Photodegradation leads to increased carbon dioxide losses from terrestrial organic matter

Abstract

CO 2 production in terrestrial ecosystems is generally assumed to be solely biologically driven while the role of abiotic processes has been largely overlooked. In addition to microbial decomposition, photodegradation -the direct breakdown of organic matter (OM) by solar irradiance -has been found to contribute to litter mass loss in dry ecosystems. Previous small-scale studies have shown that litter degradation by irradiance is accompanied by emissions of CO 2 . However, the contribution of photodegradation to total CO 2 losses at ecosystems scales is unknown. This study determined the proportion of the total CO 2 losses caused by photodegradation in two ecosystems: a bare peatland in New Zealand and a seasonally dry grassland in California. The direct effect of solar irradiance on CO 2 production was examined by comparing daytime CO 2 fluxes measured using eddy covariance (EC) systems with simultaneous measurements made using an opaque chamber and the soil CO 2 gradient technique, and with nighttime EC measurements under the same soil temperature and moisture conditions. In addition, a transparent chamber was used to directly measure CO 2 fluxes from OM caused by solar irradiance. Photodegradation contributed 19% of the annual CO 2 flux from the peatland and almost 60% of the dry season CO 2 flux from the grassland, and up to 62% and 92% of the summer mid-day CO 2 fluxes, respectively. Our results suggest that photodegradation may be important in a wide range of ecosystems with exposed OM. Furthermore, the practice of partitioning daytime ecosystem CO 2 exchange into its gross components by assuming that total daytime CO 2 losses can be approximated using estimates of biological respiration alone may be in error. To obtain robust estimates of global ecosystematmosphere carbon transfers, the contribution of photodegradation to OM decomposition must be quantified for other ecosystems and the results incorporated into coupled carbon-climate models.

The Effects of UV-A on Dry Rice Straw Decomposition under Controlled Laboratory Conditions

“…UV light accelerates lignocellulosic material decomposition (Rutledge et al 2010). A dramatic increase of more than 20% straw mass loss occurs after UV-B irradiation for up to 1.8 years.…”

Photodegradation alleviates the lignin bottleneck for carbon turnover in terrestrial ecosystems

“…Moreover, exposure to sunlight stimulated biological degradation of biorefractory compounds (6,8) and enhanced the production of dissolved inorganic carbon in water (9) and of gaseous carbon monoxide (CO) (10,11). In terrestrial ecosystems, accumulating evidence has demonstrated the importance of photochemical mineralization of organic carbon compounds (photodegradation) as a control on organic matter turnover in arid and semiarid ecosystems (12)(13)(14)(15). Losses of volatile carbon compounds, including CO 2 , CO, and methane (CH 4 ), from recently senesced plant material exposed to natural or artificial radiation have been documented (16)(17)(18) and have been linked to photochemical degradation of cellulose (16) and partial or complete degradation of lignin (19)(20)(21).…”

The role of photo- and thermal degradation for CO<sub>2</sub> and CO fluxes in an arid ecosystem

“…Photodegradation is known to be an important pathway in aquatic ecosystems (Zepp et al, 1998). Recently, the possible importance of photodegradation in terrestrial ecosystems has been suggested (Austin and Vivanco, 2006;Brandt et al, 2010;Friedlingstein et al, 2006;Rutledge et al, 2010). Photodegradation can play an important role in arid ecosystems, where microbial decomposition is restricted (Austin and Vivanco, 2006;Brandt et al, 2010;Lee et al, 2012;Lin and King, 2014;Throop and Archer, 2009).…”

“…In recent studies, the possible importance of abiotic degradation for arid regions, such as photo-and thermal degradation, has been recognized (Austin and Vivanco, 2006;King et al, 2012;Rutledge et al, 2010).…”