Nitrogen accumulation and distribution in Danthonia richardsonii swards in response to CO 2 and nitrogen supply over four years of growth

Summary 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 CO 2 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 − 2  y − 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 CO 2 enrichment only at the highest rate of mineral N supply. Levels of 15 N natural abundance in green leaf (δ 15 Ν) of − 2‰ (amb low‐N) and of below − 4‰ (enr low‐ & mid‐N) suggest that absorption of atmospheric NH 3 may have been a source of some of the extra N in the low and mid‐N treatments. Biological N 2 fixation, of up to 2 g m − 2  y − 1 was hypothesized to form the remainder of the environmental N source. Microcosm C:N ratio was higher under CO 2 enrichment. Nitrogen productivity of microcosm carbon gain (g C accumulated g − 1 leaf N day − 1 ) was increased (up to 100%) by CO 2 enrichment at all rates of mineral N supply. Green leaf %N was reduced by CO 2 enrichment, and there was less nitrogen in the green leaf pool under CO 2 enrichment. Less, or the same amount of nitrogen was present in senesced leaf, surface litter and root under CO 2 enrichment while more nitrogen was present in the soil in organic forms, and as NH 4  +  at the highest rate of mineral N supply.