Recovery of soil carbon and nitrogen stocks following afforestation with xerophytic shrubs in the Tengger Desert, North China

Abstract

Afforestation, as a key measure to curb land degradation and to drive the changes in soil C and N stocks and cycles, has been increasingly implemented in drylands worldwide. However, the long-term effects of this restoration practice on C accumulation in soil are still uncertain, especially because C-N interactions are poorly understood, which constrains the accurate assessment of potential soil C sequestration. In this study, we investigated the soil organic C (SOC) and soil total N (STN) storage in artificial shrublands in chronosequences in the Tengger Desert to address the hypothesis that afforestation with xerophytic shrubs significantly enhanced SOC and STN stocks in desert regions and that SOC and STN stocks would be positively correlated during the restoration processes. Our results showed that afforestation significantly enhanced SOC and STN accumulation. The SOC and STN stocks in 17-, 52- and 60-year-old artificial shrublands were 1.22, 2.67, 2.80 and 1.21, 2.33, 2.47 times greater than those in mobile sand dunes, respectively, with increased rates of 5.61 g C m−2 yr−1, 14.25 g C m−2 yr−1 and 13.30 g C m−2 yr−1 and 1.27 g N m−2 yr−1, 2.61 g N m−2 yr−1 and 2.50 g N m−2 yr−1, respectively. The significant enhancement of SOC and STN stocks primarily occurred in the upper 0–20 cm soil layer. SOC stocks were positively correlated with STN stocks and the C:N ratio, and the regression slopes increased with vegetation age, indicating a coupling relationship between SOC and STN stocks. Additionally, this finding suggests that the increase in STN stocks was slower than that of SOC during restoration, leading to an asynchrony in N supply and the demand and subsequent N limitations on SOC accumulation. It would take approximately 139 and 177 years to fully recover SOC and STN stocks in the 100 cm soil profile, respectively, in mobile sand dunes through afforestation with xerophytic shrubs in the Tengger Desert, which implied that afforestation-induced soil quality improvements were a long-term process and that soil conservation was a key issue in water-limited ecosystems; therefore, optimized management strategies are urgently needed.