Controls on soil organic carbon stability and temperature sensitivity with increased aboveground litter input in deciduous forests of different forest ages

A previous 5-year long field litter manipulation study at the Smithsonian Environmental Research Center (SERC) in coastal Maryland demonstrated that forest age controls the chemical trajectory of litter decay and the extent and source of litter incorporation into soil physical fractions among young (60–74 yrs) and old (113–132 yrs) successional stands. To investigate if these ecosystem-level differences influence soil organic carbon (SOC) stability and temperature sensitivity, and to infer differences in stabilization mechanisms, a six-month laboratory incubation (15 °C and 25 °C) of soils from the experimental plots was conducted. The results showed that: 1) C mineralization of wood amended soils was lower than control soils in all forests with young and old forests exhibiting distinct, early vs. late, CO2 efflux profiles over the time course of the incubation; 2) Soils from leaf-amended old forests exhibited a proportional increase in their active SOC pool but with shorter mean residence times (MRT) and a decrease in slow pools with longer MRTs, while SOC of young forests proportionally shifted to more slow cycling SOC pools with MRTs that were unchanged from controls. Structural equation modeling combining previous field and soil property data with laboratory incubation results indicated that temperature sensitivity of the active SOC pool was related to the microbial community and lignin content, while temperature sensitivity of the slow pool was related to chemical protection from silts and clays, environmental factors like pH, and soil C/N ratio. Our results underscore how successional forests of differing age can exhibit dramatically different controls on SOC-litter dynamics, through the protection and accessibility of C, that must be taken into account when predicting forest ecosystem response to future climate change.