There is increasing concern over the impact of atmospheric nitrogen (N) deposition on forest ecosystems in the tropical and subtropical areas. In this study, we quantified atmospheric N deposition and revealed current plant and soil N status in 14 forests along a 150 km urban to rural transect in southern China, with an emphasis on examining whether foliar d 15 N can be used as an indicator of N saturation. Bulk deposition ranged from 16.2 to 38.2 kg N ha À1 yr À1 , while the throughfall covered a larger range of 11.7-65.1 kg N ha À1 yr À1 . Foliar N concentration, NO 3 À leaching to stream, and soil NO 3 À concentration were low and NO 3 À production was negligible in some rural forests, indicating that primary production in these forests may be limited by N supply. But all these N variables were enhanced in suburban and urban forests. Across the study transect, throughfall N input was correlated positively with soil nitrification and NO 3 À leaching to stream, and negatively with pH values in soil and stream water. Foliar d 15 N was between À6.6% and 0.7%, and was negatively correlated with soil NO 3 À concentration and NO 3 À leaching to stream across the entire transect, demonstrating that an increased N supply does not necessarily increase forest d 15 N values. We proposed several potential mechanism that could contribute to the d 15 N pattern, including (1) increased plant uptake of 15 N-depleted soil NO 3 À , (2) foliage uptake of 15 N-depleted NH 4 1 , (3) increased utilization of soil inorganic N relative to dissolved organic N, and (4) increased fractionation during plant N uptake under higher soil N availability.

Nitrogen (N) deposition levels and frequencies of extreme drought events are increasing globally. In efforts to improve understanding of plants' responses to associated stresses, we have investigated responses of mosses to drought under elevated nitrogen conditions. More specifically, we exposed Pogonatum cirratum subsp. fuscatum and Hypnum plumaeforme to various nitrate (KNO 3) or ammonium (NH 4Cl) treatments, with and without water deficit stress and monitored indices related to carbon (C) and N metabolism both immediately after the stress and after a short recovery period. The results show that N application stimulated both C and N assimilation activities, including ribulose‐1,5‐bisphosphate carboxylase, glutamine synthetase/glutamate synthase (GS/GOGAT), and glutamate dehydrogenase (GDH) activities, while water deficit inhibited C and N assimilation. The mosses could resist stress caused by excess N and water deficit by increasing their photorespiration activity and proline (Pro) contents. However, N supply increased their sensitivity to water stress, causing sharper reductions in C and N assimilation rates, and further increases in photorespiration and Pro contents, indicating more serious oxidative or osmotic stress in the mosses. In addition, there were interspecific differences in N assimilation pathways, as the GS/GOGAT and GDH pathways were the preferentially used ammonium assimilation pathways in P. cirratum and H. plumaeforme when stressed, respectively. After rehydration, both mosses exhibited overcompensation effects for most C and N assimilation activities, but when supplied with N, the activities were generally restored to previous levels (or less), indicating that N supply reduced their ability to recover from water deficit stress. In conclusion, mosses can tolerate a certain degree of water deficit stress and possess some resilience to environmental fluctuations, but elevated N deposition reduces their tolerance and ability to recover.

Editor: Linda E. Graham Premise of research. It has been well documented that secondary metabolites in mosses can protect mosses from pathogen or herbivore attacks, but their functions in abiotic stress are poorly understood. Thus, we have investigated effects of water deficit and high nitrogen (N) on secondary metabolism in two moss species.Methodology. KNO 3 or NH 4 Cl was supplied to Pogonatum cirratum subsp. fuscatum and Hypnum plumaeforme at rates of 20-60 kg N hm 22 for 1 yr, water was then withheld for 12 d, and finally a 10-d recovery treatment was applied. Indexes of oxidative stress (superoxide dismutase [SOD], malondialdehyde [MDA]) and secondary metabolism were determined after the water-withholding and recovery treatments, respectively.Pivotal results. Both ammonium and nitrate supply stimulated SOD activity and MDA accumulation as well as the synthesis of phenylpropanoids, triterpenes, and total alkaloids. Water-deficit stress induced oxidative stress in both moss species, and N application exacerbated the oxidative stress. Water deficit inhibited SOD and L-phenylalanine ammonia-lyase activities and changed their phenylpropanoid profiles (with interspecific differences). Triterpene accumulation was stimulated by water deficit in H. plumaeforme (but not in P. cirratum), while alkaloid accumulation was stimulated in both mosses. After the 10-d recovery period, the N treatments generally still had significant effects on SOD activity and secondary metabolite contents, and water deficit still had effects on contents of some phenolics and alkaloids. However, SOD activity in the waterdeficit-treated P. cirratum samples was restored after recovery, indicating that it may play an important role in recovery from stress induced by water deficit.Conclusions. The secondary metabolism of both mosses is intimately linked to their antioxidant mechanisms. Phenylpropanoids, triterpenes, and alkaloids may mark moss responses to stress and therefore present a useful method for environmental monitoring in the mosses.

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Background: Both nitrogen deposition and low-temperature stress (LTS) affect the secondary metabolism of plants, but few were known about the responses of secondary metabolism to combined LTS and nitrogen supply, especially for mosses. Aims: To investigate the effects of LTS on secondary metabolism in two mosses, Pogonatum cirratum subsp. fuscatum and Hypnum plumaeforme, under varied nitrate or ammonium supply rates. Methods: Indices of oxidative stress and secondary metabolism in two mosses that were subjected to different N application and LTS treatment were measured both before and after a period of 10-day recovery. Results: LTS-induced oxidative stress was exacerbated by combined N application. LTS inhibited superoxide dismutase (SOD) activity in P. cirratum subsp. fuscatum, but in H. plumaeforme SOD activity it was insensitive. N application induced increases in L-phenylalanine ammonia-lyase activity in both mosses, but the consequent changes in phenylpropanoid profiles in the mosses differed. LTS only changed the phenylpropanoid profiles. Levels of triterpenes were also affected by both N supply and LTS. In addition, levels of some secondary metabolites remained relatively high after 10-day recovery from LTS. Conclusions: N supply altered the responses of secondary metabolism to LTS. Some secondary metabolites may be stress memory substances associated with LTS of the mosses, with between-species differences.