While it is well established that ecosystems display strong responses to elevated nitrogen deposition, the importance of the ratio between the dominant forms of deposited nitrogen (NH(x) and NO(y)) in determining ecosystem response is poorly understood. As large changes in the ratio of oxidised and reduced nitrogen inputs are occurring, this oversight requires attention. One reason for this knowledge gap is that plants experience a different NH(x):NO(y) ratio in soil to that seen in atmospheric deposits because atmospheric inputs are modified by soil transformations, mediated by soil pH. Consequently species of neutral and alkaline habitats are less likely to encounter high NH(4)(+) concentrations than species from acid soils. We suggest that the response of vascular plant species to changing ratios of NH(x):NO(y) deposits will be driven primarily by a combination of soil pH and nitrification rates. Testing this hypothesis requires a combination of experimental and survey work in a range of systems.

Summary• Atmospheric nitrogen deposition can cause major declines in bryophyte abundance yet the physiological basis for such declines is not fully understood. Bryophyte physiological responses may also be sensitive bioindicators of both the impacts of, and recovery from, N deposition.• Here, responses of tissue nutrients (nitrogen (N), phosphorus (P) and potassium (K): NPK), N and P metabolism enzymes (nitrate reductase and phosphomonoesterase), photosynthetic pigments, chlorophyll fluorescence, sclerophylly and percentage cover of two common bryophytes (Pseudoscleropodium purum and Rhytidiadelphus squarrosus) to long-term (11 yr) enhanced N deposition (+3.5 and +14 g N m −2 yr −1 ) are reported in factorial combination with P addition. Recovery of responses 22 months after treatment cessation were also assessed.• Enhanced N deposition caused up to 90% loss of bryophyte cover but no recovery was observed. Phosphomonoesterase activity and tissue N : P ratios increased up to threefold in response to N loading and showed clear recovery, particularly in P. purum. Smaller responses and recovery were also seen in all chlorophyll fluorescence measurements and altered photosynthetic pigment composition.• The P limitation of growth appears to be a key mechanism driving bryophyte loss along with damage to photosystem II. Physiological measurements are more sensitive than measurements of abundance as bioindicators of N deposition impact and of recovery in particular.

Bryophyte establishment represents a positive feedback process that enhances soil development in newly exposed terrain. Further, biological nitrogen (N) fixation by cyanobacteria in association with mosses can be an important supply of N to terrestrial ecosystems, however the role of these associations during post-glacial primary succession is not yet fully understood. Here, we analyzed chronosequences in front of two receding glaciers with contrasting climatic conditions (wetter vs drier) at Cordillera Darwin (Tierra del Fuego) and found that most mosses had the capacity to support an epiphytic flora of cyanobacteria and exhibited high rates of N2 fixation. Pioneer moss-cyanobacteria associations showed the highest N2 fixation rates (4.60 and 4.96 µg N g−1 bryo. d−1) very early after glacier retreat (4 and 7 years) which may help accelerate soil development under wetter conditions. In drier climate, N2 fixation on bryophyte-cyanobacteria associations was also high (0.94 and 1.42 µg N g−1 bryo. d−1) but peaked at intermediate-aged sites (26 and 66 years). N2 fixation capacity on bryophytes was primarily driven by epiphytic cyanobacteria abundance rather than community composition. Most liverworts showed low colonization and N2 fixation rates, and mosses did not exhibit consistent differences across life forms and habitat (saxicolous vs terricolous). We also found a clear relationship between cyanobacteria genera and the stages of ecological succession, but no relationship was found with host species identity. Glacier forelands in Tierra del Fuego show fast rates of soil transformation which imply large quantities of N inputs. Our results highlight the potential contribution of bryophyte-cyanobacteria associations to N accumulation during post-glacial primary succession and further describe the factors that drive N2-fixation rates in post-glacial areas with very low N deposition.

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