Afforestation with xerophytic shrubs accelerates soil net nitrogen nitrification and mineralization in the Tengger Desert, Northern China

Afforestation with xerophytic shrubs in drylands to curb desertification and land degradation has been increasingly implemented worldwide. However, the effects of this restoration practice on soil nitrogen mineralization and availability in desert areas remain poorly understood. We investigated the seasonal patterns of soil inorganic N pools and net N nitrification and mineralization in mobile sand dune and three shurblands (afforested in 1956, 1964 and 1981, respectively) in the Tengger Desert by using the in situ incubation method. Our results showed that afforestation significantly increased the soil inorganic N pools, and net nitrification and mineralization rates. Annual average values of NH4+-N, NO3−-N and total inorganic N were 2.64-4.06, 3.37-7.74 and 3.06-6.12 times greater in afforested sites that those in mobile sand dunes, with NO3−-N being predominated in all sites and its proportion being increased with stand age. In mobile sand dune, annual soil net N nitrification and mineralization rates were 1.25 and 1.87 mg N m−2 d−1, respectively, while those in afforested sites were 5.32-11.89 and 5.98-15.16 times higher, respectively, with nitrification predominated the N mineralization process. Marked seasonality in inorganic N pools, net N nitrification and mineralization were observed with the highest values in summer and the lowest in winter, which were closely linked to afforestation-induced changes in soil temperature and moisture as well as surface litter, content of clay and silt, C: N and OC of soil. The contrasting results for soil N transformations were observed during non-growing season, that was net N immobilization for the mobile sand dune without afforestation and the shrubland established in 1981, while net N mineralization for the shrublands established in 1964 and 1956, the latter two sites account for 2.65% and 9.68% of their total mineralization, respectively. These results suggested that afforestation with xerophytic shrubs has positive effects on soil nitrogen availability and cycling in desert region. It also implied that the afforestation-triggered recovery of biogeochemical cycling of N was a long-term process, and therefore soil habitat conservation is a vital issue in desert ecosystems.