Free-air CO2 enrichment (FACE) net nitrogen fixation experiment at a paddy soil surface under submerged conditions

The aim of this study was to evaluate the cumulative net free-living nitrogen (N) fixation and its response to elevated atmospheric carbon dioxide (CO₂) in the submerged surface of paddy soil. The study site was an actual paddy area in central Japan that was equipped with a free-air CO₂ enrichment (FACE) facility and was composed of four bays, where each bay had an elevated CO₂ plot (FACE plot) and an ambient plot. Field incubation was conducted at subplots without N fertilizer application from May 28 to August 18, 2012 (82 days). The CO₂ enrichment was an average of +528 ppm during the study period at the subplots. The changes in total N (TN) content in the surface soil (0–1 cm) were determined in comparison to the initial TN content. The cumulative net N fixations during the 82-day study period were 47.1 ± 3.7 and 43.3 ± 5.8 (standard error) kg N ha^?1 in the FACE and ambient plots, respectively. The difference between the FACE and ambient plots was not significant (p = 0.05). However, these values were partly affected by the charophyte blooms, which are not involved in N fixation. The results are not conclusive, and the one bay without charophyte blooming displayed a significant increase in the cumulative net N fixation (approximately 10 kg N ha^?1) with CO₂ enrichment (p < 0.001).