Soil genesis as a control on the dynamics of associations between measured soil carbon fractions and modeled soil carbon pools

Recent studies have sucessfully linked measured soil carbon (C) fractions to modeled soil C pools. This is a major step forward in bringing direct experimental data on soil C turnover into a modeling framework. However, most studies on C turnover in soils are conducted on time scales ranging from 1 to 100 years, and in this context the soil mineral matrix, an important factor for C stabilization in soils, is often considered static. In contrast, soil genesis (i.e. soil weathering) is traditionally studied at longer time-scales (centuries to millennia) by using soil chronosequences. This temporal discrepancy between C sequestration (short-term process) and soil genesis (long-term process) represents a major gap in our understanding of the interactions between mechanisms that can stabilize C in soils over longer timescales. In this study we show that the distribution of C among different soil fractions, as well as the potential turnover of these fractions, is highly variable along chronosequences as a consequence of changes in the mineral matrix. Our analysis shows that while soils developed in younger deposits show an increased ability to stabilize C with the mineral phase, soils developed in older deposits lose the ability to form aggregates and provide physical protection for C by occlusion with increasing age, leading to lower soil C content and potentially higher temperature sensitivity and faster turnover. This finding leads to the conclusion that, at larger spatial and temporal scales, associations of measured fractions to modeled pools in C turnover models need to consider changes in the mineral matrix in order to accurately reflect C turnover on the long term.