Synchronous sequestration of organic carbon and nitrogen in mineral soils after conversion agricultural land to forest

•Conversion of agricultural land to forest accelerated aggregation of soil particles.•Afforestation on farmland could sequestrate synchronously OC and N in mineral soils.•Macroaggregates dominated the accumulation of OC and N in bulk soils.•OC and N increased most during the first 20 years of afforestation. Agricultural land-use change is a global issue with significant implications for global warming and ecosystem functionality. Uncertainty regarding carbon (C) and nitrogen (N) sequestration and their dynamics after land-use change hampers an accurate understanding of the C and N cycles. To address the influence of converting agricultural land to forest on organic carbon (OC) and N sequestration and their coupling relationships, we collected topsoil (0−10 cm depth) and subsurface soil (10−20 cm depth) in afforested woodlands 10, 20, and 35 yrs after the establishment of Robinia pseudoacacia in abandoned farmlands on the Loess Plateau, China. We analyzed the concentrations and stocks of OC and N in bulk soils and water-stable aggregates. We found that afforestation of farmland resulted in a relative increase of 30 % in the proportion of macroaggregates (8 – 0.25 mm) but a relative decrease of 45 % and 30 % in the proportions of microaggregates (0.25 - 0.053 mm) and silt + clay (< 0.053 mm), respectively. The respective OC and N stocks increased by 87 % and 74 % in bulk soils and by 278 % and 159 % in macroaggregates after 35 yrs of afforestation. Macroaggregates accounted for 69 % and 68 % of the OC and N stocks, respectively, in bulk soils at the 0−20 cm depth. However, the OC and N stocks in microaggregates and silt + clay were only slightly affected. These results indicated that the conversion of agricultural land to forest could sequester OC and N in both bulk soils and aggregates, mainly macroaggregates. In addition, the dynamics of OC and N were significantly correlated, implying synchronous OC and N sequestration in soils after converting agricultural land to forest.