Accumulation of soil organic carbon in aggregates after afforestation on abandoned farmland

Abstract

To understand how organic C (OC) accumulates in afforested soils and to quantify the contribution of aggregate-associated OC to OC accumulation, we investigated the changes in soil structure, total soil OC, and aggregate-associated OC from 0- to 10- and 10- to 20-cm depths in afforested forests and adjacent farmlands of northwestern China. We assessed the contribution of macroaggregate-associated OC increase to total soil OC accumulation. Afforestation increased macroaggregate amount, mean weight diameter, and mean geometric diameter but decreased the amount of microaggregate and silt + clay-sized fractions. The improvement of soil structure was greater in surface than subsurface soils and was greater in soils afforested with white birch than in soils afforested with other tree species. Fifty years after afforestation, total soil OC concentrations and stocks and aggregate-associated OC concentrations increased depending on soil depth and tree species. Afforestation increased macroaggregate-associated OC stocks but decreased microaggregate- and silt + clay-associated OC stocks. Soil OC stocks and changes in OC stocks after afforestation mainly depended on macroaggregate-associated OC stocks and their changes. The results from this study suggest that OC accumulation in afforested soils is due to the accumulation of OC in macroaggregates and the redistribution of OC from fine particles to coarser fractions.