Afforestation Improves Soil Organic Carbon and Total Nitrogen Stocks Mainly Through Increasing > 2 Mm Aggregate Fractions and Stimulating Carbon and Nitrogen Transformations Within Aggregates in Subtropical Karst Region

Vegetation restoration can significantly increase soil organic carbon (SOC) and total nitrogen (TN) stocks, however, the response of SOC and TN stocks, as well as their distribution, transformation, and stability within aggregates, to afforestation in fragile karst ecosystems remains unclear. In this study, soil samples were collected from orchards, 10-, 20-, and 40-year Dodonaea viscosa plantations after afforestation following orchard abandonment in a karst rocky desertification area. A natural succession shrubland soil served as the control. Aggregate size distribution, SOC and TN contents, and δ13C and δ15N values in bulk soils and aggregate fractions were determined. The results showed that 0.25–2 mm aggregates were the main contributors to SOC and TN stocks, amounting to 47.7 %-55.6 % and 43.0 %-51.7 %, respectively. Compared to orchards, D. viscosa afforestation significantly increased SOC and TN stocks by 160 %-393 % and 184 %-416 %, respectively, more obviously with prolonged afforestation. This effect was mainly attributed to the increases in > 2 mm aggregate fractions and their associated SOC and TN contents. In addition, δ13C and δ15N values decreased with decreasing aggregate size in afforested and shrubland soils. The C flow pathways followed the trend from < 0.053 mm to 4–8 mm following afforestation, but this was not the case for orchard soils exhibiting pathways from 2-4 mm to < 0.053 mm progressively. These results indicated that large aggregates showed a faster turnover rate and contributed to higher SOC and TN contents following afforestation. The significant and positive relationships between SOC and TN contents and magnesium, aluminium and iron oxides suggested the stimulatory effect on SOC and TN accumulation. Noticeably, although SOC and TN stocks in bulk soils following 40-year afforestation reached the level of shrubland, the proportion of > 2 mm aggregates and the geometric mean diameter remained significantly lower than those of shrubland soils. Soil aggregate stability following D. viscosa afforestation may take longer to reach natural restoration levels.