PARENT SESSION
Thursday, August 10, 8:00-11:30 am
COS 78 - Nitrogen cycling I
L-4, Lobby Level, Cook Convention Center
Presiders: K Buckeridge

Nitrogen uptake preferences by plants in African savannas.

Wang, Lixin^*,1, Macko, Stephen¹, D'odorico, Paolo¹, ¹ University of Virginia, Charlottesville, VA

ABSTRACT- In arid and semiarid ecosystems like African savannas, nutrient availability varies spatially and temporally. Preference for different nitrogen forms presumably enhances the survivorship and fitness of plants since the relative abundances of nitrate and ammonium vary between drier and wetter areas. To test the hypothesis that species developing in dry areas will prefer nitrate whereas species growing in wet areas will prefer ammonium, a controlled experiment using a greenhouse was undertaken. Six native African grass species from different precipitation regimes were used in this study. Two species were from relatively wet areas (Pandamatenga, Botswana, precipitation = 698 mm/year), two were from dry areas (Tshane, Botswana, precipitation = 232mm /year) and other two were from intermediate environments (Ghanzi, Botswana, precipitation = 400mm /year). The grass seeds were collected in the field during the 2004 dry season and germinated and grown in a greenhouse. When individuals were mature, they were transferred into plastic pots (one individual per pot) containing commercial sand. After one week period of adjustment, a ¹⁵N labeled fertilizer (NH₄NO₃) was applied. A pair of individual plants was treated as an experimental unit. Each plant received the same amount of total N fertilizer, but one was ¹⁵NO₃ labeled and another was ¹⁵NH₄ labeled. Nutrient uptake preference was determined by the ¹⁵N difference between pairs. The results show that, the individuals from relatively dry areas have significantly higher foliar ¹⁵N signatures in the ¹⁵NO3 labeling treatment. Whereas individuals from wet areas showed significantly higher root ¹⁵N signatures in the ¹⁵NH₄ labeling treatment but were not different in foliar ¹⁵N signatures. In addition, when a higher concentration of ammonium nitrate was applied to Pogonarihria squarrosa and Leptocarydion vulpiastrum, the nutrient uptake preference did not change. Our results provide direct evidence of plant nitrogen uptake preference and suggest the possibility that this preference is associated with the abundance of the different forms of nitrogen in the different habitats.

Key words: Nitrogen uptake preference, Stable isotope, Arid and semi-arid environment