Increased soil moisture content increases plant N uptake and the abundance of ¹⁵N in plant biomass

Dijkstra, Feike A; Cheng, Weixin

Increased soil moisture content increases plant N uptake and the abundance of ¹⁵N in plant biomass

2008

Dijkstra, Feike A | Cheng, Weixin

The natural abundance of ¹⁵N in plant biomass has been used to infer how N dynamics change with elevated atmospheric CO₂ and changing water availability. However, it remains unclear if atmospheric CO₂ effects on plant biomass ¹⁵N are driven by CO₂-induced changes in soil moisture. We tested whether ¹⁵N abundance (expressed as δ¹⁵N) in plant biomass would increase with increasing soil moisture content at two atmospheric CO₂ levels. In a greenhouse experiment we grew sunflower (Helianthus annuus) at ambient and elevated CO₂ (760 ppm) with three soil moisture levels maintained at 45, 65, and 85% of field capacity, thereby eliminating potential CO₂-induced soil moisture effects. The δ¹⁵N value of total plant biomass increased significantly with increased soil moisture content at both CO₂ levels, possibly due to increased uptake of ¹⁵N-rich organic N. Although not adequately replicated, plant biomass δ¹⁵N was lower under elevated than under ambient CO₂ after adjusting for plant N uptake effects. Thus, increases in soil moisture can increase plant biomass δ¹⁵N, while elevated CO₂ can decrease plant biomass δ¹⁵N other than by modifying soil moisture.

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