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 (KNO₃) or ammonium (NH₄Cl) 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.

1 Introduction The overall trend of global climate change is warming; however, the accelerated melting and disappearance of Arctic Sea-Ice has also led to increasing local occurrences of abnormally low temperatures (Mori et al., 2014; Overland et al., 2015). Low temperature stress (LTS) damages plants by inducing oxidative stress, hindering photosynthesis, and indirectly inducing osmotic stress (Nuccio et al., 1999; Allen and Ort, 2001; Beck et al., 2007). Increasing nitrogen (N) deposition is another environmental problem which attracts global attention (Stevens et al., 2004; Galloway et al., 2008; Liu et al., 2013). Excess N deposition leads to nutritional imbalance, cell membrane destruction and oxidative stress in plants (Pearce and Van der Wal, 2002; Koranda et al., 2007). N in wet deposition mainly occurs in the forms of NO3- and NH4+, but they have different effects on plants. On the one hand, as H+ is generated during NH4+ assimilation, whereas OH- is generated during NO3- assimilation, NH4+ is more apt to decrease tissue pH and disrupt membrane function (Pearce et al., 2003, Paulissen et al., 2004). On the other hand, NH4+ is more readily available than NO3- for utilization by plants as less energy is needed during its assimilation (Flaig and Mohr, 1992). Moreover, excess N deposition can reduce the resistance of plants to LTS (Fenn et al., 1998); this phenomenon merits attention, although the underlying mechanisms have seldom been investigated. The resilience of plants to stress, namely the ability of plants to recover growth after removal of stress factors, is vital for their survival in natural environments. This phenomenon, referred to as the “compensation effect” (Belsky, 1986), exists widely in various plants, and has been studied extensively in agriculture and stockbreeding. However, the effects of N deposition on compensation effects after LTS in plants is poorly understood, despite the importance of precise evaluation of the effects of global change on plants and ecosystems. Mosses, a group of primitive land plants with small stature that mainly acquire their nutrients from air, are relatively sensitive to environmental change, and, therefore, generally more vulnerable to global changes than vascular plants (Pearce and van

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.