Off-season uptake of nitrogen in temperate heath vegetation

Abstract

In this field study we show that temperate coastal heath vegetation has a significant off-season uptake potential for nitrogen, both in the form of ammonium and as glycine, throughout winter. We injected 15N-ammonium and 15N 2x(13C)-glycine into the soil twice during winter and once at spring. The winter temperatures were similar to those of an average winter in the northern temperate region of Europe, with only few days of soil temperatures below zero or above 5 degrees C. The vegetation, consisting of the evergreen dwarf shrub Calluna vulgaris, the deciduous dwarf shrub Salix arenaria, and the graminoids Carex arenaria and Deschampsia flexuosa, showed high root uptake of both forms of nitrogen, both 1 day after labelling and after a month, in species specific temporal patterns. Plant uptake of 13C was not significant, providing no further evidence of intact uptake of glycine. Translocation of the labelled nitrogen to shoots was generally evident after 1 month and increased as spring approached, with different translocation strategies in the three plant functional types. Furthermore, only the graminoids showed shoot growth during winter. Increasing plant nitrogen concentration from fall to spring at temperate heaths may, hence, be due to nitrogen uptake. Our results suggest that the potential for nitrogen uptake in plants at winter is of the same order of magnitude as at summer. Hence, winter nitrogen uptake in ecosystems in the temperate/boreal region should be considered when making annual nitrogen budgets of heath ecosystems, and the view of plant nutrient uptake as low in this climatic region during winter should be revised.

Uptake of pulse injected nitrogen by soil microbes and mycorrhizal and non-mycorrhizal plants in a species-diverse subarctic heath ecosystem

“…The differing uptake patterns of the plant functional types agree with earlier observations of high springtime uptake rate and allocation to leaves in graminoids, and slower uptake and allocation in the woody ericoid species (Andresen and Michelsen 2005). The investigated 15 N acquisition took place on a time scale much shorter than the leaf longevity of 1.8-3.2 (Empetrum) and 1.5 (Rhododendron) seasons (Karlsson 1992) and that of Carex of less than a growing season (Aerts and de Caluwe 1995).…”

“…The revealed niche differentiation of plant species in temporal (Andresen and Michelsen 2005;Grogan and Jonasson 2003;Jaeger et al 1999;McKane et al 2002) and spatial (McKane et al 2002;Sorensen et al 2008) N uptake patterns is complementary to the differentiated N form preference of species or organism groups (Cheng and Bledsoe 2004;Falkengren-Grerup et al 2000;Kielland 1994;Lipson et al 1999;Xu et al 2006). Field studies in natural ecosystems with concomitant measurements of N uptake by soil microorganisms and plant species and their relative uptake of N from different sources are few (Grogan and Jonasson 2003;Hofmockel et al 2007;Nordin et al 2004;Schimel and Chapin 1996;Sorensen et al 2008).…”

Contribution of winter processes to soil nitrogen flux in taiga forest ecosystems

“…Under-snow microbial metabolism may represent an important sink for nitrogen, but during winter the apparent gross N release exceeded microbial nitrogen demand. The accumulation of mineralized N during winter may be used by plants and microbes during the transitional seasons if soil temperatures remain close to 0°C (Andresen and Michelsen 2005), or be lost from the system through denitrification, leaching, and runoff. The recent observation that some taiga ecosystems appear to lose more nitrogen than is produced during the growing season (Jones et al 2005), may in part be explained by these winter processes.…”

Ecosystem respiration depends strongly on photosynthesis in a temperate heath

“…C. vulgaris has previously been suggested to be dormant from October to February based on the cessation of growth (Miller 1979), irregular stomata opening and accumulation of sugar in the leaves (Miller 1979;Kwolek and Woolhouse 1981). Recently, however, both C. vulgaris and D. flexuosa have been shown to absorb considerable amounts of nitrogen during the cold season (Andresen and Michelsen 2005;Larsen et al unpublished data). Although mosses and graminoids may have contributed to the observed photosynthesis during the cold season, the dominance of C. vulgaris, the fact that is evergreen, and that mid day flux rates were considerable, indicate that it was photosynthetically active throughout the year.…”