Organic carbon, nitrogen accumulation, and soil aggregate dynamics as affected by vegetation restoration patterns in the Loess Plateau of China

Afforestation and natural succession on former croplands are typical measures used for revegetation in ecologically fragile areas worldwide. However, there are few studies regarding the differences between natural grasslands and artificial plantations in terms of soil aggregate dynamics, organic carbon (OC), and total nitrogen (TN) accumulation. We sampled soils from farmland (FL) and from two vegetation restoration patterns [Robinia pseudoacacia plantations (RP) and natural grasslands (NG)] of two age classes (18 and 42 years) on the Loess Plateau of China. The size-distribution and stability of soil aggregates, as well as OC and TN stocks in bulk soil and aggregates were measured. Revegetation of FL resulted in considerable increases in bulk soil OC and TN stocks at depths of 0-40 cm: OC increased by 101.3% in NG and 75.5% in RP, and TN by 77.5% in NG and 106.1% in RP. Bulk soil OC stocks in 18- and 42-year-old NG at depths of 0-40 cm were 9.0% and 17.4% higher, respectively, than those in RP of the same age. Conversely, the 18- and 42-year-old RP had 18.6% and 16.4% higher TN stocks than those in NG of the same age. Microaggregates dominated aggregate-size distribution in all land uses and had the highest OC and TN stocks. NG soils had higher proportions of macroaggregates and mesoaggregates and greater aggregate stability than RP soils of the same restoration age. However, there was no significant difference in macroaggregate- and mesoaggregate-associated OC and TN stocks between NG soils and RP soils of the same age. Furthermore, bulk soil OC and TN stocks were strongly associated with aggregate stability. Bulk soil OC stocks were positively correlated with mesoaggregate-associated OC stocks, whereas TN stocks in bulk soil were closely related to macroaggregate-associated TN stocks. Overall, the results of this study suggest that restoration with RP favors soil N accumulation, whereas NG may be optimal for increasing C sequestration and stabilizing soil aggregates in the Loess Plateau.