Effects of vegetation restoration on soil aggregates, organic carbon, and nitrogen in the Loess Plateau of China

Understanding the responses of soil aggregate stability and nutrient stoichiometry to vegetation restoration is essential for the ecosystem in the Loess Plateau, China. To explore the influence of vegetation restoration on soil aggregate stability and nutrient stoichiometry, we established a series of sample plots across five vegetation restoration types [sloping farmland (SFL), woodland (WL), grassland (GL), shrub land (SL), and terraced fields (TCE)], and measured the particle size distribution, stability, and organic carbon (OC) and total nitrogen (TN) content of soil aggregates. Vegetation restoration significantly affected particle size, stability, and OC and TN stocks of soil aggregates at depths of 0-40 cm. Compared with that in SFL, the quality fraction of macroaggregates in WL, GL, SL, and TCE was 215.47%, 245.82%, 189.83%, and 95.06% greater, respectively, whereas the aggregate stability (indicated by the mean weight diameter) was 109.71%, 22.83%, 71.84%, and 77.67% greater, respectively. Vegetation restoration in WL, GL, and TCE significantly increased the OC content at 0-40 cm depth and the TN content at 0-60 cm depth. Aggregates > 0.25 mm dominated the particle size distribution in all vegetation restoration types, and had the highest OC and TN stocks; minor aggregates were the main source of OC and TN accumulation. A simulated rainfall experiment confirmed that an increase in soil aggregates (>0.25 mm) was associated with greater aggregate stability. Our findings suggested that soil aggregates > 0.25 mm are optimal for stabilizing soil aggregates, and increasing OC and TN stocks, in the Loess Plateau.