Root nitrogen uptake capacity of Chinese fir enhanced by warming and nitrogen addition

There is a knowledge gap in the effects of climate warming and nitrogen (N) deposition on root N absorption capacity, which limits our ability to predict how climate change alters the N cycling and its consequences for forest productivity especially in subtropical areas where soil N availability is already high. In order to explore the effects and mechanism of warming and the N deposition on root N absorption capacity of Chinese fir (Cunninghamia lanceolata), a subtropical arbuscular mycorrhizal conifer, the fine root (NH4+)-N-15 and (NO3-)-N-15 uptake kinetics at a reference temperature of 20 degrees C were measured across different seasons in a factorial soil warming (ambient, +5 degrees C) x N addition (ambient, +40 kg N ha(-1) yr(-1)) experiment. The results showed that (i) compared with the control, warming increased the maximal uptake rate of NH4+ (V-max,V-20 degrees C-NH4+) in summer, while N addition enhanced it in spring and summer; compared with non-warming treatments, warming treatments increased the uptake rate of NO3- at a reference concentration of 100 mu mol (V-100,V-20 degrees C-NO3-) in spring. (ii) The analysis of covariance showed that V-max,V-20 degrees C-NH4+ was positively correlated with root mycorrhizal colonization rate (MCR) and V-100,V-20 degrees C-NO3- was positively correlated with specific root respiration rate (SRR), whereas no N uptake kinetic parameter was correlated with specific root length, root N and non-structural carbon concentrations. Thus, our results demonstrate that warming-increased root NH4+ uptake might be related to warming-increased MCR, whereas warming-increased root NO3- uptake might be related to warming-increased SRR. We conclude that root NH4+ and NO3- uptake capacity of subtropical Chinese fir can be elevated under warming and N deposition, which could improve plantation productivity and mitigate N leaching loss and soil acidification.