Dynamics of new- and old- organic carbon and nitrogen in bulk soils and aggregates following afforestation on farmland

Soil organic carbon (OC) and nitrogen (N) dynamics following afforestation are crucial for evaluating the balance of OC and N and their feedback to ecosystem functionality. However, the variations in newly input and old OC and N among soil aggregates are rarely studied. This knowledge gap hinders our understanding of the mechanisms behind OC and N responses to land-use changes. Herein, we measured the natural abundance of delta C-13 and delta N-15 and soil OC and N stocks in bulk soils and aggregates (> 0.25 mm, 0.25-0.053 mm and < 0.053 mm) in afforested land to identify the dynamics of newly input and old OC and N after land-use change. Soils were collected from farmlands (age 0) and afforested woodlands ranging in age from 10 to 35 years in China's Loess Plateau. We showed that afforestation decreased the natural abundance of delta C-13 and delta N-15 in bulk soils and aggregates, and delta C-13 and delta N-15 were mostly enriched in the silt + clay fraction. Afforestation resulted in an increase in total and newly input OC and N; however, the old OC and N remained stable in the bulk soils and macroaggregates. Increases in new OC and N were the main contributors to the accumulation of soil OC and N pools in the topsoil. Furthermore, the OC and N associated with macroaggregates accounted for 76.5-84.0% and 42.2-83.5% of the new OC and N in bulk soils, and 52.7-79.7% and 64.3-86.6% of the old OC and N in bulk soils along the 10-35 years afforestation. The coupling effects of OC and N existed in new- and old- OC and N stocks, regardless of bulk soils or aggregates. These results indicated that both macroaggregates and newly input OC and N contributed to the synchronous sequestration of OC and N in soils after afforestation.