Nitrogen accumulation and distribution in Danthonia richardsonii swards in response to CO₂ and nitrogen supply over four years of growth

Nitrogen-stressed microcosms of the C3 grass Danthonia richardsonii gained nitrogen from the environment when grown under ambient or enriched (359, 'amb' or 719 μL L^-1 'enr', respectively) atmospheric CO₂ concentrations over a 4-y period. This gain was apparent at all rates of supplied mineral N (2.2, 6.7 or 19.8 g N m^-2y^-1 - low-N, mid-N or high-N), although it was small at high-N. Small losses of N occurred from the microcosm as leachate, while gaseous losses of N were estimated to be between 10% and 25% of applied mineral N. Losses of applied mineral N were slightly lower under CO₂ enrichment only at the highest rate of mineral N supply. Levels of ¹⁵N natural abundance in green leaf (δ¹⁵N) of -2‰ (amb low-N) and of below -4‰ (enr low- and mid-N) suggest that absorption of atmospheric NH₃ may have been a source of some of the extra N in the low and mid-N treatments. Biological N₂ fixation, of up to 2 g m^-2y^-1 was hypothesized to form the remainder of the environmental N source. Microcosm C:N ratio was higher under CO₂ enrichment. Nitrogen productivity of microcosm carbon gain (g C accumulated g^-1 leaf N day^-1) was increased (up to 100%) by CO₂ enrichment at all rates of mineral N supply. Green leaf %N was reduced by CO₂ enrichment, and there was less nitrogen in the green leaf pool under CO₂ enrichment. Less, or the same amount of nitrogen was present in senesced leaf, surface litter and root under CO₂ enrichment while more nitrogen was present in the soil in organic forms, and as NH₄⁺ at the highest rate of mineral N supply.