Abstract
AbstractPhotodegradation has been recognized as a significant driver of plant litter decomposition in drylands. Another potential driver is the thermal emission of trace gases that occurs in the absence of solar radiation and microbial activity. Most field assessments documenting photodegradation have employed filters that absorb solar radiation, along with transparent filter controls; faster litter decay under transparent filters is taken as evidence of photodegradation. However, the temperature of litter under transparent filters is often higher, and its faster decay might conceivably stem from greater thermal emission, rather than photodegradation. If true, the growing consensus that photodegradation is a significant driver of litter decay needs rethinking. We assessed the contribution of thermal emission of CO2 and CH4 to the C loss of 12 litter types over a 34‐month photodegradation study in the Sonoran Desert by quantifying thermal emission responses and using field litter temperatures to estimate emissions. Emission of both gases from litter increased exponentially with temperature. Emission of CO2 was much greater than CH4, but their rates were strongly correlated. Concentrations of surface waxes and dissolved organic C in litter were strong predictors of emission of both gases. Emission declined from dried green leaves to naturally senesced litter, and as litter decayed. Diurnal litter temperature averaged 39.8°C under transparent filters over the field experiment and averaged 1.7°C higher than that of litter under filters that absorbed UV through blue solar wavelengths. Through all mechanisms, litter lost an average of 77.8% of its original C under transparent filters and 60.8% under filters that absorbed UV through blue wavelengths. However, thermal emission of these gases accounted for only 0.8% of the original C in litter under transparent filters and 1.0% under filters that absorbed UV through blue wavelengths, corresponding to only 1.2% and 2.0% of the total C lost from litter. While litter temperatures were higher under transparent filters, thermal emission losses from this litter were lower because emission from this litter declined faster with decay. We conclude that thermal abiotic emission was a minor C loss pathway and that photodegradation was responsible for the faster decay of litter in sunlight.
Highlights
Decomposition of plant litter represents a substantial pathway for C flux from land to the atmosphere, but our understanding of the mechanisms driving this process is limited in drylands
A notable exception is drylands, where these models consistently underestimate decay (Adair et al 2008). These shortcomings have led to suggestions that additional drivers of litter decay are at work in drylands. One such driver is photodegradation, which we define as the decay of litter by exposure to solar radiation caused by abiotic photolysis, along with any subsequent effects this may have in accelerating decay, such as through faster degradation by microbes
We found that litter exposed to full sunlight lost on average 1.5 times more mass than litter filtered from UV through blue solar wavelengths, implying that photodegradation is a significant driver of litter decay, consistent with past studies at this site (Day et al 2007, 2015)
Summary
Decomposition of plant litter represents a substantial pathway for C flux from land to the atmosphere, but our understanding of the mechanisms driving this process is limited in drylands. These shortcomings have led to suggestions that additional drivers of litter decay are at work in drylands One such driver is photodegradation, which we define as the decay of litter by exposure to solar radiation caused by abiotic photolysis, along with any subsequent effects this may have in accelerating decay, such as through faster degradation by microbes. Several field studies have demonstrated that exposure to solar radiation accelerates the decay of terrestrial plant litter (reviewed by King et al 2012, Barnes et al 2015, Wang et al 2015) Most of these studies have been conducted in drylands where high solar irradiance and low moisture availability are perceived to favor photodegradation over other mechanisms such as degradation by microbes or leaching. This includes photochemical emission of CO2 (Brandt et al 2009, Rutledge et al 2010, Lee et al 2012), CH4 (Keppler et al 2006, Vigano et al 2008, 2009, Bruhn et al 2009, Lee et al 2012), and CO (Tarr et al 1995, Schade et al 1999, Lee et al 2012)
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