Wheat leaves emit nitrous oxide during nitrate assimilation

Abstract

Nitrous oxide (N2O) is a key atmospheric greenhouse gas that contributes to global climatic change through radiative warming and depletion of stratospheric ozone. In this report, N2O flux was monitored simultaneously with photosynthetic CO2 and O2 exchanges from intact canopies of 12 wheat seedlings. The rates of N2O-N emitted ranged from <2 pmol⅐m ؊2 ⅐s ؊1 when NH 4 ؉ was the N source, to 25.6 ؎ 1.7 pmol⅐m ؊2 ⅐s ؊1 (mean ؎ SE, n ‫؍‬ 13) when the N source was shifted to NO 3 ؊ . Such fluxes are among the smallest reported for any trace gas emitted by a higher plant. Leaf N2O emissions were correlated with leaf nitrate assimilation activity, as measured by using the assimilation quotient, the ratio of CO2 assimilated to O2 evolved. 15 P lants play a critical role in regulating the chemical and physical state of the atmosphere through the exchange of biogenic greenhouse gases. Most notable are plant-atmosphere exchanges of CO 2 , O 2 , and H 2 O, but leaves also emit a variety of carbon-and nitrogen-based trace gases involved in climate alteration processes (1). One such trace gas is nitrous oxide (N 2 O). Plants-either aerenchymous (2) or nonaerenchymous (3, 4)-can serve as conduits for N 2 O between the soil and atmosphere. They transpire significant quantities of N 2 O when its concentration in the soil solution greatly exceeds the solution equilibrium concentration with ambient N 2 O, currently at Ϸ315 nmol⅐mol Ϫ1 (5).The global N 2 O budget is beset by uncertainty, and sources of N 2 O have historically fallen short of the primary sink, photolysis in the upper atmosphere (6-9). The primary biogenic N 2 O sources are from soils (70%) and involve the microbial nitrogen transformations brought about by nitrification and denitrification (6). Although nitrification and denitrification are the major N 2 O sources, several microbial organisms that do not nitrify or denitrify can also produce N 2 O during NO 3 Ϫ assimilation (10, 11). These observations have led to the general hypothesis that any enzymatic nitrogen transformation through the ϩ2 to ϩ1 oxidation state may generate N 2 O (12). One such transformation in higher plants is NO 2 Ϫ assimilation in chloroplasts. Nitrite assimilation in chloroplasts can generate intermediates capable of reacting to produce N 2 O, including NO 2 Ϫ (as HNO 2 ) with hydroxylamine (13) or reaction of NO released during NO 2 Ϫ reduction (14, 15) with ascorbate (16). Nonetheless, early attempts to observe N 2 O production by higher plant tissues were not successful (10) and were probably limited by lack of an analytical method capable of detecting plant N 2 O emission at the exceptionally slow rates reported here. We developed an analytical approach by using cryogenic trapping (17) and gas chromatography coupled to high-precision isotope ratio mass spectrometry (18). This approach resolved leaf N 2 O emissions at more than six orders of magnitude lower than photosynthetic gas exchanges of CO 2 and O 2 (Table 1), placing such fluxes among the smallest ever reported for any trace gas...

Methodical study of nitrous oxide eddy covariance measurements using quantum cascade laser spectrometery over a Swiss forest

“…We were only able to find four other publications that emphasize the role of N 2 O emissions from plants (Chang et al, 1998, Rusch and Rennenberg, 1998, Smart and Bloom, 2001, and Pihlatie et al, 2005a. Chang et al (1998) postulated that significant amounts of N 2 O may also be emitted via herbaceaous plant transpiration, but also found that watering the soil with an N 2 O rich solution immediately increases N 2 O emissions from the above-ground parts of the plant, which suggests that N 2 O is conveyed to the leaves via the transpiration stream.…”

“…Rusch and Rennenberg (1998) found similar conditions in trees. Smart and Bloom (2001) used 15 N labeled fertilizer to be able to more specifically find out where plant emitted N 2 O is actually produced. In contrast to the previous two studies they found that labeled NH + 4 fertilizer did not increase N 2 O emission significantly, whereas NO − 3 fertilizer did.…”

Evidence for biological nitrification inhibition inBrachiariapastures

“…Nitrous oxide (N 2 O) is one of the three major biogenic greenhouse gases contributing to global warming, produced primarily from denitrification processes in agricultural systems (5,7). Also, assimilation of NO 3 Ϫ by plants can result in further N 2 O emissions directly from plant canopies (8). The low agronomic N-use efficiency (NUE) found in many agricultural systems is largely the result of N losses associated with nitrification (i.e., N losses from NO 3 Ϫ leaching and denitrification) (9)(10)(11).…”

Potential for biological nitrification inhibition to reduce nitrification and N2O emissions in pasture crop–livestock systems

“…1 , results in the direct emission of N 2 O from crop canopies, further reducing nitrogen use efficiency (NUE; Smart and Bloom, 2001). Thus, maintaining soil N in NH 4 1 form is advantageous even after taking into consideration the potential negative effects of rhizosphere acidification from its uptake and assimilation (caused by H 1 excretion).…”