Lecture series in Ecology and Environmental Research SS 2009

15N natural abundance in soil-plant systems of the alpine communities in the Northern Caucasus

Eingeladen durch Prof. Zech.

We studied 15N natural abundance in different soil N pools (total N, NH4+–N, NO3––N, DON) and plant leaves in the toposequence of alpine ecosystems in the Northern Caucasus. The toposequence was represented by (1) low-productive alpine lichen heath of wind-exposed ridge and upper slope; (2) more productive Festuca varia grassland of middle slope; (3) most productive Geranium gymnocaulon/Hedusarum caucasicum meadow of lower slope; (4) low-productive snow-bed community of the slope bottom.

We found that δ15N of soil N pools correlate with N mineralization and nitrification, and concluded that these values of soil N are useful to evaluate the relative N transformation activity in alpine soils of the Northern Caucasus. To study the role of different mycorrhizal symbionts in control N isotopes composition of alpine plants we studied 25 plant species from 8 functional/mycorrhizal groups from a multi-species N-limited alpine lichen heath.

Different groups were represented by ericoid mycorrhizal species, ectomycorrhizal species, arbuscular mycorrhizal forbs, arbuscular mycorrhizal grasses, arbuscular mycorrhizal nodulated legumes, non-mycorrhizal graminoids (sedges and rushes), non-mycorrhizal hemiparasites, and orchids.

Investigated functional groups of alpine plants can be combined into two groups: (1) ericoid mycorrhizal and ectomycorrhizal species, arbuscular mycorrhizal forbs and grasses, and non-mycorrhizal hemiparasites were characterized by relatively low 15N enrichment, while (2) non-mycorrhizal sedges, N2-fixing legumes and orchids were more enriched with heavy N isotope.

Nitrogen isotopic composition of 22 plant species from all investigated communities demonstrated significant response to input of different N sources with different δ15N into the soil. All investigated species showed increase of foliar δ15N when 15N-enriched NO3––N (δ15N = +9.5‰) was added. Using isotopic mass balance we quantified NO3––N contribution to mixed plant nutrition from natural soil and artificial NO3– sources. All grasses and sedges (with exception of Festuca varia) demonstrated lower NO3––N uptake (20–40%) relative to all other species of corresponding community (40–60%). Decrease of leaf δ15N following treatment with 15N depleted NH4+–N (δ15N = –8.0‰) was less pronounced. Significant δ15N decrease was observed for grasses and sedges only, indicating the preference of grass and sedge alpine species to NH4+–N uptake. However, the experiment with simultaneous NH4+ and NO3– addition confirmed these results only partially.

Our results demonstrate that δ15N of alpine plants may reflect the δ15N of soil N sources to some degree.

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