Controls on labile and stabilized soil organic matter during long-term ecosystem development

A better understanding of factors controlling the distribution of soil organic matter (SOM) in differently stabi-lized pools in soils during ecosystem development is essential to more efficiently manage soil as a C sink and aid global warming mitigation. We, thus, investigated C and N stocks in bulk soil and differently stabilized soil fractions (free particulate organic matter (fPOM), micro-and macroaggregates, and mineral-associated organic matter (MAOM)), in seven chronosequences comprising primary and secondary successions at various locations in Europe and tested the influence of multiple environmental parameters on these C and N stocks.Best-subset multiple regression indicated that plant litter production and succession age were of minor importance. Above a certain threshold of organic matter inputs to the soil, changes in the quantity of such inputs may have little direct effect on the partitioning of organic matter to various soil fractions. Instead, bio-physicochemical soil properties (such as pH and PLFA and clay contents) were the best predictors for both C and N stocks. We also found a shift in the relevance of biotic and abiotic factors from fPOM and the various aggregate fractions to MAOM, the latter being exclusively associated with pH and the clay content. This indicates that sites dominated by MAOM likely more slowly react to management interventions, such as increasing or diversifying organic matter inputs, than sites dominated by fractions more responsive to biotic factors, such as fPOM. Finally, as compared to primary successions, secondary successions were associated with higher C and N stocks in aggregate fractions, pointing towards better conditions for soil structure formation in the latter. Our results highlight the need to elucidate the form in which C and N exist in the soil for selection of proper management strategies and that soil biophysicochemical parameters may have a disparately larger impact on the partitioning of C and N to variously stabilized soil fractions than the rate of organic matter inputs.