Deep soil dissolved C and N stocks and stratification affected by afforestation along a restoration chronosequence of Robinia pseudoacacia plantations on the Loess Plateau, China

Abstract Soil dissolved carbon (C) and nitrogen (N) are early sensitive indexes to evaluate the changes in C and N pools in terrestrial ecosystems. However, there are few studies on the changes in soil dissolved C and N with revegetation age and their stratification ratios (SRs), especially for deep soil. The changes in soil dissolved C and N and their SRs in one farmland and five Robinia pseudoacacia plantations (RP: 15a, 20a, 25a, 30a and 40a) were studied. The results showed that RP plantations significantly increased the soil organic C (SOC) and total N (TN) stocks with increasing restoration age for 30 years in the shallow and middle soil layers and improved the stocks of dissolved total C (DTC), dissolved organic C (DOC), dissolved total N (DTN), dissolved organic N (DON) and dissolved inorganic N (DIN) in most soil layers, especially in the 20a, 25a and 30a plots. In the 0–200 cm soil layer, the middle and deep soil accounted for 50%–82% of the SOC and soil dissolved C stocks and 63%–84% of the soil TN and dissolved N stocks, respectively. At all soil depths, the SRs of SOC, TN, DTC, DOC and DIN in RP plots were greater than those in farmland plots, and the SRs of SOC and TN in almost all RP plots were >2. These results demonstrated that restoration age and soil depth had significant influences on SOC, TN and soil dissolved C and N, and RP forestation increased the SOC and TN stocks in the shallow and middle soil layers, and the soil dissolved C and N stocks in 20a, 25a and 30a plots were the largest; additionally, the middle and deep soil layers played important roles in estimating the soil C and N pools. In summary, RP afforestation significantly improved soil C and N and their dissolved fractions and the SRs, thus soil quality and nutrient availability consistently improved.