Off-season ecosystem processes is becoming an area of increasing interest, being

important when considering annual nitrogen and carbon budgets. The general assumption that

physiological activity in soil microorganisms as well as vegetation is low during winter may

not be justified. In this field study we show that northern temperate coastal heath vegetation

has a significant uptake potential for nitrogen, both in the form of ammonium and as glycine,

throughout the non-growing season.

We used 15N ammonium and 2*(13C)15N glycine as nutrient tracer. This was injected

into the soil two times during winter and once at spring. The winter temperatures were similar

to those of average winter in the northern temperate region of Europe, with only few days of

soil temperatures below zero and a winter mean of 2.4 °C. The vegetation, consisting of the

evergreen dwarf shrub Calluna vulgaris (L.), the deciduous dwarf shrub Salix arenaria (L.)

and the graminoids Carex arenaria (L.) and Deschampsia flexuosa (L.), showed root uptake

of both forms of nitrogen, both one day after labelling and after a month. Translocation of the

labelled nitrogen to shoots was generally evident after one month and increased as spring

approached, with different translocation strategies in the three plant functional types.

Furthermore, shoot total nitrogen concentration increased in all plant types, but only the

graminoids and, eventually, S. arenaria showed shoot growth during winter. Our study

suggests that plant nitrogen uptake can cause increasing nitrogen concentration in shoot tissue

from fall to spring.

To our knowledge this is one of the first studies of plant nitrogen uptake during nongrowing

season at a temperate heath. Our results show that the nitrogen uptake is in the same

order of magnitude as summer uptake in other types of ecosystems in the temperate/boreal

region. These results suggest that plant nitrogen uptake during winter should be included in

the annual nitrogen budgets of heath ecosystems, and that the view of plant nutrient uptake as

low in this climatic region during winter should be revised. Furthermore, these results should

be taken into account when ecosystem responses to climatic changes such as warming are

evaluated.