 in N 2, indicating no interaction between the O 2 and clay particles. Based on the field and laboratory results, the respiratory discrimination in the soils studied is 15-31‰, with the higher values associated with colder soils. The high discrimination found for cold (<6C) soils indicates that AOX is a major respiratory pathway in these soils. This relationship between soil temperature and discrimination can be used in future interpretations of Dole Effect variations. © 2012. American Geophysical Union. All Rights Reserved.