Disentangling the changes of newly input and original organic carbon (OC) and nitrogen (N) after agricultural land-use transition is crucial for understanding the mechanisms behind soil OC and N dynamics in terrestrial ecosystems. However, these dynamics have received little attention so far due to the lack of an approach separating them. In this study, we determined the natural abundance of ¹³C and ¹⁵N in soils from afforested forestlands and adjacent croplands, the forestlands were established at different times (10, 20 and 30 years) at four sites with different soil textures across the Loess Plateau of China. The objective was to explore the changes of new- and old- OC and N along with soil clay gradient, and to examine the proportions of these changes in total OC and N along with the clay content. We showed that the conversion of cropland to forestland increased total- and new- OC and N stocks by 0.69 kg m⁻² and 0.10 kg m⁻², but decreased old OC and N stocks by −0.20 kg m⁻² and −0.04 kg m⁻², respectively. When averaged across afforestation age, the proportions of new OC and N increased, but the proportions of old OC and N decreased with the clay content. In the soils that have a higher clay content, the accumulation of N rates were greater than that of OC in the new stock, and the loss rates of N were higher than that of OC in the old stock after afforestation. In contrast, in the soils that have a lower clay content, the accumulation rates of N were lower than that of OC in the new stock, and the loss rates of N were smaller than the loss of OC in the old stock. These findings indicated that soil texture regulates the changes of OC and N in new and old soil organic matter pools following cropland afforestation.