Nitrogen‐regulated effects of free‐air CO₂ enrichment on methane emissions from paddy rice fields

Abstract

Using the free-air CO 2 enrichment (FACE) techniques, we carried out a 3-year monofactorial experiment in temperate paddy rice fields of Japan (1998)(1999)(2000) and a 3-year multifactorial experiment in subtropical paddy rice fields in the Yangtze River delta in China (2001China ( -2003, to investigate the methane (CH 4 ) emissions in response to an elevated atmospheric CO 2 concentration (200 AE 40 mmol mol À1 higher than that in the ambient atmosphere). No significant effect of the elevated CO 2 upon seasonal accumulative CH 4 emissions was observed in the first rice season, but significant stimulatory effects (CH 4 increase ranging from 38% to 188%, with a mean of 88%) were observed in the second and third rice seasons in the fields with or without organic matter addition. The stimulatory effects of the elevated CO 2 upon seasonal accumulative CH 4 emissions were negatively correlated with the addition rates of decomposable organic carbon (Po0.05), but positively with the rates of nitrogen fertilizers applied in either the current rice season (Po0.05) or the whole year (Po0.01). Six mechanisms were proposed to explain collectively the observations. Soil nitrogen availability was identified as an important regulator. The effect of soil nitrogen availability on the observed relation between elevated CO 2 and CH 4 emission can be explained by (a) modifying the C/N ratio of the plant residues formed in the previous growing season(s); (b) changing the inhibitory effect of high C/N ratio on plant residue decomposition in the current growing season; and (c) altering the stimulatory effects of CO 2 enrichment upon plant growth, as well as nitrogen uptake in the current growing season. This study implies that the concurrent enrichment of reactive nitrogen in the global ecosystems may accelerate the increase of atmospheric methane by initiating a stimulatory effect of the ongoing dramatic atmospheric CO 2 enrichment upon methane emissions from nitrogen-poor paddy rice ecosystems and further amplifying the existing stimulatory effect in nitrogen-rich paddy rice ecosystems.

Influences of free-air CO2 enrichment (FACE), nitrogen fertilizer and crop residue incorporation on CH4 emissions from irrigated rice fields

“…A long-term FACE study of temperate paddy rice fields in Japan showed increases (26-51 %) in CH 4 emissions as a result of long-term CO 2 enrichment (Inubushi et al 2003;Tokida et al 2010). Another FACE study of subtropical paddy rice fields that were subjected to a rice-wheat rotation regime in China showed a significant stimulatory effect (53-188 %) on CH 4 emissions without or with a medium addition rate (1,000 kg C ha -1 ) of wheat straw, but there was no significant effect when the conditions included a high addition rate (2,000 kg C ha -1 ) of wheat straw (Zheng et al 2006). The FACE studies that were simultaneously performed at five sites along a transect of oligotrophic peatland mires in Europe demonstrated no significant CO 2 effects on CH 4 emissions (Silvola et al 2003).…”

“…This demand will most likely lead to increased CH 4 emissions if no appropriate mitigation strategies are developed and applied. The increasing atmospheric carbon dioxide (CO 2 ) concentration is hypothesized to amplify the level of CH 4 that is released from not only natural (Dacey et al 1994;Hutchin et al 1995) but also managed wetlands, including irrigated rice fields (Inubushi et al 2003;Tokida et al 2010;Zheng et al 2006;Ziska et al 1998).…”

“…Free-air CO 2 enrichment (FACE) techniques have been developed in the past two decades (Kimball 1992;Okada et al 2001) and have recently been applied to investigate the effects of the atmospheric CO 2 enrichment on the CH 4 emissions from managed (Inubushi et al 2003;Tokida et al 2010;Zheng et al 2006) and natural (Silvola et al 2003) wetland ecosystems. The FACE techniques can facilitate undisturbed field conditions and produce more reliable measurements in comparison with the use of environmentally controlled facilities (Long et al 2006;Okada et al 2001).…”

Application of a process-based biogeochemistry model, DNDC-Rice, to a rice field under free-air CO2 enrichment (FACE)

“…It has been also shown that increased [CO 2 ] can significantly enhance CH 4 emission from rice fields, presumably by increasing the C supply from rice roots to the soil, plant conductance of CH 4 (increased transport to the atmosphere), and other factors (Ziska et al, 1998;Allen et al, 2003;Inubushi et al, 2003;Cheng et al, 2006;Zheng et al, 2006). Therefore, there is strong concern that increasing [CO 2 ] will also stimulate global CH 4 emission from rice fields, creating a positive-feedback effect on global warming.…”

How does elevated ozone reduce methane emissions from peatlands?

“…A positive link between soil inorganic N availability and methanogenic activity is plausible, and CH 4 emission in paddy soils has been positively correlated to levels of soil mineralisable N (e.g. Zheng et al, 2006). However, there may be important differences between these heavily fertilised systems and natural wetlands.…”