Above- and belowground carbon stocks are decoupled in secondary tropical forests and are positively related to forest age and soil nutrients respectively

Reducing atmospheric CO₂ is an international priority. One way to assist stabilising and reducing CO₂ is to promote secondary tropical forest regrowth on abandoned agricultural land. However, relationships between above- and belowground carbon stocks with secondary forest age and specific soil nutrients remain unclear. Current global estimates for CO₂ uptake and sequestration in secondary tropical forests focus on aboveground biomass and are parameterised using relatively coarse metrics of soil fertility. Here, we estimate total carbon stocks across a chronosequence of regenerating secondary forest stands (40-120 years old) in Panama, and assess the relationships between both above- and belowground carbon stocks with stand age and specific soil nutrients. We estimated carbon stocks in aboveground biomass, necromass, root biomass, and soil. We found that the two largest carbon pools - aboveground biomass and soil - have distinct relationships with stand age and soil fertility. Aboveground biomass contained ~61-97 Mg C ha^-1 (24-39% total carbon stocks) and significantly increased with stand age, but showed no relationship with soil nutrients. Soil carbon stocks contained ~128-206 Mg C ha^-1 (52-70% total stocks) and were unrelated to stand age, but were positively related to soil nitrogen. Root biomass carbon stocks tracked patterns exhibited by aboveground biomass. Necromass carbon stocks did not increase with stand age, but stocks were held in larger pieces of deadwood in older stands. Comparing our estimates to published data from younger and older secondary forests in the surrounding landscape, we show that soil carbon recovers within 40 years of forest regeneration, but aboveground biomass carbon stocks continue to increase past 100 years. Above- and belowground carbon stocks appear to be decoupled in secondary tropical forests. Paired measures of above- and belowground carbon stocks are necessary to reduce uncertainty in large-scale models of atmospheric CO₂ uptake and storage by secondary forests.