Primary limitation on vegetation productivity shifts from precipitation in dry years to nitrogen in wet years in a degraded arid steppe of Inner Mongolia, northern China

Abstract

Arid steppes in northern China have degraded severely in recent decades due to frequent human activities, resulting in poor soil quality and thus low productivity. The objective of the current study was to investigate whether nitrogen addition was a useful approach to improve productivity of these degraded steppes in Inner Mongolia. In the current study, severely degraded arid steppe was fenced in June 2014 and then fertilized for consecutive 3 years, 2014, 2015, and 2016. There were four nitrogen fertilization rates, 0, 50, 100, and 150 kg N ha−1, and two phosphorus rates, 0 and 60 kg P2O5 ha−1. Each treatment replicated three times, with each plot size reaching 400 m2 (20 m × 20 m). The annual precipitation in 2014 and 2016 were 255 and 309 mm (dry years), respectively, lower than that (412 mm) in 2015 (wet year). The results indicated that aboveground biomass in wet years was significantly higher than that in dry years, suggesting that water is the most important limiting factor influencing steppe productivity. Plant nitrogen concentration in Stipa krylovii (dominant species) was positively correlated with the concentrations of soil available nitrogen and nitrogen use efficiency (NUE), confirming that the plant adsorbed more nitrogen under fertilization and thus increasing the NUE. The NUE and water use efficiency (WUE) in wet year were higher than those in dry years and a positive correlation was also observed between WUE and NUE, confirming that the NUE was relied mainly on precipitation. Nitrogen fertilization was effective in increasing grassland production in wet years but not in dry years, suggesting that the primary limitation on grassland productivity in this ecosystem might shift from precipitation in dry years to nitrogen in wet years. Higher NUE could be obtained under low nitrogen rates in wet years. Therefore, in degraded arid steppe, low nitrogen rate (50 kg N ha−1) was recommended in wet years to improve steppe productivity.