Effects of nitrogen addition on root traits and soil nitrogen in the long-term restored grasslands

Fine root traits are plastic and responsive to increased nitrogen (N) deposition. However, with the restoring of the ecosystem after grain for green, little research has been reported about the response of root traits in a long-term restored ecosystem to increased N deposition. Therefore, a successive N addition experiment was conducted in a long-term restored grassland on the Loess Plateau to analyse the effects of different N addition levels (0, 2.5, 5, 10, 20 g N/m2/year) on root morphological traits, soil carbon (C) and N. Our results showed that root morphological traits (except for root diameter) firstly increased and then declined, with the maximum in the N level of 5 g/m2/year. N addition significantly increased soil organic carbon, total nitrogen, ammonium nitrogen (NH4+-N) and nitrate-nitrogen (NO3 --N) with the increasing N addition level, especially in the soil surface layer. Specific root length and specific root area had remarkable negative correlations with NO3--N, while root diameter and root length density had positive correlations with soil availability N and soil microbial biomass carbon. This study indicated that plants could have the threshold response to adapt to the N addition and prefer to slowly grow rather than quickly invest and return in order to adapt to the environmental stress.