Effects of past, present and future atmospheric CO 2 concentrations on soil organic matter dynamics in a chaparral ecosystem

Abstract

Owing to the continuously increasing concentration of atmospheric CO 2, it has become a priority to understand if soil organic matter (SOM) will behave as a sink or a source of CO 2 under future environmental changes. Although many studies have addressed this question, a clear understanding is still missing, particularly with respect to long-term responses. In this study, we quantified soil C stores and dynamics in relationship to soil aggregation and pool composition in a Californian chaparral ecosystem exposed for 6 years to a gradient of atmospheric CO 2 concentrations, ranging from pre-industrial levels 250 to 750 μl l −1 CO 2. Fossil fuel-derived CO 2 depleted in 13C was used for the fumigation, thus providing a tracer of C input from the vegetation to the soil. Long-term CO 2 exposure invariably affected soil aggregation, with a significant decrease in the macroaggregate fraction at highest CO 2 levels relative to the other two size fractions (i.e. microaggregates and silt and clay). This soil structural change most likely reduced the stability and protection of SOM, and C content generally decreased in most fractions over the CO 2 treatments, and induced faster turnover of recently fixed C at high CO 2 levels. The strongest response was found in the C content of the microaggregates, which decreased significantly ( P < 0.05 ) with rising levels of CO 2. We conclude that increasing atmospheric CO 2 concentrations will decrease soil C in chaparral ecosystems, and that the microaggregate fraction is the most responsive to increasing concentrations of atmospheric CO 2.