R. S. P. van Logtestijn

Nitrogen supply differentially affects litter decomposition rates and nitrogen dynamics of sub-arctic bog species

High-latitude peatlands are important soil carbon sinks. In these ecosystems, the mineralization of carbon and nitrogen are constrained by low temperatures and low nutrient concentrations in plant litter and soil organic matter. Global warming is predicted to increase soil N availability for plants at high-latitude sites. We applied N fertilizer as an experimental analogue for this increase. In a three-year field experiment we studied N fertilization effects on leaf litter decomposition and N dynamics of the four dominant plant species (comprising >75% of total aboveground biomass) in a sub-arctic bog in northern Sweden. The species were Empetrum nigrum (evergreen shrub), Eriophorum vaginatum (graminoid), Betula nana (deciduous shrub) and Rubus chamaemorus (perennial forb). In the controls, litter mass loss rates increased in the order: Empetrum < Eriophorum < Betula < Rubus. Increased N availability had variable, species-specific effects: litter mass loss rates (expressed per unit litter mass) increased in Empetrum, did not change in Eriophorum and Betula and decreased in Rubus. In the leaf litter from the controls, we measured no or only slight net N mineralization even after three years. In the N-fertilized treatments we found strong net N immobilization, especially in Eriophorum and Betula. This suggests that, probably owing to substantial chemical and/or microbial immobilization, additional N supply does not increase the rate of N cycling for at least the first three years.

Similar cation exchange capacities among bryophyte species refute a presumed mechanism of peatland acidification.

Fen-bog succession is accompanied by strong increases of carbon accumulation rates. We tested the prevailing hypothesis that living Sphagna have extraordinarily high cation exchange capacity (CEC) and therefore acidify their environment by exchanging tissue-bound protons for basic cations in soil water. As Sphagnum invasion in a peatland usually coincides with succession from a brown moss-dominated alkaline fen to an acidic bog, the CEC of Sphagna is widely believed to play an important role in this acidification process. However, Sphagnum CEC has never been compared explicitly to that of a wide range of other bryophyte taxa. Whether high CEC directly leads to the ability to acidify the environment in situ also remains to be tested. We screened 20 predominant subarctic bryophyte species, including fen brown mosses and bog Sphagna for CEC, in situ soil water acidification capacity (AC), and peat acid neutralizing capacity (ANC). All these bryophyte species possessed substantial CEC, which was remarkably similar for brown mosses and Sphagna. This refutes the commonly accepted idea of living Sphagnum CEC being responsible for peatland acidification, as Sphagnums ecological predecessors, brown mosses, can do the same job. Sphagnum AC was several times higher than that of other bryophytes, suggesting that CE (cation exchange) sites of Sphagna in situ are not saturated with basic cations, probably due to the virtual absence of these cations in the bog water. Together, these results suggest that Sphagna can not realize their CEC in bogs, while fen mosses can do so in fens. The fen peat ANC was 65% higher than bog ANC, indicating that acidity released by brown mosses in the CE process was neutralized, maintaining an alkaline environment. We propose two successional pathways indicating boundaries for a fen-bog shift with respect to bryophyte CEC. In neither of them is Sphagnum CE an important factor. We conclude that living Sphagnum CEC does not play any considerable role in the fen-bog shift. Alternatively, we propose that exclusively indirect effects of Sphagnum expansion such as peat accumulation and subsequent blocking of upward alkaline soil water transport are keys to the fen-bog succession and therefore for bog-associated carbon accumulation.

Nitrogen content in fine roots and the structural and functional adaptations of alpine plants

A difference in the adaptation of plants to rich and poor soils is observed at the intraregional level. The nitrogen and carbon content in fine roots and its relation to the taxonomic position and ecologo-morphological traits of a plant has been studied in 92 alpine plant species of the northwestern Caucasus. The nitrogen content varies from 0.43% (Bromus variegates) to 3.75% (Corydalis conorhiza), with the mean value equal to 1.3%. The carbon content varies from 40.3% (Corydalis conorhiza) to 51.7% (Empetrum nigrum), with the mean value equal to 43.4%. The average C: N ratio in fine roots is 34: 1, which is higher than the same value observed in other regions. Roots of dicotyledons have a higher nitrogen content than those of monocotyledons. The highest and lowest values of this parameter are observed in Fabaceae (2.1%) and Poaceae (1.3%), respectively. The highest and lowest carbon concentrations were observed in roots of Ericaceae (47.2%) and Ranunculaceae (42.1%), respectively. Among the species studied, the carbon content in roots increases in the following order: geophytes < hemicryptophytes < chamephytes. The specific root length positively and negatively correlates with the nitrogen and carbon content in roots, respectively. Large-leaved species, which have a higher specific leaf area, are characterized by higher nitrogen and lower carbon content in their roots. The content of nitrogen and carbon in fine roots positively correlates with their content in leaves. This fact confirms the association between the ecologo-morphological traits of plants and the chemical composition of their fine roots. Plant species with a higher growth rate are characterized by a higher nitrogen and lower carbon content in their roots; they are also characterized by the arbuscular mycorrhyza, a higher seed production, and larger leaves with a higher water content and a higher specific leaf area. These traits correspond to the plants of the competitive and ruderal strategies. On the other hand, stress-tolerant (patient) plants with adaptations to low nutrient levels and low growth rate are characterized by higher carbon and lower nitrogen content in fine roots, smaller leaves with a low specific leaf area, and a low seed production.