Can differences in microbial abundances help explain enhanced N₂O emissions in a permanent grassland under elevated atmospheric CO₂?

Abstract

Long-term effects of elevated atmospheric CO 2 on the ammonia-oxidizing and denitrifying bacteria in a grassland soil were investigated to test whether a shift in abundance of these N-cycling microorganisms was responsible for enhanced N 2 O emissions under elevated atmospheric CO 2 . Soil samples (7.5 cm increments to 45 cm depth) were collected in 2008 from the University of Giessen Free Air Carbon dioxide Enrichment (GiFACE), a permanent grassland exposed to moderately elevated atmospheric CO 2 (+20%) since 1998. GiFACE plots lay on a soil moisture gradient because of gradually changing depth to the underlying water table and labeled as the DRY block (furthest from water table), MED block (intermediate to water table), and WET block (nearest to water table). Mean N 2 O emissions measured since 1998 have been significantly higher under elevated CO 2 . This study sought to identify microbial and biochemical parameters that might explain higher N 2 O emissions under elevated CO 2 . Soil biochemical parameters [extractable organic carbon (EOC), dissolved organic nitrogen (DON), NH 4 + , NO 3 À ], and abundances of genes encoding the key enzymes involved in ammonia oxidation (amoA) and denitrification (nirK, nirS, nosZ) depended more on soil depth and block (underlying soil moisture gradient) than on elevated CO 2 . Ammonia oxidation and denitrification gene abundances, relative abundances (ratios) of nirS to nirK, of nosZ to both nirS and to nirK, and of the measured soil biochemical properties DON and NO 3 À tended to be lower in elevated CO 2 plots as compared with ambient plots in the MED and WET blocks while the DRY block exhibited an opposite trend. High N 2 O emissions under elevated CO 2 in the MED and WET blocks correlated with lower nosZ to nirK ratios, suggesting that increased N 2 O emissions under elevated CO 2 might be caused by a higher proportion of N 2 O-producing rather than N 2 O consuming (N 2 producing) denitrifiers.

Trace gas fluxes of CO<sub>2</sub>, CH<sub>4</sub> and N<sub>2</sub>O in a permanent grassland soil exposed to elevated CO<sub>2</sub> in the Giessen FACE study

“…However, the total denitrification (N 2 O + N 2 ) and the ratio of N 2 -to-N 2 O were significantly higher (36 % and 33 %) under elevated CO 2 compared to the ambient treatment. In general the ratio under elevated CO 2 (average 1.358) was similar to the ranges reported by Rolston et al (1976) (0.1-40) but were lower than the ratios (345 and 410) reported by Baggs et al (2003b).Thus, under elevated CO 2 and under non-N limiting conditions most likely a stimulation of the N 2 O reductase occurred while under N limiting conditions the higher N 2 O emissions under elevated CO 2 may be related to a higher nirK/nosZ ratio (nitrite/nitrous oxide reductase) as observed by Regan et al (2011) for this soil. Thus our studies provide indirect evidence that the kinetics of the reductase systems during denitrification in this grassland soils are linked to the enhanced C input in connection to Noxide availability (Dendooven et al, 1994).…”

“…However, the relative rate of N 2 O from denitrification and the N 2 -to-N 2 O ratio changed under elevated CO 2 . Thus, elevated CO 2 appears to have an impact on the denitrification kinetics in this grassland soil which was also confirmed by molecular studies of this soil (Regan et al, 2011) The enhanced CH 4 oxidation under elevated CO 2 is surprising and shows that the potential for CH 4 oxidation may increase in this soil. However, this effect was not observed in the field suggesting that the combination of N application in combination with the environmental regulators (e.g.…”

“…The higher N 2 -to-N 2 O ratios under elevated CO 2 emphasize the need for the consideration of N 2 measurements in future denitrification studies and showed that despite a non significant response to N 2 O total denitrification may be altered. This might be due to a shift of the denitrifier community under elevated CO 2 which exhibit different of the different reductase dynamics during denitrification (Regan et al, 2011).…”

Effects of soil temperature and moisture on methane uptake and nitrous oxide emissions across three different ecosystem types

“…Morley and Baggs, 2010;Pilegaard et al, 2006) or microbial community dynamics (e.g. Regan et al, 2011) exert a significant control on the temporal dynamic of nitrous oxide fluxes as well. This lack of predictive power of simple relationships between environmental drivers and nitrous oxide fluxes for long-period datasets, spanning at least one year, has been observed for other natural and seminatural systems as well, e.g.…”

Soil N cycling processes in a pasture after the cessation of grazing and CO 2 enrichment

“…We conclude that factors other than soil moisture content had a greater influence on N 2 O emissions at eCO 2 at the soil moisture conditions in the present study. Such factors might include fungal co-dentrification (Kammann et al, 2008) and N 2 O reductase activity (Abbasi and Müller, 2011;Regan et al, 2011). It should be noted that an increased soil moisture content was apparent during the period of active CO 2 enrichment (Ross et al, 2013) as a consequence of CO 2 effects on plant physiology.…”